Wall insulation question
Hi,
We opened two exterior walls in our 1850s farmhouse in upstate NY (climate zone 6) to stop cold air in the winter and prevent rodents from entering the walls. The wall is now more rodent proof and we are moving on to insulating, but I have some questions.
Outside one of the walls is an uninsulated attached shed with an earth floor – this wall is constructed of (from out to in): 1″ plywood, thin (about 1 mm) styrofoam double-faced w/ foil (perforated on one side, solid foil on other), tar-paper (about 1 mm thick), 1″ wood board, rotting paper, 1″ wood board with gaps between each board, then an ~8″ deep stud bay. For this wall (8′ x 13′) I’m thinking of two options: 1) 8″ rockwool, CertainTeed Membrain, taped drywall. Or alternatively, starting with rigid foam at the back of the stud bays – sealing the rigid foam with canned foam, then rockwool, membrain and sheetrock.
The adjacent exterior wall (8×9′) does not have an attached shed outside of it. This wall from outside to in contains vinyl siding, untaped 1″ extruded foam panel, wood lap siding, then a 6″ deep stud bay. Here I was planing on adding R23 rockwool. and vapor retarder. Alternatively, I could also seal the back of the bays with rigid foam.
We don’t want to opt for expensive alternatives on such a small project (e.g., closed cell spray foam).
Thanks – hoping to close these walls up soon!
Dave
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Replies
For you shed side:
The simplest way to treat this is to assume the attached shed is similar to an attached garage for a house. Garage walls are treated the same as any outside wall, so in my area, that means interior vapor barrier with batts.
The issue in your case is the thin layer of foil faced foam. This creates a true cold side vapor barrier, which is a bad idea in heating dominated climates. Your choice is either take out the foam and go for a standard wall assembly (drywall, VB, batts, sheathing, siding) or add on enough rigid insulation on the outside (meaning inside the shed) for condensation control. In zone 6, this ~R15 if you use R30 batts. You can reduce the amount of foam by going with thinner batts (say R15 batts, R8 foam), which would still be a pretty good wall for Zone6.
Cut and cobble the foam to the inside of the cavity won't work, the foam needs to go on the outside. If you don't want this, about 1.5" of closed cell spray foam would also work. I've used one of the two part DIY kits for this type of assembly before.
For the other wall, you assembly will work fine. Make sure you get the drywall air sealed as you are relying on the lack of air flow and warm side vapor barrier to keep the moisture out of the wall. The existing exterior rigid foam does allow for a bit of drying, so as long as you watch these details, the wall should work great.
P.S. When dealing with old walls with uneven spacing and real 2" studs, I find it is better to get 24" batts, turn them sideways and cut to the width of the bays. Much easier than trying to trim 1/2" off the sides.
Thanks so much. Maybe removing the outer plywood and then remove the foam would be easiest - should I just put the plywood back as the inside-shed exterior wall, or would another covering be better? Should I also remove that thick tar paper when I remove the foil?
If the plywood is in good shape, you can definatley put it back. Something like densglass or fiberboard would be "better" as it is more permeable, but I don't think you need that in this case.
The tar paper is an excellent WRB. Older paper tends to be quite brittle and easily damaged, so most likely you'll have to replace it. You can use felt or one of the many synthetic house wraps.
Note that he probably doesn't need a WRB inside the shed, though an air sealing layer would still be nice, and of course the wall should be rugged enough to withstand whatever the shed is used for.