>>344161The R-value is a thickness / thermal conductivity ratio, so if you stack it, yes, it simply adds up.
But then a R-value isn't exactly interesting, it doesn't say much. If you want to get an idea of the lowest temperature you should use some pad or mat, here's a good way to calculate it:
Tlimit (°C) = 16 - R-value * 6.78
So with a cheap foam pad with a R-value of 1.4, it's fine until a temperature of 6.5°C, which sounds about right- well, it's the lowest, so obviously you'd be cold as fuck at 6.5°C.
The "16" in this formula refers to the lowest temperature without a sleeping pad, estimated to be of 16°C.
