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Vented vs. unvented roofs

GBA Editor | Posted in Energy Efficiency and Durability on

I just talked to an insulation contractor who told me that if I do a proper job air sealing my house, I would be better off NOT venting the roof (this is with cellulose, not foam).

What do people think? I’m in Vermont, and I will have 14′ cavity, 2×10 rafters with 2x4s on edge running perpendicular underneath.

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Replies

  1. Riversong | | #1

    You must mean a 14" cavity (or actually 12.75" if it's KD lumber). In fact, if you do a proper job air-sealing the house, particularly at the critical ceiling plane, then you're far better off venting the roof. The only liability to a vented roof is that if there is greater exhaust (high) net free vent area than intake (low), you can create negative pressure above the ceiling which will exacerbate any ceiling air leakage.

    Most advocates of unvented roofs are focused on saving time and money rather than long-term durability.

    A properly-vented roof, which means approximately 1:150 vent/ceiling area ratio, split evenly between soffits and ridge, and with external wind baffles at the ridge to prevent convective reversal and rain/snow infiltration, serves these critical functions:

    - reduces peak roofing and sheathing temperatures (though not as much as very light colored roofing)
    - eliminates any moisture that does exfiltrate (the original purpose)
    - allows sheathing to dry after a weather-related wetting event (happens to every house at some time)
    - prevents ice dams, which can cause devastating leakage and tear down gutters

    Even those building scientists who advocate "hot" roofs, generally acknowledge that a vented roof is superior.

    In a cathedral ceiling, studies have proven that a vented roof is more durable. And the continuous vent channel can serve as a secondary drainage plane to keep potential roof leaks from soaking the cellulose. A vented roof is an absolute necessity if an impermeable membrane - like Ice & Water Shield - is placed above the roof sheathing.

  2. Danny Kelly | | #2

    My first question would be - do you have your HVAC system in your attic. If not, no reason to really consider this option. I have looked into ways to do an unvented attic with cellulose but cannot come up with an affordable way to provide the code required air barrier on the attic side.

  3. HDendy | | #3

    Many energy professionals will argue that it's better (more efficient) to stop the heat at the roof and contain all volume within the thermal boundary, insulating at the roof deck to create a conditioned or semi-conditioned attic. A product like ventilated nail base might provide the best of both worlds.

  4. homedesign | | #4

    Hunter:

    Many energy professionals will argue that it's better (more efficient) to stop the heat at the roof and contain all volume within the thermal boundary, insulating at the roof deck to create a conditioned or semi-conditioned attic.

    Hunter,
    I agree with your statement
    I do not agree with the "Many energy professionals"
    I think we should Isolate Living Enclosures from Attics and just say "NO" to storage and HVAC in Attics.

  5. HDendy | | #5

    John, in some parts of the country that would be possible and probably a wise recomendation (assuming your closing the crawl). I design quite a few houses that are on the coast in flood plains and we elevate them to a foot or so above the flood line so sometimes the only option is to run HVAC in the attic. In that case I would rather not have the ducts sitting in a 120 degree attic during the summer. We have to have a min of R13 (I prefer much higher) wall between the 72 deg indoors and the 90 deg outdoors, but only R8 between the 120 deg attic and 60 deg supply air.

  6. homedesign | | #6

    Crawlspace! We don't need no stinkin Crawlspace ;-)

    Per Dr Joe
    "The Best CrawlSpace is No CrawlSpace".

    Why does everyone(in the South) think it is IMPOSSIBLE to design a home without a ginourmous HVAC in the Attic?
    I think the key is to provide a LOW Calorie Enclosure
    Like the 1976 LOW-CAL recipe (precursor to passivhaus)
    mentioned in Martin's History Presentation
    http://sites.google.com/site/phconferenceoct172009/session-i-history-and-systems-design/track-1-us-history

    Exteremely Good Enclosures would not require Ginourmous mechanical equipment or large diameter ductwork.

  7. HDendy | | #7

    Yes, but even small/proper sized ducts have to go somewhere. And if the crawl space isn't an option...
    Dr. Joe does recommend closing the crawls...unless your in an area prone to flooding.
    Not everyone in the South thinks like that. Some of us down here do realize that different locales and site conditions call for different solutions ; )

  8. homedesign | | #8

    I will grant you that in coastal neighborhoods where floodstage may be many,many feet above grade...then a house on stilts may be the best ticket.

    Otherwise why not just build a slab on grade?
    It works in Texas,Vermont,Germany and most parts of Florida.

