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

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

Tim Hanrahan Aug. 20, 2020Last updated: Sept. 25, 2020

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 (6)

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

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 Aug. 31, 2020, 12:12 p.m.

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. 

Shed Tools
Shed Tools Dec. 30, 2020, 1:21 p.m.

I know this is a few months ago but i have a similar question,

We only use our basement for laudry and storage and over time our stuff smells like the tipical basement! I can see white residue on different areas of the foundation but no actual water or puddling anywhere. 

Am i best to just drylock foundation or can i just install 1" xps sealing it on top and bottom? 

I did air seal all sill plates already and installed new windows. My dehumidifier is an 80gallon heat pump hot water heater which makes the room colder, i figure this unit could work better having the basement walls warmer with the xps installed. The boiler is down here also but i am a big wood burner so it doesn't come on very long. 

Basement walls are 77' total and 7½ high, i figure the xps board would be cheaper and actually insulate.

Let me know your thoughts! Im on east coast longisland, thanks, Chris

Jie Li
Jie Li Feb. 20, 2021, 11:22 p.m.

The best solution suggested in this diagram is using Mineral Board (Vapor permeable and water resistant)? If it's true, no wet wall?

 

Mike Reynolds Feb. 22, 2021, 9:14 a.m.
No, the choice of insulation is not what will determine if the wall is dry or wet. If your basement has no exterior vapor or water protection (including a barrier between the footing and wall) then you are more at risk of having wet walls. On the inside you want to ensure that the wall assembly is not sealed in with concrete with a poly vapor barrier. The reason we like mineral wool insulation is that in the event that there is poorly managed moisture, because it handles it better than fiberglass batts.