DIY breathing polyester tent for backpacking

The making of an experimental breathing polyester tent for lightweight backpacking.

Background to the development of my polyester tent

Note: This is a work in progress as I learn from others and test my tent during field use, as Covid19 permits.

On my daily walks, I take an umbrella as a precaution if I think there might be a chance of rain. You know how it goes, most of the time this precaution is enough to stop it from raining. However, it sometimes rains, and my simple, old and cheap umbrella keeps my head and upper body quite dry. There is no sign of water leakage, not even through the many fine lines of holes where the umbrella panels are sewn together.

My ultralight pyramid tents have been made with ripstop nylon woven fabrics that have been impregnated or coated with silicone rubber silnylon from both sides. This is a wonderfully waterproof fabric for light hiking tents. A reasonable expectation with this fabric with both sides coated with hydrophobic silicone rubber is that it would repel water and be quick to dry. As an example of this expectation here is a quote from goodcampingtents (I think it may be in Chinglish, but I am sure you will catch the drift):

Silicon foil is not getting wet, the water does not enter into the fiber, it remains only on the surface of it. This type of material is not deep watering. These foils are flick of water or dry in the Sun in a few minutes. Unlike PU, silicon is hydrophobic, not hydrophilic. This coating is not permeable to water or water vapor. In the case of these tents must be ensured the ventilation through an appropriate aeration and, often, by opening entries

goodcampingtents

Excessve condensation

My extensive, cold climate camping experience with pyramidal bell tents that are made with silnylon, with numerous long nylon guy cords, has been counter to these expectations, apart from the condensation issue with which I agree.

Silnylon wetting can happen without any rain or precipitation, but entirely by condensation of water from the air. The condensation forms predominantly on the inside of the tent and is the most problematic for tent comfort and slowness of drying before packing up. Some condensation also forms on the outside of the tent, so both sides need to be dried.

The major drawback of this wonder fabric is that it is a ‘condensation magnet’ or more properly a condensation trap. The fabric slowly becomes wetter and wetter during cold nights. Then with buffeting winds or heavy raindrops, the condensation from the inside of the tent can be bumped off and can feel like fine rain falling within the tent.

Under freezing conditions, the condensation forms a frost layer and buffeting of wind can give you a kind of snow in the tent. This removes some water for a lighter pack weight, but the fabric is far from dry and it makes for an unpleasant night and damp gear.

Excessive stretching

My silnylon bell tents go slack, apparently as condensation builds up. This makes the internal condensation “rain or snow’ issue worse. For years, I have assumed that the tent stretch was due to the silnylon fabric stretching, but I, like others, was always perplexed about how the hydrophilic nylon fibres could wet out and stretch so much when impregnated or sealed within silicone rubber. While I don’t quite believe it, the apparent stretching of my tents could be due to the stretching of my long and, numerous guy cords that are made of braided nylon (brickies line) without waterproofing coatings. This possibility was pointed out by Mike at TheTentLab Polyester vs Nylon Fabrics — When Wet. This property of the nylon cord is also confirmed by Tim at SlingFin (pers. com.). “I have an alternative polyester cord ordered to eliminate this possible factor.”

For early morning packing up, the silnylon tent canopy can be made ‘less wet’ by vigorous shaking (if the overnight wind has not done it already), but a residual ‘wetness’ remains and it results in carrying extra weight and a backpack unfriendly load. “I usually pack the tent on the outside of my pack and look for a drying opportunity along the trail to progress the drying quickly.”

Tim at SlingFin has this to say about nylon in an article titled Tent Fabrics 101:

Nylon absorbs water, which poses several problems. In addition to weighing more, wet nylon will expand. In the field, this means that a nylon fly will sag when exposed to moisture (from precipitation or high humidity) which translates to late-night trips outside to tension the fly to keep it from sagging onto the tent body. Again, coatings can help mitigate this issue (look for both side silicone coated fabrics instead of PU; more on this in future articles). While polyester also absorbs some water, it absorbs much less than nylon, and will maintain tension better when wet.

Tim from SlingFin

The photo below kindly provided by Tim from SlingFin shows a new tent fly, that is made of both sides coated silnylon, staying taught after 2 hours of heavy rain simulation. So this may mean that the silnylon on my tents is not stretching. “It may be that my nylon guy lines are doing the stretching?”

Polyester as an alternative tent fabric

The nylon wetting-out phenomenon is described in a rather forthright article Polyester vs Nylon Fabrics — When Wet by Mike at TheTentLab where he shows multiple wet tent flys sagging down to rest on the inner tent. He advises that polyester fabrics are largely free of these unwanted characteristics and should be used for ultralight backpacking tents.

All our current fabrics (polyester presumably) are sil/pu with silicone on the outside plus polyurethane on the underside. They repel water really well and they don’t let it through.

Mike at ThetentLab

My excitement of finding Mikes post combined with my past addiction to silnylon (coated on both sides) resulted in me failing to initially realise that the ‘sagging’ tent flys were made from nylon that was either uncoated or coated only with polyurethane (PU on one side). “None were actually coated on both sides with silicone rubber (As used by SlingFin)”. By my assessment, all of Mike’s fabrics would allow water absorption into the nylon fabric, whether it was directly into the nylon or through the PU coating that is hydrophilic and permeable to water vapour. PU water vapour permeation is described in Roger Caffin’s informative post on various waterproofing coatings for fabrics.

While nylon is the predominant fabric of backpacking tents, polyester is the preferred fabric for large tents, because of its minimal moisture stretching, according to the article by The Tent Lab. Mike indicate that backpacking tents would be much more functional if made with waterproofed fabrics, such as polyester with PU on the inside plus more recently silicone rubber on the top side.

Mike also indicated that the water moves through the silicone as vapour (per.com. Mike).

