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Any vented attic air sealing tricks? And recommended R-values? Blown fiberglass?

Kail_Z | Posted in Energy Efficiency and Durability on

I am building a home in climate zone C4 and am getting ready for insulation at last. I have wrapped the house with a layer of 1″ xps already, but am getting ready for the rest. The local insulation contractor is pushing blown fiberglass over blown cellulose, what do you guys think about this, I have heard it settles less. Most of the house has cantilevered scissor trusses. Do you guys have any good practices for air sealing around all of the electrical lighting boxes (not cans) that I have installed in the ceiling? I was originally planning on installing a 1/2″ layer of polyiso on the face on the second floor ceiling before sheetrock and taping and air-sealing this layer, but now I am thinking this may be too expensive and time consuming. Do you folks know any good tricks for air- sealing thats easier? What about using Sill Gasket at the corners of the sheet rock? I heard this is a good trick. And finally if I am in climate C4 and will probably have R 28 exterior walls, what should the R-Value of my vented attic ceiling be?

-Thanks so much for any advice-

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Any vented attic air sealing tricks?"

    A. Here is a link to an article to guide you: Air Sealing an Attic.

    Q. "And recommended R-values?"

    A. In Climate Zone 4, the 2012 IRC calls for a minimum R-value of R-49 for ceilings. More is better, of course. It's relatively cheap to upgrade to R-60.

    Q. "The local insulation contractor is pushing blown fiberglass over blown cellulose. What do you guys think about this? I have heard it settles less."

    A. I prefer cellulose. All reputable cellulose installers plan for settling. Cellulose does a better job than fiberglass at reducing air leakage and convective looping. For more information on this issue, see Blown Insulation for Attics: Fiberglass vs. Cellulose.

    Q. "Do you guys have any good practices for air sealing around all of the electrical lighting boxes (not cans) that I have installed in the ceiling?"

    A. Airtight electrical boxes are available, but if you forgot to install them, you can air seal ordinary electrical boxes by (a) sealing leaks on the attic side with canned spray foam, and (b) sealing the crack between the ceiling drywall and the electrical box from below, with caulk.

  2. Expert Member
    Dana Dorsett | | #2

    Dense packed fiberglass (1.8lbs per cubic foot, minimum) in wall cavities is somewhat higher performance than dense packed cellulose (3.2lbs per cubic foot, minimum, in your climate) but the effect on total comfort and energy use is "in the noise". I woudn't pay any extra for the fiberglass.

    In open blown loose fill attics cellulose is hands-down the performance winner.

    A layer of polyiso in the ceiling isn't buying you anything. Air sealing the ceiling gypsum isn't usually very tough. For those who love broad sheet flexy stuff 6 mil polyethylene could be safely used at the ceiling, as long as the attic is vented.

  3. Kail_Z | | #3

    Thanks for the advice guys. The local insulation contractor is really pushing fiberglass. He says that the fire retardant in cellulose breaks down over time and it becomes a fire hazard and that is is also corrosive. Is there any truth to this? He also says it will settle no matter what. Is that true?

  4. GBA Editor
    Martin Holladay | | #4

    In some regions of the country, contractors are simply unfamiliar with cellulose. All they know is blown-in fiberglass. It can be hard to resist these prejudices. (And in some areas, the prejudice is so deep that there aren't even any local sources of cellulose at a decent price). You may have to lose this battle.

    If I had no choice but to install blown-in fiberglass instead of cellulose, I would get a little extra -- more than my R-value target -- to cover for the deficences of the fiberglass. And I would be very, very conscientious with my air sealing.

    Cellulose insulation is not a fire hazard. Of course cellulose insulation settles (when installed on an attic floor); that's why the initial installation of cellulose is measured in a way that accounts for anticipated settling.

    For more information on cellulose, see How to Install Cellulose Insulation.

  5. charlie_sullivan | | #5

    The corrosion concern is the only one of those mentioned that has any basis in fact. But that's really only true if you have ammonium sulfate in the fire retardant mix, and you have high humidity conditions. With an all-borate formulation (which is a good idea anyway), there is no concern about corrosion.

  6. Expert Member
    Dana Dorsett | | #6

    The notion that the fire retardents break down over time is simply untrue. It is true that sulfated fire retardents are corrosive (only when WET), but the up-charge for all-borate fire retardent material is negligible (and desirable.).

    If using blown fiberglass in the walls, it has to be at least 1.8lbs density to be as air-retardent as cellulose, and that needs to be specified. Newer types of fiberglass can be installed at 1.0-lb density without settling in walls, but at low density is an air filter, not an air retarder.

    The better fiberglass materials for attics is basically chunked-up high density batt shreds. It's more air retardent than lower density blowing wools, but still not as convection-resistant as cellulose, and it's usually more expensive than cellulose.

    Also just to clarify, while you CAN use 6 mil poly as an air barrier for the ceiling plane, you don't actually NEED a low vapor permeance air barrier in your climate. In the Canadian midwest it's cold enough to need a ceiling vapor retarder in a vented attic or even the zone 7 parts of the US, but not in marine zone 4C (even if it's on the Canadian side of the border, though local code officials may still demand it.)

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