Passive House Design: Thermal Bridging

| 3/16/2011 1:46:21 PM

Mark A. Miller is a practicing architect/builder/developer living in Chicago who designs projects around the country. His studio, Mark A. Miller Architects + Builders, designs and builds high-performing, energy-efficient homes that speak to the soul. Mark recently co-founded the Passive House Alliance Chicago and is lecturing about the Passive House standard throughout the Midwest. You can learn more about his unique approach to designing thoughtful homes at his websites: Zen + Architecture and Passive House Midwest.  

I wanted to continue to go through some of the techniques that help one achieve the Passive House Standard. Today we're going to talk about thermal bridges. Like a road bridge assists in the transport of cars from one location to another, a thermal bridge assists in the transport of thermal energy from one temperature zone to another different temperature zone. The big difference is that the transport of cars is a benefit, whereas the transport of thermal energy in our homes from outside to in, or inside to out, is not.

Well, hey, that’s why we have insulation, right?  Yes. 

The nature of thermal energy transfer is to flow from hotter temperatures to cooler temperatures. The role of insulation is to impede or slow down the time it takes the thermal energy to transfer through the building envelope. Think of it as an hourglass. The better the insulation, the smaller the middle area of the hourglass, the slower the sand takes to go from the top to the bottom. The less insulation, the larger the middle of the hourglass, and the quicker the sand transfers. Because of the laws of physics, you can’t completely stop the transfer, so let’s be conscious of our limits.

In a typical American wood-framed home, we usually find insulation in-between the wall studs. So when we think about how well we are stopping the flow of energy through our envelope we tend to measure at this insulated cavity area. However, up to 15 percent of the building envelope is comprised of the wood stud structure. This is the weaker situation as our insulation is not in the stud, it’s in the cavity between studs. A 2-by-4 cavity insulated with fiberglass batts typically can achieve an R-13 in this area. The R-value for framing lumber is around 1.25 per inch, which gives a total R-value through the stud of 4.375 (3.5-by-1.25 inches)—a decrease in insulative performance by about 66 percent!  Not to mention the colder stud will affect the performance of the adjacent insulation material, reducing its actual performing R-value.

A great example of this phenomenon is to look at a photograph from a thermal imaging camera. The brighter the color, the more thermal energy is transferring through the building envelope. We can see the wood structure as being brighter than the cavities in-between the wood structure, illustrating this point about the losses to a home through these “thermal bridges.”