Typical Rafter Insulation Detail

Here is a typical insulation detail for a rafter framed roof.  We’re showing a 1” polystyrene baffle to provide airspace between the roof sheathing and rigid insulation for required roof ventilation.  Under that is a 2” spray foam insulation with an R-6.9 per inch thermal value.  The remaining rafter bay is filled with Spider fiberglass batt insulation which has a built-in adhesive, and has an R-value of 4.3 per inch.  In a typical 11-7/8” rafter bay, this entire assembly provides an R-value of 52,  well above the min. R-45 we show on our plans and the min. R-38 required by code.


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