As you try to understand outdoor gear’s performance, you’ll notice it’s hard to wade through all the grandiose marketing. Naturally, every company wants you to think their materials are excellent and without flaw, but that results in grand but vague descriptions (e.g., “super light,” “stormproof,” “bomber”). Advantages get touted while weaknesses go unmentioned.
That’s understandable – but problematic if you’re really trying to find the best gear and materials. The only way around this is to take a deep dive into the materials to understand the trade-offs involved. Here we’ll do just that for tent fabrics and coatings.
X-Mid fabric under the microscope
In particular, the X-Mid tents have been a pioneer and advocate for the use of polyester in lightweight tents. Since the X-Mids are the best known and likely best selling polyester tents, they’ve had to swim upstream against a lot of misinformation on the topic. Thankfully that has died down with polyester now getting more popular and many companies like Black Diamond, Six Moon Designs, Mountain Laurel Designs, TrekkerTent, TarpTent, MEC, Yama Mountain Gear, Lightheart Gear, and others now making the switch. But it’s still worth taking the time to explain why polyester is better and also explain the rest of the materials choices we use in the X-Mid. As you might expect, all the X-Mid materials choices are reasoned from first principles to deliver the highest all-around performance. That’s not to say there aren’t compromises, though – the lightest fabric will never be the most durable.
20D Sil/PEU Polyester
The X-Mid uses a 20 denier polyester fabric for the fly and floor, which is sil/PEU coated. Denier (or “D”) refers to the thread weight (in grams per 9km), where lightweight tents range from 7-30D. At 20D, the X-Mid fabric is a bit more durable than most lightweight tents from mainstream companies (e.g., Big Agnes, MSR), which are typically 10-15D, but it’s still a light fabric and needs to be treated with some care. We think 20D fabric is a well-rounded choice because it is durable enough to use on most surfaces without a groundsheet with proper care (e.g., checking camp spots for sharp sticks and rocks). In contrast, lighter floors such as 10D commonly require a groundsheet and thus ends up just as heavy and more complicated when you add that. 30D is a good choice too, but we can save some weight with 20D, and it holds up well.
Polyester vs Nylon
The most contentious fabric topic these days is polyester or, more specifically, polyester vs. nylon. This topic is a bit of a red herring because there are so many variables that go into a fabric that are similar or more impactful than the fiber type (e.g. weave type, threadcount, calendaring, how heat is applied, coating thickness, coating type etc) such that arguing over the fiber type while ignoring all of that is looking at a very narrow slice of the big picture (i.e. either of these fibers could perform much better or worse than you might expect if you are only considering the fiber type). Nonetheless, we will dig into the main differences.
Lightweight polyester is relatively new and has had an unfortunate amount of misinformation written about it by otherwise reputable tent companies who have dismissed it too quickly. For example, one competitor writes:
What is the difference between Nylon and Polyester?
"These are both synthetic fabrics, and polyester does have some subtle advantages in UV performance but Nylon is ounce for ounce tougher. Therefore a shelter made of Nylon can be lighter than one made of Polyester. Nylon is also more supple and packs smaller. Polyester is usually a lot cheaper and more common in lower end products."
That makes nylon sound pretty good, but these points are either false (e.g. nylon being smaller packing, polyester being cheaper) or an over simplification (nylon being “ounce for ounce tougher”). More problematically, this paragraph is very one sided as it skips all the main advantages of polyester: no-sag in wet conditions, fast dry, and minimal water weight gain and instead tries to make it look bad by falsely associating it with low end products.
