UV and You: Tent Fabrics and Sun Damage
Tents have it pretty rough. Their purpose is to keep you protected from the elements, so by definition, they’re exposed to nature’s fury so you don’t have to be. Season after season, your trusty tent is subjected to wind, rain, dust, snow, hail, animals, and (sometimes) careless users. But for a tent that has been treated well over the years, the thing that will eventually kill it is something you might not expect: the sun.
In this article we'll go through some fabric basics and the reasoning behind our experimental design before we dive into the results. We discuss which fabrics outperformed expectations and which fabrics fell short. This includes a discussion of polyester vs. nylon and the performance of different fabric coatings. Finally, we'll talk about what this means for the user in practical terms, and how to mitigate the harmful effects of UV on your tent to maximize your tent's lifespan.
LFD on Everest. Overexposed for dramatic effect.
In addition to visible light, the sun emits a massive amount of ultraviolet (UV) radiation, which has a shorter wavelength than visible light and is present wherever there is sunlight. While inorganic materials (like silicone and metal) are largely unaffected by UV radiation, organic compounds (i.e., anything containing carbon, not just your fancy kale) can be damaged by exposure to UV. This includes all organic polymers, whether synthetic (like nylon and polyester) or naturally occurring (like cotton and wool).
Synthetic polymers (e.g., all plastics) are ubiquitous in the tent world. By the time modern geodesic tents hit the scene in the 1970s with the North Face Oval Intention, pretty much all technical lightweight tents have been made of synthetic polymers, usually nylon or polyester. Historically, nylon has been far more popular, with polyester relegated to the realm of low-end price point tents. Recently, however, a new crop of high-end shelters has emerged using higher quality polyester, which has become popular for its ability to retain its shape when wet and its reputation for superior UV resistance. For some background on tent fabrics, check out our tent fabric article here.
Here's a quick terminology explainer to make sense of the abbreviation soup in our fabric names. For example, let's look at our Portal/2Lite/SplitWing/Flat Tarp fly fabric: 10D*450T NY66 RS SIL/SIL.
10D- "D" refers to the denier (pronounced DEN-yer) of the fabric, which is basically the weight of the individual fibers that make up the fabric. Technically, 10D means that a 9000 meter length of one of the individual yarns weighs 10 grams. But for the purposes of this article, it's enough to know that a small denier means a lightweight fabric and a larger denier is a heavier fabric. Larger denier fabrics tend to perform better in UV because not as much UV penetrates the yarns.
450T- "T" is the thread count of the fabric, or the number of fibers in one square inch of the fabric. We didn't isolate this as a variable so it's not particularly relevant in this article.
NY66- "NY" means nylon. "66" means it's type 66 nylon. Type 66 is much less common than type 6 so if it's not specified NY66 then it's assumed to be type 6. If the fabric is polyester, it will say "Poly" here instead of "NY".
RS- Denotes a ripstop fabric. Ripstop fabrics have a grid of larger denier fabrics woven into the fabric to stop tears from propagating through the fabric. That's the square grid you can see in many technical lightweight fabrics. Almost all tent fabrics are ripstops. The alternative to ripstop is taffeta, denoted by "Tf". We only included one taffeta in this test.
SIL/SIL- This describes the fabric coating. Sil/sil means the fabric is coated on both sides with silicone. This is unlike sil/PE or sil/PU fabrics, which are coated on one side with silicone and with polyether urethane (PE, also called PeU) or polyester urethane (PU) on the other. "TiO2" means the fabric coating contains titanium dioxide. More on that later. For a more detailed explanation of the pros and cons of each coating, check out our fabric coatings article.
Synthetic Polymer Chemistry
Now, let’s hold our noses real quick for a bit of chemistry. It’ll be over soon, I promise. Polymers are large molecules (called macromolecules) composed of chains of smaller components called monomers. These monomers are linked together in chains held together with different types of chemical bonds. In nylon, these are called amide bonds, which is why nylon is also known as polyamide. Polyester is similar in that it is made of monomers linked by ester bonds. Poly-amide, poly-ester… get it? I really thought I would never have to think about this again after I ditched pre-med, but I just can’t seem to escape organic chemistry entirely.