I found this reference about silicone rubber being a liquid water barrier, but not a water vapour barrier Versaperm:

It (silicone rubber) is an excellent barrier against liquids, but many vapours – such as oxygen, water vapour and hydrogen can pass through most types of silicone almost as if it was not there.

Versaperm

Wet silnylon is slow to dry on cold sunless mornings

For an early morning pack up, the weight of my 1,000g silnylon tent for two (or three) can increase by 50% if good drying conditions are not available, as is often the case when you desperately need a tent. Under such circumstances, I shake the tent and slap it to remove some of the surface water, but it seems that some water will stay attached and only a good airing in the sunshine will remove it. “Running the tent stove for a little while works a treat too if I have time.”

Can a wet tent breath out water vapour to reduce condensation significantly?

It has always perplexed me as to why the water vapour inside a tent with 100% humidity within it would be able to significantly permeate water vapour through the tent fabric to the outside of the fabric into a world that is also at 100% humidity and wet with rain or condensation.

More on water vapour permeation

I should understand the permeation process (such as dialysis). I used it in a laboratory for many years to separate biological molecules from most fluids from dairy cows as part of their analysis. For this to work, there must be a concentration gradient where there is a lower concentration of the permeating entity on one side of the membrane (essentially distilled water) than on the other. This difference provides the driving force for the permeation of small molecules through the membrane into the fluid with a much lower concentration. “It always struck me as a miracle that this process could work so well to separate these small molecules from the ‘big junk’ through a sheet of ‘cellophane paper’ and that it could lead to quantitative analysis.”

For water vapour molecules, the gradient is apparently provided by differential vapour pressure on each side of the membrane. However, when the air is saturated with water on both sides of the tent fabric, this pressure differential is mediated by the warmer temperature inside of the tent fabric when compared to the outside temperature. “This explains why a tent stove can so quickly dry a soggy silnylon tent.”

This is described in an interesting article Use of Water Vapour Permeable Fabrics in Tents by Brad Cain. He details complex mathematical models of tent fabric moisture permeation coupled interactively with a heat transfer model. He modelled combinations of temperatures and relative humidities (RH); 15C & 100%RH, 5C & 100%RH and 15C & 50%RH (inside) with ambient conditions at 0C & 100%RH, -10C & 100%RH and -40C & 100%RH (outside); and three diverse fabric water vapour resistances (the inverse of permeabilities).

The above temperatures and RHs were run on the models with fabrics of three different permeabilities. The most permeable was equivalent to Ripstop nylon, 100% nylon, uncoated, 92 gsm, 0.18 mm thick. (This fabric is about twice as heavy as any fabric that I have used for a backpacking tent canopy. The ambient temperatures are considerably lower than I normally experience)

Amongst other things, the modelling predicted the rates of condensation accumulation inside the tent and the rate of transpiration of water vapour from the inside through the fabric to the outside of the tent.

To me, the most concise expression of the condensation management issue is provided in the following chart. The rate of water vapour transpiration from inside the tent to the outside world is expressed as a fraction of the rate of potential condensation water that would form if the tent fabric was totally impermeable. These relationships are shown for three fabrics covering a 15 fold change of vapour resistance (the inverse of permeability).

Note: The value of vapour resistance of one (1) in the above figure is representative of the water vapour permeability of uncoated 92gsm nylon ripstop. This value of one is very low resistance (or high permeability) as it represents the rate of movement of water vapour through a still film of air that is only 1mm thick. I have not been able to find the vapour resistance value for an equivalent polyester (PET) fabric. However, a chart of comparative water vapour permeabilities of various barrier materials from Versaperm shows that polyester (PET) is about half as permeable as nylon (uncoated), so its vapour resistance should be about 2 mm of still air. This indicates that polyester (PET) fabric is very permeable.

Back to the results of the modelling

If I understand the results of the modelling correctly, for 100% RH inside and out, the mystery water vapour permeation driving force is the differences in water vapour pressure on the inside and outside surfaces of the tent skin fabric. When the outside reaches -40C there is little vapour transport as the vapour pressure difference across the skin is very small. Furthermore, when relative humidity is 100% inside and outside the tent (as it often is in my cold climate), then the differential water-vapour pressure is mediated exclusively by temperature differences on the inside and outside of the tent fabric.

In other words, in saturated conditions, inside and out, a higher inside temperature that is above freezing will be needed to get useful permeation of potential condensation water from the inside of the tent to its outside. Even the most permeable fabric will be of little value if these conditions are not met. “

I never experienced -40C ambient temperatures while tent camping. Probably -10C is the typical minimum in Australia). However, I often experience a frozen tent on the inside and outside (As experienced on a recent ski tour and described below. I expect that the temperature would be essentially the same on both surfaces.

When I later discuss the relatively low condensation rates for my polyester tent under such field conditions I am still perplexed by the apparent better handling of the condensation problem by my polyester tents when compared to a silnylon equivalent. “Maybe there is considerable water vapour lost through the tent canopy before the tent finally freezes in the early hours of the morning?”

Back to my experimental polyester tent design

I wished to try a tent fabric that would:

• Be strong enough to survive the rigours of alpine camping,
• Water repelant enough to keep the rain and snow off me,
• Not accumulate condensation on the inside, and
• Be quick to dry

Consequently, my Covid19 lock-down project is radical, quixotic or even stupid to some critics. It will use an unproofed polyester fabric. “That’s right unproofed umbrella fabric to keep me drier!” However, after reading the following words in a blog titled Tents 101 by Bogong Equipment I am a little more heartened:

Tent fly fabric doesn’t need to be extremely waterproof, because it doesn’t have to handle the same amount of pressure that a tent floor does (e.g. you won’t be kneeling on your tent fly) – the tent fly only has to deal with falling rain. What’s more, if tent fly fabric is tensioned, rain just bounces off it. (Think of how rain bounces off even crappy umbrella fabrics with very low waterproof ratings.)……So, it’s not the tent fly fabric, but the tent floor fabric that you should pay most attention to…..

