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Air space or rigid foam to insulate on exterior of block wall that will have furring strips for vinyl siding.

I've been googling and cannot find info close to my situation. So I am hoping to get some ideas here. I have purchased an old building with block walls and no windows. It is the corner eyesore but is useful as a secondary shop for me, plus it zoned as commercial property, which you never know when having a commercial address can be beneficial. Anyway, I only plan to use it as a workshop, so optimal heating isn't crucial. However, being an eyesore, and my experience with painted exteriors in Michigan, I plan to vinyl side the building, which will include furring strips. I was thinking of double reflective polyethylene roll insulation after the furring strips. Now, some sales pitches for various items claim air space is insulation. I was wondering the effectiveness of the airspace vs filling it with 3/4 inch rigid foam. Or would I be better off putting the polyethylene on first, then furring strips. Also, would I gain much to insulate the interior as well as exterior of the block walls. I plan on using wood burner heat, so it seems to me once I get the block warm, it would really hold the heat. Any help will be greatly appreciated. Thank You

Asked by Eric Wheeler
Posted Aug 3, 2014 9:53 PM ET


2 Answers

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While it's possible to use a radiant barrier adjacent to an air space to insulate your wall, the R-value of the air space is low -- about R-3. You'll be better off installing real insulation. For example, 2 inches of EPS would give you about R-8, or 2 inches of polyisocyanurate would give you R-12 or R-13.

If you want to meet minimum code requirements for wall R-value, you would need thicker foam.

For more information on installing vinyl siding over furring strips, see Can Vinyl Siding be Applied Over Furring Strips?

Answered by Martin Holladay
Posted Aug 4, 2014 5:04 AM ET
Edited Aug 4, 2014 5:06 AM ET.


Q. "Would I gain much to insulate the interior as well as exterior of the block walls?"

A. It's best to install all of your wall insulation on the exterior of the walls, if you possibly can. You should be able to install 4 inches of rigid foam on the exterior, if you are willing to do so. The vertical furring strips can be attached with long TapCon screws through the foam to the block wall.

Answered by Martin Holladay
Posted Aug 4, 2014 6:26 AM ET

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