Exterior XPS and 2″ support at bottom of wall
idahobuild
| Posted in General Questions on
Hey all,
Our house calls for exterior, rigid insulation with a 2×2 support at the bottom (see atch’d image). In some areas the exterior insulation is over the conc. stem wall, while in other areas it will hang over open air (where the conc. slab isn’t square). The rigid insulation will be screwed into the walls with those large, flat washers.
Is the 2x support at the bottom really necessary?
Does the 2x perhaps serve some other purpose — protecting the bottom edge of the foam, etc.?
Thanks.
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Replies
idahobuild,
You don't need it for support, but you do need something to protect the bottom edge. That can be a variety of materials. A 2"x2" is probably the easiest.
Thanks Malcolm - it does seam like the 2x would create a thermal bridge at the bottom plate. Any ideas for different materials - L shaped iron/aluminum etc.?
idahobuild,
You could use a U-shaped piece of perforated flashing that extends out to cover the bottom of the rain-screen cavity, but I don't think that thermal bridge is particularly troubling. You already have a worse condition with your bottom plate in contact with the stem-wall. That 2"x6" plate has about R-3 outside it, and about R-1 below it.
I haven't been able to find a U-shaped flashing, but I did find a 1" x 2" galvanized flashing that is 28 Gauge steel (see atch'd). It seems like it would work well. Any thoughts on this?
idahobuild.
Perforated L-flashing will work fine. With 28 ga I would tack the front edge to the bottom of your furring strips to make sure there is no gap for pests.
When I did this I used wood blocking
Malcolm - Am I reading this right -- you used wood blocking to protect the bottom edge of the insulation?
idahobuild,
Yes. The situation was a bit different in that the sub-floor had a layer of foam on it, but as I said in post #3, I don't think the difference between using wood and flashing is worth worrying about.