Bogong Equipment

Polyester should also be low-stretch and easy to dry. I hope that its hydrophobic and porous nature will provide high outwards water vapour permeability to reduce condensation. I hope it will shed the rain well as my polyester umbrella does. “I will not know until I try.”

Starting small- scale model silnylon vs breathing polyester tent comparison

I started with two little ~1/4 scale model tipi style tents made from a sample of 15 D silnylon or an old polyester umbrella canopy. They had no floor, door opening or venting so as to provoke condensation formation. I pitched them side by side on my lush moist lawn during winter weather. This was my invitation for making plentiful condensation formation via soil water evaporation and transpiration from the grasses.

Every morning after several consecutive days of cold but humid nights, I observed the expected heavy condensation in the silnylon tent, while at the same time, the inside of the umbrella tent remained dry. Both tents kept natural rainwater out. Additionally, heavy sprinkling with water from a garden hose did not enter either tent.

I even put a big dinner plate of water in the polyester tent and still, there was no condensation inside.

Full-scale breathing polyester tent

The success of the scale model tent comparison meant that it was time to ‘bite the bullet’. I ordered some custom-made polyester umbrella fabric with no waterproofing and with one side coated with aluminium to provide a reflective surface. I settled on putting the silver-reflective coating on the outside surface of the tent. My thoughts were that it would make the tent cooler in the sunshine, but warmer in cold weather, thinking that it would provide considerable UV protection for the polyester.

This polyester is 58.5 gsm (measured by me) which is a little heavier than many silnylons used for tents and tarps these days. However, I was happy with this as the thicker fabric would compensate for the somewhat lower strength of polyester when compared to nylon. “It was also just what was available to me. The tent design meant that the weight of the tent canopy would only be about 1000g. If shared by two or three people the extra weight becomes inconsequential.”

Square tent design

I used the square pyramid tent design that is described in my post; ultralight tent. The bell wall (300mm high) on the current tent was, for the first time, cut as a continuous part of the roof panel, as were the snow skirts (or bug-flaps) at the bottom of the wall. This was instead of sewing these separate components onto the roof and wall panel, as I had done previously with tents of this design.

This meant that all the critical tent panel seams were sewn with continuous flat felled seams from apex to ground level. Also, each seam combined a straight fabric grain and a bias-cut grain, so that any seam stretching was limited and uniform for all seams. I have subsequently found that the seam is more correctly named a Mock Felled Seam according to ISO 4916 stitch number 2.04.06. (ISO seam naming)

Glueing before sewing

While making the tent, I used my experimental glueing with silicone rubber as an alternative to pinning and this was a great success while sewing the large tent panels together with straight grain being joined to a bias grain on every seam. It made the sewing very relaxed and easy. For the first time, I made only one small sewing blunder (catching an unwanted bit of another tent panel in the line of stitches.) for the many meters of seams.

Very few of those ‘machine gun’ lines of blunder holes for the sun to show up from inside the tent on a sunny morning (Shown in the video below). They are ugly reminders of my crap sewing skill, but they never have become a source of rain leakage, even without silicone rubber sealing. Maybe one day I will dare to run the sewing machine (without thread) on the embroidery setting and write the words in needle holes ‘Mothy The Crap Sewer Did This!’

Mothy The Elder

Load-bearing apex hem

The tent apex finishes in a radical load-bearing hem that I now use on all my tents. It is strong, versatile, easy to sew and elegantly covers the area where all the tapering tent panels vanish to a disappearing sort of nothing with no width to form a seam or strength to hold up anything.

In use, the hem and drawcord just close up, under load, around the tent pole or any sized bush pole. If suspended by an apex cord that is tied to a tree branch the hem ring closes up altogether.

Observations so far

The tent has no zipper at this stage, as I am waiting for it to arrive. The canopy weight is about 800g and I have deliberately not silicone sealed the seams yet as I wish to observe the rain repellency with and without such sealing.

The tent has been pitched for many days and nights on my winter wet green and grassy lawn. I wished to impatiently see if it sheds rain well without seam sealing and also what inside condensation forms over cold and damp nights and mornings with the door opening overlapped to make it close.

So far, the rain shedding is excellent as I expected from my umbrella tent experiment.

Similarly, the condensation build-up has been very light and very much less than I would expect in my equivalent silnylon tent under the same conditions. The condensation has been so light that I doubt that it could be shaken off to form ‘tent rain’ under windy conditions. Even if it did shake off it would be barely noticeable and would do minimal harm. “It appears that the polyester tent can transpire moisture at the same rate that the wet grass and soil do.”

Backpack weight and volume

The polyester tent canopy is slightly heavier (~100g) and is significantly bulkier than the silnylon one when stuff packed. However, I found that I could systematically fold all the triangular tent panels over one another and roll up the tent as a tight bundle and it’s volume is the same or slightly less than the stuffed silnylon tent (More details on this later).

If it stays condensation free I will be very happy. This extra weight of the tent will be greatly exceeded by the weight of residual condensation water that I must carry (after a good shaking) when I pack up my silnylon tent on many cold backpacking mornings.

Dry/wet stretching

My first impressions are that there is minimal dry to wet stretch with the polyester (as expected). Any stretch is very much less than I have experienced with equivalent silnylon tents (both with nylon guy cords). This bodes well for simple pitching without fiddling to adjust for weather changes.

My light guy out cords are made of nylon and they will also be replaced when the stretch free polyester cord arrives. “I may even be able to remove my little bungee stretch adjustment devices that I put in each line. That will be a nice simplification as the splicing connections to the bungees corrupt the otherwise tangle-free guy design.”

