BackPage Primer from Environmental Building News
July 1, 2009
Thermal Bridging
The purpose of insulation is to slow down the movement of heat, and most of that effect occurs by slowing
conductive heat transfer. The higher the conductivity of a material, the more quickly heat can move through it. Insulation materials are rated by their
resistance to heat transfer, a term we call
R-value. The higher the R-value, the lower the heat transfer.
When different materials with different thermal conductivities span from the warm side to the cold side of an assembly, more heat flow occurs through the material with higher conductivity. If the differences are large enough, that heat flow essentially short-circuits the insulation—a process referred to as
thermal bridging.
Consider steel framing and fiberglass insulation. The thermal conductivity of steel is 1,200 times as great as that of fiberglass insulation. What this means from a practical standpoint is that if you have an insulated wall that’s constructed of standard thin-gauge steel studs and insulated with fiberglass, even though there is a relatively small cross-sectional area of steel in the wall, thermal bridging through the steel will have a huge effect on the average—or unit—R-value of the wall.
Wood also causes thermal bridging in a wall that’s insulated with fiberglass. The conductivity of softwood (spruce) is a whole lot lower than steel, but still nearly three times greater than fiberglass.
Let’s look at a real example: a 2x6 wall insulated with R-19 fiberglass. If that wall is built with softwood studs 24"(610 mm) on-center, the
unit R-value for the wall will be R-16. If the wall is built with steel studs with the same spacing, the unit R-value will be R-8.6—less than half the nominal R-value of the insulation.
The conductivity through steel studs in a wall may be so great that condensation occurs (moisture from the air condenses out as liquid on cold surfaces). One occasionally sees “shadows” of the studs on such a wall created by mold growth due to that conductivity. To reduce thermal bridging in walls, exterior rigid insulation is often used—covering the studs entirely with foam.
Wall insulation isn’t the only place in buildings where thermal bridging is a major concern. It also comes into play with slab edges in both commercial and residential buildings and with glazings. Any time a concrete slab or roof deck extends to the outer surface of the exterior envelope, significant heat loss can occur.
Metal glazing extrusions for windows and curtainwall systems are often made from aluminum, which is even more conductive than steel. Thermal bridging through the aluminum not only causes excess heat loss, but the cold surfaces often also result in condensation, which can cause moisture problems. Glazing extrusions with integral polyamide (nylon)
thermal breaks will reduce that thermal bridging, with an effectiveness that increases as the space between interior and exterior metal elements gets bigger.
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Illustration: Julia Jandrisits
