Exterior Wall -insulation retrofit “Retrofit for the Future”- FHB 242
greid
| Posted in Energy, Heating & Insulation on
I’m thinking of doing a retrofit to some of our exterior walls.
Currently have 2 layers of vinyl siding with thin (maybe 1/2 inch) Styrofoam in-between the 2 layers. Plan was to take down both layers, build out & insulate with spray foam, reinstall vinyl siding.
In the recent FHB issue (Apr-May 2014) p 65 – it shows an exterior wall retrofit. I was confused by the description/figure. It shows 2×3’s – 16 oc held off existing sheathing by screws & PVC with closed cell spray foam. I was surprised that exterior sheathing & housewrap is not shown. Siding is just attached onto 2×3’s.
https://www.finehomebuilding.com/design/articles/a-retrofit-for-the-future.aspx
Shouldn’t sheathing & housewrap been added?
Graham
London ON, Canada
Replies
It should be noted that the "insulation" between the vinyl layers isn't there for insulation purposes (even though it may be advertised as that), but is simply the cheapest, fastest way to achieve a (relatively) flat surface for installing the second layer. It's insulation value is negligible.
Although it's not called out on the drawing, the exterior section shows what appears to be 1/2" sheathing. Its the layer immediately exterior to the cellulose insulation. I'm guessing that the spray foam is acting as the housewrap.
Another, possibly less involved, way to add exterior insulation is to use rigid foam sheats. The Green Building site has many articles about this. I've done the foam sheet method on my own home. It works well but is labor intensive. The method in the article seems even more so. Plus, spray foam isn't cheap.
Another consideration is that some insects, carpenter ants in particular, like to burrow through foam for their nests. The articles house doesn't seem to allow for any provision for this.