As you try to understand the performance of outdoor gear, you’ll soon notice it’s hard to wade through the thick layer of marketing that gets applied. Naturally every company wants you to think their materials are great and without flaw. That results in grand but vague descriptions (e.g. “super light”, “stormproof”, “bomber”). Advantages get touted (e.g. DCF having a 8000 mm HH) while weaknesses go unmentioned (e.g. DCF forms microcracks and pinholes quite easily and thus is one of the first materials to leak).
That’s understandable – but problematic if you’re really trying to find the best gear and materials. The only way around this is to actually understand the materials, so here we’ll take a deep dive into fabrics and coatings as it pertains to tents.
X-Mid fabric under the microscope
In particular, the X-Mid tents have been a pioneer in the use of polyester in lightweight tents. Since they 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 polyester is getting more popular and looks to be taking over with companies like Black Diamond, Six Moon Designs, MEC, Yama, Lightheart Gear etc recently making the switch. But it’s still worth taking the time to explain why polyester is so much better and explain the rest of the materials choices we use in the X-Mid. As you might expect, all the materials choices in the X-Mid are reasoned from first principles to deliver the highest all-around performance. That’s not to say there aren’t compromises though – the most durable fabric will never be the lightest.
20D Sil/PEU Polyester
The X-Mid uses a 20 denier polyester fabric for the fly and floor, which is sil/PEU coated. 20 denier (or 20D) refers to the thread weight (in grams per 9km), where lightweight tents range from 7-30D. At 20D, the X-Mid 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 responsibly. We think 20D fabric is a nice well rounded choice because it is a durable enough to use without a groundsheet if you use proper care (e.g. checking camp spots for sharp sticks and rocks) where as 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 we’ve never had a failure with it.
Polyester vs Nylon
The most contentious fabric topics these days is polyester, or more specifically polyester vs nylon. There has been an unfortunate amount of misinformation about polyester with otherwise reputable tent companies making unfortunate statements about it.
For example, one competitor writes:
These claims are misleading (nylon being “ounce for ounce tougher”) or false (nylon being smaller packing, poly being “a lot cheaper“), and this paragraph omits all the key benefits of polyester (no-sag, fast dry, minimal water weight gain, and superior UV resistance).
The truth is that nylon comes in a variety of formulations, where the best possible nylon (“nylon 6,6”) only has a slight edge in strength over modern polyester (there are new polyester formulations in the lab that are much stronger yet, but not commercially available). Instead of believing a tent company with a vested interest in promoting one type of fabric, we can look to DuPont who actually design and produce these fibers. In their documents here, they provide the strength of polyester vs. nylon 6,6 and give that 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% advantage are fairly common). So let’s grant nylon 6,6 a 15% advantage. That still leaves it ounce for ounce inferior in the field for two reasons. First, is that it 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 it’s weight in water pretty easily), slow to dry (since the water is in the fibers), and weaker by about 10% (since the swelling process stretches the molecular bonds). Most noticeably though, as the material expands by 2-4% it ends up with several inches of slack over the arch of the tent, which is why most tents look limp after rain with the fly stuck to the inner tent. All of those are pretty serious issues for a tent fabric. Focusing on strength vs weight though, this means that any advantage nylon has in dry conditions will disappear in wet/stormy conditions (where you most need that strength) because it gives up 10% strength while potentially doubling in weight. The next day you’re carrying around a heavy shelter for no strength advantage. You can read more about those hydrophilic chemical properties here. 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 a nice taut pitch. Outstanding qualities for a tent fabric. Even if nylon had a sustained 15% strength advantage, the no-sag and fast dry of poly would be worth it.
The second big weakness of nylon is UV degradation. Nylon is highly susceptible to UV degradation, where even if you break camp early and set up late, your tent will degrade in strength over a few years. A new nylon tent can lose 10% strength in a weekend of pure sun, or in roughly a season of regular use if you take it down in the daytime. Hence why 5-10 year old nylon tents are commonly under 50% tear strength. This was less of an issue for older, thicker nylon because UV can only penetrate so deep, but ultralight nylon fabrics of 7-30D are very thin where UV can penetrate a larger % of the way through the material and thus cause proportionately higher strength losses. Thus the naturally UV resistant polyester is particularly well suited to very light materials where it enjoys a much longer lifespan. The initial strength of nylon might get the marketing hype, but what actually matters is how strong a tent is throughout it’s lifespan. To conclude, polyester offers no sag performance, fast dry, and good strength throughout it’s life, while nylon offers an initial 15% strength advantage that disappears in a few months or if it rains, and then leaves you with a saggy, slow drying, heavy tent that is steadily getting weaker.
Why then do some companies say nylon is far stronger, with some claims up to 100%? Partially they aren’t considering these other factors but mostly it’s because they are conflating differences between coatings (silicone vs PU vs PEU) with fibers. Silnylon will be much stronger than a PU poly, but mostly because of the sil vs. PU difference and not the fibers (see below for discussion on coatings). That’s why silpoly is also much stronger than PU nylon. Coatings matter far more for strength than fiber type.