It is early days but the results look very promising.

More polyester tent performance observations

The tent has been pitched for about two weeks and has survived wintery heavy rain and high winds. It continues to shed rain well and accumulates only slight condensation that dries off quickly with daylight. The photos below show the condition of the tent in the morning.

Still waiting for the zips to arrive before fitting them and testing the tent with a sleepover.

Here is a little video tour of the tent:

Side-by-side comparison of silnylon and breathing polyester tents with door way zippers

After fitting the full-length zipper tent door closure to the polyester tent it was pitched side-by-side with a similar silnylon tent on a wet and soggy lawn at midday. There was no rain for the monitoring period. The difference in the condensation between the two tents was monitored at about 8:30 pm, 10:pm, 11:30 pm and then at 8:00 am the following morning. The silnylon tent had heavy condensation on the inside and light condensation on the outside at every observation. The polyester tent had no significant condensation.

Note: The silnylon tent has a netted vent low down on the bell wall and another hooded vent up near the apex. In contrast, the polyester tent has no such venting (yet).

To make some objective measure of the condensation problem in the silnylon tent, I placed a 200*200mm square of two-ply paper tissue on the inside of the tent and allowed it to soak up the condensation from an equivalent square on the inside of the tent. I measured the weight gain of the tissue and estimated that the condensation water loading was equivalent to 28gsm (grams/square meter). This is about equal to the gsm of the lightest tent fabrics or about half that of more traditional modern lightweight backpacking tent fabrics.

While much of this water could be shaken off at pack-up time, it does present the potential risk of the water that could be shaken off, as tent rain, on the tent occupants during a windy night and early morning in a single skin tent or tarp.

On a particularly bad condensation night, I later recorded values of 28 and 53gsm for the polyester and the silnylon. Using the estimated area of the tent canopy of 9.7 square meter, the 53gsm equates to about 500g. That is a lot of water to create discomfort inside a tent. Any significant reduction of this would be very welcome by most campers.

Field testing of the polyester tent

I did not find the opportunity to sleep test the tent on a rainy night at home. However, our release of Covid19 lockdown made a week-long late spring ski trip possible at last. It was an excellent opportunity to test the tent under harsh, cold, and condensation forming conditions that I would have experienced with my equivalent silnylon tent.

I saw no evidence that the seams or needle holes left by my sewing blunders leaked any rainwater. However, the night before the trip, I hastily applied my DIY silicone rubber seam sealing liquid to the stitches from both outside and inside the tent. The smell of turpentine was detectable when I packed the tent, but the smell dissipated after I pitched the tent in the sunshine and wind before dashing off for a day of spring skiing.

The ambient temperature dropped well below freezing on two nights where permeation should theoretically cease. On the last night, those harsh conditions eventually combined with powerful and gusty alpine winds that eventually blew us home a day earlier than planned.

The tent performed well and was not tested by any rain (That will have to wait for another day.). However, it kept the two of us and our equipment protected from the elements. At no time did the tent pitch need adjustment to keep the canopy taut as my silnylon tent would have needed. “It was my first set and forget pyramid tent.”

Until the inside of the tent froze on two nights, there was minimal condensation that made only a small amount of mist that could be felt on exposed skin when the tent was shaken by wind gusts. “I can assure you there was very little skin exposed!” The mist was not a real problem to us. When the tent froze, the mist changed to fine ice crystals that again were not a real problem.

Condensation drying for packing up

After our last night out with high gusting winds, the tent was still frozen, inside and outside. As the dawn broke, we made a hasty decision to cancel a planned leisurely breakfast by the fire before packing up. Instead, we had a quick hot drink and raisin bread in the tent as we quickly packed up everything inside the tent before pulling the tent down with a little ice still adhering to the bell wall and snow skirts.

Most of that ice flaked of as sheets during the hurried packing up. Otherwise, it was delightfully dry in comparison with what I had experienced with a similar silnylon tent during ‘desperation morning-packups at sparrowfart’. “The tent was so dry that I did not feel the need to take it out to air or dry immediately upon my return home as was my past practice with my silnylon tents.”

I still don’t quite understand why the polyester tent did not accumulate a thicker ice layer when the tent froze inside? The moisture from our breathing would continue. Maybe the flapping and pumping of the tent panels extracted the moist air? Maybe the ground froze so that the vapour pressure from it did not make condensation? Maybe running the stove to make coffee helped, but it was a quick cuppa? Who knows? It was certain that the polyester tent did not stretch, was drier to sleep in and quick to dry in the mornings when compared to my equivalent tent made from silnylon.

Me

Tent packing

For packing, the polyester tent was so dry that it could have been carried within my pack if needed. However, it had an outside stowage place allocated as a contingency in case the expected fast-drying did not eventuate.

Prior to leaving on the trip, I tested stuffing the new tent into the original bag that I made for the silnylon tent (that I always stuff packed). The relative stiffness of the polyester meant that it fitted in easily enough but could not easily be compressed to become as small as the stuffed and compressed silnylon tent in the same bag.

Next, I tied down the apex cord and rapidly folded each of the 8 tent panels neatly on top of one another. I did more folds to make the width of the triangle smaller so that when rolled up it would fit inside the bag. This method was very simple and very suited to packing up in windy conditions. When the roll was formed, I wrapped and tightly bound the roll with the apex cord that I always leave attached. This held it securely together and started the compression process. The pack volume was equivalent or slightly less than that of the stuffed and compressed silnylon tent and was a very suitable shape for stowing in an outside pocket of a pack.

Addendum 1. Summary of ongoing tent testing

In my experience, thorough testing of innovative DIY backpacking devices is a slow process. “How does that old supposedly Russian saying go? One swallow does not a summer make.” Mother nature throws up a variety of combinations of environmental challenges, sometimes rarely, particularly under field conditions.

