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How best to seal and insulate cantilevered extensions?

mathiasx | Posted in General Questions on


I’ve got a 1925 bungalow that has a few original room extensions out over the foundation, and one addition from the 50’s in the kitchen that also extends out over the foundation. The previous owner stuffed the cavities between the joists with fiberglass, and the bottom surface is simply beadboard and trim. Above the joists is just subfloor and hardwood floors; nothing to seal. This makes for cold floors in the winter that extends into the room, as verified by a Flir camera. My guess is the fiberglass isn’t blocking any wind coming in, and the joists are a great thermal bridge to the hardwood floors. When it is particularly windy it also brings the cold into the basement.

How should I insulate these cavities to bold stop air flow and to install a correct air/moisture barrier? Drainage on the horizontal surfaces is one of my main concerns — how would you seal and flash this?

Thanks in advance,


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  1. GBA Editor
    Martin Holladay | | #1

    Start by reading this article: How to Insulate a Cold Floor.

    After you have read the article, feel free to post any follow-up questions here.

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