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Best way to insulate attic

My attic currently has R19 foil faced fiberglass batts installed. I would like to air seal the entire attic and re-insulate with blown in cellulose.
The original plan was to cut EPS to fit between the stud bays and seal with great stuff, followed by the fiberglass and blown cellulose. From my understanding, EPS or any rigid foam is not worth the labor for this particular application.

Should I remove the fiberglass batts, caulk/foam the seams and openings, and then reuse the fiberglass followed by cellulose?

My concern is the vapor barrier. I'm up in the Michigans Upper Peninsula and it gets brutally cold up here during the winter months.

Any help planning/recommendations is greatly appreciated.

Thank you!

Asked by craziekeiichi
Posted Nov 13, 2017 10:01 AM ET


3 Answers

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This article will answer most (maybe all) of your questions.


Answered by Steve Knapp CZ 3A Georgia
Posted Nov 13, 2017 1:24 PM ET


Thanks Steve,
The article did have a lot of useful information. Should I approach this as having a newly built house, and remove/discard all the old fiberglass batts?
I was thinking about removing them, sealing up the area, and reinstalling them with blown in cellulose on top. Is it worth the effort?

Answered by craziekeiichi
Posted Nov 15, 2017 9:54 AM ET


If using EPS is better to keep the EPS in large sheets, and detail it as an air barrier (even after detailing the ceiling gypsum as an air barrier), so that it's performance isn't undercut by the thermal bridging of the joists.

How deep are the joists?

In da Yoop you're either in zone 7A though the middle third (east to west) is zone 6. In zone 6 you'd need at least half the R at center cavity to be the EPS for dew point control at the foam/fiber boundary if there is fiber insulation between the joists, and in zone 7 that rises to 60%.

So if the joists are 2x10s you could install R30s between the joists and 7-8" of EPS above the joists and still be fine in zone 6, but you'd be looking at 10" + for dew point control in zone 7. But in either climate zone you could skip the fiber and still hit code min on a U-factor basis with just 9" of continuous EPS.

If the joists are 2x8s, installing low-density R25s yields R24 if compressed to the depth of a 2x8, and 6" of EPS would be sufficient for dew point control in zone 6, 9" in zone 7 (but again, 9" of continuous EPS on it's own makes code-min, the batts would still be a good idea to limit thermal bypass air flows in the joist bays though.)

If going with a cellulose solution, scrap the low density R19s with the foil facers- it's not worth it. In zone 7 paint the attic side of the ceiling gypsum with half-perm "vapor barrier latex" after air sealing, and heap on 15-16" of low density cellulose (raked flat, full depth everywhere) to hit code-min. In zone 6 you probably won't need the paint, but it doesn't hurt.

Answered by Dana Dorsett
Posted Nov 15, 2017 4:38 PM ET

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