What is Thermal Bridging?
Why R-19 is never R-19
Like wind washing, 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. is something folks mention all the time during audits (meaning they never ask about it). But what is thermal bridging, and why do I keep bringing it up when my customers just want new windows?
To understand thermal bridging, you need to understand your home’s wall assembly and the various materials used in its construction.
My house is well insulated, right?
The exterior walls of your home are (hopefully) well insulated. The building shell of your home provides the structural barrier between your dream home’s interior and the rest of the world. It keeps out unwanted air infiltration (mostly), moisture (again, fingers crossed) and retards the flow of heat through the wall. If well insulated, your home ought to stay warmer during cold winter months and cooler in the summer.
R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. is the measure of how well your insulation resists heat flow across the material in question. (For more information on R-value, see Understanding R-Value.) In theory (quite literally — a material's theoretic R-value is tested under lab conditions), R-value is supposed to be an objective comparison of the thermal resistance of different materials.
Unfortunately, this just isn’t so. As I explained in a previous post, we all wish there was a magic, universally applicable number like car gas mileage with which to compare various insulation. But there isn't.
BLOGS BY ERIC NORTH
Thermal bridging is one reason you can’t blindly judge insulation based on its R-value.
Standard wall assembly construction (at least in the Northeast) consists of drywall on 2x6 studs, 6-inch-thick fiberglass batts within the wall cavities, plywood or OSB sheathing, Tyvek building wrap as a water control plane, and siding. The R-value for any cross-section of the wall can be found by adding the material R-value of each layer.
A boring definition of thermal bridging
Thermal bridging occurs when a more conductive (or poorly insulating) material allows an easy pathway for heat flow across a thermal barrier. The most common form is probably within the eyesight of every reader of this article: wall studs.
Suppose your walls have 6″ fiberglass batts. (I’m sorry. We’re working to update the building code.) Every 16 inches on center in that wall is a 2x6 or 2x4 stud. The fiberglass has an R-value of around 3.5 per inch, and the stud is around R-1.2 per inch. The wood studs allow heat to flow through the wall assembly at a rate that is 3 times faster than the heat flow through the surrounding insulation.
While the advertised R-value for a 6-inch fiberglass batt is R-19, the building assembly’s effective R-value is about R-3 lower. Bummer.
Cures for the thermal bridging blues
The question then becomes… What to do?
Simple enough: you need to eliminate or reduce the thermal bridging. There are many approaches. In new construction, you can build the walls SIPs (structural insulated panels) or use advanced framingHouse-framing techniques in which lumber use is optimized, saving material and improving the energy performance of the building envelope. techniques which reduce the number of wall studs.
A newer approach involves applying strips of insulation over the wood studs to provide a thermal break.
In retrofit scenarios, sheets of polyisocyanurate foam, high-density rock wool, or Larsen trusses can be applied on the exterior side of the wall sheathing as a thermal break.
If your home has aluminum-framed windows, the addition of insulated shades can help reduce the severity of the thermal bridging through the aluminum during the summer.
Similarly, you can reduce the effect of thermal bridging through a metal-framed door if you install a storm door.
Thermal bridging is one of the hidden heat-loss paths that a home energy auditEnergy audit that also includes inspections and tests to assess moisture flow, combustion safety, thermal comfort, indoor air quality, and durability. can uncover. Keep it in mind when thinking about the heat loss (and the heating bills) at your home.
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