Fundamentals - Material Properties
Every material has a specification to allow you to properly select.
Permeability and insulation spec
How permeability is measured is a bit variable and involves some estimating and even guesswork, but here are the figures that I use. Also below are the R values.
6 mil Polyethylene sheet
Expanded Polystyrene (EPS)
15psi EPS is 1.5 - 3.5 perms per inch (in practice a worst case
of 3 perms per inch).
3" thick EPS is therefore 1 perm.
8" thick EPS (as I use on my walls) is therefore 0.375 perms. I judge this to be sufficient to avoid needing a separate vapor barrier membrane in the wall.
15psi EPS is about R4.5 per inch. (The actual R value varies with temperature.)
Polymer-modified Stucco with sealant
2 perms for 3/4" thick.
35 perms for 5/8".
R0.1 for 5/8".
No 15 Asphalt saturated felt
House wrap (Tyvek)
R0.5 for 8" thick.