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Conditioned attic with vented roof deck

Jeffrey_Savage | Posted in Energy Efficiency and Durability on

I am planning on building a conditioned attic space in my new home. I was going to vent the roof deck by adding 1×4 furring strips and then a second OSB (with radiant barrier) to the furring strips. Vented soffit to ridge. Is the 0.75 inch airspace large enough to be effective for the radiant barrier? Or should I spec 2×4’s?

Thanks.

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Replies

  1. Jeffrey_Savage | | #1

    Im in Phoenix, BTW.

  2. GBA Editor
    Martin Holladay | | #2

    Jeffrey,
    You'll get better air flow with a 1.5 inch deep ventilation channel than with a 3/4-inch deep ventilation channel. But the radiant barrier will work in either case.

    What problem are you trying to solve? If you live in Phoenix, I doubt that you are worried about ice dams.

    My guess is that the money you are about to spend on two layers of sheathing could be better spent on something else -- perhaps thicker insulation.

  3. Expert Member
    Dana Dorsett | | #3

    If you are building an insulated unvented roof insulated to the IRC 2015 R38 code min for US climate zone 2 (Phoenix is in zone 2B) the additional benefit of radiant barrier is somewhat academic, and certainly not worth paying extra for. It's worth adding an inch of foam for wintertime dew point control (foil faced polyiso is fine) above the roof deck, and using a higher solar reflective index roofing (look up third party tested SRI specs on the CRRC site: http://coolroofs.org/products ), but building secondary vented channel just for the radiant barrier isn't worth it.

    With R5 above the roof deck you'd be good to go without interior vapor retarders (other than standard latex paint on ceiling gypsum). See Table R806.5:

    http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%208.html

    If your rafters are 2x10s, and you can only get R30 fiber in there, then an exterior inch of polyiso still gets you to code compliance for thermal performance on a U-factor basis. See Tables N1102.1.2 and N1102.1.4:

    http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%2011.html

  4. Jeffrey_Savage | | #4

    I'm just trying to lower the temperature entering my attic space. I've seen several sources tout the benefits of venting the decking of an unvented attic space. Claiming significant cooling due to the venting, longer shingle life, etc. I figured the radiant barrier would be a plus.

    Above sheathing insulation is cost prohibitive. The foam itself cost more than the OSB and furring strips. In materials alone it would be >$4,000, then I would have to add a second layer of OSB prior to shingling. So really I'm just paying for furring strips to create the ventilation. Maybe none of its worth it and I should just add more insulation.

    I plan on reaching my R value with open cell spray foam.

  5. GBA Editor
    Martin Holladay | | #5

    Jeffrey,
    Before settling on open-cell spray foam for this application, you may want to read the warnings in these two articles:

    High Humidity in Unvented Conditioned Attics

    Open-Cell Spray Foam and Damp Roof Sheathing

    One way to mitigate the risks described in these articles is venting above the open-cell foam, so your plan isn't crazy (although your stated objective -- "to lower the temperature entering my attic space" -- is much less important than the structural objective I'm talking about: "preventing rot in the OSB layer").

    Usually, it's cheaper to ventilate under the lowest level of sheathing rather than above the lowest level of sheathing. If you install stiff ventilation baffles in the rafter bays before the spray foam is installed, you'll save money, because you will only need one layer of OSB instead of two.

    Either venting strategy -- below the sheathing or above the sheathing -- will only work if you have a roof without dormers, skylights, valleys, or hips. If you have a chopped-up roof with the type of impediments I've listed, you need an unvented approach (and in that case, closed-cell spray foam is much less risky than open-cell spray foam).

    If you go ahead with your plan, make sure that you leave out the roofing underlayment above the lowest level of OSB. That way, you'll get some outward drying to mitigate the OSB rot that plagues some roofs with open-cell spray foam.

    Finally, if you have a roof geometry that allows venting, you might reconsider using open-cell spray foam, and just switch to dense-packed cellulose. That way, you avoid the risks associated with open-cell spray foam.

  6. charlie_sullivan | | #6

    Martin suggests, "switch to dense-packed cellulose. That way, you avoid the risks associated with open-cell spray foam."

