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Musings of an Energy Nerd

Site-Built Ventilation Baffles for Roofs

If you want sturdy baffles that create deep ventilation channels, you’ll probably have to make your own

Site-built ventilation baffles can be made out of rigid foam. They can also be assembled from thin plywood or fiberboard sheathing. Site-built baffles have several possible advantages over manufactured baffles: they can create deeper ventilation channels, for example, and can be sturdier.
Image Credits: Image #1: Fine Homebuilding

Roofs often require ventilation channels directly under the roof sheathing — either for a short section of the roof (for example, near the eaves) or for the entire roof, from soffit to ridge. When the wind is blowing, these ventilation channels allow air to move from the soffit vents to the ridge vents.

To establish a ventilation channel, it’s usually necessary to install some kind of baffle. A baffle performs three functions: it defines the depth of the air gap; it keeps insulation from intruding on the air gap; and (if properly installed) provides an air barrier to limit wind-washing of the insulation. (Wind-washing degrades the thermal performance of air-permeable insulation materials like fiberglass, mineral wool, and cellulose.)

The value of providing ventilation channels under roof sheathing is often exaggerated. To a limited extent, these air channels can help keep roof sheathing dry and can reduce the chance of ice dam formation. However, ventilation channels aren’t able to perform the miracles that some ventilation advocates claim. For more information on this issue, see All About Attic Venting.

Where the baffles belong

In a house with a vented unconditioned attic, ventilation baffles are only needed near the eaves. The baffles should begin above the ventilated soffit — just to the exterior side of the plane of the wall sheathing — and should extend far enough into the attic that they terminate above the top of the insulation on the attic floor. The deeper the insulation, the longer the baffles have to be.

In a traditional Cape Cod house with 4-foot kneewalls, there are two ways to install ventilation baffles. The preferred way is for the baffles to begin above the ventilated soffit and extend all the way to the small attic above the second-floor ceiling. (This approach…

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21 Comments

  1. Kris Knutson | | #1

    Brentwood industries
    Frustrated with common cardboard and foam baffles not being job-site tough, I was impressed when I saw this product at a tradeshow.

    I never used them in the field, but they were impossible to tear, puncture and were very resilient, even when crumpled up and stepped on. They have one specifically for deeper attic insulation, "The AccuVent High Energy."

  2. User avater GBA Editor
    Martin Holladay | | #2

    About the AccuVent
    Kris,
    I agree that the AccuVent is a good product. That's why I mentioned it, and provided a web link, in my article.

  3. Lucy Foxworth | | #3

    Soffit dam question
    Martin,
    I've been working on my attic insulation lately so your topic is very helpful to me. The problem is that I cannot physically get to the eaves to install a baffle because of the impossibly shallow roof with these funny 2 x 6 supports that I cannot crawl over. I was going to glue the spacer sticks made out of offcuts of polyiso on the rafters as far down as I can reach. Then apply foam sealant to a piece of polyiso cut to fit the rafter bay and slide it down as far as it can go as well. Does that sound reasonable even if I can't get a good seal at the soffit side of the rafter bay?

    I've got a relatively new metal roof (about 10 years old), new Hardiboard siding and soffits, gutters (about 4-5 yrs old) so I really don't want to take the soffits down to install the baffles. The guys who did the work did put a piece of those pretend (meaning they only pretend to do the job) foam baffles in there, but they aren't secured to any thing. So I think they are useless. I didn't know better when I had that work done. I was new to GBA at that time.

  4. User avater GBA Editor
    Martin Holladay | | #4

    Response to Lucy Foxworth
    Lucy,
    Q."Does that sound reasonable even if I can't get a good seal at the soffit side of the rafter bay?"