UV radiation breaks down the bonds between these monomers, leading to the disruption of the macromolecules. In practice, this means nylon and polyester fabrics become brittle, weak, and unsuitable for use in tents. Fabric left exposed to sunlight for too long starts to look, feel, and tear like tissue paper.
While it has been generally accepted by the outdoor industry that polyester has better UV resistance than nylon, the experimental evidence supporting this comes predominantly from other industries and generally involves much heavier fabrics in a variety of bizarre testing conditions. We haven’t been able to find any studies directly examining the effects of sun exposure on the lightweight fabrics used in outdoor equipment. So in the interest of making informed design decisions, we decided to conduct our own dubiously scientific tests to investigate the effect of UV on tent fabrics. We wanted to know just how big of a role UV plays in a fabric’s lifespan, and what other factors contribute to a fabric’s UV resistance, like weight, coating, and fabric type.
From our experience making tents, we know that UV is the only thing that will inevitably kill a properly cared-for tent (as long as you didn’t buy a tent with PU-coated fabrics), but we wanted to know just how quickly that destruction takes place. Is UV really all that much of a factor in tent lifespan? How much do you have to use a tent for it to get UV-cooked? Is polyester really better than nylon, and if so, by how much?
EXPERIMENTAL DESIGN AND DATA COLLECTION
The primary goal with our experimental design was to find a way to compare fabrics side-by-side in the same conditions, and to reveal broad trends in how fabrics react to UV exposure. These tests are not to any standard, ASTM or otherwise, so we caution against extrapolating these results too broadly. The purpose of this experiment was to compare a variety of fabrics to each other, some field-proven and some new, to inform our fabric selection and design process.
Test apparatus and procedure
To begin, we had to design our testing facility. We wanted our testing to simulate real-world conditions as closely as possible. And since we don’t have access to an ASTM-compliant UV testing facility but we do have a nice roof above our design studio, we decided to put the fabrics outside and let the sun do its thing, since that’s a better simulation of real world tent use anyway. We assembled some industrial racking, oriented southwest for maximum daily sun exposure. We sewed together panels of up to six fabric swatches, all side-by-side and facing the same angle.
Our UV testing setup after the first test strips were removed. From left to right: 20D nylon sil/PE, 20D nylon sil/sil, 40D nylon 66 sil/sil, 70D nylon sil/sil, 300D polyester PE, 10D nylon 66 sil/sil
Prepping samples of the fabrics pictured above
Pulling apart our 40D*260T NY66 RS SIL/SIL
The first thing we noticed when we started tearing fabric apart is that UV damage occurs rapidly to tent fabrics of all weights. After one month of exposure, all the fabrics except one exhibited a measurable drop in tensile strength, with losses ranging from about 5% to a whopping 47%. After 100 days, all the fabrics (besides our three titanium dioxide-coated fabrics, which we’ll discuss later) had dropped below 70% of their original tensile strength, and most of the fabrics lighter than 40D (as well as some of the heavier fabrics) had lost over 50% of their strength. The weathering was visually obvious as well. The colors faded, the whites turned yellow, and there was general discoloration.
Before and after:
Day 0: So fresh, so clean
Day 98: Not so much. Note that the white TiO2-coated fabric in the upper left is relatively unchanged.
INTERPRETING THE DATA
It’s important to note that obviously, not all fabrics start out with the same tensile strength. A 50% reduction in strength may not be a big deal or it may render a fabric totally useless, depending on how strong it was to begin with. To help visualize this, we made two graphs of the same data: one is each fabric’s tensile strength in pounds, and the other is each fabric’s tensile strength as a percentage of its original strength. When we talk about the rate of strength loss, we’re usually referring to the rate at which it loses strength compared to where it started. We think the rate of strength loss is more indicative of how well a fabric stands up to UV exposure.
