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Community and Q&A

Roxul Comfortboard to create cathedral ceiling baffle?

bwc209 | Posted in Green Building Techniques on

Hi all — thanks for taking the time to read this.

I’ll be building a new home this spring in zone 6 (northern Vermont) that will have cathedral ceilings. I’ve read a lot about the best way to do this in my climate (thanks for all the info on this site, Martin) and I’ve settled on what seems like a safe bet: taped drywall and vapor retarder paint, dense pack cellulose (16″ or so), homemade baffle, 2″ airspace, plywood sheathing, underlayment, metal screw down roof.

From what I understand, it’s nice for the material that is between the cellulose and airspace to be somewhat permeable so any moist interior air that does get through the drywall can escape to the ventilation channel. My question: in there any reason not to use rigid mineral wool (roxul comfortboard) as the baffle (instead of foam board) since it is REALLY permeable? It’s more expensive than foam, but I’m thinking it could give more peace of mind since it’s so permeable. Anyone out there tried this or can you think of any reason it wouldn’t work?


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  1. charlie_sullivan | | #1

    That's an interesting idea. There are three possible drawback I can think of. The one that I think is most serious is that I doubt it would be strong enough to hold back dense-pack cellulose. I think it's a little too flexy for that. I suppose you could add lots of spacer strips until you have it supported well enough to avoid that, but then it's extra work as well as extra cost.

    One might also wonder about wind washing, but tests have shown mineral wool boards do OK in that respect on the exterior of walls, so it should be fine.

    My final question would be about air leakage. The drywall is the primary air barrier, but if you can air seal the baffle well too, that would reduce the impact of any small leakage in the drywall. So that's not a problem with using mineral wool, but just a lost opportunity if you were to use something else that could be made air tight.

    So I would tend to opt for something more rigid and cheaper. Plywood or fiberboard are cheaper and still quite permeable.

  2. GBA Editor
    Martin Holladay | | #2

    Charlie nailed the answer with his list of three potential problems. I don't think that mineral wool panels make sense for ventilation baffles.

    As I wrote in my article on the topic, Site-Built Ventilation Baffles for Roofs, I don't think that you need to worry about the vapor permeance of the material you choose as your ventilation baffle. We aren't receiving any reports of failures due to the use of vapor-impermeable baffles, for the reasons explained in my article:

    "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."

    -- Martin Holladay

  3. STEPHEN SHEEHY | | #3

    Blake- since you are still in the design stage, let me suggest another alternative, that we used for our pretty good house.
    We used raised heel scissor trusses, 24" o/c. The roof pitch is 8/12, the ceiling pitch is more like 4/12. From the ceiling up:
    1x4 strapping
    air barrier- we used Siga majpell, but there are lots of alternatives.
    approximately 24" of blown in cellulose
    flash coat of closed cell foam at the eaves (doing it again, we'd use deeper heels and skip the foam)
    insulation baffles running 4' up from the eaves
    Advantech sheathing
    underlayment (Titanium)
    standing seal metal roof

    The cellulose was blown in through an access port in the gable end. It was a simple roof, about R 70 or so.

  4. aaronbeckworth | | #4

    Our roof design sounds a lot like Stephans. We are located in a high elevation area of SW New Mexico (4B), and our design has evolved to include a 550 square foot loft over the 850 square foot main floor. The design includes raised heel scissor trusses, blown in cellulose, and metal roof. Our design/builder does not seem to think that we need soffit vents or ventilation baffles. His idea is to size the gable ends vents to provide all attic ventilation. I would greatly appreciate any comments supporting or questioning this thinking.

  5. GBA Editor
    Martin Holladay | | #5

    Check your local codes. Most building codes require soffit vents. Here's the relevant section in the 2009 IRC:

    Section R806.2 Minimum area
    The total net free ventilating area shall not be less than 1⁄150 of the area of the space ventilated except that reduction of the total area to 1⁄300 is permitted provided that at least 50 percent and not more than 80 percent of the required ventilating area is provided by ventilators located in the upper portion of the space to be ventilated at least 3 feet (914 mm) above the eave or cornice vents with the balance of the required ventilation provided by eave or cornice vents. As an alternative, the net free cross-ventilation area may be reduced to 1⁄300 when a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.

    -- Martin Holladay

  6. aaronbeckworth | | #6

    I will revisit this topic at our next design meeting. Thank you.

    If not for the code requirement, wouldn't eliminating the soffit vents and ventilation baffles greatly simplify the roof design and construction?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "If not for the code requirement, wouldn't eliminating the soffit vents and ventilation baffles greatly simplify the roof design and construction?"

    A. Yes, they would. But you don't want to simplify things to the point that the roof assembly is at risk of moisture accumulation or rot.

    There are lots of factors here. In general:

    1. Well ventilated roof assemblies are less risky than unvented assemblies.

    2. You really don't want any cellulose touching your roof sheathing -- you want an air gap between the top of the cellulose and the roof sheathing.

    3. Your attic might perform well, even without soffit vents, if (a) you had an air gap between the top of the insulation and the roof sheathing, and (b) as long as you were sure that your ceiling was airtight. But soffit vents lower the risk.

    -- Martin Holladay

  8. bwc209 | | #8

    Thanks everyone. I think I'll go with plywood based on your responses!

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