    A. Under the circumstances, that sounds entirely reasonable.

  5. Andy Kosick | | #5

    Our method might help Lucy
    A technique I developed for low slope retrofits uses 3/4" polyiso and vinyl F-channel cut into 4" pieces. The 4" clips, as I call them, fit tightly over the polyiso. Put the clips at the heel end of the baffle up so the nail fin acts as a 1" stop and the top clips down so you can staple the baffle in place. Measure and score a flap on the end to make up the remaining heel height as a wind-washing dam. Leave this flap folded while sliding the baffle in place, staple the topside nail fins, and then carefully wedge the flap snug using the end of a long barrel foam gun (or stick) at the very outside of the top plate. Then seal in place with the same foam gun. I realize the long barrel gun is the key to this working really well, but even with out it you can reinforce the flaps with tape and wedge them in place tight with a stick and it still does a better job than anything else I've used in a tight heel. Either way you can do the whole thing from 5 feet away. Seal the plate to wall board connection with duct mastic and a long handled trowel, it's the most reliable thing I've found at such a tight angle. A crew of two makes this much easier with one in the heel and one in the middle clipping and trimming (16 o.c. is apparently nominal?). We've installed these as long as 8 feet in a really low slope with success. There's my trade secret, good luck.

  6. Andy Kosick | | #6

    Pictures
    Here's some picture for clarity.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Andy Kosick
    Andy,
    I'm grateful for your comments and photos. This type of tip, shared by someone with lots of field experience, is very valuable. Thanks.

  8. Armand Magnelli | | #8

    a variation on Mike Guertin’s technique
    Martin, thanks for a very informative article. Would you comment specifically on the YouTube video showing a variation on Mike Guertin’s technique. I'm concerned about the fact that the baffles do not span the entire width of the rafter cavity, and the lack of air sealing at the top plate. I was however very impressed with their assembly line technique! Andy's approach seems both more effective and practical to me, and if you had access from the soffits it could go quickly with someone working at both ends of the baffle.

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Armand Magnelli
    Armand,
    Although shows an installation that doesn't follow my advice, there is always something to learn from observing other people's methods. The installation tools developed by Andy to hold the baffles in place as the glue dries are clever.

    It's possible to learn a few tricks from this video, and use the tricks in a different way -- ideally, to install baffles that span the entire width of the rafter bay and that are installed in a more airtight manner.

  10. Armand Magnelli | | #10

    Agreed
    Martin, you are of course correct. I appreciated both the assembly line approach and the use of simple tools to get the adhesive to hold the baffles in place.

  11. Stewart Akerman | | #11

    In an attic used for storage,
    In an attic used for storage, do baffles built with rigid foam meet code? A similar question was asked in a 2009 GBA Q&A (link below) but wasn't addressed, other than a general reference to Dow Thermax.

    https://lakesideca.info/community/forum/energy-efficiency-and-durability/14545/fire-code-concerns-rigid-foam-panels

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Stewart Akerman
    Stewart,
    Q. "In an attic used for storage, do baffles built with rigid foam meet code?"

    A. The brief answer to your question is yes. That said, I'm not exactly sure what your concern is.

    Some building inspectors do not allow rigid foam to be left unprotected, and insist that it be covered by other materials for fire safety reasons. But attic ventilation baffles are always covered by insulation.

    If you install fiberglass batts between your rafters, the fiberglass batts protect the rigid foam baffles from igniting. That said, it's always a good idea to have an air barrier on the interior side of the fiberglass batts, and it is illegal to leave kraft facing exposed.

  13. Stewart Akerman | | #13

    Thanks for the reply, Martin.
    Thanks for the reply, Martin. I was asking about the part of the rigid foam baffle that extends *above* blown attic insulation and is not covered.

    I'm sorry I wasn't more clear. The storage I was envisioning was in an attic accessed by pull down stairs, where a portion of it is a decked plywood storage platform. The remainder of the attic floor is insulated with blown cellulose.

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Stewart Akerman
    Stewart,
    I have never heard of any code official objecting to a little bit of rigid foam peeking out above the insulation on an attic floor. But the only person who can definitively answer your question is your local code official -- since he or she is the one who interprets the code locally.

  15. Adam Emter | | #15

    Site build or accuvent?
    I'm getting ready to install site-built or Accuvent baffles. My house is new construction and my trusses are 16" heels. I took the sheathing all the way up to the bottom of the top truss chord. I have used the pre-made plastic baffles before, but they seem flimsy. Thinking about ripping 7/16 OSB and building baffles myself. My question is whether I need to air-seal those site-built baffles if it is a vented attic with continuous soffit and ridge vents. I will be very meticulous with attic air sealing. Any advice would be appreciated!

  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Adam Emter
    Adam,
    If you are building a vented unconditioned attic, these ventilation baffles will be short. They will extend from your wall sheathing to a few inches above the top of the insulation installed on your attic floor.

