Site-Built Ventilation Baffles for Roofs

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Site-Built Ventilation Baffles for Roofs

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

Posted on Apr 24 2013 by Martin Holladay

Roofs often require ventilation channels directly under the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. — 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 barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. 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 damA ridge of ice that forms along the lower edge of a roof, possibly leading to roof leaks. Ice dams are usually caused by heat leaking from the attic, which melts snow on the upper parts of the roof; the water then refreezes along the colder eaves working it's way back up the roof and under shingles. 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 is used when the triangular attic behind the kneewalls is incorporated into the home’s conditioned envelope.) The traditional (less effective) approach is to install baffles only in the section of sloped ceiling between the kneewall and the horizontal ceiling. (This approach is used when the builder attempts, usually unsuccessfully, to keep the triangular attic behind the kneewalls outside of the home’s conditioned envelope.)

In a house with vented cathedral ceilings, the baffles should begin above the ventilated soffit and extend all the way to the ridge.

Manufactured ventilation baffles

Most builders install manufactured ventilation baffles. The most popular baffles are made from polypropylene, vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate)., expanded polystyrene, or cardboard.

The ideal ventilation baffle:

  • Is durable and stiff — stiff enough to resist the pressure of dense-packed insulation;
  • Extends the entire width of the rafter bay;
  • Is relatively easy to air seal; and
  • Creates a ventilation channel that is at least 2 inches deep. (While building codes require that ventilation channels be at least 1 inch deep, most experts advise that 2-inch-deep channels perform better than 1-inch-deep channels.)

As far as I know, the only commercially available baffle that fulfills all of these criteria is the . Another product — the — comes close, but the depth of the ventilation channels created by the AccuVent is only 1 3/4 inch instead of 2 inches.

Most manufactured baffles made of expanded polystyrene or cardboard aren't sturdy enough to resist the pressure of dense-packed insulation.

Site-built baffles

The best-performing ventilation baffles are site-built. It’s possible to make site-built baffles that are sturdier, more airtight, and more vapor-permeable than any manufactured baffles.

[Photo credit: Graham Fisher]

In most cases, you want to create a ventilation channel that is between 1 inch and 2 inches deep. A 1-inch-deep channel barely meets code, while a 2-inch-deep channel allows for more air flow and probably performs better. If you’re working in a tight area, however, you may prefer to build 1-inch-deep or 1 1/2-inch-deep channels to leave more room for insulation.

The first step to assembling site-built baffles is to install “sticks” in the upper corners of each rafter bay. These “sticks” are the stops that establish the depth of the ventilation channel. For 1-inch-deep channels, most builders rip 1"x1" pieces out of softwood lumber. These sticks are then tacked into place with a few finishing nails.

Needless to say, the sticks don’t have to be continuous; short lengths separated by gaps will also work.

For deeper channels, it’s possible to rip sticks that measure 1"x2". It’s also possible to cut rectangular pieces of rigid foam to any desired dimension, and to use those pieces as stops.

If you are planning to use foil-faced rigid foam as your baffle material, you can score the foil facing on one side to create a hinge, and fold the foam over so that you create integral stops that are oriented 90 degrees to the baffle.

A video shows Mike Guertin demonstrating this method: How to Ventilate Rafter Bays When Adding Insulation. One disadvantage of Mike Guertin’s approach: unless shims are added to the folded-over stops, the channels are only as deep as the thickness of the rigid foam.

A shows a variation on Mike Guertin’s technique.

What material should use use to create the baffles?

The most common materials used to create ventilation baffles are thin plywood, rigid foam, fiberboard, and stiff cardboard.

Lots of installers are fairly casual about airtightness when installing these baffles. The sloppy technique is sometimes justified by this argument: “It’s OK if the baffle is a little leaky — it will help let the moisture escape.”

I disagree with this approach, instead embracing the Energy Star HomesA U.S. Environmental Protection Agency (EPA) program to promote the construction of new homes that are at least 15% more energy-efficient than homes that minimally comply with the 2004 International Residential Code. Energy Star Home requirements vary by climate. principle calling for air-permeable insulation to be enclosed by an air barrier on all six sides. I advise builders to install ventilation baffles in an airtight manner; this is especially important for ventilation baffles installed in a cathedral ceiling. The perimeter of each baffle (including the seams where a baffle butts up to an adjacent baffle) should be sealed with high-quality tape, canned spray foam, or caulk.

Do baffles need to be vapor-permeable?

One of the reasons that builders install ventilation channels is to help damp roof sheathing dry out. Researchers now realize that ventilation channels can help a little bit at this task, but not as much as some people think. (In fact, during the summer, ventilation channels can actually add moisture to roof sheathing rather than remove it; for an example of this phenomenon, see Image #4, below.)

