Basement rigid foam
I have a newer home (completed Sep 2023) in Climate Zone 4 (Maryland) and just received my building permit to finish my basement. I am a little bit confused regarding conflicting advice for basement wall insulation. For context, I have a poured concrete foundation with exterior waterproofing applied below grade, an exterior drain along the footing, and an interior drain to a sump. Three sides of the house have concrete walls with the back being a wood framed walkout area.
What perm rating is best for my application? I see conflicting information with respect to “drying in”. I am specifically looking at 2″ Foamular 250 or 2″ R-tech from Home Depot. The R-tech is half the cost, but the facer polymeric confuses me with respect to proper drying. I intend to attach unfaced fiberglass insulation (R-11), currently attached with vapor barrier to all concrete walls, inside of the studs when I frame.
Thanks!
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Replies
TaylorD,
There is no benefit to giving your concrete foundation walls the ability to dry inwards. Use the faced foam, but no vapour-barrier or retarder inside that beyond painted drywall.
Malcolm,
Really appreciate your response. My confusion lies with the advice that poly sheeting should never be used in the basement wall assembly because it can trap moisture. Isn’t a polymeric facer doing essentially the same thing since it is not vapor permeable?
Also, is the Henry R-tech (data sheet attached) a good choice for my application? Not sure why the vapor perm is listed as <1.
TaylorD,
The interdiction is really against poly on the interior face of framed walls with permeable insulation. Martin made this clearer in his comments #12 and #23 in this link:
https://www.greenbuildingadvisor.com/article/three-ways-to-insulate-a-basement-wall
And if you look at the assemblies he recommends, some use completely vapour-closed foam.
Concrete doesn't need to dry, and moisture on its interior face isn't trapped, it can stay happily where it is, or go back the way it came. There is no benefit to allowing that moisture into either the framed wall, or the interior of the house.
Thanks for the explanation Malcolm. Makes sense. So as far as specific product between foil-faced polyiso, foil-faced eps, or XPS, just use the best r-value for the dollar essentially?
TaylorD,
Yes. The only other consideration would be the global warming potential of the various types of foam if that is important to you.
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