Amount of exterior foam for a remodel in zone 6a?
Hi,
Here’s my situation: I am remodeling a ca. 1830 farmhouse in the northern adirondacks zone 6a. It is a timber frame with barn-board sheathing, lath and plaster inside. The interior plaster walls are in mostly great shape and I would like to keep them. The house needs new siding, so my plan was to remove all old siding down to the barn-board also remove all the “add-on” overhangs/rafter tails on the house and wrap it with polyiso foam. 90% of the exterior walls inside are original plaster. A few have been altered so there is 2×4 framing in between the timbers. I plan on leaving those walls free of insulation and putting it all on the outside of the house. My question is: What is the proper amount of foam for this application? I have looked at charts on this website and found the appropriate thickness for a 2×4 wall in my climate, but the majority of my walls are 2 – 2.5″ thick with no airspace whatsoever. I also am planning to foam the roof as well but that will be a later project. Any help would be much appreciated.
Thanks,
Michael
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Michael,
My usual answer is that you should install at least the code-minimum level of insulation. According to the 2009 IRC, homes in Zone 6 need at least R-20 wall insulation and R-49 roof insulation.
Of course, in a retrofit situation, many homeowners conclude that achieving new-construction R-values is too difficult or expensive, so they choose an R-value that falls short of the code. But it's a good goal to aim for -- at a minimum. Builders of superinsulated houses often aim to double the minimum code requirements. In your case, that would mean aiming for an R-40 wall.
To achieve R-20, you would need about 5.5 inches of EPS, 4 inches of XPS, or 3 inches of polyiso. To achieve R-40, you would need about 11 inches of EPS, 8 inches of XPS, or 6 inches of polyiso.
Many builders have successfully installed 6 inches of exterior foam on the walls of an existing house. Beyond 6 inches, however, you may encounter difficulties driving the long screws necessary to secure the furring strips on the exterior side of the foam.