XPS rigid foam insulation and moisture
Valgal
| Posted in Energy Efficiency and Durability on
I have read that XPS extruded pink and blue polystyrene loses a “significant” amount of insulation value when wet. Is this correct and would anyone have a percentage amount? This question is in realtion to an ecoroof underlayment in a marine climate.
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Replies
Valerie,
I'd like to know the source of your information.
According to the information I've seen, XPS retains its R-value even in a damp environment. That's why it is commonly used below grade.
Define "significant". If you trust DOW, read this: http://building.dow.com/eu/gbr/en/pdf/291-18225.pdf
As one small anecdote, I have submerged a piece of "normal" blue board (approx 25 psi) and a piece of similar density EPS for over a year now. I took the lead off of both pieces a month ago and both float like corks. All that says for sure is that they are not sponges, but it sure looks like water has minimal effect on them.
If I am reading it correctly, in the "Revised Builder’s Guide to Frost Protected Shallow Foundations" (http://www.toolbase.org/PDF/DesignGuides/revisedFPSFguide.pdf), they derate the R valueof XPS by 20% for horizontal underground installation. EPS get derated ~ 35%.
Good article. I'd be curious to know the source of ".... so accepted design values for FPSF insulation materials were conservatively calculated to be approximately 10% less..."
My understanding is that XPS works well as an insulator even when damp. And it even blocks water to some extent. After all, they make coffee cups out of styrofoam. And XPS is used below grade.
But, I also understand that ants like wet XPS. See http://www.buildingscience.com/documents/insights/bsi-056-leiningen-versus-the-ants-redux. What I have seen recommended is to use metal flashing and bug barriers to stop ants.
Clement: If you ask the EPS folks that I spoke w/, they claim that both are very water impervious. Thorsten Chlupp, and many PassivHauses (apparenlty), use EPS. ICF's are EPS, I believe. I wish I knew The Answer on this one. BTW: Coffee cups do not need to hold liquid for 100 yrs, so that may or may not be a good analogy. A pro bug killer told me that ants will burrow into any foam quite happily. Bugs are pretty good at doing what bugs have been doing for eons.