Deterioration of material performance with age and days of usage and abuse are other factors that can not easily be sped up. It is only by performing well in the worst of combinations of these conditions over considerable time that the testing can be said to be done. However, I think that it will be useful to update the tents performance testing as I encounter these various conditions either by misfortune or deliberately over time. “And no to the ‘nit-pickers’, I will not apologise for not doing backpacking field tests with an adjacent silnylon tent as a control. That will be limited to tests on my lawn at home. I will list the test results below:

September 15-16, 2021 (early spring Bogong High Plains, Australia, altitude 1635m above sea level ) testing during ski touring camping.

Conditions: On the tents maiden trip, it was pitched beside the snow on luxurious wet deep snow grass that had been nourished and rendered flat by aged horse pooh (Brumby’s for the romantics who don’t think that these are introduced environmental vandals.) and topped off by a smattering of chainsaw chips. “Perfection!”

The tent was left pitched for two days and two sub-zero Alpine nights. There was no requirement to adjust the pitch of the tent as would have been the case for my similar silnylon tents. There were two tent occupants. The tent eventually froze inside and outside (when the tent stove was extinguished). “I wore full merino thermals, a down puffer jacket and down pants and maintained a comfortable temperature within my sleeping bag that is rated to -7C (comfort).”

There was light water condensation on the inside that produced a very fine ‘tent mist’ during strong wind gusts. On the 16th, forecasted strong buffeting wind gusts started before midnight and persisted through beyond the morning pack up. The condensation formed ice and similarly a little light ‘tent snow’ fell. The ‘tent mist or snow’ could just be detected on bare skin (on our faces). It was not an issue of discomfort or cause of sleeping bags or equipment getting noticeably wet.

The tent was essentially dry for a very rapid early morning pack upon the last day of the trip before the sun was fully up. Small patches of ice flicked off easily in the wind as the tent was flogged by the strong wind during pack up. Any condensation issues were very much less than I would have expected when using my equivalent silnylon tent under the same conditions. “For the first time, I did not feel like I was carrying tent water home in my winter backpack.”

September 21-22, 2021 (early spring Australia, 153m above sea level), Testing at home .

Conditions: The tent was pitched on my lush green lawn at home with the forecast of persistent rain and there was one tent occupant (me) who slept in the tent for the evening. The soil was saturated with water so that any significant rain produced over-land flow. The 21st was a cold spring night (not freezing), a light breeze and steady light rain fell from about midday and continued throughout the night until morning.

This was the first time that I had seen the polyester tent develop a significant condensation problem. The absence of strong wind meant that the condensation did not fall off as ‘tent rain’, but I am certain that it would have with strong wind or large heavy raindrops. The condensation dried quickly in the morning. Under these conditions, the polyester tent was no better or worse than my silnylon one, except that it did not go saggy and dried much more quickly even when the doorway was left essentially closed.

On the 22nd, the rain had cleared to just a few occasional short showers. The ground inside the tent was saturated and formed mud with every footstep. The tent stayed essentially condensation-free with only a slightly damp feel to the inside of the tent. I doubt that strong wind or heavy raindrops could shake off any ‘tent rain’. The tent stayed comfortably dry and was much drier than I would have expected from my silnylon equivalent.

September -28-30, 2021 (early spring Australia, 153m above sea level), side-by-side testing at home

Conditions: The polyester and silnylon tent were pitched on my lush green lawn that had been freshly mown with the forecast of persistent rain on the way. Just minor showers on the 28th but termites were flying and water birds were flying to the West to greet the rain and indicated that a significant rain event was on the way.

28th. Both tents were slightly wet on the outside and stayed essentially condensation-free inside for the day and the following night.

29th. Only an occasional light shower up until 10:00pm, so I did not do a sleep test in the tent, but slept in my own bed. “It would have to be good rain to do the sleep test!” The rain became steady soon after 10:00 pm and lasted until about 8:00 pm when I inspected the tents. The outsides of the tents were very wet and the insides were essentially condensation free. However, the silnylon tent was very saggy and the polyester tent pitch stayed tight as shown in the photos below. For this testing, I deliberately did not adjust the pitch of both tents after the initial dry pitching ( As I would have done with real bush camping.).

The following photos and video tell the story better than my words can:

Here is a little video compilation of the tents in the rain:

30th. I slept in the polyester tent overnight while the already very wet tent received rain showers as predicted throughout the night. The inside of the tent stayed dry and comfortable and had a very light layer of condensation in the morning. This condensation was considerably lighter than that in the adjacent unoccupied silnylon tent and there was very little sagging as for the silnylon tent.

30th ~2:00 pm. There was a substantial break in the rain and a brief 15 min period of sunshine that substantially dried the silnylon tent at last. The tent had taken up its good pitch once more. “I was pleased with this as it showed that I had not simply botched the original pitching!” I dashed to get the camera to record the transformation (photo below).

As I examined the taut tent more closely, I could see pimples in the silnylon that looked as though they were stretch marks from residual raindrops that had not yet dried. They could have been from a recent short shower, but their scarcity on the sunny North side of the tent indicated that this was not likely.

Mike at TheTentLab has reported on these water pimples ( Polyester vs Nylon Fabrics — When Wet ), but his observations were with nylon that was coated with silicone rubber on one side and PU on the other. My observations are with nylon coated with silicone rubber on both sides.

I finally put a big drop of water onto the drier Northside and within about 5 minutes I could see a deformation or pimple forming around it. Also, I noticed small bulges or raised bumps at the intersection points of the ripstop threads. It will be interesting to see if these change when the tent is bone dry?

Lastly, mother nature sent down a gentle short shower that immediately started to turn the silnylon tent into a sad and sorry-looking thing once more.

