Polyester vs Nylon Fabrics — When Wet

We use 100% polyester fabrics on the MoonLight tents. They cost more than similar nylon fabrics so we better have a DARNED GOOD reason to use them.

We do: it makes the tent almost completely
sag proof.

When it gets wet,
nylon absorbs water and expands 3.5% in length (2% for nylon 6,6 going from fully dry to fully wet). Polyester is unchanged for all practical purposes*. That doesn't sound like much, but if you do the math it's dramatic. Small tents have at least one fabric dimension over 120" long that will shrink and expand 4” from full dry to full wet. Little dome tents measure more like 140" over their tops, so that's 5” of expansion when wet. Big tents, well, the numbers get so large that all big tents have been forced into polyester for quality control reasons some years ago. You can't even make a large nylon tent in China that will set up correctly in the Western US. Take a look, every quality manufacturer of large tents uses polyester for at least their rainflies. 
Another really nice thing about polyester is that since it absorbs less water, it
dries faster AND, of course, it weighs less when it is wet.
I really think that manufacturers that use nylon for floors really should re-evaluate what they’re doing because it’s those tents that look all tippy-toed and scrunched when they’re set up in a dry area. Not to mention that their advertised square footage numbers are about 7% higher than what the customer actually gets…that's kind of alarming (and maybe even illegal, come to think of it).
* Polyester absorbs just a few tenths of a percent water and doesn't seem to grow or shrink at all.
What this all means is that when you pitch a NYLON tent and it looks beautifully setup and taut like this:
Nylon sil/PU rainfly on a prototype MoonLight 4 - DRY

When it rains, it sags and looks like this, causing massive condensation, dripping on the inside, and flapping in winds:
Nylon sil/pu coated rainfly on the same prototype MoonLight 4 (after just 4 1/2 minutes of spraying down with a hose)

the same tent with a polyester rainfly doesn't sag when wet:
A Polyester rainfly before and after (8 min hosing) - boring but good. By the way, the polyester fabric on this particular prototype didn't have an exterior silicone treatment. It was just polyurethane on the underside. All our current fabrics 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.

Talk about stoic!
 Backpackers have put up with this sagging for DECADES. This probably explains a lot to you experienced tent users out there. You knew something like this must be going on. But you dutifully went out to re-tighten everything in the middle of the night so it wouldn't flap and drip. But it wasn't your pitch job coming loose or fabric stretch or the weight of the water on the fly or anything like that – it was just the nylon expanding because it got wet (probably also made worse by nylon guyline cords). Pretty shocking and completely unacceptable for our purposes here: to have the ultimate no fuss, set-it-and-forget-it tent. 
Now that sagging and stretching nylon is outed, it can be understood as a tradeoff that ultralight backpackers must make (in 2016) to have the absolute lightest tents possible. Tent-worthy polyesters currently only go down to 20D fabric weights. Nylon fabrics are available all the way down to 7D.

Why don't other brands use and promote lightweight polyester fabrics?

I’ve been thinking about how to say this exactly right and I think I’ve finally got it:
There are two kinds of brands, ones that "go with the flow" and do what is normal and expected (usually for a good price) and ones that are pushing the marketplace with their voice to champion certain features and benefits (for which they rightfully get a higher price). Right now the ones pushing are selling the lightest tents possible which are therefore made from 7D, 10D and 15D nylon fabrics. The weights of these nylon fabrics are lower than available lightweight polyesters – currently 20D is the lightest for polyester. So the lightest polyester fabrics are in a no-man’s-land marketing-wise. The feature being talked about by brands that push is the lowest possible weight. Polyester fabrics don’t offer that advantage so they’re out. Plus there’s the problem that if a push-brand decided to sell the advantages of polyester, they would be also be saying that their top-end tents were actually not that great.
Meanwhile, go-with-the-flow brands mostly sell tents in the $200 range made out of 68D and 75D polyester fabrics because of the excellent value those fabrics offer. For their marketing and customer, tents made from 20D polyesters are far too difficult and too expensive to sell. You see how this plays out: no 20D polyester for anyone but fanatics (me).

There's one more reason why some manufacturer's stay away from polyester: the specter of color transfer. It's kind of a boogeyman thing – no-one points to a specific reason it happens, only that it could, especially if a dark color is put next to a light one. Yet, despite it's monster-under-the-bed status, ALL the big tents in the camping world use polyester and they seem to do just fine. We've taken the approach that sagging when wet is completely unacceptable and we're willing to manage our fabric suppliers to ensure that the colors don't bleed (and we also have chosen colors that would be hard to even know if there was any bleeding of one onto the other).

Here's the video that the photos above are from. BTW this video shows what about 2-3% expansion does. I didn't start with 100% dry fabric nor finish it 100% soaked (which would have taken longer):

I’m the first to admit that this page might come off a little strident. I mean, how bad could nylon be if people have been using it without complaint for many years? From what I’ve seen the answer, frankly, is “pretty bad.” Take a look below at the photos below which show some regular nylon fabric tents (not 6,6 nylon or sil/pu). I don’t know about you but I’m not putting up with it any more.

Nylon rainfly wet-dry2 small

Nylon rainfly wet-dry1 small

Scenes like this happen everywhere when it rains. It's the nylon…
sagging 1 P1160102 772px