What is a thermal bridge?

Submitted by Andrea on April 3, 2011

My current enormous task is to design a complete framing plan. I am literally mapping out exactly where every single stud and beam will go, which I hope will save us a lot of effort when we're working with actual lumber. I'm keeping a running list of questions to ask Marc and our structural engineer, just to make sure we're not doing anything too stupid. Obviously our primary goal here is structural soundness, but my driving obsession is to Avoid Thermal Bridges. This is one of the central tenets of Passivhaus construction, so I thought I'd tear myself away from SketchUp for a few minutes and explain what this actually means. To paraphrase Homes for a Changing Climate, thermal bridges are the path of least resistance for heat to flow out from a house. They occur when an element in the house has higher heat conductivity than the surrounding materials. For example, a balcony slab that isn't thermally isolated from an interior concrete floor can suck the heat right out of the house. The most common thermal bridge in a wood-frame house might be the wall studs themselves. In a 2x6 wall, studs extend through the thickness of the wall. The inside of a stud wall is normally covered by drywall sheets on the inside of the house and cladding outside the house. In the diagram at the right, you can see that the wall is full of fiberglass insulation, except for where the studs are. So the insulated parts of the wall will have an R-value of, say, R-19, but the studs themselves are only about R-6, meaning that much more heat will escape through the studs than through the insulation batts. We plan to address this in several ways. One is to raise the wall's overall R-value by putting additional rigid foam insulation outside the stud assembly, beneath the exterior cladding. Another is to use as few studs as we can get away with. To accomplish this we are using Optimum Value Engineering, which does all sorts of clever tricks to minimize the amount of lumber used in construction. So in a nutshell that's what I've been doing, trying to design our house frame with as few thermal bridges as possible. It's a little trickier than it sounds, at least for a construction neophyte like me. Incidentally, here's a peek at the framing plan so far. It's missing most of the windows and, notably, a roof, but you get the idea.

Submitted by Andrew (not verified) on July 3, 2011

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Just found your site and I am enjoying the read. I am surprised that you do not mention considering a double stud wall to reduce thermal bridging. It has always seemed an elegant solution , particularly when using loose insulation. You may have considered this approach and discarded it for your design. If so I am curious why. (my apologies if this is covered else where. I am reading through chronologically. Thanks.

Sorry about the slow reply -- the "new comment notification" feature seems to be on the fritz.

Ted originally wanted to do a double-stud wall, but we had some concerns about moisture would accumulate, with one cold set of studs and one warm set. Marc quickly talked us into using exterior polyiso, along with dense-pack cellulose. Maybe it's just his particular prejudice, but we stuck with it.

I'm hoping to get some insulation quotes this week, so I'll have much more information about our exact cellulose plans soon.

Cheers,
Andrea