I always read that if a self inflatable mat feels too hard we should blow extra air in it (with the mouth) to make it softer, more comfortable.
My 7.5cm thick Vango Adventure feels way too hard after blowing extra air in it though, and even theoretically it makes no sense to me: more air = more air molecules packed together = harder not softer!
Is there a way, and if so what is the way to make a self inflatable mat a bit less firm?
It depends on the thickness of the mattress and the ground below it. With a very thick mattress and a level ground, it's the opposite: Less air is softer/more comfortable.** The thinner the mat or the less even the ground (e.g. roots), the bigger the chance that you "touch" the ground (i.e. only compressed foam in between) - usually that happens at your hip (highest load) or wherever the protruding feature (e.g. root) is. In that case more air means more load distribution over the mattress and thus more "ground-clearance", meaning you are less likely to "touch" the ground. So while the mat is stiffer, it is still a lot softer/more comfortable than "touching" the ground.
** That's obviously an oversimplification: There is an optimal point in this case. At some point less air will also feel less comfortable. When that point is reached is individual, as with regular mattresses too.
You need the right balance of air pressure. Both too much and too little can reduce comfort.
This can be seen by thinking about the limiting cases. With no air, you're directly on the ground. If you could put very high pressure into the mattress without it exploding, the mattress would itself be hard, and you're effectively back to being on hard ground.
You want something in the middle. The best for distributing your weight over the most body area is when the lowest point on you body just touches the ground. That is usually the hip when lying on your side. With the hip not quite touching the ground, you don't feel the hard ground, but the mattress is selectively squished and conforms to your body up to the depth of the mattress.
I've got one of those inflatable pads that's maybe 1 inch or a little more thick, and I have found the above works just like the theory suggests. I usually over-inflate the mattress when setting up the tent, then let a little air out when I lie down. I do this while lying on my side since my hip in that position is the first thing that touches the ground when the mattress is under-inflated.
Extra air works if the reason it feels too hard is that you're compressing the filling (these mats have an open, springy foam inside). But of course there's a point where adding air makes it firmer even though the air moves so you're supported over a larger area. Just add with old-fashioned air mattresses, you need to optimise them for your own use.
You can choose not to blow into it. You can close the valve when it first fills out, and you'll have less air in there than if you left it a few minutes with nothing on top. You can allow it to inflate itself with a sleeping bag on top.
An underinflated mat means you're on the ground. A firm (enough)a mat means you're not.
The greater air doesn't make the mat softer, it makes the sensation of the ground under the mat softer.