    It's always a good idea to pay attention to airtightness. In this region of your attic, you want to limit the effects of wind-washing. (Wind-washing is the degradation in your insulation's thermal performance due to the movement of outdoor air through the fibers of your insulation.)

    If I were installing these ventilation baffles, I would try to seal the perimeter of each baffle. If you don't do the air sealing, it's not the end of the world. But if you are up there in the attic installing baffles, and you have the materials you need to seal the edges of the baffles, you might as well do a good job.

  17. Adam Emter | | #17

    Thanks for your advice,
    Thanks for your advice, Martin. I didn't fully understand the reason behind air sealing the baffles, but now I know. Whether I build my own baffles or opt for the Accuvents, I'll be sure to use sealant.

  18. Charlie Sullivan | | #18

    Cathedral ceilings and baffle material
    The article says,

    "There really aren’t any reports of failures or problems resulting from the use of vapor-impermeable materials — for example, polypropylene, vinyl, or foil-faced polyiso — to make ventilation baffles."

    People considering impermeable baffles in cathedral ceilings might want to know about a failure reported in a recent Q&A discussion. A homeowner reported major condensation problems that resulted from the combination of impermeable baffles and poor air sealing of the ceiling drywall. (Comment number five in this thread https://lakesideca.info/community/forum/gba-pro-help/99296/insulation-vaulted-ceiling)

    The primary advice to the homeowner was that air sealing of the drywall is critical, and that advice would hold whether or not the baffle was permeable. So that doesn't necessarily invalidate the position taken by the article --most of us would primarily attribute the failuer to inadequate air sealing, not to the use of impermeable materials. But if it can be that bad with major air leaks, it seems like it could be a little bad with a little air leakage. And one of the themes that I hear repeated here a lot is that a good design is one that takes into account the fact that perfect construction is unlikely in practice, and designs that depend on it aren't very wise.

    I think that if I were building a vented cathedral ceiling, I'd want to either use a vapor permeable baffle material, or use a thick enough foam layer to meet the guidelines for exterior rigid roof foam.

    Of course, that doesn't contradict the article, which also says:

    "Anyone worried about this possibility should probably make their ventilation baffles out of a vapor-permeable material."

    So I'm not disagreeing with the article--I'm just saying that this story is a little more reason join the camp of people who do worry about it and do use vapor permeable materials. I think that's especially true in cathedral ceilings because there are other ways for moisture to escape in an attic with insulation on the floor and baffles only up a few feet on the sides.

  19. Daniel F. Vellone | | #19

    Benefit of vents
    My project has 6' of slope insulated from the top plate to the collar ties where the insulation continues on the flat. I installed site built rafter vents (2") rather than filling the entire cavity because I can inspect the vented space for leaks from the deck installed on the collar ties. Having built two homes that are both capes I've come to see the greater benefit of insulating the flat for several reasons, one of which is the ability to detect any leaks which are impossible to see prior to damage that eventually reveals itself in the living space once it's too late to prevent. I can't imagine filling the rafters to the peak, closing it all in from below and resting comfortably, assured that I'd suffer no water damage in the closed off and inaccessible space.

  20. Jon R | | #20

    > "...embracing the Energy
    > "...embracing the Energy Star Homes principle calling for air-permeable insulation to be enclosed by an air barrier on all six sides."

    Would be interesting to see more discussion of this. Most discussions on GBA refer to single air barriers in a partition.

    OK, I do see this:

    GBA: musings/one-air-barrier-or-two

    And this (singular):
    GBA: questions-and-answers-about-air-barriers

  21. User avater GBA Editor
    Martin Holladay | | #21

    Response to Jon R
    Jon,
    I guess you found the answer to your own question. In case you have any lingering confusion: The Energy Star requirement (namely, that in most cases fiberglass insulation needs to be enclosed by an air barrier on "all six sides") was developed to address insulated kneewalls and skylight shafts (where many builders were omitting the attic-side air barrier).

    The exception to the "all six sides" rule is for insulation on an attic floor, where no top-side air barrier is required. (Researchers and builders agree that it is cheaper to blow a little more cellulose on top of the insulation than it is to unroll a layer of Tyvek.)

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