If nothing else, these observations show that some form of wet stretching is at play with the silnylon tent. It is reversible and can happen quite quickly even with a light shower of rain or condensation. Even the setting of the sun starts the saggys.

October 6-8, 2021 (early spring Australia, 153m above sea level), side-by-side testing at home

6th. Both tents were dry inside at 9:00 pm and the air temperature was 9C. There was on and off light rain up until 3.00 am when the rain became heavy for an hour before returning intermittent very light rain. At 6:00 am the polyester tent had light condensation while the silnylon was somewhat heavier (3.0 and 7.5 gsm respectively).

7th. The tents were dry at sunset and there were a couple of very light showers overnight. At 8:00 am the tents were very wet on the outside and inside there was significant condensation. Using the tissue paper test (described above), I was rather disappointed to find that the polyester had 28gsm. However, I was somewhat heartened to find that the silnylon had 53gsm. So this means that the polyester tent was still performing much better under bad condensation forming conditions.

General discussion and conclusions

General criticism and investigative philosophy

I have had some negative feedback on this post. The worst was from an irate engineer/scientist who said he was personally offended (by my post). Here is the link if you would like to read Motormouth’s full rant. I don’t know how this could be so? Putting aside the criticism of the misuse of scientific methodology and citations, their argument runs like this: umbrella fabric is not waterproof (I agree 100% and say so above). Then they go on to say that because of this a polyester tent WILL therefore leak rainwater. I quote them verbatim; (After their critique of my poor logic and science…….”Then you end with you accepting that uncoated poly isn’t waterproof and that’s ok? Why would anyone want a tent that they know is going to leak?”

This seems to disregard my empirical evidence (however pathetic) that the polyester tent simply DOES NOT LEAK rainwater when tested on many occasions under very rainy conditions where the fabric was wetted by rain for many hours. Not having a silnylon control tent while evaluating the tent on a real ski trip is no scientific sin. I was there to ski! I could not manufacture rain and it would have ruined the skiing! The observation of a tent not leaking while it is raining does not need a control measure. It either does or it doesn’t leak when it is raining.

However, for the other issues that have less discrete evaluation measures such as condensation, stretching and speed of drying observations. These evaluations do benefit from such controls even if they are very subjective. The critic seems to conveniently disregard the fact that I frequently used a control tent for the test models and the full-scale tent when I did my testing at home. The observations that I report would have been valid and useful to others even if controls were not used.

The reporting of my project is not a paper for a peer-reviewed science journal. “I have finished with that since retiring.” It is a bit of fun about exploring the possibility of making very much better backpacking tents from breathing, un waterproofed, cheap (USD 1.00/M at 1.5m wide) polyester umbrella fabric.

With no apologies, I have used science citations and theory to help to explain my reasoning for making the tent and the observations that I made. That is my lifelong background and that will not change. However, the simple results irrefutably stand without this.

I appreciate analysis, speculations and explanations in quality posts that are written by others. Consequently, I expect that most of my genuinely interested readers will do the same and I also invite questions and logical criticism.

“I had a hunch that was informed by curiosity, many years of backpacking experiences and a background in science. No harm or offence can come from a serial innovator like me giving a polyester tent a crack and openly sharing my findings with others.”

This critic is not alone and others have more pleasantly question why I would bother making a backpacking tent from a fabric that we all know has a very low hydrostatic water head test value. “Yes, I would not dream of making a rain cover for my backpack from it, but a tent is very different!”

I too have a little mean streak as I will enjoy seeing some people eat their words. Probably will not happen! It will more likely be skulking off into silence. That is the nature of believing in dogma, there is not any room to be wrong or achieve real learning.

Mothy The Elder

Ranwater leaking

Despite the above incredulity, the polyester tent showed no evidence of leaking. It may in time start to leak as the fabric ages/degrades (see comment below). However (sorry engineer/science Zealot), my ancient and much-used umbrella, which was second hand to me, is still shedding water perfectly!

Ultraviolet light (UV) resistance

Polyester is intrinsically more UV resistant (Tent Fabrics 101). Also, concerning potential degradation, this polyester fabric does not have other compounds (other than the silver aluminium reflective coating) added to improve its functionality for tent making as silnylon does. This COULD mean that there are fewer components to degrade with time? Also, I chose a silver-coated polyester fabric thinking that this would largely reflect the UV radiation that is the main cause of the degradation of tent fabrics. Only time will tell?

“Yes, I am having an each-way bet by choosing to use silver coated polyester to provide UV protection! Just keeping in touch with my indecisive side.”

Mothy The Elder

I have started some simple accelerated ageing tests and will report them in another post: Ageing test for tent fabrics.

Durable water repellent (DWR)

When I ordered the fabric for the experimental tent I did it with some care as it involved purchasing a rather large amount to comply with MOQs. “There was great potential for a big stuff-up or a possible success!” I went to some effort to ensure that the polyester fabric had no DWR treatment. “My confirmation messages were met with what seemed to me little indignation. It was probably caused by communicating in Chinglish as I have no Chinese language skills!”

The primary reason for requesting no applied DWR compound/chemical application was to prevent potential interference with the silicone rubber bonding that I have found so useful in my tinkering with DIY backpacking gear making. “All that was in vain, as the bonding to my new polyester is useful, but mediocre at best when compared with bonding to silnylon. And the reasons for this will probably be considered in another post.”

A secondary reason for eliminating DWR was to be sure that there was no DWR that could decay, get soiled or otherwise become defective, as you have probably experienced, and as is reported in the DWR treatments link.

The third reason was that it would not allow the potential sceptics to make the argument that an applied DWR treatment was the reason for the initial success and that water leakage would be inevitable in time as the effectiveness of DWR wears off over time, as we all know it does.

The last reason is that the best DWR treatments make breakdown chemicals that are toxic and are persistent in the environment.