The truth is that nylon comes in a variety of formulations, where the best possible nylon (“nylon 6,6”) does have an edge in strength over modern polyester, but it is modest. Instead of believing a tent company with a vested interest in promoting one type of fabric, we can look to DuPont who actually produce all of these fibers. In their documents here, they provide the strength of polyester vs. nylon 6,6 and give the latter a 1% advantage in ounce for ounce tensile strength (circled below):
Of course, there is more to fabrics than simply fibers, and other sources do give nylon 6,6 fabrics a larger edge (reports of 10-15% are fairly common). So let’s grant nylon 6,6 a 15% advantage (which is far from the “2x stronger” marketing claims you might run into). That still can leave nylon no stronger in the field because nylon is a “hydrophilic” (water-loving) molecule, so when you camp in wet conditions, it absorbs water and swells up. That makes it heavy (it can gain 100% of its weight in water), slow to dry (since the water is in the fibers), and weaker by about 10% (since the swelling process stretches the molecular bonds). Thus, its strength:weight in rainy or even humid conditions is lower than its lab specification. You can read more about those hydrophilic chemical properties here. A related and more noticeable issue is that in wet conditions nylon expands by 2-4%, which is why most tents look limp after rain and with the now loose fly commonly stuck to the inner tent. 2-4% may not sound like much, but it can be 6″ of slack over the arch of a tent which is a major issue for tent fabric.
Water absorption means that any strength advantage nylon has in dry conditions can disappear in wet/stormy conditions (where you most need that strength) because it gives up 10% strength while potentially doubling in weight. When you set up a nylon tent, it might be 10% stronger for the same weight, but once the rain starts, there’s little strength advantage yet you’re carrying around a heavier shelter.
What about poly? It is hydrophobic (repels water) and thus does none of that. In wet stormy conditions, poly remains strong, light, fast-drying, and retains an excellent taut pitch—outstanding qualities for tent fabric. Even if nylon had a sustained 15% strength advantage, the no-sag and fast dry characteristics of poly would be worth it.
To conclude, polyester offers you no sag performance, and fast dry while nylon offers an initial advantage in strength that roughly disappears when it rains, leaving you with a saggy, slow drying, heavy tent that is steadily getting weaker.
The inescapable conclusion here is that nylon is less well suited to tents – so why is it popular? After poly and nylon were invented in world war II, most tents did use polyester. With the movement to lighter tents in the ’80s, backpacking tents switched to nylon simply because only nylon was available in lighter weight versions (due to demand from other industries with more sway than the small tent industry). Meanwhile, poly remained only available in heavyweight versions and thus relegated to use solely in heavier mountaineering and car camping tents. In the last decade that has finally changed with lightweight poly now commercially available. Thus, lightweight tents are switching to polyester, with many brands making the switch each year. It’s a slow process because there are still a lot more nylon fabric options than poly in the 10-30D range but a long list of companies are now using polyester for their ultralight tents.
Lastly, what about the claims that polyester is cheap? This is hardly a good critique since it would be wonderful if the superior tent material was also lower cost, but unfortunately, that is just misinformation meant to make poly look bad. The situation is that lighter fabrics use smaller threads so they have a higher thread count, making them more expensive to weave. Since historically only heavier/low thread count poly was available, it was relatively affordable. Now that lightweight/high thread count poly is available, it costs about the same as high thread count nylon. Browse the fabric selections fabric retailers like RipstopbytheRoll.com and you’ll see the prices are roughly the same.
The traditional way of coating tent fabrics is with a PU (polyester urethane) coating, which is highly problematic stuff. It substantially weakens the material and slowly breaks down from water (seriously). Worse, it absorbs water inside so it feels dry when it’s not (much like nylon). Then you store your tent thinking it’s dry while both the nylon and PU are hiding moisture inside, and then when you pull it out 6 months later you find the coatings have broken down and are delaminating.
A much better coating is silicone. It’s solidly waterproof (when applied heavily enough) while actually strengthening the fabric instead of weakening it, plus being largely immune to degradation. It’s way better than traditional PU but still has a couple of downsides: you can’t seam tape it, and it’s so slick that tent floors become awkwardly slippery if you’re camped on any sort of a slope.
