Commonwealth Property Group

Contributor Randy Williams, , is writing a series for GBA about exterior insulation, also called continuous insulation (CI) and “outsulation.” He talks about using it in new construction and adding it to existing houses. It is a building assembly he favors. So, what are some benefits of putting insulation on the exterior of walls and roofs?

Having rigid insulation over sheathing and framing reduces thermal bridging and air infiltration. That means reductions in heat loss and a higher whole-assembly R-value.

For example, take a 2×4 wood-framed wall with R-13 fluffy stuff in the cavity and R-5 board insulation on the exterior. The combined R-value is R-17.26, while a 2×6 wall without exterior insulation measures R-15.67. So the 2×4 wall with continuous insulation performs better than the 2×6 wall without it.

Other benefits include reducing the chance for condensation on the inside face of the sheathing, which happens when the sheathing temperature is below the dew point. Insulation outboard of wood-based sheathing keeps the sheathing warm and dry by keeping it above dew point temperature. Moisture in the cavity can lead to rot and mold, or at the least, a diminished R-value.

Also, because continuous insulation reduces air infiltration, it typically means a smaller, less expensive HVAC system.

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