Adding foam board to unfunished concrete basement walls

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01_Sentea2.0

New Member
Sep 7, 2021
35
WV
I have a unfinished basement where my wood furnace is currently and plan on putting up 2" eps foam board on my concrete walls to help keep the cold winter air off my furnace. The problem I'm having is blocking concrete foundation seems to have pink insulation under the sill plate and not the correct sill sealer that most house have. I can stick my hand in the hollow cores of the block. I'm worried about putting the foam board over the hollow gap to the wall foam board and causing moisture to wick through to the pink insulation to my sill plate. How about you go about this?
 

NickW

Minister of Fire
Oct 16, 2019
567
SE WI
Home Depot carries pink Owens Corning sill sealer, so it may be OK. In discussing doing the same thing in the future Phase 2 of my cabin build with my insulator, he was adamant that no air circulation be allowed between the sheets and concrete at the sides and top. They must be caulked/sealed.

I will also comment that PL300 creates an extremely strong bond between foam board and concrete. I had a carpenter drop a ladder on an exterior sheet and the spot hit broke into multiple pieces and ripped off the wall leaving the adhesive and thin coat of foam on the wall. If using adhesive, use vertical lines and no horizontal to allow any moisture to drain down.
 

01_Sentea2.0

New Member
Sep 7, 2021
35
WV
Home Depot carries pink Owens Corning sill sealer, so it may be OK. In discussing doing the same thing in the future Phase 2 of my cabin build with my insulator, he was adamant that no air circulation be allowed between the sheets and concrete at the sides and top. They must be caulked/sealed.

I will also comment that PL300 creates an extremely strong bond between foam board and concrete. I had a carpenter drop a ladder on an exterior sheet and the spot hit broke into multiple pieces and ripped off the wall leaving the adhesive and thin coat of foam on the wall. If using adhesive, use vertical lines and no horizontal to allow any moisture to drain down.
I bought this house and I guess the inspector didn't catch it till I was doing some researching and happened to look on how to go about putting up the foam board and seen there wasn't a sill sealer under the seal plate. Putting on a sill sealer right now would require me to jack my whole house house and I could currently only do one half because the other half is finished.

I have pl300 for my foam board. Never thought about the way the adhesive was applied but that makes alot of sense putting it on in vertical lines.

I'm just worried about trying to seal the top of the hollow block with foam board to sill plate. One part of my basements leaks when it rains. There is a couple holes in the block to allow the water to come through so it won't fill up the block. There is drains around front of the house to a manage the water but doesn't entirely fix it.
 

NickW

Minister of Fire
Oct 16, 2019
567
SE WI
How are the sill plates attached to tihe concrete block? Is it possible that there is a sill sealer that is exactly as wide as the sill plate but not as wide as the blocks? 2x4 sill plates usually use 3 1/2" sealer, 2x6 sill plates use 5 1/2" sealer; so the sealer can be hard to see. I assume the sill plates are treated lumber?

I just did a quick search on alternatives to sill sealers, and it appears there are some options; but water in a foundation is no bueno.
 

01_Sentea2.0

New Member
Sep 7, 2021
35
WV
How are the sill plates attached to tihe concrete block? Is it possible that there is a sill sealer that is exactly as wide as the sill plate but not as wide as the blocks? 2x4 sill plates usually use 3 1/2" sealer, 2x6 sill plates use 5 1/2" sealer; so the sealer can be hard to see. I assume the sill plates are treated lumber?

I just did a quick search on alternatives to sill sealers, and it appears there are some options; but water in a foundation is no bueno.
Attached with bolts. I guess there could be but I stuck my hand back in there and I did feel anything but pink insulation. Not sure if it's treated or not. I think this house was built in the 80s.

That's disappointing
 

NickW

Minister of Fire
Oct 16, 2019
567
SE WI
Definitely disappointing...

Concrete will absorb and release moisture. If you seal the cavities it can't go into the air, so it can only go out or into the wood (IF there's no barrier). I'd almost consider leaving the cavities open at the top to allow air circulation and hopefully reduce how much moisture goes into the sill plate, but with that open the insulation can only do so much... And it could also lead to more moisture in the concrete due to condensation.
 
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01_Sentea2.0

New Member
Sep 7, 2021
35
WV
Definitely disappointing...

Concrete will absorb and release moisture. If you seal the cavities it can't go into the air, so it can only go out or into the wood (IF there's no barrier). I'd almost consider leaving the cavities open at the top to allow air circulation and hopefully reduce how much moisture goes into the sill plate, but with that open the insulation can only do so much... And it could also lead to more moisture in the concrete due to condensation.
From what I've been reading eps is very vapor permeable so it should still come through the foam board to allow it to breath. Your supposed to seal all cracks with seal/foam to not allow the cold concrete to hit the warm air and allow it to condensate. If I do all of this everything should be good right or do I have something wrong?
 

NickW

Minister of Fire
Oct 16, 2019
567
SE WI
I decline to affirm or rebuke...

I would expect moisture to try to balance within the saturated material and release via the path of least resistance or most absorptive contacting material...

You may want to consult with an expert. Not saying hire someone, but have some conversations.
 

Bad LP

Minister of Fire
Nov 28, 2014
1,800
Northern Maine
Just put a larger sized dabs of PL on the board, apply it to the wall and hold it in place till cured. It’s not going anywhere.
No need to worry about vertical or horizontal runs of PL.
 
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monteville

Member
Nov 23, 2019
152
Dallas
Insulation should be non combustible. EPS is very flammable. Think why houses are not insulated by cheap styrofoam and bubble wraps.
 

Highbeam

Minister of Fire
Dec 28, 2006
19,338
Mt. Rainier Foothills, WA
Insulation should be non combustible. EPS is very flammable. Think why houses are not insulated by cheap styrofoam and bubble wraps.
Houses are absolutely insulated with "cheap" EPS and XPS. You just need to cover it with sheetrock or something else that meets the requirements.
 

velvetfoot

Minister of Fire
Dec 5, 2005
10,094
Sand Lake, NY
I did this and I was glad when it was done. I did a thread on it.

I had visions of making it living space, but the pellet boiler and attendant 6 tons of pellets scotched that.

If you cover the walls with foam insulation (I used the Owens-Corning stuff), you have to put sheet rock over it.

The house came with fiberglass insulation in the first floor joists and the rim joist bays. I left the floor joist (basement ceiling) in place and took out the rim joist fg.

I put in multiple layers of cut foam in the rim joist space, until it was at the wall, foamed or forced fit and caulked the pieces. What a PITA.

I put two layers of foam on the wall (4" total). Put on furring strips to strap it to the wall as well as provide support to the drywall. Machined slots in the second layer to fit over the furring strips. Another PITA.

PL300 was involved too, but I forget where. I remember it's a PITA to hold the board up while it cures enough-I think I made a jig of some sort.

You can run into the finished wall and just bounce off, it's so tough.

We don't have termites here, knock on wood, and I don't know if that could be a factor.

For an unfinished and unconditioned space, I'm of the opinion you can spend your time and money better elsewhere. Maybe basement ceiling fiberglass and ductwork insulation and stuff the rim joist wells with plastic bags filled with fiberglass (easily removed for inspection). I don't think you want to cover up a leaky wall if you can help it.
 
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