The most important way to prevent the migration of moisture from the interior of a house to the roof sheathing is have a good air barrier at the ceiling. The reason is simple: the usual transport method for this moisture is air leakage, not vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. . Because of this fact, installing ventilation baffles that are airtight makes this type of roof assembly more, not less, robust.

What if interior moisture is able to reach the underside of a ventilation baffle — isn’t it possible that the moisture might condense against the baffle (especially if the baffle is cold)? If so, isn’t this a good argument in favor of using vapor-permeable materials (for example, fiberboard, cardboard, or thin EPS) for ventilation baffles?

The answers to both questions is a qualified yes. Anyone worried about this possibility should probably make their ventilation baffles out of a vapor-permeable material.

That said, 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 polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. — to make ventilation baffles. The main reasons:

  • Not much moisture manages to make its way to the ventilation baffles (especially in homes that pay attention to airtightness);
  • The air in the ventilation channels is often warmer than outdoor air, a fact which limits condensation; and
  • Any moisture that does make its way there seems to be incorporated into the rafters via sorption. The ventilation channels are able to remove a limited amount of moisture from the rafters, and it appears that the rate of drying exceeds the rate of wetting.

Wind-washing dams

If you are installing ventilation baffles near the eaves of a house with a vented unconditioned attic, it’s important to install wind-washing dams between the top plateIn wood-frame construction, the framing member that forms the top of a wall. In advanced framing, a single top plate is often used in place of the more typical double top plate. of the exterior wall and the underside of your ventilation baffles. These wind-washing dams need to be installed in the same plane as the wall sheathing. (That way, you can be sure that the attic insulation covers the wall’s top plate.)

The easiest material to use for these dams is foil-faced rigid foam. Once the insulation dam is installed, seal the perimeter of each dam with canned spray foam so that the installation is airtight. A method for installing these wind-washing dams is demonstrated in a GBA video, How to Ventilate Rafter Bays When Adding Insulation.

Martin Holladay’s previous blog: “Fixing Attics With Vermiculite Insulation.”

Tags: , , , , , ,

Image Credits:

  1. Image #1: Fine Homebuilding
  2. Image #2:
  3. Image #3: CurbDog on
  4. Image #4: William Rose

Apr 24, 2013 1:04 PM ET

Brentwood industries
by Kris Knutson

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."

Apr 24, 2013 2:05 PM ET

About the AccuVent
by Martin Holladay

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

Apr 24, 2013 10:13 PM ET

Soffit dam question
by Lucy Foxworth

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.

Apr 25, 2013 7:20 AM ET

Response to Lucy Foxworth
by Martin Holladay

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.

Apr 27, 2013 10:33 PM ET

Our method might help Lucy
by Andy Kosick

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.

Apr 27, 2013 10:39 PM ET

by Andy Kosick

Here's some picture for clarity.

IMG_3350.jpg IMG_5985.jpg IMG_3824.jpg

Apr 28, 2013 4:17 AM ET

Response to Andy Kosick
by Martin Holladay

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

Apr 30, 2013 10:18 AM ET

a variation on Mike Guertin’s technique
by Armand Magnelli

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.

Apr 30, 2013 10:41 AM ET

Response to Armand Magnelli
by Martin Holladay

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.

May 5, 2013 10:05 AM ET

by Armand Magnelli

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.

Jul 13, 2013 10:28 PM ET

In an attic used for storage,
by Stewart Akerman

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.

Jul 14, 2013 7:51 AM ET

Edited Jul 14, 2013 7:53 AM ET.

Response to Stewart Akerman
by Martin Holladay

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.

Jul 14, 2013 12:53 PM ET

Edited Jul 14, 2013 12:54 PM ET.

Thanks for the reply, Martin.
by Stewart Akerman

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.


Jul 14, 2013 2:36 PM ET

Response to Stewart Akerman
by Martin Holladay

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.

Aug 25, 2013 11:59 PM ET

Site build or accuvent?
by Adam Emter

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!

Aug 26, 2013 5:12 AM ET

Response to Adam Emter
by Martin Holladay

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.

Aug 26, 2013 7:20 PM ET

Thanks for your advice,
by Adam Emter

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.

Jan 15, 2017 12:42 PM ET

Cathedral ceilings and baffle material
by Charlie Sullivan

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

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.

Sep 10, 2017 10:59 AM ET

Benefit of vents
by daniel f. vellone

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.

Sep 10, 2017 12:06 PM ET

Edited Sep 10, 2017 12:10 PM ET.

> "...embracing the Energy
by Jon R

> "...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

Sep 11, 2017 7:48 AM ET

Response to Jon R
by Martin Holladay

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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