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Best way to insulate attic

craziekeiichi | Posted in Energy Efficiency and Durability on

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!

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  1. user-2310254 | | #1

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

  2. craziekeiichi | | #2

    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?

  3. Expert Member
    Dana Dorsett | | #3

    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.

  4. GBA Editor
    Martin Holladay | | #4

    In addition to the article that Steve Knapp linked to, you may want to read this relevant article: Air Sealing an Attic.

  5. craziekeiichi | | #5

    Thank you so much for the great recommendations! I have 2x4 rafters up there.
    The temps here are already below freezing most of the time, but I hope to be able to paint in the attic as recommended this year and blow in the new insulation.
    I'm going to reuse the old batts after I paint (I don't think it'd hurt, right?) and blow R48's worth of cellulose on top of everything.

    Thanks again!

  6. Expert Member
    Dana Dorsett | | #6

    What are you going to paint in the attic? If you're re-using the foil faced batts, forget the half-perm paint- just install them with the foil facer down, next to the conditioned space, not the attic. Vapor diffusion is a permeance x area issue, and with extremely low permeance foil between the conditioned space and the insulation over the lion's share of the area there won't be enough moisture getting through via vapor diffusion to really matter.

    The more important factor is to make it AIR tight. A square inch of air leak moves more moisture than a whole attic's worth of vapor diffusion through and around the edges of foil faced batts.

    It only takes ~3" of open-blown cellulose over the top of that low-density fiberglass is sufficiently air retardent to restore the thermal performance of the batts, and you'll be adding more than that.

    A foot of cellulose would be enough to cause some compression of the batts, which is fine- keep track of the total fiberglass + cellulose depth and assume an average of ~R3.5/inch for estimating the final R-value.

  7. craziekeiichi | | #7

    In order to resolve the air leakage problem, I was going to remove the foil faced batts and air seal with caulk and great stuff. As per your recommendation, I planned to spray vapor barrier primer after air sealing. Then use some of the batts again and blow cellulose on top.
    There are many openings through the ceiling leading to pretty severe air leakage. If the vapor paint doesn't make a difference, I'd gladly skip that step. I would still have to remove the foil faced batts to seal it up.

  8. Expert Member
    Dana Dorsett | | #8

    That's right, the batts need to be pulled for air sealing, but if you're re-using them with the foil-side down there's no point to the vapor barrier latex.

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