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Musings of an Energy Nerd

All About Roof Venting

Not all roofs need to be vented—but if you decide to vent your roof, get the details right

Airtightness matters. Whether you are building a vented attic or a vented cathedral ceiling, your roof venting strategy won't work unless your ceiling is airtight. [Image credit: Fine Homebuilding]

There are several reasons why builders have so many roof-venting questions. For one thing, building codes concerning roof venting are confusing. For another, builders are often uncertain about when unvented roofs are acceptable. If they are carefully detailed, either vented or unvented roofs perform well—but whether vented or unvented, poorly detailed roofs can fail quickly, especially in a cold climate.

Why do we vent roofs?

Builders include a ventilation channel under roof sheathing in hopes that moving ventilation air will remove moisture that has escaped from the house through air leaks in the ceiling. In some climates, we include a ventilation channel under roof sheathing in hopes of reducing the frequency of ice dams.

According to building scientists, roof ventilation is less effective at drying roof sheathing or reducing ice dams than commonly thought. Moreover, some building codes imply that indoor air travels to roof sheathing via vapor diffusion. (These codes are a legacy of an earlier era—the benighted era when vapor barriers were mistakenly seen as the best solution to moisture problems.) In fact, vapor diffusion has very little to do with the types of moisture transport that affects roof sheathing. In most homes with damp roof sheathing, the moisture reaches the roof sheathing via air leaks from the interior, not via vapor diffusion, so the presence of a vapor retarder or vapor barrier near the insulation is far less important than the presence of an air barrier.

If you are concerned about keeping roof sheathing dry or reducing ice dams, make sure that your house has an airtight ceiling, and verify your air-sealing techniques by performing a blower-door test. For dry roof sheathing, an airtight ceiling is more important than the effectiveness of your vent channel.

For more information on roof-venting effectiveness, see “All About Attic…

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13 Comments

  1. Russell Bounds | | #1

    Martin - great post - as usual - however, "If you are building an unvented cathedral ceiling or an unvented attic, and you are installing air-impermeable insulation only" and I use standing seam metal roof - do or don't I - need to provide an air space (slats or 2x4) between the roof underlayment panels and the metal. I have read and read - and still not certain, seems some do some don't. I have basically a 6/12 shed roof - will be fully sealed envelope. Thanks RB

    1. GBA Editor
      Martin Holladay | | #2

      Russell,
      The answer to your question depends on the type of standing-seam roofing you will be installing. Contact the roofing manufacturer or the roofing installer to determine the exact requirements of the manufacturer and installer.

  2. Expert Member
    Malcolm Taylor | | #3

    If widely read this blog would forestall about half the posts on GBA's Q&A section.

  3. Jasiu77 | | #4

    Thanks, Martin! I have a question about spray foam being in direct contact with the underside of the sheathing in an unvented attic. In a previous discussion about insulating unsheathed walls in old homes, it was mentioned that foam board can be put against the lap siding and then insulated behind, with the space between the foam board and the lap siding acting as an air gap. Curious, does the same apply to roof insulation? For example, if the home has slats underneath the sheathing (left over from old cedar roofing), would it still be better to spray foam directly against the slats and the sheathing, or would it be better to line space between the joists with asphalt paper and then spray foam, leaving the air gap between each slat in place?

    1. GBA Editor
      Martin Holladay | | #6

      Jasiu,
      There is no definitive answer to your question, but my advice would be to include the asphalt felt layer. The reason is that closed-cell spray foam glues everything together, and closed-cell foam is hard to remove. If you ever have sheathing rot in 20 years, the roofer who makes repairs will be grateful if you include the asphalt felt, or will curse your name if you don't.

      1. Jasiu77 | | #9

        Thanks so much, Martin! That was my plan, so just wanted to double check I wasn't overlooking something. Really appreciate it, as always!

  4. optimax | | #5

    Martin,

    I have a historic home in Redlands California that needs re-roofed. It is a 2-story home with a floored, unfinished attic. It currently has 1 x 4 skip sheathing with wood cedar shingles and no insulation at all. I was hoping to finish the attic into a living space, but leave the open rafter appearance (1 x 4 and wood shingles) from inside the attic space. Therefore, any insulation and venting (if necessary) would be done from the outside. Possibly considering an unvented cathedral assembly. Was planning to use rigid insulation and a metal standing seam roof.

    Need to make sure this is done right. Do you know of a consultant that can help me design it correctly; or perhaps a GBA contact.

    Thanks Martin.

    Rick in Redlands

    1. GBA Editor
      Martin Holladay | | #7

      Rick,
      GBA has a "Green Building Bulletin Board" with a list of green building consultants. The most likely one for you to call is Ann Edminster of Pacifica, Ca. Her contact info:

      Ann Edminster
      Design Avenues
      115 Angelita Ave.
      Pacifica, CA 94044
      650-355-9150

      If she can't help you, she may be able to suggest someone who can.

      1. optimax | | #8

        Martin,

        Thank you very much, I have been looking for someone for quite awhile.
        Will give her a call.

        Rick in Redlands

  5. Jim D | | #10

    I'm building a pole barn house with a metal roof on purlins and no sheeting. The attic will be insulated with cellulose loose fill. It is wildland fire prone and very windy when it rains here. The climate zone is 6. I'd like to avoid vents. Maybe just vented foam closure at the ridge. It seems like without wood sheeting the metal should be able to bake mousture out of the attic and through just a vented ridge? And there is less worry of rotting wood.

    Thanks

    1. GBA Editor
      Martin Holladay | | #11

      Jim,
      I'm not a fan of metal roofs over purlins without sheathing, unless you're talking about a barn or an equipment shed, because under some weather conditions, you'll get condensation on the underside of the metal roofing, and this condensation will drip on the insulation below. Usually, this moisture evaporates before you get a ceiling leak, but in general, an insulated house deserves roof sheathing.

      As long as you install an airtight ceiling, and the house is tested with a blower door to make sure that your ceiling really is airtight, you don't have to worry too much about attic venting. If you have wildfire risks, you should skip the soffit vents.

      For more information, see "Insulating a Pole Barn."

      1. Jim D | | #12

        Hi Martin, thanks for the fast reply,

        Trusses are already ordered for 4ft on center and this roof assembly was chosen due to the high sheathing costs.

        The ceiling will be tongue and groove that can span the 4 ft spacing. And a poly vapor barrier above the T&G. Considering having poly vapor barrier behind the drywall exterior walls as well. As this insulation is mostly only for heating days and very few cooling days.

        Our insulation supplier has vinyl faced fiberglass that comes in an unrated R value thickness of about 1in that is used specifically for condensation under metal roofs. Not sure if you think this is a useful product? It would also dampen any roof noise. Or I have seen projects where people put synthetic roofing underlayment directly on purlins

        Thanks
        Jim

        1. GBA Editor
          Martin Holladay | | #13

          Jim,
          This is a big topic. The basic story is that you have chosen a construction system developed for agricultural buildings, and you are trying to use the same system for a residence -- and that may lead to problems.

          For more information, see "Insulating a Metal Building." (I know that you aren't planning to build a metal building, but there are issues with metal buildings that also apply to pole barns.)

          The short answer is that vinyl-faced fiberglass insulation installed between your metal roofing and your purlins will reduce condensation problems. Make sure that your roofer is on board with your plan before you order the insulation, though.

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