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Fundamentals - Material Properties

Star 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

0.05 perms.


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.


Porcelain tiles

1 perm.


Gypsum Drywall

35 perms for 5/8".

R0.1 for 5/8".

Latex paint

5 perms.


No 15 Asphalt saturated felt

6 perms.


House wrap (Tyvek)

35 perms.



R0.5 for 8" thick.