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-   -   Insulation for Concrete Basement Walls (http://www.diychatroom.com/f19/insulation-concrete-basement-walls-47447/)

soundman79 06-25-2009 10:16 AM

Insulation for Concrete Basement Walls
 
OK, I need to know if there is still hope for me here.
I have a walk-out basement w full-sized windows. Half above grade, half below. On two of the four basement walls, I have already built a wooden frame up against the concrete. KD studs sitting on a PT plate. Seeing how it is too late to build the recommended sealed polystyrene wall, I wanted to know what my best options are. I have read that a vapor barrier up against the concrete can actually ruin the walls because the moisture can't go anywhere. I have also read that fiberglass batting can hold moisture and grow mold. Please help! Thanks!

Gary in WA 06-25-2009 03:35 PM

Rigid foam board glued to the concrete below grade, none above grade. Stud wall on p.t. plate, 1" away from foam, with fiberglass insulation batts (no vapor barrier on below grade area). Read this: http://www.buildingscience.com/docum...gs?full_view=1 Be safe, G

soundman79 06-25-2009 03:43 PM

Right, but my wall frame is already built and flush to the concrete wall. So, is the next best thing to stick the rigid foam board between the studs?

jogr 06-26-2009 11:36 AM

If the concrete walls are bone dry and stay that way then the studs will be ok. The trouble is exterior concrete walls rarely stay bone dry and if they are right now they might not be in 5 years. So before putting a lot of extra money into the walls take the time to move them out 1".

Depending on how they were attached it could be as simple as backing out some tapcons from the floor plate and cutting the nails between the top plate and joists. In corners one of the walls will need to be cut back 1".

soundman79 06-26-2009 11:51 AM

What about slipping a moisture barrier behind the studs?

Paragon 06-26-2009 12:16 PM

I think you will find it a nightmare to try and weave the vapor barrier. Just bite the bullet and remove and move or take your chances. That is what professionals are well worth the money but this is a mistake you will probably never make again, correct?

Good luck and be safe!

Gary in WA 06-26-2009 12:36 PM

That's the way I would do it now. Buy some 6" wide window bituminous roll flashing, cut it to 3" wide by stud length, pry the middle of studs out- slip it in sticky side to stud. Install foam board against the 3/4" tape overage next to the studs on each side. Or just use 4" tape. Then batts as suggested before. If you do remove the wall, install a thermal break material (sill sealer) under bottom plate to stop moisture there. Be safe, G

RyanD 06-30-2009 09:08 AM

I'm in a similar situation so I want to clarify before I make the same mistake(sorry OP). My exterior walls(in the basement) are half walls and half foundation. The full top part of the walls are insulated with batts and I was going to frame out the bottom concrete parts and do the same but after reading this that might be a bad idea. Whats the best solution for this, glue rigid foam board to the concrete and then frame around it? Install a vapor barrier over the concrete and then frame it as usual?

superfly 07-15-2009 01:54 PM

Quote:

Originally Posted by RyanD (Post 295084)
I'm in a similar situation so I want to clarify before I make the same mistake(sorry OP). My exterior walls(in the basement) are half walls and half foundation. The full top part of the walls are insulated with batts and I was going to frame out the bottom concrete parts and do the same but after reading this that might be a bad idea. Whats the best solution for this, glue rigid foam board to the concrete and then frame around it? Install a vapor barrier over the concrete and then frame it as usual?

I'm in the same boat--I've uninsulated pony walls and thinking about addressing that.

http://www.buildingscience.com/docum...n-systems/view

Has an interesting summary of various insulation systems. My read on it is that it's the concrete, being porous to water, that's the problem, and why you can't just slap some vapour barrier around it. I'm planning on using a foam board over the concrete, with some drywall for fire protection, leaving an air gap. I'll leave the wooden frame as is, with fibreglass batts.


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