Insulation options CMU Walls above grade
I am currently remodeling a few rooms in a house where the outside walls are 8″ concrete blocks, no insulation. The siding on the outside is furred out 3/4″, no insulation. On the inside the drywall is also furred out 3/4″, but no insulation. The drywall will be removed from the inside of the block walls as part of the work, so this seems like a good time to add some insulation. I haven’t found a lot of information, however, on insulating above grade CMU walls. If it was below grade, the problem is fairly easy, using rigid foam such as the Dow WallMate or Owens Corning Pink stuff (I found some great info on this at the Building Science web site for this situation).
So, my original plan was to add about 3″ or so of the rigid foam board, and drywall over top, just like I was finishing a basement wall, except that it is above grade. The only downsides I can see, other than losing a few square feet of floor surface, is the cost (foam board is expensive), and that I will lose the thermal mass of the concrete. Searching for “best practices” for this situation, however, didn’t turn up much, except for the following message in these forums, which said that Ownes Corning does not recommend foam above grade, but there wasn’t any additional information as to why:
http://forums.taunton.com/tp-breaktime/messages/?msg=40378.3
Can anyone explain why foam is a bad idea in this situation, and perhaps discuss what a better method is to insulate these walls?
Many thanks,
Frank
Replies
One thing I can think of is the foam would prevent any water vapor that gets into the wall from getting out if the exterior also had some effective vapor barrier. You don't have that issue.
Another consideration is fire, but you can use type X drywall over it. Foam burns and will produce toxics as products of combustion. FG doesn't burn.
"Let's get crack-a-lackin" --- Adam Carolla
Thanks, hanging fire rated gyp. is a good thought. I still haven't found any good information on the insulation, but I have a request in to one of the manufacturers to see what they say. No response yet, but hope to have one soon. I've also contacted a couple of local contractors - they say stud out, fill with fiberglass batts. When asked why, well, that's just how it is done. One contractor said to only put 3/4" foam on the outside and reside, because that will give me all the r-value I "need", and don't put anyting on the inside. Hmmmm ... I'll keep searching.Some other interesting suggestions I have found online-Fill cavities with perlite by pouring in from top of finished wall.
-Get a foam insulating contractor to come in and foam the interior of each of the blocks by drilling a small hole in each block (actually 2, one for each cavity) and injecting foam.I may explore perlite, in addition to any other insulation I do, but I doubt I'll get total coverage since the wall is already built. Outside insulation is a nice idea, but not practical right now - the siding is not being replaced at this time, but the inside is being gutted.
Question about your old post
I saw your 2006 post about your house with CMU walls that you wanted to insulate. I have the same circumstance (8" CMU with plaster on inside and stucco on outside) that I would like to insulate ... Thinking it would be best on the outside. What did you do?
Thanks