Working With Thick Layers of Exterior Foam

With extra thick layers of exterior foam, keeping a consistent “water control layer” can be complex. Building America’s latest “measure guideline” provides step-by-step guidance for solving water-sealing details around all types of wall penetrations, including windows. In the examples shown, the water-control layer is applied over the foam. Details for applying it under the foam for both “innie” and “outie” windows are also provided.

The details for using thick layers of foam on a wall include how to detail side-wall flashing. This one depends on the ingenious use of foam blocking behind the flashing.

This detail for attaching and flashing a deck ledger doesn’t completely solve thermal bridging issues. But given the risk of a deck collapsing from a ledger attachment failure it’s the safest approach offered in the new guidelines.

This table is adapted from Table R601.3.1 “Class III Vapor Retarders” from the 2009 IRC and Table R702.7.1 “Class III Vapor Retarders” from the 2012 IRC. It provides guidance for the minimum R-value needed on exterior walls to control wall-cavity condensation for climate zones 5 to 8 and Marine 4.

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