Why insulation should not end with filling framing cavities
Everything is relative — especially when it comes to 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. . Thermal bridging occurs wherever assembly components with low R-values relative to surrounding materials span from the inside to the outside of a building assembly. Thermal bridging takes place in wood-framed assemblies because, although wood is a pretty good insulator at about R-1 per inch, it is at least three times more thermally conductive than any cavity insulation, which start at about R-3.5 per inch.
With steel framing at about R.04 per inch, it’s thermal bridging on a whole different scale. Steel framing typically reduces the in-cavity R-value by as much as 50%, while wood framing reduces in-cavity R-value by a bit less than 10%.
Wrapping a building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. with exterior rigid insulation cuts off the thermal bridging. Rigid insulation as exterior sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. is a great idea for wood-framed assemblies, imperative for steel. But as in all things hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out., you get a double benefit when you reduce or eliminate thermal bridging — you save energy and reduce the potential for condensation, mold, and rot.
The main reason we put insulation in framing cavities is because there is space — it’s the easiest and least expensive place to put the stuff. But it is not the best place, in large part because of thermal bridging. In what Joe Lstiburek of Building Science Corporation calls the "perfect wall” (or roof), all of the insulation is on the exterior. This means that the building frame experiences nearly the same conditions as the occupants, a real recipe for happy, comfortable, and long-lived people and buildings.
To gauge the energy implications of framing type, spacing, and thermal bridging, try the ORNL Whole Wall R-value Calculator.
- Dan Morrison
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