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All self-inflating mats I have seen are self-inflating in the sense that you’re supposed to inflate them yourself. [That is, the mat self-inflates a bit, but mostly you have to breathe the air in.]

Given this, what purpose does the foam inside the mat serve? If there was none, wouldn’t the mat be lighter?

My hypothesis is that the foam prevents the air from moving inside the mat, thus greatly increasing the heat resistance. Is that the case? (Do you have some dependable sources?)

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FYI, blowing into the mat is not advisable as the moisture in your breath causes the foam to decay and then the mat is less able to self-inflate. If you need to boost the self-inflation ability, close the valve about an hour before bed and roll the air into the foot of the mat, then reopen the valve. Repeat once again right before you get ready for bed. –  Pulsehead Feb 5 '13 at 19:43
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My decade old thermarest says this is not a huge problem! –  Loofer Feb 15 '13 at 11:39
    
You shouldn't have to inflate them. We have a range of ones for the family, and as long as you lay them out an hour or so before you intend to lie on them they just require that you close the valve. No blowing required. –  Rory Alsop Oct 3 '13 at 14:02
    
I seldom have the patience :) –  zoul Oct 3 '13 at 15:14
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2 Answers

up vote 9 down vote accepted

I have a self-inflating mat, and I think the foam serves three purposes. In order of importance:

  1. It sets the shape of the mat when inflated. Think of the foam as limiting how far the opposite walls of the mat can be in any one place. If you didn't have this, the mat would become more of a circular tube as it is inflated. For cushioning, the foam does little compared to the air pressure.

  2. It increases the insulation value of the air inside the mat, as you say, since it keeps the air from easily circulating. Note that these mats are warmer than just inflatable mattresses.

  3. It provides for some self-inflation. Actually I think this is merely a mildly useful by-product of having the foam there for the other reasons, but if you're selling these mats you might as well feature this. I find that my mat self-inflates most of the way, but I still have to blow into it for the right pressure. When you blow into it, you get moisture from your breath in there, which can't be good from a standpoint of mold growth and the like. Letting is self-inflate to the extent it can minimizes this.

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I agree in particular with #3. If you prefer a softer mattress, the self-inflation from the foam can be enough by itself. –  Kevin Feb 1 '13 at 15:41
    
@Kevin: It depends on how much you weigh. The way these pads work is that the inflation pressure effectively sets the firmness. You want the pressure such that the point on your body that pokes into the pad the most (usually hip when lying on your side) just touches the ground. Less inflation and it's hitting the presumably hard ground. More inflation and the pad starts approaching hard ground itself. These pads seem to be just thick enough for this to work. –  Olin Lathrop Feb 1 '13 at 22:23
    
The shape of the nylon casing also affects the comfort/firmness and thickness required to get off the ground. For example, I have self-inflatable pad that is hourglass-shaped with a wide area under shoulders and hips. It is very comfortable considering its modest thickness because the air cannot escape from where you put pressure under the pad. –  nsandersen Feb 6 '13 at 13:11
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The foam inside the mat prevents heat loss through convection which would be the case with the air if there was nothing to prevent it moving. This forms part of the open cell vs closed cell argument for sleeping mats.

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