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This question about how not to make anything touch the outer wall of the tent implies that doing so lets the outer tent leak water through. I never head this problem with my vango mirage tent during a recent trip in heavy rain and wind even though my bag touched the outer tent and the wind pushed the outer tent onto the lower parts of the inner tent on the sides. Still from scouting I know that with this Spatz tent the outer tent will leak when touched from the inside.

What materials/surface treatments of tent outer walls do/don't leak water when touched from the inside?

As a bonus but not really outdoor related: What is the reason for one material to (not) leak?

EDIT: The distinction that it is not leaking before touching and leaking after touching is important. It is obvious that when it leaked before and you touch the outer, that the inner gets wet.

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    Growing up in Western Washington, I learned two very important and inviolable rules about camping. 1) It will rain, no matter what the weather forecast said. 2) Waterproof tents... aren't. If someone has somehow managed to produce a tent that truly is, something is very wrong with the world. What's next? Human Sacrifice, dogs and cats living together, mass hysteria? – Mason Wheeler Aug 21 '15 at 19:44
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Snow Crash's and WedaPashi's answers explain well, how water leaks through the fabric by capillary action and how touching the inside increases water flow through the fabric. I will try to answer the first part about why this effect is apparent in some fabrics and not so much in others. I base this on two excellent but sadly deleted answers by Dynadin and Aravona. While naturally by rewriting my own interpretation will impact the result, it is still their work who made it possible.

The question of whether it leaks on contact is mostly related around the difference between modern and older tents. This has to do with the hydrostatic head (HH) of the material. So the higher this rating on a material, the more water it can withstand before becoming permeable. Older tents usually have lower hydrostatic head and in addition, they were often untreated materials. Newer tents have materials whose outer surface is treated. This coating prevents water from getting through by capillary action when touched from inside. Without this coating, the only thing resisting the water going through is a higher HH.

As an example a recent tent (Dynadin's): It has three sections: Outer, inner and ground sheet, which are all made from polyester. The inner has no HH, any water will pass straight through. So if you touch it it, you will get wet quickly. The outer has a 5000 HH. Even when you touch it on the inside while it is completely wet on the outside no water gets through. The ground sheet has a 8000 HH. So even with constant pressure on it there will not pass any water through.

Modern tents made of polyester or nylon are most often treated and have a high HH, so you usually do not need to worry about touching the inside - they still wont leak. Cotton and canvas tents (older) are usually untreated and have a lower HH, so they will leak via capillary action when touched from the inside.

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Physics-wise, this is called Capillary Action. i.e. touching the surface changes the physical characteristics.

Simpler explanation - take a look at water in a glass. Notice the meniscus (the way water seems to rise at the edges)? That's capillary action caused by the water in contact with the glass.

Don't think the hydrostatic head measures this per se.

UPDATE

In answer to questions about how touching changes the structure - e.g.

That conduit is there before and after the touch. It didn't change, but flow did. That's why I would not attribute the flow change to a structure that did not change.

The structure has changed. When you look at the water in a glass it rises at the edge because it's in close proximity to the glass. When your finger touches the inside it is exactly the same as that glass touching the water. The structure now has an additional surface at the end of that (rather short) tunnel/conduit formed by the threads.

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    I think capillary action is why the tent lets water through when you touch it. Whereas the hydrostatic head measures how easily the material gets wet, rather than just having water on the outer surface. – nivag Aug 20 '15 at 15:35
  • I don't see how this would be capillary action, as that requires some kind of narrow conduit, like a capillary. But the structure did not change during the finger touch. The touch would break the surface tension, however. – donjuedo Aug 20 '15 at 21:44
  • @donjuedo the type of tents the OP is talking about are made of fabric. this is made up of interwoven threads. the square space between the parallel threads is the narrow conduit you're asking about. – Snowcrash Aug 21 '15 at 19:44
  • @SnowCrash, Right. That conduit is there before and after the touch. It didn't change, but flow did. That's why I would not attribute the flow change to a structure that did not change. – donjuedo Aug 21 '15 at 19:53
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Yes, it is indeed due to Capillary action. As there are great answers already by Dynadin and Aravona and Snow Crash, I would strictly limit the scope of my answer to why it happens and how, just to clarify how the Capillary Action comes in place.

The basic property of water molecules is staying together, we refer that as Cohesion, and those molecules also tend to stick to whatever that comes in contact, thats what we call Adhesion. The Physics defines Capillary action as the movement of water molecules (for layman, water as whole) within the spaces of a porous material or surface due to adhesion, cohesion, and surface tension. Cohesive property of water is what causes to inhibit Surface Tension which is a property inhibited by liquids to acquire surface area as less as possible.

Best example to explain this would be half a glass water on a hard flat surface and you wiping it with tissue paper. Tissue paper is fibrous and hence porous.

As Dynadin explained (quoting from his answer)

In older tents the hydrostatic head was usually quite low. In addition they were quite often base untreated materials, cotton, nylon, etc. When touched there was nothing to stop capillary action from happening so the inside of the material became wet.

which clearly explains why water drips in through. The only assertion that I think everybody missed is Surface Tension that comes in picture along with the Sorptivity of the tent fabric. The technical term (when specifically considered with Capillary Action) to what we are vaguely referring as permeability or amount of leaking that can take place is Sorptivity, which wikipedia defines as:

measure of the capacity of the medium to absorb liquid by Capillarity

Mathematically (and in the harder way to look at it),

V = AS(t1/2)

where, V is volume of the water intake, A is the surface area subjected to water, S is Sorptivity of the tent surface and t is the time for which you have pitched the tent while something is touching the wall.

One question: Have you ever bought a cheap rain-jacket? Gets wet at the elbows first than belly.

Try this, take a cloth which is not really rainproof. Put it onto a vessel as a filter. Pour water quickly, faster and in more volume than what cloth can filter and drip below. You should see water being gathered, slowly filtered and then collected in the vessel below. Now, somehow, put your hand in the vessel and poke the filter-cloth from inside, water shall be collected faster than before. End of play, throw the water away to the plants.

So, What Capillary Action has got to do with something touching the tent from inside?
It has all got to do with the Porosity and the Sorptivity of the surface and the Surface Tension of the tent fabric.

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