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Cathedral ceiling, adding insulation….

brazosdog02 | Posted in Energy Efficiency and Durability on

I have posted here before about retrofitting my house with spray foam and turning it into an unvented attic. I have since realized that this will be a huge waste of money since my home is already set up and insulated with batt insulation. So, I would like to add to or correct what I have and go with it. Ive talked to a few pros and the only ones suggesting I do spray foam are…well, the spray foam companies. Everyone else actually suggests I add to what I have or don’t add anything.

I have a few areas in my unfinished second story that are cathedral or cold cieling setups. The house has a metal roof with tech shield radiant barrier. Between the 2×10 roof rafters in these areas is R19 insulation that is installed in a ‘cupped’ fashion to preclude the use of baffles and facilitate air flow from the soffits to teh attic.

The issue is that I live in Zone 2, near Houston, TX and I think this is technically under insulated. My living room is done this exact same way, but its already finished and Im not in the mood to pull drywall down to re-do it.

So, in the areas I still have control over, how can I address this? Does it even need addressing since it has a metal roof and radiant barrier? Should I simply add foam board and get close to R30 as possible? If so, should I remove the facing from the batts before putting rigid foam board on the rafters?

Any advice is welcome. I would like to to this as correctly, but price effectively, as possible.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jason,
    Those fiberglass batts aren't doing much, especially if there is no air barrier above them, and no air barrier below them.

    Probably the easiest way to proceed is to install at least two layers of 2-inch-thick polyisocyanurate under the rafters. If you use Thermax, your building inspector may let you leave the Thermax exposed (assuming it's in an attic).

    More information here: How to Build an Insulated Cathedral Ceiling.

  2. Expert Member
    Dana Dorsett | | #2

    Assuming it's a kraft facer and not a shiny-foil facer, in Houston you can just go ahead and wet-spray or dense-pack cellulose into 2x10 rafters compressing the batts right up to the roof decking, as long as you make the interior gypsum reasonably air-tight. Alternatively (and this may be easier/cheaper), you can go ahead and spray open cell foam directly on the facers of the R19s to whatever dept it ends up being when trimmed flush to the interior side of the studs.

    Read & understand: http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems

    See Table 3, p11 (p12 in .pdf pagination) . The most-problematic roofing in the Houston case is tiles, which are a "reservoir cladding", that produces high moisture drives when the sun hits wet tile. With a metal roof you don't have that issue.

    Unless there's an air gap between the r.b. and the metal &/or roof deck it's doing very little for you performance-wise.

    Even if there is some gap remaining between the batts & roof deck after adding more fiber or o.c. foam below it's not a huge hit to the thermal performance.

    In the living room, if there's a gap between the gypsum and batts that gap can be safely filled with cellulose without having to pull the gypsum, but it does involve drilling 1.25" holes in the gypsum that would have to be patched and painted over.

    If you sandwich a layer of foam board between the new gypsum and rafters in addition to a full (or almost-full) cavity fill you'll get a performance boost, but unless you have reliable roof-deck ventilation it must be at least somewhat permeable to avoid creating a moisture trap with the impermeable radiant barrier above. That means NO FACERS on the rigid foam, and if XPS, keep it at 1" or less.

  3. brazosdog02 | | #3

    There is an air gap between the R19 batt insulation and the roof decking. With 2x10 rafters, it leaves NO ROOM for additional insulation as it is. I want to know if it is suitable as is, or if I need to add to it? Not really interested in spray foam any longer. With my location, Im having an incredibly difficult time finding reputable contractors of any trade.

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