Retrofit rigid foam in existing wall?
I currently am in a house where I am getting a lot of air infiltration on a particular outside wall. The wall is 2×4, fiberglass insulation, homasote on outside, covered by aluminum siding. Whenever the wind blows you can feel the air blowing in through the outlets, and at the bottom of the wall in that small gap between the bottom of the drywall and the subfloor. Very cold in the winter time!
I am about to renovate the kitchen which will give me an opportunity to strip the drywall, and try to fix the problem from the inside (I’m not in a position to reside at this time, so I can’t address the problem from the outside). I am considering stripping the drywall, removing the fiberglass, adding ridgid foam planks in the stud walls and sealing around the edges with spray in foam sealant, and then rehanging the drywall.
Here is a link to the product I was thinking of using (STYROFOAM* PERIMATE). It’s available in 2″ & 1.5″ thickness (3.5″ total to match stud depth):
http://www.dow.com/styrofoam/na/res-us/products/perimate.htm
Insulation is not my strong point, so any thoughts from the pros? Bad idea? Good idea? Any strong negatives or other gotchas? Other than a bit of work and mess stripping and rehanging drywall, am I setting myself up for trouble down the road?
Replies
The only thing I can add is to caulk all your joints
like between slab/bottom plate,between top plate and double plate etc.
also if your opening up to a intersecting wall usually in behind
the"T"there is a 2" dead space that probably isn't insulated that I
would see if I could inject some canned foam into via some drilled
holes in to the side of the stud at various heights.
I have taken some small clear tubing and added that to a can of
foam to extend the "reach".
If you have a full fiberglass batt (R12) in the wall and the drywall is in good shape, then I wouldn't strip the wall to gain the extra R. I would concentrate on the airsealing for comfort and heat savings for now and add the exyterior when you get to re-do the sding.
Remember that we use insulation mainly to save $$$$$, right??? But insulation investment $$ and savings are governed by "The Law of Diminished Returns":
Going from equals % reduction of heat flow from original R1
R1 to R2 50%
R2 R3 another 16.6%
R3 R4 " 8.3%
R4 R5 " 5%
R5 R6 " 3.4%
R6 R7 " 2.3%
R40 R41 " .024%
So, If the cost of each extra R is the same as the one before it, at some point the rate of return by adding more R gets ridiculous like 25-50 years. Don't go beyond the economic level for your area. Take the money and find something else to do on your conservation project that has a better or more reasonable payback.
If you're going to take out a good but cheap insulation (about $.03 per R) and then replace it with a product which supplies R at a cost of $.10 per R, it may not be the best economic decision....but it's your money!
Frank, you might try to get Mike Smith to give you a rundown on the Mooney wall he likes to use. He combines rigid foam with blown in cellulose.
He had mentioned it in a few of his threads in the photo gallery, but I don't have one handy to link.
jt8
"Real difficulties can be overcome; it is only the imaginary ones that are unconquerable. " --Theodore N. Vail
First off, welcome to BT.
If you are already committed to stripping the DW, then I'd look into getting either dense-pack cellulose or spray-in foam. This might not be as expensive a situation as it might, if it's a relatively small project.
Hmm, I just flipped over to the building science site, which shows DC as "mixed humid." Looks like you ought to have an external VB. That kind of suggests using foam, as you get some seal against the homasote sheathing.
Now, with the wall open, you are also more likely to properly address the window jambs and behind electrical boxes (where you are getting the sensible infiltration). Cutting down infiltration is a big way to increase comfort in the house.
I'd recommend against the rigid in your case. While it would be better than the FG that's there, sealing it in with spray-can foam is really making a VB on the wrong side for you. And leaving a reversed VB situation until the siding gets replaced does not have a lot to recommend it.
But that's just me, others differ.
When the insulation doubles as the vapor barrier then the rules about which side to put the vapor barrier on change. It is better to think of it in terms of the temperature of the first condensing surface. If you can be sure that the first surface on which moisture might condense is either warmer than the dewpoint or moisture tolerant, then the assembly is safe.
In the winter the first condensing surface would be the inside face of the foam which would be warm. In the summer, the first condensing surface would be the siding. Moisture that got past the siding would pass through the homosote and come in contact with the foam which would again be warm.
Many foam insulations are not vapor barriers but are instead vapor retarders. To be considered a vapor barrier, the perm rating of the material must be less than .1 perm.
There is only one concern that I have related to FrankinDCs plan. If the current wall assembly allows water to enter the wall in even small amounts, the foam will slow the drying potential of the wall to the point that rot may occur. This would happen because of a reduction of air and temperature flows. These are the mechanisms that allow for drying and are what allowed us to build leaky homes that would last for a hundred years before we started using insulation.
Be sure that there is no evidence of water penetration before you make the change. I'm sure you will be fine from a water vapor stand point.
Ah, yes, quite. I was first thinking of the air permeability, and should have thought of the moisture too.
Makes for a difficult situation, feeling drafts and having no budget.
Hmm, my remodeling reflexes are trying to kick in here. If the walls are admiting air at various humidities, is it better to insulate, then VB under the DW (and in just the one room "for now"), if it could be 4-5 years before the house can get re-sided?
I'm not sure there is a one good answer, so some of this is an academic expercise in which compromise is least "bad" (like most compromises with buildings, c'est la vie . . .)Occupational hazard of my occupation not being around (sorry Bubba)
Thank you all for the very thoughtful replies. I see that the issue is a bit more complicated than I first thought - it not simply a case of dropping in some foam, caulking, and hanging a few sheets of drywall. I suspected there might be vapor barrier-type issues, and you folks have explained the issues well. The last thing I need is a rotting wall!
It looks like the most sensible course of action for now is to just seal up as best as I can - along the sill plate, rim joist, footer, around electrical boxes, etc., and live with it as best as I can.
Thanks again, y'all saved me a bit of work, and more importantly, the concerns of rot and mold!