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.”
Other areas notorious for acting as a thermal bridge are: where the wall sits on the concrete foundation; where the floor structure intersects the wall structure; and where the roof structure intersects the wall structure. We can see these brighter bands on the photo as well.
Doors and windows are another weak link in the system, as these building elements typically have a very small value R-value. Both doors and windows have solid wood frames that act as thermal bridges. Most doors are 1 3/4-inch-thick with no or sometimes little insulation. I am looking forward to the day our exterior doors take on the performance characteristics of a door found on a commercial "walk-in-cooler" while maintaining the aesthetics found on entry doors. (I’m speaking to you U.S. door manufacturers; let’s get going here).
Passive Houses take these elements very seriously. The Passive House Institute does its own testing on the thermal performance of windows, with very high benchmarks to be met regarding the frames, seals, glass, glass spacers and solar heat gain. We will devote another blog entry just to this topic of windows soon.
Another element contributing to thermal bridging are the pipes that penetrate the building’s shell. These can be found in electrical conduits feeding exterior lights and interior outlets/switches, hose bibs, plumbing pipes and more. Passive House architects design these elements to be anywhere but in the exterior envelope of the home. If one can design this way, it makes it much easier to calculate the intended thermal performance of a Passive House. If you are stuck with a few thermal bridge situations, the use of a piece of software called “Therm” can be used to calculate these, as they are a drawback to our Passive House goals.
Finally, foundation walls, footings and basement slabs are a big source of thermal bridging. Passive Houses spend a good deal of effort alleviating this problem with many inches of foam insulation. We can do a few blogs on this area in the future.
So let's discuss some solutions to the thermal bridging problem. For several years now I have been adding a layer (or two) of insulation to the outside of the sheathing that is over the exterior face of the wall studs. This provides a protective “thermal break” over the weak links in the framing system we discussed: studs and the perimeter band board of the floors and roof where they meet the walls. One needs to be careful with the type of insulation chosen here to make sure the wall can still dry out to the exterior or the interior.
Dow has a newer product called SIS panels, which offer structural sheathing and insulation performance in one product. Other companies have similar products. If detailed properly, using ICFs (Insulated Concrete Forms) can assist in the halting of thermal bridging activity in walls. Double stud walls or staggered stud walls also are a design technique where continuous, uninterrupted insulation acts as a thermal break from outer studs to inner studs.
As our thermal imaging camera reminds us of the unseen effects of this topic of thermal bridging, proper design and detailing can eliminate a significant source of energy loss in our building structures. Only recently is this topic gaining momentum in the discussion of building community professionals. We can thank the Passive House movement for raising the banner for all to see.
Don’t forget to visit www.passivehouse.us as they have recently posted some of the lectures from the recent 2010 North American Passive House Conference.