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Can we replace fiberglass insulation with XPS?

Hi!

We have a 33 year old townhouse that has a brick exterior on top of wood frame. There is fiberglass insulation in the wood frame, but we have lots of air leakage.

Would there be a problem with replacing all of the fiberglass insulation with XPS rigid insulation or spray foam insulation? If we did, would the XPS and or spray foam be an adequate air and vapor barrier?

Our goal is to reduce the air leakage and heat loss/need for air conditioning.

Thanks . . .

Phil

Asked by Phil Troy
Posted Thu, 07/03/2014 - 19:15
Edited Fri, 07/04/2014 - 04:37

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1 Answer

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Phil,
Q. "Would there be a problem with replacing all of the fiberglass insulation with XPS rigid insulation, or spray foam insulation?"

A. The work you describe is possible but expensive. To do the work, all of the stud bays would have to be open, which in this case means removing all of the gypsum drywall or plaster on the interior. Hopefully, your walls include good wall sheathing (boards, plywood, or OSB) protected on the exterior by a properly lapped WRB (asphalt felt or housewrap).

If you really want to insulate the walls with XPS, you can; the method is called "cut and cobble." For more information on this method, see Cut-and-Cobble Insulation.

Professional insulation contractors never use the cut-and-cobble method; they would be more likely to recommend spray polyurethane foam.

Because the work you contemplate is so expensive, it's worth taking a step back to decide whether the work makes any sense. In most cases, it's more cost-effective to perform other retrofit measures, such as improving the thickness of your attic insulation (to reduce heat loss in winter and heat gain in summer), and to provide window awnings or window film (to reduce heat gain in summer).

Q. "If we did, would the XPS and or spray foam be an adequate air and vapor barrier?"

A. If properly installed -- yes.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 07/04/2014 - 04:35

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