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Community and Q&A

Unvented attic and wall?

Coopsdaddy | Posted in Energy Efficiency and Durability on

Zone 3, 12/12 pitch roof, Galvalume color roof.
Talked to insulation contractor and discussed options.

2×6 walls = Hardie lap siding, 3/8″ rainscreen, Tyvek, OSB, air sealing, BIB insulation (R-23), 5/8″ sheetrock.

2×8 cathedrial roof, unvented, with total elect units and ducting in kneewall area and spare room in the center = standing seam, membrane?, 7/16″ OSB, 2 inch polyiso, membrane or taped joints?, 7/16″ OSB, 7 inch open-cell or BIB insulation.
No venting.

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  1. GBA Editor
    Martin Holladay | | #1

    Your questions are cryptic, but I guess your questions concern the membranes in the roof assembly.

    The membrane between your roofing and the top layer of OSB is called "roofing underlayment." You can choose from traditional asphalt felt or one of the new synthetic roofing underlayments. (Note, however, that many manufacturers of synthetic roofing underlayment forbid the use of their products in unvented roof assemblies.)

    Between the polyiso and the lower layer of OSB, you can install an air barrier membrane if you want -- some builders buy a European membrane from 475 High Performance Building Products for this purpose -- or you can tape the sheathing joints with a high-quality tape to create your air barrier. In either case, attention to airtightness at this layer is essential (although if you install open-cell spray foam underneath the sheathing, the need for perfection is reduced).

  2. Coopsdaddy | | #2

    Martin,here is our issue,when we meet with designer me and my wife mentioned using the 3d floor attic as a extra space with maybe a ships ladder etc,just for a hang out for kids and maybe a place to read.Well it is turned into a full blown conditioned 400 sf space with extended stairs to 3rd floor.
    My issue is the risk i guess,nobody in our area really has a clue on doing it right,the insulation to r38,standing seam roof unvented with polyiso or vented with vented nailbase,plus all that adds up to 20 plus thousand for this extra space.
    What if we just put insulation in second floor attic floor and use gable vents or soffit to ridge vent with standing seam and called it good.But what about the second floor duct,do we put them in first or second floor joist,also how much room for total elec central heat an air.
    We just need some advise.Seems the money would be better spent on new barn instead of 3rd floor bonus room. This is what the house will look like.

  3. GBA Editor
    Martin Holladay | | #3

    Your designer and builder can provide you options and cost estimates. Then it's up to you to decide what you want. Only you can decide whether it's worth $20,000 for a conditioned space on your third floor. If it isn't, don't do it.

    If you have chosen a builder (or a roofing contractor) who is reluctant to build an unvented insulated roof assembly, in spite of detail drawings which I assume will be provided by your designer, then it may be time to look for a different builder or roofing contractor.

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