# Calculating perm rating at increased thickness.

| Posted in Energy Efficiency and Durability on

If a product has a perm rating of 5 at a 1″ thickness. What is the perm rating at 10″ thick? Is the math as simple as taking the perm per inch and dividing by thickness or is the relationship non-linear? What happens when you add a coat of primer and two coats of latex paint?

## Join the leading community of building science experts

### Replies

1. GBA Editor
| | #1

Jim,
As far as I understand it, the best way to determine the permeance of a sample of a given thickness is empirically. In other words, you have to measure it (or find the permeance in a chart or table that lists the thickness of the material you are interested in).

If you have two materials layered on top of each other, the theoretical permeance of the two layers is:
perm[total] = 1 / (1 / perm[layer 1] + 1 / perm[layer 2]). Actual measured permeance may differ from the permeance derived from this formula, however,

2. Jim Swenson | | #2

So if a material with a perm rating of 2.3 had a material with a perm rating of 1.5 layered on top of it the total perm would be 1/ (1/1.5) + (1/2.3)=0.907 Does that sound about right?

3. GBA Editor
| | #3

Jim,
It looks like your math is correct.

• |
• |
• |
• |