UV resistance and tensile strength
When we say that a fabric is UV-resistant, we mean that the rate of strength loss is slow compared to its initial strength. For example, after 208 days of UV exposure, a 300D polyester was tearing at around 64lbs. Our 10D nylon was tearing at around 32lbs. However, the 10D started at 72lbs (that’s exceptional for a 10D, by the way), or about a 65% loss compared to its original tensile strength, whereas the 300D started at a whopping 334lbs, meaning it had lost 81% of its strength by the end of the test. Thus, we say the 10D is much more UV-resistant than the 300D because it was closer to its initial strength, even though the 300D was still technically twice as strong by the end of the test.
Of course, we still factor tensile strength into our fabric selection process. One way to increase a tent’s lifespan is to use a fabric that’s initially much stronger than it needs to be, so it will take longer to reach the point where the UV degradation becomes a functional issue. But at a certain point, very UV-resistant fabrics will eclipse less UV-resistant fabrics that started out stronger. For instance, after about 150 days, the 10D nylon that I mentioned above actually surpassed the strength of all three of the 20D fabrics that we tested, as well as a 40D nylon that started out 76% stronger than the 10D. So while the 10D started out with a lower tensile strength, its UV resistance was so much better than the other fabrics that eventually the 10D was the strongest of the five.
Without further ado, here are the results:
WINNERS AND LOSERS
There were a few fabrics that stood out for both good and bad reasons. Several fabrics performed surprisingly well; far better than their weight would suggest, whereas others significantly underperformed expectations. We’ll start with the overachievers.
The biggest surprise of our testing was our 10D*450T NY66 SIL/SIL, which is the flysheet fabric we use in our Portal, 2Lite, SplitWing Tarp, and Flat Tarp. Generally, such lightweight fabrics have relatively poor UV resistance because the sun penetrates them more easily. However, our 10D nylon was exceptionally UV-resistant. It could be because it's a type 66 nylon, but the 40D NY66 that we tested did significantly worse, so it's hard to say exactly why our 10D did so well. In terms of the rate of strength loss, it outperformed all the non-titanium dioxide coated fabrics. In terms of absolute tensile strength, by the end of our test period, it was actually stronger than several of the fabrics that started out with a higher tensile strength by virtue of its slow rate of strength loss.
The other top performers were titanium dioxide (TiO2) coated fabrics, which was not surprising as the UV mitigating qualities of TiO2 are well-established. Titanium Dioxide is a UV blocker commonly used in sunscreens and paints. This stuff is great, and has absolutely unbelievable UV resistance. It can be mixed in with some coatings specifically to increase their UV resistance, which is what we do with our LFD and BFD expedition basecamp domes. The Everest season is 2-3 months, during which the tents are left up continuously between 15k and 20k feet in elevation where the UV index is about 20% higher because of the thinner atmosphere. These are absolutely brutal conditions for fabric. Guides using our domes on Everest report getting 5-8 seasons of use out of our LFD/BFD. Other companies’ non-TiO2 coated domes generally last 1-2 seasons, according to those guides.
The best performing fabric was our ET70 fabric, which is a TiO2-coated 70D nylon. Unfortunately, we’ve had to move on from it because of supplier QC issues. However, we’ve found a great alternative in a 250D TiO2-coated polyester ripstop, which is much stronger by virtue of its being heavier. So while the UV resistance of the TiO2-coated polyester is slightly lower (although we’re working with our new supplier to add more TiO2 to the coating to get it up to the level of ET70, because overkill is underrated), its strength started out so much higher that at the current rate of strength loss, it would take about 460 days of exposure to reach the initial tensile strength of the ET70. That’s four seasons on Everest, even when adjusting for the higher UV index at altitude. Not too shabby. But I digress. Long story short, TiO2 makes a BIG difference. To visualize the effect of the TiO2 coating, compare the graph of the 250D Poly RS TiO2 with the 250D Poly RS without TiO2, which is the exact same fabric without a TiO2 coating. Notice a difference? Thought so.
It wasn’t all fun in the sun, though. There were a few fabrics that on paper should have performed very well but failed to meet expectations. The most egregious was the 300D solution-dyed polyester. 300D is quite heavy by tent standards (which usually translates to better UV resistance), and because solution-dyed fabrics tend to do better in UV*, we were expecting it to be a top performer, especially given polyester’s reputation for UV resistance. It lost a whopping 38% of its tensile strength in the first month of testing. The only fabrics that lost more strength in that time were two lightweight 20D fabrics; a PE-coated nylon and a sil/PE-coated polyester. After 132 days, the 300D poly had lost almost 75% of its initial strength. By contrast, in that same time period, our 10D NY66 only lost about 37% of its initial strength.
