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Community and Q&A

Air-Sealing a Bay Window

severaltypesofnerd | Posted in Energy Efficiency and Durability on

How might you go about air sealing a semi-enclosed bay window floor from below?  In the past it looks like someone used duct seal, but it cracked.  The subfloor and the finish hardwood floor run the same way, so you can literally see sunlight through the floor. Zone 3 marine.


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  1. Expert Member
    1. severaltypesofnerd | | #2

      That's a great article, unfortunately it's a 1926 house and dense packing while great for controlling airflow, feels risky due to moisture.

      Like many such bay windows the bottom was originally stucco, and it... you guessed it... trapped water and rotted. The older designs are a great reason for vent strips and Exterior Elevated Element inspections. But not so much for energy efficiency.

      Short of using a time machine to secret modern designs to the 1920's: what's the retrofit option here?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        I would treat the underside of the floor the same as the blocking between the joists, with pieces of EPS foamed into place. Completely fill the cavities with whatever batt insulation you prefer, and finish the underside with cement based board, or plywood.

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