Wednesday, December 22, 2010

Thermablok and Thermal Bridging

While mold most often occurs where there are roof or siding leaks, it also occurs when warm, moist air contacts cold structural members, causing water to condense out. We found mold on the face of a stud in the south hallway wall. The south hallway wall receives no sun in winter so the outside wall doesn't warm up during the day and, as a result, likely gets much colder at night. When mold occurs on a stud and there is no evidence of any siding leakage (as in this particular case) it is probably a result of thermal bridging. Insulation won't help because it is installed between the studs, and it won't stop heat transfer through the face.

I discussed thermal bridging in this previous post. The conclusion was that it would be too expensive and difficult to treat the whole house for thermal bridging, besides which, the insulation and drywall contractors probably wouldn't know what to do. However, I do want to treat the south wall of the hallway, to eliminate the condensation problem. So I ordered 50 strips of Thermablok , which I described in my previous post, enough to face the studs on the south hallway wall.  Thermablok is an aerogel insulation that comes in 4' by 1.5" strips. The back of the strip has adhesive on it. Installing it couldn't be easier: you peel off the plastic strip and paste it onto the face of the stud (making sure of course there is no drywall dust or other dirt on the stud to clog up the adhesive). Aerogel is the most effective insulating material after vacuum, at around R-10/inch. These strips will add R-4 to the studs, effectively doubling the insulating value of a 2x4. But aerogel isn't cheap, the strips are about $4 apiece.

On Fri., a package arrived with the Thermablok. The guys at Acoustiblok (the company that sells Thermablok) like packaging. The entire thing was packed in heavy cardboard with duct tape on both ends. Inside that, the Thermablok strips were bundled together in plastic garbage bags that were also duct-taped:
Inside these, the Thermablok strips themselves were packages in shrink-wrap:
Here's a closer view of the label:
I took the strip and set it against a stud to see how it would look when installed:
Here is a closer view of the bottom:

The old drywall was not removed under the molding, so you can see how the Thermablok strip will line up with areas where there is still old drywall. Only about 0.1" of space separates the face  of the old 0.5" thick drywall from the 0.39"  thick (after the plastic strip is removed) Thermablok strip.

This is a problem. The thinnest drywall you can get is 0.25" and that is not recommended on framing with 16" centers The recommendation is for 0.5" thick drywall, probably because drywall has very little structural strength, and 0.25" therefore has much less strength that 0.5". So if 0.5" drywall is installed, the new drywall will stick up 0.4" beyond the existing drywall and various other stuff on the wall (electrical sockets, window sills, etc.). I looked around for alternative wall covering material that is thinner than drywall,  but there doesn't appear to be much of anything. If we were planning on doing thermal bridging treatment for the whole house, this problem could cause a lot of additional work. All the molding would need to be taken off, maybe the electrical boxes would need to be moved, etc. But since we are only planning on doing one wall, the amount of work should be fairly limited.

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