*solution-dyed fabrics are dyed early in the production process, before the raw material is spun into yarns. The pigment fully permeates the yarns, making them less penetrable to UV. For the same reason, darker fabrics tend to be more UV-resistant because they're less penetrable to light.
The other biggest flop was the 20D*420T Poly RS sil/PE, which is a popular fabric in some “silpoly” shelters (even though it’s not sil/sil, more on that in our fabric coatings article). It lost almost half its strength in the first month, and after 104 days was down to only 14% of its original tensile strength. It was so bad by this point that we could barely get it into our tear tester intact, and it was easy to pierce with a finger. We call this “the finger test” and when a fabric fails the finger test, it’s totally cooked. It performed comparably to a 20D sil/PE nylon, which I expected to be the least UV-resistant fabric of the test. I didn’t have many good reasons to believe the 20D polyester would do better than the 20D nylon other than what I’ve read in other folks’ marketing materials, but I guess I’m a sucker, and the 20D polyester’s poor performance was therefore more noteworthy to me.
20D polyester sil/PE finger test
20D*420T Poly RS sil/PE at the end of the testing period. Total devastation. Where did the rest of it go? The answer is blowin' in the wind.
POLYESTER VS. NYLON
My expectations for the polyesters were higher than for our nylons because of polyester’s widespread reputation for excellent UV resistance. But if you’ve been paying attention, you may have noticed that two of the three least UV-resistant fabrics were polyester. That may sound like a glaring indictment of all polyester, but there are some important caveats.
Let’s compare our two worst-performing fabrics, in terms of relative strength loss. One was the 20D*420T Poly RS sil/PE I mentioned above, and the other was the 20D*330T NY RS sil/PE floor fabric we use in our lightweight tents. Both started out at basically the same strength and have the same type of coating. Since this polyester is already in use in a variety of other flysheets and tarps, we expected it to fare significantly better than the comparable weight nylon. However, compared to the nylon, the polyester performed slightly worse. The difference wasn’t big enough to say definitively that the nylon is more UV-resistant, but I feel pretty good about saying that there’s no significant increase in UV performance by switching to polyester in this particular case. Either way, I wouldn’t feel comfortable speccing either of these fabrics in a flysheet, given their poor performance in UV. That’s one of the reasons we only use this particular nylon in our floors, tent body reinforcements, and the WebTruss of our CrossBow tent (20D provides a balance of weight and abrasion/puncture resistance and the PE coating gives the fabric excellent seam slippage, which is important for the high tension seams on the WebTruss and reinforcements).
However, we observed several instances of polyesters falling short of comparable (or even lighter) nylons. One example is the non-TiO2 coated 250D Poly RS. With a Titanium Dioxide coating, this fabric outperformed everything except our original ET70. But without the TiO2 coating, it really was nothing special. While it did better than the terrible 300D solution dyed polyester (as it should; it ain’t cheap, even without the TiO2), it was beaten by our 70D nylon sil/sil expedition series flysheet fabric (used in our Indus, ProGuide 3, Arctic 6, and Kahiltna Dome), which is about 40% lighter, and yes, also by our 10D nylon 66 sil/sil flysheet fabric (seriously, that stuff is incredible).
There are limitations to the conclusions we can draw from these results. There is a lot of variation even between fabrics of the same spec; every fabric is an individual, so apples-to-apples testing is very hard, and we’re limited to whichever fabrics we’re able to get our hands on for testing. As I’ll discuss in the next section, the type of coating seems to have an effect on UV resistance, and we didn’t have access to any sil/sil polyesters for testing. Generally, sil/sil fabrics outperformed comparable sil/PE fabrics, and all our polyesters were sil/PE, PU, or PE only. So at this point, I wouldn’t go so far as to assert that nylon is more UV-resistant than polyester as a rule, but what is clear that polyester is not necessarily more UV-resistant than comparable weight nylon, at least at the weights and with the coatings typically found in tents. Even if there is a slight advantage to polyester, there are other factors (primarily coating type and individual variations in fabrics) that overshadowed the difference between polyester and nylon in this test. It's safe to say polyester’s blanket reputation as a UV heavyweight seems to be overhyped.
