Can you offer feedback on an idea for insulating my home's walls?

Asked by Ron Beck
Appleton, WI

Exterior walls are 2x6 with fiberglass insulation. On the interior side of these walls I installed 1" double-foil Styrofoam insulating board (had to bring out all electrical boxes an extra inch). Taped all the joints with duct tape. Exterior has tar paper with wood siding. Have you heard of this procedure before and was it a good idea on my part as far as vapor barrier protection and insulating value?


Danny Kelly

Answered by Danny Kelly

Charlotte, NC

Kelly McArdle Construction

December 2, 2010

Your wall assembly should perform fine, although most people are installing the foam on the exterior in cold climates these days.

  • The interior foam is now your air barrier so it's important to tape all seams and be sure your air barrier remains continuous.
  • Tricky points will be transitions at rim joists, etc.

The concern is if your warm air gets past your foam and into your wall cavity, the warm air will condense on your cold exterior sheathing. Exterior foam (or "outsulation"), if installed at the proper thickness, will prevent condensation on cold wall sheathing.

Be sure you have a tight seal everywhere and you should be okay. I would recommend running a blower door test to see if there are any hidden leaks you should attend to.

Tagged In: insulation

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