Is rigid foam insulation foam good on basement walls or does it trap moisture?


Can basement walls to dry to the inside with foam board?

Tim Hanrahan Published: Aug. 20, 2020, 12:34 a.m.Last updated: Sept. 25, 2020, 8:41 a.m.

I'm planning to install and seal 2 inch eps foamboard to my basement foundation walls. From all of my reading it says that your basement foundation walls can only dry to the inside. If I put foamboard on the concrete and seal it everywhere, when moisture comes through the concrete it will have nowhere to go.

Perhaps I am missing something here but seems to me over time behind the eps will be wet as this moisture has nowhere to go?

I have also read attach it directly to the concrete with no air gap, lately I'm reading leave a tiny air gap. Looking for thoughts on these 2 options.

Responses (3)

Tim Hanrahan Aug. 28, 2020, 4:02 p.m. Reply

Thanks for the great info Mike.

I am planning to use a good dehumidifier as part of my basement solution so I can monitor humidity and keep it within a good range.

would there be any point in cutting maybe 6x12 inch squares out of the bottom of the foam about the size of a cold air return. In winter I would put the cut piece of foam back in place and seal it up tight but in summer I could remove it and allow any moisture back there to dry to the interior of the basement with my dehumidifier system?

Mike Reynolds
Mike Reynolds Aug. 31, 2020, 12:12 p.m. Reply

No, nothing good will come from that I think.  Without inspecting it I would still say it is almost a foregone conclusion that your concrete walls will always stay wet, so any drying that may happen by you letting moisture into the basement (which will be an anecdotal amount at best anyway)  will just be replaced by more as it is surrounded by an endless supply. All that would serve to do would be give your dehumidifer a bit of extra work to do for no reason. Best is to seal it up so the moisture stays on the other side and leave it.