EFFECTS OF COATINGS ON UV RESISTANCE
As we note in our fabric coatings article, the waterproof coating that’s applied to the fabric has almost as much effect on the properties of the fabric as the fabric itself. This is true for UV resistance as well. Our three lowest-performing fabrics were all sil/PE-coated (two polyesters and a nylon), rather than sil/sil. Since we pretty much only use PE coatings in our floor fabrics, UV isn’t a big factor, but it’s worth noting if you’re shopping for tents elsewhere (though why would you do that?), as most tent companies use sil/PU or sil/PE flysheet coatings.
By far the most significant variable as far as coatings are concerned was the presence or absence of titanium dioxide. The three clear winners in our testing all had TiO2 coatings. This is what we expected, since the TiO2 was added to the coatings specifically to increase their UV resistance. To the best of our knowledge, no lightweight tent fabrics use TiO2 (it adds significant weight and cost to the coating), so for most people this won’t be an option in their tent purchasing decisions.
When deciding on a lightweight backpacking or mountaineering tent, a more likely scenario would be a choice between sil/PE or sil/sil flysheet fabric. While we didn’t test many sil/PE fabrics as our flysheets are all sil/sil anyway (except our LFD and BFD, which use TiO2 with PU for waterproofing) we tested both a sil/sil and a sil/PE 20D nylon and the sil/sil nylon was clearly superior. Our bottom three performers were all sil/PE coated. We’re currently doing more testing to investigate the degree to which the coating formulation affects UV resistance, but the current trend is that sil/sil coatings perform better in UV than PE or PU. If you didn’t skip the chemistry bit at the beginning of this article this should make sense, because silicone is inorganic whereas PU and PE coatings are organic. Since organic compounds are more susceptible to UV damage, PE/PU coatings become brittle and delicate, just like the fabric underneath them, which decreases the tensile strength of the fabric.
NYLON 6 vs. NYLON 66
Generally, we prefer nylon 66 to nylon 6 where feasible because of its lower water absorption and increased abrasion resistance. Also, our anecdotal experience has been that nylon 66 tends to perform better in UV than nylon 6, but the results of this round of testing are inconclusive. We only tested two varieties of nylon 66 in this test, our incredible 10D and a 40D, both with sil/sil coatings. The 10D (have I mentioned how great it is? It’s really great) did unbelievably well, whereas the 40D didn’t do as hot. However, the 40D used a slightly different formulation of silicone coating that our current round of UV testing seems to be indicating is not as resilient as the coating used on the 10D. Even if that is the case, it would mean that the coating composition was a bigger factor than the type of nylon. Based on the results of our first round of testing, it’s hard to say with any confidence if there’s a practical difference in UV resistance between nylon 6 and nylon 66, though we’ll have more results in a few months that will hopefully provide more conclusive data.
So, what does all of this mean for you in practical terms, and how can you mitigate the impact of UV damage on your tent to maximize its lifespan? Leaving these fabrics on the roof of our studio day in and day out is a pretty extreme scenario that (hopefully) is more than most tents will experience in their normal life, but the results can inform both our tent selection, use, and care and storage.
How trip style affects UV exposure
Your backpacking style plays a big role in the amount of UV your tent gets exposed to. If you tend to move camp every day, your tent will only be up during the day for a few hours, and it will be inside your pack when the UV index is highest during the middle of the day. In this scenario, such as during a typical thru-hike, you’ll only get an hour or two of sunlight on your tent a day, and those will be the mildest daylight hours in terms of UV exposure. In this kind of use, a typical 5-month PCT hike (and this is a very rough estimate, don’t @ me) might translate to one month’s worth of our testing (assuming 14 hours of daylight on average during our test and just under 3 hours of daylight tent time per day on a 150 day thru hike), before taking into account the fact that our fabrics were on our roof even during the strongest periods of sunlight. This isn’t enough to fatally compromise any of the fabrics we tested, though it was enough to weaken some of them significantly. Take into account bad weather and tree cover (if you’re an AT hiker, that is) and the fact that the UV index is often several times higher around solar noon than in the hours preceding and following it, and the chances are if you’re thru-hiking and moving camp every day, you’re probably going to break something else on your tent before your flysheet fails the finger test. But take too many zero days or bring your tent on more than one thru-hike, and you’re into UV damage territory, and there’s a better chance that a gust of wind will rip your guy points off.
