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Remodel insulation retrofit, mixed approach

bennettg | Posted in Energy Efficiency and Durability on

Hello everyone. We’re remodeling our home on the coast of NC in warm-humid zone 3.

Our house is a 1.5 story with 8″ (7.25″ actual) rafters which currently has a vented roof structure insulated to ~ R-19 in the cathedral areas, “some” behind the knee walls and about R-30 in the attic areas. We will be re-roofing. I’m considering all the possibilities and their pros and cons.

For the questions here, I’m considering going to an unvented roof with some rigid foam on top of the existing roof sheathing and augmenting the current insulation. The eaves / gables and ridges would be sealed with either spray foam or foam blocking and can foam. I think this might balance the amount of demolition, materials to the landfill, use of plastics, blowing agents and direct cost to me.

From my reading here, once we have an unvented attic with more than the minimum foam above the roof deck , the interior, air/vapor permeable insulation should be in contact with the underside of the roof deck and the underside covered with an air barrier. Correct?

What would be the most effective approaches to augmenting / replacing the existing fiberglass batts / blown in cellulose insulation?

1) In the current cathedral ceiling rafter bays, which have ~ R-19 fiberglass batts covered by sheetrock. Would dense-pack cellulose or fiberglass be the route to go here?
2) In the rafter bays behind the knee walls and above the existing attic areas. I’ve seen rigid foam detailed for the underside air barrier. If so, would it need to be a fire-rated material like Thermax, since it is exposed? I’ve seen Thermo-Ply sheathing mentioned here as well. Would that be a good alternative, particularly in terms of ease of installation and sealing?

I would like any answers to the above, but I think I’ve just made a case for all rigid foam on top of the existing roof sheathing, as it would be way simpler to install and the sealing at the eaves would have to be done in either case.

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Bennett,
    Q. "Once we have an unvented attic with more than the minimum foam above the roof deck, the interior, air/vapor permeable insulation should be in contact with the underside of the roof deck and the underside covered with an air barrier. Correct?"

    A. Correct.

    Q. "What would be the most effective approaches to augmenting / replacing the existing fiberglass batts / blown-in cellulose insulation in the current cathedral ceiling rafter bays, which have ~ R-19 fiberglass batts covered by sheetrock? Would dense-packed cellulose or fiberglass be the route to go here?"

    A. Assuming that you plan to install an adequately thick layer of rigid foam above the roof sheathing, and assuming that you plan to seal the soffit vents and ridge vents, the best approach would probably be to fill the ventilation channels from above (by drilling through the roof sheathing) with cellulose insulation (if available) or with blown-in fiberglass insulation (if local contractors don't offer cellulose).

    Q. "What would be the most effective approaches to augmenting / replacing the existing fiberglass batts / blown in cellulose insulation in the rafter bays behind the knee walls and above the existing attic areas? I've seen rigid foam detailed for the underside air barrier. If so, would it need to be a fire-rated material like Thermax, since it is exposed? I've seen Thermo-Ply sheathing mentioned here as well. Would that be a good alternative, particularly in terms of ease of installation and sealing?"

    A. Assuming that you're talking about sealed attics, most building inspectors allow exposed rigid foam in these areas. (There is no need to specify Thermax or to protect the rigid foam with drywall, since you already have drywall on the interior of the kneewalls and the interior horizontal ceiling.)

    In these areas, it would probably be easier to install fiberglass batts or mineral wool batts rather than blown-in insulation. You can use almost any type of rigid foam as the interior air barrier, but I would probably use foil-faced polyiso because it is easy to tape. If you want an interior air barrier that is vapor-permeable, housewrap or EPS would work.

  2. bennettg | | #2

    Thanks very much, Martin. I hadn't thought of blowing insulation through the roof deck. It's steep, they'll need rigging or a lift. Thanks also for the detail on the foam within the attic and behind the kneewalls. That is probably a detail to check with the local building department on.

    Bennett

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