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Community and Q&A

Foam rafter baffles with interior foam as well

caryfitzhugh | Posted in Green Building Techniques on

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!

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Replies

  1. User avater GBA Editor
    Martin Holladay | | #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."

  2. User avater
    Dana Dorsett | | #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-rigid-insulation

    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.)

  3. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #3

    Martin, you write:

    ...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.

    This is what's killing me... So the roofing manufacturer is requiring (for warranty) a vapor impenetrable underlayment. This gives the sheathing only one way to dry... down. You're saying essentially that vapor-closed baffles are no big deal because moisture generally doesn't get to this location... I guess I'm coming to the slow realization that everyone can't be right and that I have to assimilate all this information myself.

    Should I agree with the Metal Roofing manufacturer and install a vapor impenetrable membrane over the plywood? Or... (don't shoot the messenger!) should I become inspired by 475's "Vented High Performance Standing Seam Metal Roof"

    https://foursevenfive.com/blog/vented-high-performance-standing-seam-metal-roof/

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

    Ethan,
    If you have a ventilation channel under your roof sheathing, it doesn't matter if your roofing underlayment (or, for that matter, your roofing) is vapor-impermeable, since the roof sheathing (if damp) can dry downwards to the ventilation channel.

  5. User avater GBA Editor
    Martin Holladay | | #5

    Ethan,
    Yes, roof sheathing can dry upwards, as long as:

    1. The roofing underlayment is vapor-permeable, or has a variable vapor permeance, like #15 asphalt felt (note that the higher the perm rating of the underlayment, the faster the rate of upward drying), AND

    2. There is a vented air space between the roofing underlayment and the roofing, OR the roofing is vapor-permeable (e.g. concrete tiles, cedar shingles, or slate).

  6. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #6

    Martin, I get that, I just wonder if it can also dry upwards if the underlayment ISN'T impermiable (as shown in the detail from Aurora Builders via 475). I just wish I could get water column data for impermeable underlayments so I could compare apples to oranges or apples to apples as it may be.

  7. User avater
    Dana Dorsett | | #7

    Ethan: It's more than just about having a mere path to drying. The rate of drying and the amount of moisture that it has to move are also very important.

    The reason roof decks are best vented from below is that the wintertime moisture drive is from below, particularly severe in heating dominated climates. With the vent channel between the cold roof deck and the more humid building interior, any air leakage or diffusion moisture hits the vent channel first, which is constantly being replenished with convecting outdoor air. In this configuration the only moisture that has to move through the roof deck is whatever leaks through the WRB above.

    But if the roof deck is only vented from above the moisture from interior drives has to pass through the roof deck to reach the ventilation channel. Even without roofing materials applied the roof deck is fairly vapor retardent, and will slow down the transfer of moisture from the interior side into the vent channel above. The vapor redardency of 3/4" plywood is less than 1 perm when dry- a class-II vapor retarder.

    In a zone 5 wall wood structural sheathing can be "rainscreen" vented on the exterior without an interior side vapor retarder tighter than standard latex interior paint, but not in zone 6 or higher. But the same is not true for roof decks, which run colder than wall sheathing due to both the higher R values, and the lower wintertime temperature due to nighttime radiational cooling taking the roof temperature well below the outdoor air temperature at night. In warmer climates such as zone 3 or lower you might be able to get away with venting the roof deck on the exterior side without interior vapor retarders, but in zone 5 even Intello on the inside of the roof assembly might not cut it, even with only higher-permeance underlayments up top and a generous vent channel (not shown in 475's drawing.) It might work in zone 5 if perfect, but doesn't seem very resilient, not robust enough for the real world construction errors or damage over time.

    If vented on the underside of the roof deck you wouldn't even need the Intello- the air in contact with the roof deck is outdoor air- much drier in winter, making it more tolerant of incidental air leaks between the vent channel and home's interior.

  8. Malcolm Taylor | | #8

    Dana,

    Thank you, thank you and thank you. I've been trying to understand the difference between locating the ventilation channel above and below the roof sheathing and with your concise but clear explanation its finally clear in my mind. I'll sleep better tonight.

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