Thermal Barriers - Exterior Wall Assembly

Saturday, October 28, 2017

“ Thermal barrier” is a fancy term for insulation. In a high-performance home, insulation should be installed on all exterior surfaces in an unbroken sequence. Any gaps, voids or breaks in the insulation coverage of the entire building assembly can result in heat loss or gain.  is is shown in the image below with the use of a thermal imaging camera. In a color photo, heat loss shows up as warm yellow or orange and cool well-insulated areas are blue or black. In this black-and-white rendition, light and bright areas indicate heat loss.

Thermal bridging is the rapid transfer of heat through a build- ing component when that component has less thermal resistance (R-value) than materials surrounding it. Framing materials o er a good example of thermal bridging through the building envelope. Wood has an R-value of a little less than one per inch, so a typical 2 × 4 stud has an R-value of around 3.5. Compared to the surround- ing insulated wall cavities, if perfectly installed to manufacturers’ specifications to achieve R-13 or R-19, that’s quite a difference. So, if you look closely at the image above, the thermal movement through the wood-framing members allows you to see all of the studs and even the roof rafters glowing with the heat they are losing. Thermal bridging can greatly reduce the effective insulation value of a wall, floor or ceiling.

Another place where poorly insulated wood is typically used is for structural headers to displace the vertical loads over windows and doors, as was mentioned previously.  is volume of uninsulated wood creates large areas of thermal bridging, significantly reducing the overall thermal performance of the entire wall assembly. As was mentioned earlier, it’s unfortunate that many framing crews are taught to install headers over every window and door, even when they are located in non-load-bearing walls.  is usually hap- pens due to a lack of framing detail provided to them by the structural engineer or truss designer. Best practices are to install headers only where they are required structurally, to size them only for the actual load they are to carry and to insulate them. Many insulated structural header products are available on the market, or you can make your own by sandwiching a rigid foam board panel between two layers of wood (or structural wood product) to create a thermal break.

By adding a layer of rigid board insulation on the exterior of our entire wall assembly, we can reduce or eliminate thermal bridging, as this material provides an insulated break between the wood framing and the exterior heat source. By sealing the attic and insulating over the exposed roof rafters, we can reduce or eliminate thermal bridging there using the same approach. We could also
choose to use an alternative building system, like SIPs, ICFs, AAC block or a natural material, which could significantly reduce thermal bridging in the building assemblies.

We recommended raised heel or energy truss de- sign as a remedy for insulation gaps between the top of the wall assembly and the edge of the roof assembly. It is evident from Figure  that this home suffers from poor insulation in the so t area, a significant source of heat loss in the winter. These are like holes in the thermal envelope, and so the walls perform as if someone has left a window or door open, putting additional strain on the air conditioner or heater as it attempts to provide comfort under these conditions.

Finally, note the heat loss through the foundation or basement perimeter.  is has become a more important issue as we built tighter thermal envelopes, which should enable us to reduce the size of air conditioning and heating systems required to keep them comfortable. However, this heat loss through the foundation assembly can result in raising the heating loads, negating any savings achieved in the main building assembly. In fact, we have seen in- stances in the last couple of years where heat pump system sizing is being determined by these heat losses, driving up heating loads even in cooling-dominated climates with very mild winters.  is means that although we did a good job reducing the cooling loads through building science and envelope improvements, we were forced to install a larger HVAC heat pump system to handle the heat loss through the foundation in the few very cold days of winter that occur.  is is the best argument for insulated slabs in any location that has any chilly winter days. An insulated slab or basement can reduce the heating load on the home by as much as 25 percent or more depending on your climate and house plan.

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