Replies

1. GBA Editor
   Martin Holladay | Sep 05, 2017 02:10pm | #1

   Peter,
   The best retrofit is not the cheapest.

   The best retrofit from a thermal perspective will take up interior space.

   If cost is no object, you would want to aim for R-40 walls.

   The best approach to take would depend on whether there are any signs of water entry. If the sheathing seems sound, and there are no signs of water entry, you could achieve R-40 with about 11 inches of cellulose -- an approach that would require a double-stud wall -- or about 6 inches of closed-cell foam (which isn't particularly environmentally friendly).

   If you want your retrofit to be cost-effective, you'll probably have to stop short of the "best" specifications, and settle for "pretty good."

2. GBA Editor
   Martin Holladay | Sep 05, 2017 03:21pm | #2

   Peter,
   If there are no signs of sheathing damage or water entry, one "pretty good" approach would be to fill the stud bays with fluffy insulation (cellulose, mineral wool, or fiberglass), and to install a continuous layer of rigid foam on the interior side of the studs. If you choose this route, attention to airtightness is important.

   For more information, see Walls With Interior Rigid Foam.

3. 
   PeterTAnderson | Sep 09, 2017 01:50am | #3

   Martin,
   Thank you for your answers. I am confused about the location of 15# felt in the existing wall assembly I described. The felt is on the inside of the 2x sheathing. What purpose does this serve? If I were to place a VB on the room side (inside) of this wall aren't there two vapor barriers? I'd certainly settle for a pretty good wall. I am somewhat hesitant to rely on the rigid foam on the inside wall being airtight (old home, owner may be a DIY). Can you advise on a more forgiving wall specifically with regard to drying?

4. GBA Editor
   Martin Holladay | Sep 09, 2017 05:28am | #4

   Peter,
   Q. "I am confused about the location of 15# felt in the existing wall assembly I described. The felt is on the inside of the 2x sheathing. What purpose does this serve?"

   A. The asphalt felt was placed in that location to limit air infiltration. Asphalt felt is not a great air barrier, but it helps -- and better materials were unavailable when your house was built.

   Q. "If I were to place a vapor barrier on the room side (inside) of this wall aren't there two vapor barriers?"

   A. No. And in any case, you don't want to install an interior vapor barrier. Building codes do not require (nor do building scientists recommend) the installation of an interior vapor barrier. Instead, most codes require the installation of an interior vapor retarder -- a less stringent layer than a vapor barrier. Vapor retarder paint (applied to the drywall) meets this requirement.

   Asphalt felt is not a vapor barrier. It is a material with variable vapor permeance -- a so-called "smart" retarder.

   For more information on these issues, see these articles:

   Do I Need a Vapor Retarder?

   Vapor Retarders and Vapor Barriers

   Q. "I am somewhat hesitant to rely on the rigid foam on the inside wall being airtight (old home, owner may be a DIY)."

   A. No matter what type of insulation you settle on, you need to pay attention to airtightness. Most builders use the drywall as the interior air barrier. If you go this route, you need to either (a) install airtight electrical boxes, or (b) carefully seal ordinary electrical boxes to limit air leakage.

   If you prefer to keep your air barrier separate from the drywall layer, you might want to install a smart vapor retarder like MemBrain in an airtight manner, and depend on the MemBrain to be your air barrier. But MemBrain can be fragile, so I think the airtight drywall approach makes more sense.

   Q. "Can you advise on a more forgiving wall specifically with regard to drying?"

   A. Your existing exterior layers allow outward drying. If you skip the interior polyethylene, your wall assembly will also be able to dry inward -- especially if you omit the continuous layer of rigid foam. However, inward drying only matters during the summer.

5. 
   PeterTAnderson | Sep 10, 2017 03:43pm | #5

   Martin,
   Thank you for the considerate response. After I have time to digest all of your comments I may return to a discussion with a proposed assembly for your input.