I’d like some comments on this proposed detail for an existing building’s 2×6 stud exterior wall.
user-6825743
| Posted in Plans Review on
We want to add insulation value to an existing wall and be sure to have a dry wall as well. May I have your critique of this detail. How will it perform? Any suggestions for alterations?
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It would be better to install the foam as a continuous layer. Typically, the strapping goes on top of the foam to eliminate or minimize thermal bridging and create a gap that enables the siding to dry. The strapping only needs to be 3/4 of an inch. See this article for details:
https://www.greenbuildingadvisor.com/blogs/dept/musings/how-install-rigid-foam-sheathing
User-6825743, are you Jeri, the architect noted on the drawing? The GBA software is a bit buggy and doesn't always show the name you registered with.
As Steve noted, continuous exterior insulation would be better, to minimize thermal bridging as he mentions, and also because the battens you show would only be attached to sheathing--it's much better, and required by some siding vendors, for the vertical battens to be attached directly to framing.
You don't specify a rigid insulation product. There are three main types of rigid foam insulation, plus sem-rigid mineral wool, each of which have different R-values and characteristics. The most common product for this application is polyisocyanurate; at R-5.6/in, the 2" you specify would just squeak by code requirements for dewpoint control. Conscientious builders and designers don't use XPS (unless it's recycled) because its blowing agents are potent, persistent global warming agents (aka carbon polluters). If you decide to use XPS you will need at least 2 1/2" to meet R-value requirements over a 2x6 wall in Maine (Climate zone 6). If you choose EPS, which is less environmentally damaging than XPS, you will need 3".
You don't specify where your airtight layer is or where your WRB (weather resistive barrier) is. It looks like you might be planning on relying on Siga tape to foam for both of these? It's better and easier to put both at the sheathing layer. There are several products under the Siga brand name; Wigluv is probably the best cost/benefit for this situation, and Fentrim would be an upgrade.
In addition to the article Steve linked to, read this one: https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing.
Table 1405.3.2 is below. I believe that the intent there is "continuous insulated sheathing" and this is the best of the listed (and unlisted) options . Note that for many foams, your interior side perms must be between 1 and 10 (unfaced EPS might not trigger this requirement and mineral wool certainly wont).
https://codes.iccsafe.org/public/document/IBC2015/chapter-14-exterior-walls
I'd use a 10-15 perm WRB (eg, Typar) if you choose mineral wool.