I may sound smug after the success of my experiment with polyester tent without DWR. It proved to be very water repellent as well as having many other better qualities when compared to silnylon. The reason for using DWR (as described in the above link) is to increase the hydrophbic properties of fabrics that are designed to be ‘breathable’ or water vapour permeable or have low water vapour resistance.

However, by good luck or intent, my breathing polyester umbrella fabric is made of one of the cheapest and most hydrophobic polymers made by man and as such does not need DWR. I expect it will never loose its hydrophobic property because it is an integral part of the warp and weft of the polymer fibres. It needs no add-on compound to make make it repell water effectively as other inferior breathing fabrics do!

Mothy The Elder

Condensation Issues

Subjectively, the polyester tent collected much less condensation than I would have expected with my equivalent silnylon tents. It has been more comfortable to sleep in. When the tent pitch automatically stays tight, the shaking off of condensation becomes less of a problem. When I did comparative blotting tests with tissue paper, the polyester tent typically had half as much condensation as did the silnylon tent, when there were strong unavoidable condensation conditions.

Wet stretching

Under all conditions, the polyester tent was immune to any wet/stretching issues that were always observed in the silnylon control tent even on dry cold nights. It seems that moisture absorption and possibly lower temperatures (they often go hand in hand while camping) are the likely cause of this silnylon stretching problem. This is despite the common expectation that silicone rubber coating should prevent water entry to the nylon.

I have difficulty reconciling the wet stretching of my silnylon tent with the taught wet silnylon described by SlingFin above. It could be due to different silicone rubber treatments used. There may have been some improvements to the silicone treatment process that have been made since I made my silnylon tent (Est ~8 years ago).

Possible fabric degratation

Alternatively, the wet stretching of the silnylon could be due to simple fabric degradation from the many days of camping use plus days of testing that I have exposed my beloved tent to?

I show weak photographic evidence that supports the idea that aged silnylon forms water distortion (localized stretching) around residual water droplets on otherwise dry looking fabric that has been dried to recover a taut tent shape.

I put a drop of water on a very dry tent surface and the distortion started within about 5 minutes.

Even when apparently dry (after being very wet), I observe what might be ‘water swelling bumps’ at the silnylon ripstop thread intersection points.

At the risk of offending the science zealots, I have observed that the fabric of my silnylon tent is quite different (less hydrophobic) to the same fabric that has been stored, out of sunlight, in the original roll. The exposed fabric is possibly degraded but not to the point of leaking water through. However, it seems to be less hydrophobic. These issues may become the subject of another ‘less than full scientific post’.

Just for fun, I had left samples of my nylon ‘brickies line’ on my washing line and various campsites that I frequent. It was a deliberate but ‘uncontrolled’ test (sorry Zealots) of the degradability of this cheap and versatile cord with regard to use as tent guy cords and other DIY backpacking gear.

The sample that I left on my washing line at home had probably been there for ~8 years exposed to sun rain and frost and wind and largely forgotten about. That sample also had tied to it an experimental silnylon tie out tab (same as my grey fabric as in my ‘control’ tent) that had been formed with DIY silicone rubber glue. I found the cord and the tattered remnants of the tab on the washing line.

The cord looked very tatty but seems to have considerable residual strength. However, the silnylon was still visible but extremely degraded despite being factory coated with silicone rubber on both sides. I expect that it would have disintegrated to oblivion if it had not been backed up and bonded to the ultra-thin film of silicone rubber that was left from my DIY silicone rubber glueing experiment.

Accelerated aging test

Because the duration of exposure to UV light is likely the principal degradation factor for tent fabrics, I think a similar ‘un-scientific washing line test’ might be able to give us an early indication of the polyesters capacity to withstand degradation by the elements relative to silnylon.

It will be interesting to see what happens to my polyester umbrella fabric when it is formally tested in this way. I thought it would be interesting to have test pieces that have the silver side facing outwards as well as ones with the non-silver side exposed to the elements along with some of the fresh original silnylon (accelerated ageing of polyester).

Already, the post title is out of date as I have added silnylon and polyester and nylon cords to the test platform and hydraulic head test samples may soon be added. It is morphing into a post about the relative ageing of tent materials in general.

Polyester as a recyclable polymer

Polyester is a very popular textile polymer and unfortunately is not very sustainable as it is made from petroleum, but nylon is no better. So much polyester, as PET bottles and discarded clothing etc, are disposed of in landfills (along with many other polymers).

The difference for polyester is that its sustainability benefits by it being highly recyclable and contrasts with nylon that is complex and costly to recycle.

I make an exception for those that those who don’t like the inconvenient truth. For the rest of us we face the reality that we live with finite resources and a limited planet into which we can dump our waste streams. These waste streams contain the very elemental resources that we should have used in the first place. So it will be nice to think that we can go backpacking in the wilds and live in a tent that is efficiently made from discarded rubbish.

Possibly, both longevity and recyclability could be more comparative advantages to add to the already long list of benefits of polyester as a tent fabric.

Nylon guy cords are not a major cause of wet tent sag

I speculated earlier that the apparent wet stretching of my silnylon tent could have been compounded by my use of nylon guy lines that may also stretch when wet. Because both my polyester tent and the control silnylon tent have similar guy cords and bungee stretch adjusters, I think I can conclude that my nylon ‘brick-layer line that I use for guy cords is not the fundamental cause of the saggy tent.

I will still replace the nylon with a polyester cord when it arrives so that its possible wet stretching will not be involved. It may even obviate the need for the bungee devices for the polyester tent? On the other hand, these little devices may still be valuable in strong buffeting winds as shock-absorbers to compensate for polyesters lower stretch properties and lower strength when compared to nylon?

Tent drying time for backpacking packup.