All of this discussion on tear strength ignores that there are many other aspects of fabric durability, such as abrasion resistance and puncture resistance. You don’t hear about these because (1) they aren’t as dramatic and (2) no one thinks nylon has an advantage here. Abrasion resistance is very important for a tent floor – moreso than outright strength – and poly is at least equal to nylon here with most material science finding that polyester is more abrasion resistant for the weight. But all the focus is on tear strength because the fear created from the mental picture of a tent being ripped apart by a storm is a powerful way to sell tents.
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. Then with the movement to lighter tents in the 80’s backpacking tents did switch to nylon but simply because only nylon was available in lighter versions (due to demand from other industries with more sway than the small tent industry). Meanwhile poly remained only available in heavy weight versions and thus relegated to use only in heavier mountaineering and car camping tents. In the last 5 years that has changed with lightweight poly now commercially available (20D poly is now widely available at affordable pricing, while 15D poly is available but fairly expensive). Thus, lightweight tents are switching to polyester with many brands making the switch each year.
Lastly, what about the claims that polyester is cheap? It would be great if the superior tent material was also lower cost, but unfortunately that is just misinformation meant to make poly look bad. Lighter fabrics use smaller threads (e.g. 20D) so they are higher threadcount – which makes them more expensive to weave. Since historically only heavy/low threadcount poly was available, it was relatively affordable. Now that high threadcount poly is available, it costs about the same as high threadcount nylon. Browse the fabric selections at a fabric retailer 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 pretty problematic stuff. It dramatically weakens the fabric, and likes to absorb water which causes it to break down – especially when stored like that. That results in in delamination, mildew and a leaky tent. Even if you think you dried your tent, there is probably some moisture inside because both the coating and fibers (if it’s nylon) like to hold moisture inside so it appears dry when it’s not.
A much better coating is silicone. It’s also solidly waterproof (when applied heavily enough) while actually strengthening the fabric instead of weakening it, plus being largely immune to degradation other than physically being abraded away. It’s way better than traditional PU, but still has a couple 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.
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 traditional 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 now switching and older PU formulations becoming obsolete on premium tents.
With the X-Mid, we use dual coatings where the main coating is a heavy coat of silicone on the outside for high waterproofness and additional strength, and then we also apply a thinner coat of PEU on the inside so we can seam tape it for you and to make the floor non-slippery. Some companies will tell you that using only silicone is stronger yet and theoretically it is, but the key 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 even stronger yet (slightly) if that second coat was sil too but the difference is small and brings several advantages (seam taping, non-slip floors). Compared to single coated silicone poly (“silpoly”), we are paying more for the additional inner PEU coating, so we can then pay more for seam taping because that gives you a better product that is ready to go, whereas user seam sealing is a hassle for you and doesn’t work as well.
All of that ignores the most important 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 basically 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. Virtually all fabrics are waterproof enough when new but then degrade where a high initial HH might not translate to one down the road. DCF for example starts out very high (8,000mm) but quickly forms microcracks and pinholes under stress so it can soon be at 500 mm. Similarly, traditional PU would be applied very heavily to get 5000mm or even 10,000mm, but that’s because it degrades so much that it leaks – causing customers to want even higher initial HH (which lowers tear strength) when actually they just need less degradation. Sil/PU and PEU coatings can also be very high, but if they are thin coatings or not well impregnated into the fabric they will wear down quickly. All coatings degrade a surprising amount (but PU and DCF are the worst), were 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, in comparison to 4 other fabrics from popular competitors. We haven’t cherry picked which competitors to include – rather we submitted our fabric to an independent lab for testing whom had recently tested these other fabrics on their own accorded, and provided the results to us for context. The blue and grey fabrics in particular are used in by two popular lightweight tent companies for almost their entire product lines.
You’ll notice all but one of these fabrics starts out at 3500mm – 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 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 3 months of continuous high winds and heavy rain). That’s what matters. Like tear strength, it’s not about boasting the highest new spec but rather avoiding dropping too low later in the lifespan of the tent. For HH, the key thing here is how long a fabric can maintain ratings comfortably above 600mm. Here we see the X-Mid fabric is on track to do that for longer than any of these competitors.
In more detail, the blue fabric is from a competitor who claims it at 5,000mm and writes that it is “highly waterproof” and “will not degrade in it’s first year” but it actually starts out about 1,200mm and begins to decline right away so it reaches 720mm by the end, which is barely sufficient. The yellow and orange fabrics are from another company and spec’d at 2000mm and 4000mm respectively. Those ratings are fair since they do start off quite high, and yet both drop rapidly after a certain point which shows you how uninformative these new fabric specs are. Even their 4000mm one is less waterproof than the X-Mid by 16,200 cycles. Likely their coatings are thin or not that well impregnated, so once the coating starts to wear through the rating rapidly drops. The yellow one is questionable by the end while the orange one is still waterproof but on a worrisomely steep decline.
The grey fabric is the main fabric of a large competitor who uses it throughout their line and it shows good performance with the second highest end result and a moderately steep decline indicating it’ll still be waterproof for a long time. Of all these fabrics though, 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 600mm. We rate this fabric at 2,000mm as a conservative number that reflects the mid-lifespan results moreso than the new fabric, but note that it is actually more waterproof in the long run than these competing fabrics rated at up to 5,000mm. That’s really what you need to know if you’re concerned about reliable waterproofing.