# EPS Coefficient of Expansion

| Posted in Energy Efficiency and Durability on

I’m working out the detailing of a new house with exterior EPS insulation. It will be in 2 layers of 1″ thick each, with sheet joints staggered. The exterior layer’s joints will be taped.
We all know we need to gap plywood sheets around 1/8″ to allow for expansion. Should I be specifying a similar gap between insulation sheets? Or should the sheets be installed tight to each other?

I read that the coefficient of expansion of EPS is 0.000035 in/degree f.
Assuming an 8′ sheet is installed in 50 degree weather and once enclosed, on a hot day it might well reach 120 degrees, that would amount to almost 1/4″ expansion.

Would there be a benefit to bonding or NOT bonding the two layers to each other? Should I gap the sheets during install?

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### Replies

1. | | #1

You might want to take a look at your calculations again.

2. | | #2

coefficient of thermal expansion 0.000035 in/degree f
example: 8' (96") sheet
installation temperature 50 degrees
insulation temperature in full sunlight on a hot day 120 degrees (roofs can get a lot hotter)

0.000035 * 96 * (120-50) = 0.2352 inches, apx 1/4"

1. | | #5

My reply was poorly worded. Your math is correct, but what I meant was what if the sheet measures exactly 96" and is installed at 30° or at 70° or at 90°? In other words, why 50°?

1. | | #8

I had to pick some specific numbers to use as an example and these seem pretty much suitable for a typical condition across the country.

3. Expert Member
| | #3

I have never seen an issue with foam expansion causing problems, though it probably has at some point. A more common issue is that the foam shrinks over time. I recommend using a tape that can stretch, such as 3M 8067, for taping foam.

1. | | #6

Thanks for the tape recommendation. Sounds good.

4. Expert Member
| | #4

Most foam will shrink over time so you definitely don't want gaps to start. Butt them up nice and tight.

Unlike wood which is very stiff, foam is pretty soft. That 1/4" expansion on an 8' panel might only produce a couple of pounds of force (if I did the quick math right, a 1" 15PSI foam will need 18lbs along the 4' edge to compress it that much). Not something that causes any issues.

1. | | #7

Thanks. You bring up a good point about the foam being soft enough to compress. I'm confident now it's something I don't need to worry about. I've been trained to worry until I'm quite sure I don't need to be.

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