Wandering Daisy wrote: ↑
Sat Sep 08, 2018 1:10 pm
A lot of the newer UL tent floors are not waterproof, per se. The material (silicon impregnated nylon) is waterproof only under a specified pressure...
All fabrics are like that. They are "waterproof" to a certain hydrostatic head. That is, if you put a pool of water on top of the fabric it will leak at some height of water. The pressure that that height of water would create could be generated by another means, like pressing some part of your body against the fabric or from wind-driven rain.
It's a classic trade-off. I have a Hilleberg tent with a heavy duty polyurethane coated floor that's rated (when new) to be waterproof when up to 49 feet of water are on top of it. That's really good. It's equivalent to 21 pounds per square inch. It's also more abrasion resistant than a typical silnylon fabric. But it weighs 2.7 oz/yd2, roughly twice what a typical silnylon fabric weighs. A trade-off.
An elbow pressed against the floor takes up roughly half a square inch. So you'd only need to press around ten pounds with your elbow to reach the limit of waterproofness of one of the better fabrics available. Standard off-the-shelf 1.1oz silnylon fabric has about 1/10 the hydrostatic head so only about one pound of force is needed with your elbow to reach the specified limit of waterproofness. The Copper Spur floor is rated even lower, at 1200mm of water.
Given that you'd think we'd be swimming in water all the time in our tents. But exceeding the limit of waterproofness says nothing about how permeable -- how fast -- the water will actually seep. And tent site selection greatly reduces the problem as well. Condensation is generally a much bigger source of moisture in a tent. Unfortunately, while you could use thick, heavy materials that are a lot more waterproof (e.g. steel) there's only so much you can do about condensation, at least without something like a dehumidifier.
I have a couple of tents with skimpy silnylon floors. Durability is in some ways separate from waterproofness. Silnylon is fairly strong in terms of tear resistance but it can be abraded quite easily. On the other hand, it's easy to repair and re-treat (although not so conveniently on a trip).
Big Agnes isn't up front with the specifications of their proprietary fabrics so it's hard to say how durable the floor fabric is. Sometimes appearances are deceptive. But I wouldn't be surprised if they're pushing it a bit. It is the trend. It probably factored into your decision to buy the tent.