My observation of the drying properties of the polyester tent was that it was wonderful. This contrasted with my years of experience with two silnylon tents that were buggers to dry. “Sorry, not very scientific words but I am sure you will get my drift.” I offer no measurements as it all depends on the method, the extent of shaking, rain, mist, sunshine, breeze etc.

The extremely hydrophobic nature of the entire fabric structure combined with its water vapour permeability MAY be the explanation. Possibly it can dry rapidly via both sides at the same time? Not getting so damn soggy wet in the first place might be another explanation. “I am also pleased that I am not a lone voice in the wilderness bemoaning the relatively crappy performance of wet silnylon when I read the forthright article Polyester vs Nylon Fabrics — When Wet by Mike at TheTentLab.”

Is the strength of polyester fabric enough?

To me, the adequacy of the strength of polyester compared to nylon (over time) is the last remaining question. Polyester is known to have less strength than equivalent nylon fabric (Tent Fabric 101). The process of coating nylon with silicone rubber increases the strength and tear resistance. Conversely, coating nylon with polyurethane (PU) reduces its strength (Waterproof Fabric Coating 101). The question still remains, with good tent design and care, and less than extended extreme use at high altitude, how much strength is needed for a very functional backpacking tent?

As a teenage student, I made my first tent from heat-sealed polythene sheet (poly, polyethylene or PE). “The sheet was second hand and probably contaminated, as I later learned, even invisible contamination from the air can result in good looking heat seals that are actually weak. The seams failed during strong winds on the first night out on an extended walk near Mt Howitt. I shared my brother’s tent for the rest of the walk.” Consequently, all my subsequent tents have used a woven tent fabric and none have failed through lack of strength.

The polyester tent design is such that all load-bearing stress points (Tie-outs, seams and apex hem.) are simply sewn in such a way that the sewing threads pass through four layers of canopy fabric many times as ganged stitches. These four layers spread the forces over a large area to share the load with the rest of the tent canopy. The attachment is simple and cheap and has never mechanically failed or leaked over many years [Add a post when ready]. If these stress points continue to hold, then the intrinsic strength of the warp and weft of the thousands of polyester tent panel fibres are the only remaining area of possible weakness.

“If I take a tiny bundle of these polyester firbres when they have frayed from a cut fabric edge (before I started hot cutting), I always cut them off as they are so strong they would cut my finger off if I tried to use brute strength to remove them. They are strong and probably strong enough with good design. Only time and exposure to horrid winds and UV while camping will answer the question.”

Mothy The Elder

[Add the outrageous video of the polyester ribbon supporting my swinging body weight plus a couple of Covid19 kgs]

Contingency for failure

My dependency, in the wilderness, on unproven experimental tents will be considered by some of my critics as risky or even foolish. While I like to have confidence that my tent will withstand any wind, I usually choose to camp in trees for amenity and wind protection. “I also like the little dead bits of trees for stove fuel.” However, I also have the prudence (or lack of arrogance) to have contingency plans B, C and D planned and rehearsed for camping in the worst alpine conditions, if forced to by mother nature. She can be so harsh, uncompromising and unpredictable. These are listed below:

A. Camp in the protection of trees if possible. “It also keeps the glade-skiing close at hand.”

B. Design the tent to easily pitch low without the bell wall as a pure pyramid (less comfort but much less wind loading).

C. Have ultralight deep snow anchors (and connections) for camping on treeless snow plains, particularly when extensive snowmelt may happen.

D. Have a tent that can be easily used as a bothy shelter if all else fails.

The pyramid/bell tent design that I used for the experimental polyester tent has all these features.

The following photo is of a silnylon tent of a similar design (12 sided instead of 4). It was pitched low in expectation of a forecast strong wind. We built a snow wall to protect the tent from that wind, but it came from the opposite direction. The horrendous wind felt very cold, but it was above 0C and unfortunately rapidly melted the snow wall and the tent pegs started to melt out. The tent pole was bending alarmingly and we put a splint on it to make it stronger. We survived until the morning. However, we packed our packs and were ready to drop the tent pole and use the tent as a bothy shelter. We would have used our packs to provide a comfortable breathing space above our faces.

The final word

I suppose a good endorsement of this new experimental polyester tent is that it has become my go-to tent. An even greater endorsement will be that others will willingly choose to share this tent in preference to my previous silnylon ones that they so willingly shared.

Addendum 1

I have continued to use the tent for off snow extended walks and it continues to perform very well with no significant condensation problems and moisture on the outside has been quick to dry off for our early morning pack ups.

Addendum 2

Since preparing this post I have devised a tent design model spreadsheet for hexagonal tents. to be used by multiple people for snow or terrestrial (or both) backpacking camping. When I considered the usable space in a hexagonal tent, I could not help but compare it with the high efficiency of my smaller square pyramid or bell tents. Consequently, I developed a similar tent design spreadsheet model that calculates the dimensions and functional features including sleeping positions of square or rectangular pyramid tents or bell tents from height and width inputs.

The above design model shows that a tent (230cm square and 183cm high) optimally provides walk-in sleeping comfort for 3 or 2 with gear and a tent stove. It shows that increasing the width and height to just 265cm and 163cm can make the tent comfortably sleep 4 with gear or 3 people with gear and a tent stove.

Tim

You may be interested in my latest use of Hot cutting and welding to prepare tent seams for easy sewing.

Other post for pyramid tent fans

Pyramid tent design- A spreadsheet model

Polyester ageing- About as interesting as paint drying

DIY silicone seam sealer- Getting a long pot-life

Hexagonal tent design- A spreadsheet model

Tie out tabs for pyramid tents- Keeping DIY tabs cheap, small, simple, strong and light

A pyramid tent vestibule- Turn a pyramid into a winter palace, using a spreadsheet model

Square pyramid tent pitching- Getting the square pitch right every time