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Re-siding and need to re-insulate but need advice


I live in a 47 year old ranch house in Southern Maine (climate zone 6).

I plan to re-side the house in the spring and would like to improve the insulation while I have the siding off. I would especially like to improve the front of the house and air seal it well as the front faces North.

The house currently has 2 x 4 stud walls with Owens Corning kraft-faced 1965 fiberglass (I think it's supposed to be R-8), 3/4" pine 1x12 sheathing, tar paper, and cedar shingles. My wife and I have lived in it 7 years and in those 7 years I've removed all the old wood framed single pane windows and replaced them all with vinyl new construction windows with built-in J channel. I plan to re-side the house with cedar look vinyl siding. (something like Certainteed's Cedar Impressions).

Anyway, to get to my question. This is what I'm thinking of doing:

After the siding is off, rip off the 1x sheathing, and then cut and fit polyisocyanurate foam into the stud cavities. I plan to cut it about 1/4" small on all sides so it can be foamed in place with Great Stuff Pro. The rigid foam would be held back in the stud bay so it's up against the sheathing. I'm thinking of either using 1 7/8" (R 12.2) Tuff-R since Home Depot has it as a stock item.

Once the cavity foam is in place, I'm not sure what to do. Ideally I would like to wrap the outside of the house in rigid foam to get some more air sealing and cut down on thermal bridging. To avoid removing/reinstalling the windows and door, I was thinking of installing new 7/16" OSB sheathing that would be glued and nailed to the studs. Then I was thinking of installing 1/2" Polyiso on top of the OSB and taping the seams, and then installing the vinyl. This would equal 15/16" of foam/sheathing thickness, which would only be 3/16 more than the 3/4" sheathing on now. If something thicker (like 1" foam) is recommended by the experts, I would have to remove the windows and reinstall them so I'm trying to avoid doing that. But I also know that more foam on the outside is usually better. I only want to do this once and do it right, so I figure I will ask on here to get some advice from smart people.

If you've made it this far, thanks for reading my ramblings. I look forward to any advice people can offer. Thanks!

Asked by Brett Cragg
Posted Dec 30, 2012 2:26 AM ET


2 Answers

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I don't recommend that you remove your sheathing or do anything to your stud cavities. Your existing sheathing is sound -- 1x12 boards make excellent sheathing, and it's a shame to remove the boards.

Any improvement to your insulation is best done with a continuous layer of insulation on the exterior side of your sheathing. Most people would choose some type of rigid foam; I advise you to use polyisocyanurate, the thicker the better.

It's true that you may have to remove the windows, or at least find a way to flash the windows carefully without removing them. But since this job is only done once every 50 years, it's worth doing right.

Answered by Martin Holladay
Posted Dec 30, 2012 7:35 AM ET


Thanks for the reply, Martin. So just an exterior foam layer and nothing else? I was thinking I definitely had to do something in the cavities. That's good news in terms of labor required.

If nothing is recommended for the cavities, I'll probably use the 1 7/8" polyiso Tuff-R that HD sells.Would you recommend putting Tyvek on top of the foam (or under it), or just install the foam, tape the seams and put the siding on?

Answered by Brett Cragg
Posted Dec 30, 2012 9:34 AM ET

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