X-Mid coatings are ultra waterproof and durable
More recently, a new formulation of PU has come along called polyether (vs. ester) urethane that doesn’t add as much strength as silicone but doesn’t lose as much as traditional PU either, and it solves both the degradation issues of conventional PU and the downsides of silicone (slippery, can’t seam tape). Confusingly, this coating is still called PU but is also called PE and PEU. It’s hands-down better than older PU, which is why mainstream companies are making the switch. About 80% of premium mainstream tents are now switching, and older PU formulations are becoming obsolete on premium tents.
With the X-Mid, we use dual coatings where the primary coating is a heavy coat of silicone on the outside for high waterproofness and additional strength. Then we also apply a thinner coat of PEU on the inside to add even more waterproofness and so we seam tape it for you while giving a non-slippery floor. Some companies will tell you that using only silicone is stronger yet and theoretically it is, but the critical point here is that we are using a similar amount of silicone as most silpoly but then we also add a second lighter coat of PEU that has little effect on strength. Yes, it would be even stronger yet (slightly) if that second coat was sil too, but the difference is slight and PE brings several advantages (seam taping, non-slip floors). Compared to single coated silpoly, we are paying more for the additional inner PEU coating, so we can then spend more for seam taping because that gives you a better product that is ready to go, whereas user seam sealing is a hassle and doesn’t work as well.
All of that ignores the most crucial coating related topic: is it waterproof? That’s way more important than splitting hairs on tear strength. The industry-standard approach here is to provide an HH specification for the new fabric, which tells you how much water you’d have to pile onto the fabric to get enough pressure to push it through. Unfortunately, these new specifications are almost useless because it ignores wear/degradation which is the main challenge. Virtually all fabrics are waterproof enough when new but then degrade where a high initial HH might not translate to one down the road, so the key question is not how waterproof the coatings are but rather how well they hold up over time.
Similarly, traditional PU also degrades and eventually leaks – causing customers to ask for even higher initial HH like 5,000mm or even 10,000mm. Manufacturers oblige but that just lowers tear strength even more and can still degrade eventually. What campers really need is not higher HH but rather less degradation. Sil/PU and PEU coatings can also wear down quickly if they are not well impregnated into the fabric. All coatings degrade a surprising amount, where the key is to keep them above ~600mm for as long as possible because real-world leaking happens at about 600mm in heavy rain.
Below is a chart showing the hydrostatic head of the X-Mid’s fabric (green) as it is subjected to wear cycles compared to 4 other materials from popular competitors. We haven’t cherry-picked which competitors to include. Instead, we submitted our fabric to an independent lab for testing who had recently tested these other fabrics on their own accord and provided the results to us for context. The blue and grey fabrics, in particular, are used by two major competitors for almost their entire product lines.
You’ll notice all but one of these fabrics start at 3,500 mm – that isn’t the real maximum for these fabrics but just the upper limit of the testing equipment. The key takeaway from this chart is that the X-Mid fabric isn’t as high as some fabrics early on (e.g., 5,400 wear cycles), but because the coatings are deeply impregnated, it holds that waterproofing for longer. By the end (16,200 wear cycles), it is the most waterproof fabric (Note: 16,200 wear cycles roughly corresponds to many months of continuous high winds and heavy rain). That performance at the end is what matters. Like tear strength, the goal isn’t to boast the highest new spec but rather avoiding dropping too low later in the tent’s lifespan, because that’s where failures happen. For HH, the critical thing here is how long a fabric can maintain ratings comfortably above 600 mm.
Here we see the X-Mid fabric is on track to do that for longer than any of these competitors. The grey and orange fabrics perform well too, but the blue and yellow are problematic. Of all these fabrics, the X-Mid fabric shows the most gentle decline because the coatings are so heavily impregnated. At 1,600 mm after 16,200 cycles, it is on track to remain waterproof for the longest time and could likely repeat the entire test again while staying above 600 mm.
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