The Thermal Bridge to Nowhere
How poor design affects heat transfer, comfort, and moisture problems
Let's play a little game today. Take a look at that photo at right. See anything that bothers you?* Well, pretend that you're the heat in the house once everything is finished and people are living in it. Does that help? If your answer is still no, let me give you a little help. Here are the approximate R-values of wood and the standard insulation you might find in a wall (fiberglass, cellulose, open-cell spray foam):
Insulation: R-3.7 per inch
Wood: R-1.1 per inch
When it's cold outside, the heat inside the house passes through wood more than three times as easily as through insulation. In other words, the wood forms a thermal bridge, allowing the heat to escape more easily. In the summer, the heat moves from outside to inside, but it's still a thermal bridge.
The problem in the photo above is that there's too much wood. The ideal way to eliminate the thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. would be to put all the insulation on the outside and create what Joe Lstiburek calls the 'Perfect Wall.' In that type of building, you'd get continuous insulation, which can perform better overall even with less R-value.
Let's look at the trouble spots.
Trouble Spot #1
The first trouble spot is where the rafters come down onto the double top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. and even hit the header. Either the designer or the framer made the bad decision to lower the roof here, and that puts a lot of excess wood into the building enclosure. If there was a nice big space above the exterior wall, they could blow the attic insulation deep enough there and not have to worry about filling the space between the rafters. The rafters would be higher and not nearly as much a part of the building enclosure as they are here.
As it is, you've got ceiling joists and rafters nailed together in a way that will make insulating very difficult. And that constuction on the right side of this closeup has a gap that may well not have gotten any insulation at all. Air has a much lower R-value than wood, so if indeed that gap didn't get insulated, that's a serious thermal bridge. If this were a bathroom, it could create a cold spot in winter that collects moisture and starts a biology experiment.
Trouble Spot #2
The second area with a lot of thermal bridging is the solid wall of wood above the window. Section A in the photo (Image #3, below) is the double top plate, a standard part of most stick-built homes. It's possible to use advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. and build with single top plates, but most builders don't.
Section B is the structural header. That's fine. With a window below, you've got to have a way to transfer the load downward without going through the window. This is made of two 2x10s with a 1/2-inch gap that's normally filled with wood (OSB) spacers. (This home is in IECC International Energy Conservation Code. climate zone 3, where 2x4 walls are the standard.) You can double the R-value here, and reduce thermal bridging, by putting a 1/2-inch piece of foamboard between the 2x10s.
Section C containts the most avoidable problem. To fill the gap below the header, someone made the decision to just add more wood, so they stacked three 2x4s on top of a 1/2-inch piece of OSB. That's an extra 5 inches of solid wood in this wall. That may not sound like much, but underinsulated areas in the building enclosure can have a huge effect on heat loss and heat gain.
The upshot of all this is that not paying attention to thermal bridging can hurt performance. That room might not only waste energy; it also could create comfort and moisture problems. Somehow, the team that designed and built this house did not include a Building Enclosure Control Freak.
*One thing that may bother you is that the framers appear to have violated building codes and compromised the structural integrity of this home. I agree. The way they’ve cut the rafters means that little of the wood is actually being supported by the exterior wall. Although you can’t see from this photo, the poor framing may not actually bring the house down, though, because this was part of a relatively small bump-out, so the support at the sides may overcome the weakness you see here. But this article is about heat loss, so let’s ignore the framing defects here.
A tip of the hat to our former HERS rater student and current HERS rater Jonathan Goodman of Envirofoam for the expression, 'thermal bridge to nowhere.'
- Energy Vanguard
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