    You'll also save some money with that choice.

  7. Jeffrey_Savage | | #7

    Single gable roof with no dormers, skylights, valleys or hips ( I read your roofing advise column, :) ). I didn't realize dense-packed cellulose was a suitable alternative. As it is a truss roof with an attic space just for HVAC I was going to spray 5+ inches beyond the top strung (2x6) of the truss. Eliminating most of the thermal bridging. I know Owens Corning has a Blown Fiberglass system that is blown into a loose mesh that hangs beyond the rafters, but it doesn't work well with the truss' struts getting in the way. I imagine dense-packed cellulose would have similar challenges.

    I think stiff ventilation baffles seem like the perfect solution for me. They seem to accomplish my goals in a simpler system. I'm just trying for a pretty good house standard.

    I'm not so concerned about moisture issues because of our dry climate (outside of a couple months in monsoon season, and even then humidity dissipates between storms to less than most areas).

    Thanks again!

  8. user-2310254 | | #8

    You might find another of Martin's articles helpful: https://www.greenbuildingadvisor.com/blogs/dept/musings/martin-s-ten-rules-roof-design

    It sounds like you have your hvac system and ducts in the attic. Ideally you want this equipment inside the home's conditioned space. Often, the easiest way to do this is to create an unventilated attic.

  9. Jeffrey_Savage | | #9

    Unvented is what I will be doing. I am wanting to vent just the roof decking. I'm hoping for moisture control and extra efficiency by doing so. Just trying to figure out the most practical method of doing so.

    Im in zone 2B and the Delta between room temp and attic temps can be 75 degrees. My goal is to get the roof decking to cool off as much as possible when we have our "lows" of 95 degrees in the summer.

  10. Expert Member
    Dana Dorsett | | #10

    Unvented ATTIC, but vented roof deck is what you are proposing, which is fine.

    Even with a high-pitched 12:12 roof the amount of cooling you get out of venting the roof deck is pretty negligible, and at low pitches (under 3:12 ) it's effectively nil. Most of the cooling of the roof deck is via re-radiation back at the sky and convection on the exterior surface, even with a vented roof. The primary function of venting the roof deck is to purge moisture.

    The way to maximize the cooling of the roof deck is to use a high solar reflective index (SRI) roofing, and to use a roof pitch of 6:12 or higher to promote convection at the exterior. The Cool Roof Ratings Council (CRRC) maintains a searchable database of third-party verified solar reflectance data on a large number of roofing products, which I linked to in a prior post. Here is that link again:

    http://coolroofs.org/products

    An aged SRI 30 or higher would be enough for a 6:12 pitched roof, but you'll want to go much higher for a 3:12 or lower roof angle. And this is independent of whether the roof deck is vented or not, which has remarkably little cooling effect when compared to roof pitch and solar reflectance. Even an SRI of 40+ there are many color options with asphalt shingles:

    http://coolroofs.org/products/results/search&channel=products&orderby=cf_product_3yr_sri+desc&cf_product_slope=Steep&cf_product_pri_prod_type=%22Steep+Slope:+Asphalt+Shingles%22&limit=50

    But to hit the 90s you're basically looking at white mop-on coatings or membranes, or light colored finishes on metal roofing.

    http://coolroofs.org/products/results/search&channel=products&orderby=cf_product_init_sri+desc&cf_product_slope=Steep&limit=50

    Of course, the best way to cool the roof deck would be shading it from the sun with rail-mounted solar panels. (Vote "no" on the Referendum 1- it's a utility-wolf in solar-sheep's clothing: http://www.nytimes.com/2016/10/28/science/florida-solar-power-referendum.html?_r=0 )

    The short-course on roof venting vs. unvented in a Florida climate is in this literature survey piece:

    http://www.fsec.ucf.edu/en/publications/pdf/FSEC-CR-1496-05.pdf

  11. Jon_R | | #11

    In that last source you will find "...double roof configuration for sealed attics results in large improvements in thermal performance". And the drawing and picture suggest that you don't need 12:12 to get it.

  12. Expert Member
    Dana Dorsett | | #12

    That was in reference to a sealed roof deck with only R22 open cell under the roof deck, nowhere near a current code min.

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