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Unvented attic with open-cell spray foam

John Grutta | Posted in Energy Efficiency and Durability on

Newbie here with a design question. Planning a retirement home about one hour west of Ft. Worth, TX, (Gordon,TX) climate 3B I believe. Having trouble with the proper unvented roof design using, as I’m told by most of the approved builders in my development, open cell sf (spot leaks if any develop). What are my most practicable, cost efficient methods of :

1) achieving my required R code value in the roof (R38 roof/R15 walls per USDOE website)
2) eliminating thermal bridging (if that is a concern in climate 3B?)
3) Use ADA air sealing of sheetrock

A portion of the roof will be a cathedral great room, the remaining attic will have the mechanicals in it.

My understanding is I cannot use poliso (as in a vented Hunter panel) over the open cell under the roof deck. Roof will most likely be composite shingles or metal panels, if budget allows.

My real concerns is the extreme summer heat they experience in this area.

Thanks. Learning alot on this great site.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    John,
    There are many ways to build an unvented cathedral ceiling. Learn about your options here: How to Build an Insulated Cathedral Ceiling.

    Q. "What is my most practicable, cost efficient method of achieving my required R code value in the roof (R38 roof)?"

    A. You'll have to get bids from local contractors. You could build a vented cathedral ceiling insulated with cellulose, or an unvented cathedral ceiling insulated with spray polyurethane foam, or an unvented cathedral ceiling with some of the insulation (rigid foam) above the roof sheathing, and some of the insulation (fiberglass or cellulose) below the roof sheathing.

    Q. "What is my most practicable, cost efficient method of eliminating thermal bridging?"

    A. If you're talking about your cathedral ceiling, I would say: install rigid foam insulation above your roof sheathing. (Follow the guidelines for minimum insulation thickness provided in the article above.)

    Q. "What is my most practicable, cost efficient method of using ADA air sealing of sheetrock?"

    A. ADA is pretty straightforward. You don't have many choices, except you have to choose which brand of airtight electrical boxes to install, and whether to use gaskets or caulk. Other than that, it's a basic recipe without many variations. More information here: Airtight Drywall.

  2. Expert Member
    Armando Cobo | | #2

    John,
    To address thermal bridging in the DFW climate I use 1” R5 rigid foam on top of the roof decking (2” is better R10) and 8.5” R33 OC foam under the roof decking. For wall assemblies, ½” R3 rigid foam for outsulation (1” is better R5) and 3.5” R13 cellulose (5.5” is better R23).

  3. Steven Medlin | | #3

    The use of LeeCor Systems ThermaSteel Roof or Ceiling panels can provide an effective R40, eliminate your thermal bridging and eliminate the need for any roof sheathing completely if you go w/ the metal roof finish. If you select an architectural shingle finish, you would have to add some type of non-structural sheathing to serve as a nailing surface. The LeeCor structural panels can also eliminate the need for trusses and wood products in the building envelope all together.

  4. John Grutta | | #4

    Thanks for your response Martin and Dan. I recall reading somewhere (I will look for it) that I cannot put rigid foam over open cell spray foam because its vapor-permeable. But if I can, would the system from the inside out be:
    ADA Drywall - OCSF the full depth of the rafter - Roof sheathing - 2" of unfaced polyiso (not foil faced) or (2" Hunter panel or equal) - Another layer of sheathing - Roof membrane (Dupont RoofLiner?) - Shingles.
    I'm creating a hot roof, nothing vented.
    No vent space between the OCSF and the first roof sheathing. No recessed lights in the sprayed rafter bays.

    For the walls why not stay with the OCSF and 1" rigid foam over wall sheathing? Also, I plan on a
    stone exterior, does that change any of the wall assembly?
    So the wall assembly would be:
    .5" ADA Drywall - 3.5" OCSF - Sheathing - 1" Polysio foam (unfaced?) - Tyvek - Drainage plane mat - Stone
    Windows would be attached to the sheathing or brought out to the face of the foam layer? Thanks.

  5. John Grutta | | #5

    In response to Steve's suggestion, I only have a few builders I can use. Most never heard of a SIPs Panel. I really don't want to be the first house they build with any methods out of their comfort zone. I originally thought SIPs would be a great way to go, but no one uses them in my area. The LeeCor panels do look like a great solution. Any idea what the cost would be compared to systems Dan and Martin suggest?

  6. GBA Editor
    Martin Holladay | | #6

    John,
    1. If you want to install rigid foam on top of the roof sheathing, it's OK to install open-cell spray foam underneath, because open-cell spray foam is vapor permeable. Don't install closed-cell foam.

    2. DuPont RoofLiner is a vapor barrier and therefore cannot be used on an unvented roof. Here is information from a DuPont website: "DuPont RoofLiner is a vapor barrier. Ensure that the space beneath the roof sheathing is properly ventilated before installing to avoid condensation problems." (http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/pdfs/CON11_17395_RoofLiner_InstallGuidelines_REV_me05_LR.pdf). Ordinary asphalt felt is a perfectly good roofing underlayment.

    3. I don't know whether your stone exterior is natural stone, manufactured stone, or adhered stone veneer, but all three types of cladding are associated with wet-wall problems and wall-rot problems unless the installation is meticulous. A free-draining air gap between the stone and the sheathing is mandatory, and flashings must be impeccable.

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