Polyisocyanurate Insulation Below Grade
I have been a builder for many years and have never used Polyisocyanurate insulation below grade, due to the fact that it is an open cell type of insulation and will absorb water. My neighbor has just installed 2″ of this around the perimeter of his house foundation. It’s your typical Polyisocyanurate, with foil facing and exposed edges.
My questions are, will this insulation deteriorate over time? Also how much r-value will the insulation loose due to absorption of water?
Replies
polyiso is closed cell
Perhaps "open cell " was the wrong terminology.I was thinking of this type of insulation as opposed to an extruded type like Owns Corning Blue Board.
I think that the tear off informational sheets that were with these at Home Depot mention this. I recall it stating that below grade is one of the manufaturers approved uses. I used it around my foundation due to reading this. No problems yet. I've heard that bugs or ants may like the stuff to live in though. I think some company makes a borate infused version.
As far as contact with water, this foam has no issues that I know of.
EPS with borate by R-Control would be a better choice.
It's cheaper by about half & bugs no like.
Water absorbtion is not a concern they say.
Joe H
After some searching, I found the answer on Dow Chemical's Website. Dow produces Thermax Polyisocynaurate.
"No. None of the polyisocyanurate based insulation products made by The Dow Chemical Company are recommended for use below grade. STYROFOAM* XPS Insulations are more water resistant and are ideal for this application."