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Safe to dense-pack cellulose into cathedral ceiling bays and polyiso later when re-roofing?

agurkas | Posted in GBA Pro Help on

I have very high and steep cathedral roof in my living room and since that addition was built in ’78, it isn’t exactly built to some great insulation standards. Rafters are 8″ deep and currently I have windswept fiberglass there. Have no idea how bad that kills the insulation value, but looking at how fast temps drop in that room gives me sense it isn’t good.

I will be replacing the roofing in about 5-7 years. In the meantime one of insulation contractors is suggesting we cut a slit at the top and bottom of the roof, pull out all the fiberglass, and dense pack those bays with cellulose. I am actually thinking we just dense pack over the fiberglass and skip pulling it out.

Do you think I run a high risk if I do that? 5-7 years I will be putting several layers of polyiso over the roof and doing standing seam.

I am in Zone 5 (Boston)

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  1. charlie_sullivan | | #1

    When you say "windswept", do you mean that the rafter cavities are vented above the fiberglass?

    If part of the poor 1978 construction is air leaks into the ceiling, the risk might be significant. If the ceiling is pretty air tight, you might be OK, and you'd have an opportunity to check the condition of the sheathing in 7 years and replace it if necessary.

    On the other hand, the savings over 7 years might not be large enough to justify the trouble of doing it now rather than in 7 years.

    Depending on the vents, an option might be to stick a humidity probe in there partially close the vents to reduce the wind washing, and keep an eye on the humidity, opening the vents more if it gets higher than you'd like for a while.

  2. agurkas | | #2

    Yes, fiberglass is only taking up 6 or so inches of the rafter cavities. When I looked there with a camera, yellow fiberglass is gray from all the dust. Right now that room drops about 4F degrees per hour, if heating is off and it is around 34 degrees outside. If left without heat overnight, 68F ends up at about 54F when I wake up. It is 30'X21' room with almost 30' at the peak of the ceiling. Vent is like a 1" wide slit.

    If I close the vent, what moisture level I would want to keep it at?

  3. Expert Member
    Dana Dorsett | | #3

    Is there a polyethylene vapor barrier on the interior side? (That was fairly popular in the 1978 time frame.)

    With a poly vapor barrier it absolutely needs to be vented, since there is no drying toward the exterior through #30 felt + asphalt shingles, and no drying toward the interior through 6-mil polyethylene.

    The fiberglass is probably low density R19s, without an exterior side air-barrier, which means they're probably performing at about R10-R12.

    If there is no interior side poly you'd probably get away with dense-packing over it for 5-7 years without losing the roof deck to rot, but there is some risk, riskier if one of the pitches is facing due-north (getting zero direct sun heating) , and/or if the shingles are a lighter color (rejecting some of the solar heating.)

  4. GBA Editor
    Martin Holladay | | #4

    If your cellulose insulation contractor is very experienced at dense-packing this type of cavity, the risk is probably acceptable. If the cellulose isn't fully dense-packed, and there are areas with low-density insulation or voids, your suggested approach becomes riskier.

    It would certainly be safer to install the rigid foam above the roof sheathing first, and to dense-pack the rafter bays after the foam is installed.

  5. agurkas | | #5

    Roof is facing almost exactly East and North. I was looking through old blueprints and specs and only found that it was paper faced fiberglass. When I checked with my scope camera that is what I found. No poly.
    Also, interesting bit, since ceiling has not been painted since 70s, somewhere around half way up the ceiling you can see like gray ghosting in the paint. It is like a foot wide horizontal band that looks like outline of rafters and whatever ties them together.

    Included picture of where the snow was melting the most last winter on that roof. You can kind of see some nice thermal bridging.

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