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Foam rafter baffles with interior foam as well

Hello,

I have looked quite a bit but have not seen this explicitly discussed.

We are planning on using vented rafters with dense packed insulation. With 2x12s we will also need to add another R10 of insulation below the rafters in the form of rigid foam.

Planning to tape and air seal that interior foam as best I can.

The baffles will be site built, but I don't know what material to use.

1/2" OSB would be easy and I'll already have piles around the construction site.
1/2" Plywood may be better?

Air seal either one and leave it at that. I do wonder about condensation on the interior surface? It seems that enough people do vented attics this way so maybe it turns out not to be an issue if you air seal the interior correctly.

That being said, 1" poly iso foam board is about the same price or cheaper than 1/2" plywood - so I wonder if that's a good upgrade. For the same price, it's adding R.

I hesitate because I imagine that might prevent the rafter insulation from drying to the outside or the inside. Maybe foil-faced is not permeable enough and it should be something like EPS?

The problem is I probably have read just enough to confuse myself.

Thank you!

Asked by caryfitzhugh
Posted May 11, 2018 12:47 PM ET

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2 Answers

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1.

Cary,
I don't think it matters much whether you decide to use OSB, plywood, or rigid foam for your ventilation baffles (as long as you choose rigid foam that is stiff enough to resist pressure of the dense-packing process). Even if you also include a continuous layer of rigid foam on the interior side of your rafters, my opinion doesn't change.

The question was addressed in my article, "Site-Built Ventilation Baffles for Roofs." In that article, I wrote:

"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 diffusion. 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 polyiso — 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."

Answered by Martin Holladay
Posted May 11, 2018 1:07 PM ET

2.

With 24" o.c. rafter spacing 1" polyiso isn't nearly rigid enough or strong enough to handle dense packing pressures, and it will bow, narrowing the vent channel considerably, or even buckle during installation. To use 1" polyiso as a baffle, install a mid-span stripe of support spacer half way between the rafters, such as a 1" wide strip of foam-board of the appropriate vent channel depth glued to the roof deck as well as tight to the rafter on each side. That brings the unsupported spans down to about 11". It's pretty easy to cut 1-3" foil faced goods into 1" strips using a 4-5" steel drywall knife sharpened on the edges, and straight edge:

http://www.finehomebuilding.com/2009/01/29/theres-a-better-way-cutting-r...

At 1" and with R10 rigid foam (very vapor retardent) on the interior the vapor permeance of foil facers on the site-built baffle doesn't really matter. The path through the rafter is still going to be 1 perm or higher, and the cavity can dry toward the exterior (assuming full soffit-to-ridge venting.)

Answered by Dana Dorsett
Posted May 11, 2018 1:17 PM ET

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