However, if you tend to hike in to a basecamp and leave your tent up for longer periods of time, especially at altitude, UV becomes much more of an issue. The most extreme example of this is expedition-style mountaineering, where basecamps can be left up for weeks or months at a time. This is why we spec TiO2 fabric on our expedition basecamp domes. But even in less extreme conditions, like backcountry ski trips or even backpacking trips where you camp for extended periods between moving camps, a few weeks of use a year add up quickly when you’re leaving your tent set up all day, and this is where UV becomes a factor for tent fabrics. Assuming you keep your zippers clean and dry out your tent before you put it away (and you bought a tent without any PU coatings, like a SlingFin for example…) you may reach the end of your flysheet’s lifespan before other parts of your tent become worn out.
Maximizing gear lifespan
Mitigating UV exposure isn’t rocket science, and if you’ve made it this far into this article, there’s a good chance that you might be the kind of tent user for whom UV damage might actually be an issue. UV and visible sunlight go hand in hand, so anything you can do to minimize your tent’s exposure to sunlight will prolong its lifespan. If you don’t plan on moving camp, try to find campsites that are shaded, at least during peak UV hours (midday) and even consider moving your tent under tree cover after you get up in the morning. If you use a footprint, you could remove it from under the tent when you’re not inside and spread it over the top to share the load.
A quick field anecdote- mountaineers on extended trips often maintain the loft of their down bags by drying them on top of their tents in the sun every day. After seasons of use, some actually reach the point where the sleeping bag itself falls apart because of this intermittent UV exposure. While prolonging the lifespan of their flysheet is probably not top of mind in this scenario, it’s certainly a helpful byproduct of this practice.
OneUp on Shishapangma in Tibet. This particular tent was used heavily at high elevation in the Himalaya, came back to the US, did a tour of duty at Burning Man, and subsequently met its demise when a chunk of ice fell straight through the UV-cooked flysheet during a ski tour at Crater Lake in Oregon.
Taking down your tent when you’re not moving camp is kind of a bummer, but I guess if you’re really looking for something to do in camp you could try that as well. My personal recommendation would be to buy multiple tents and in so doing reduce the UV exposure to each one. I’m no mathematician, but my rough estimate is that a four tent rotation can reduce UV exposure for each tent by up to 75%. Email me regarding bulk discounts.
Storing your tent to minimize UV exposure
The sneaky UV killer is improper storage. Even window-filtered sunlight has plenty of UV. My parents store their gear in a loft in the garage directly underneath a skylight. I’ve ripped handles off duffel bags, torn stuff sacks, put my butt through fabric chairs, you name it. The sun exposure through the plexiglass window was enough to completely fry even heavy fabrics (nylon, polyester, and cotton) over a couple years of storage. If your living space is cursed with ample natural light (I can’t relate) you’ll have to figure it out. Find a dark closet or use opaque bins for gear storage. Just stick ‘em where the sun don’t shine.
UV exposure is only one of a slew of variables that determine how long your tent will last. Managing your tent’s exposure to UV is just another facet of being a good tent parent. Knowing how you plan to use your tent will help you determine how heavily to weight UV resistance when making buying decisions. That being said, all fabrics are individuals. In our tests, fabric performance was not always consistent with our expectations based on weight, color, etc. Without testing, it’s hard to predict how well a fabric will perform in real-world conditions.
Hopefully, you've found some nuggets of nerdy goodness buried in this article, whether you're just curious, looking for a new tent, or learning how to care for your current one. If there's anything else you want to learn about, check out the rest of our blog or drop us a line to ask us questions!