  9. HDendy | | #9

    Yes, slab on grade is great, where it makes sense. That might require mechanical runs in the attic though, back to the case of the location of thermal boundary. Sometimes the architectural goals and/or restrictions call for a structure that is slightly raised from grade, and sometimes we just can't work out the grading and save all the trees and get a suitable pad height, site slopes too much etc. There are just too many variables out there for one prescribed design, we need many tools in our box and asses what's best on a case by base basis. I design homes on the coast and in the mountains, in hot-humid climate and in mixed climate, so I have to be flexible. Believe me, it doesn't make the job any easier.

  10. homedesign | | #10

    Thanks Hunter
    Did not mean to change the subject on this thread.
    I am glad to see more and more members from the Hot, Humid & Mixed
    I think the Cold Climate guys have a head start.

  11. Riversong | | #11

    I think the Cold Climate guys have a head start

    We have to. We get real sluggish in the winter ;-)

  12. Riversong | | #12

    The Experts on Roof Venting

    Bill Rose, ASHRAE, Illinois Building Research Council:
    Airtight ceilings are a more reliable way to ensure a dry attic than venting, but in practice most houses fall into a middle ground where venting balances moisture input.

    Don Fugler, research director, Canada Mortgage and Housing Corporation
    Houses with ceilings tight enough to meet Canada’s strict R-2000 standard “probably could get by without roof venting.” But most Canadian houses, even new ones, don’t have such perfect ceilings.

    Ned Nisson, author, The Superinsulated Home Book, editor, Energy Design Update:
    "I hesitate recommending [unvented roofs] to clients unless I’m absolutely assured of impeccable quality control. In my opinion, roof ventilation is cheap insurance against expensive callback problems. Why gamble?"

    Wayne Tobiasson, research engineer U.S. Army Corps of Engineers, Cold Regions Research Center:
    “Because of the monumental problem of ice damming, there is no question in my mind that the ventilated roof is an order of magnitude better in cold regions.”

    Anton TenWolde & William B. Rose, members, ASHRAE:
    We recommend venting of attics in cold and mixed climates. However, if there are strong reasons why effective attic vents are undesirable, unvented attics can perform well in cold and mixed climates if measures are taken to control indoor humidity, to minimize heat sources in the attic, and to minimize air leakage into the attic from below, or vice versa. The necessity and effectiveness of vents in cathedral ceilings in cold and mixed climates is still a contested issue. Unvented cathedral ceilings can perform satisfactorily in cold and mixed climates if the cavity is properly insulated, measures are taken to control indoor humidity and minimize air leakage into the roof cavity, and a vapor retarder is installed in the ceiling.

    Paul Fisette, director of Building Materials Technology and Management at the University of Massachusetts, Amherst:
    "There are many ways to treat the symptoms [of ice damming], but proper air sealing, insulation, and attic venting are the best ways to eliminate the problem."

    Joe Lstiburek, Building Science Corporation:
    "Vented attic/roof designs have the advantage of a long, proven historical track-record. However, they work best with airtight ceiling/attic interfaces and where ductwork and air handlers are not located within attic spaces. The increase in the use of complex roof shapes and cathedral ceilings has resulted in problems with vented roofs."

    "In extreme snow regions it is necessary to add a vented air space between the roof cladding (shingles) and the rigid insulation to avoid ice damming. The vented air space is needed to flush heat away trapped by the insulating value of relatively thick snow."

    Journal of ASTM International, Volume 6, Issue 4 (April 2009)
    Peter E. Nelson P.E., Senior Principal,Simpson Gumpertz and Heger, Inc., Jason S. Der Ananian P.E., Senior Staff I-Building Technology,Simpson Gumpertz and Heger, Inc.:

    All of the unvented roof assemblies are intolerant of incidental water leakage and the moisture-sensitive layers (i.e., sheathing and gypsum wallboard (for open-cell polyurethane insulation)) exceed the threshold for decay. In hot, humid climates, the most durable roof assemblies are the vented, open-cell polyurethane systems with shorter drying time of the interior gypsum wallboard when compared to the unvented roof assembly; both the sheathing and gypsum wallboard dry out within 2-1/2 months. In an unvented assembly, the drying time for sheathing is similar but the drying time for gypsum wallboard increases to 6.5 months on average. Alternatively, unvented permeable shingled roofs are a viable option in hot, humid climates, although they are slightly less durable. The least tolerant roof assemblies in either climate are the unvented closed-cell polyurethane roof assembly due to trapped moisture and slow drying of the roof sheathing (up to 12 months in Miami, FL and 27 months in Boston, MA).

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