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Should I use baffle vents above rockwool batt insulation?

Rocky12 | Posted in General Questions on

I have a 2″ ventilation space above my rockwool insulation (below roof sheathing). This is a cathedral ceiling. Would it be a good idea to use something like Durovent rafter vents above the rockwool to ensure airflow and prevent wind-washing?

I will be installing the rockwool batts from the interior side (pushing the insulation upwards) so thinking rafter vents are a good idea to prevent the insulation potentially blocking airflow.

What do you guys think?

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Replies

  1. Rocky12 | | #1

    One other thing- is there any material I could use that is vapor permeable? Since I am using rockwool insulation, it's a must a have a vapor open roof.

    I was also thinking of using my vapor-open housewrap (for a site build rafter vent) over the rockwool.

  2. GBA Editor
    Martin Holladay | | #2

    Rocky,
    Yes, you want ventilation baffles. You can either install commercially available baffles or you can make site-built baffles.

    If is far more important that these baffles be relatively airtight than that the baffles be vapor-permeable, for reasons explained in this article: Site-Built Ventilation Baffles for Roofs.

    The article lists several materials you can use for site-built baffles, and includes installation advice.

  3. Jon Harrod | | #3

    One of the builders we work with makes site-built baffles from lauan for cathedral ceilings, using wood cleats to maintain spacing to the roof deck. I think this approach is the gold standard in that it provides a six-sided cavity for the insulation, a continuous, obstacle-free vent channel, and an airtight but somewhat vapor permeable barrier between them.

    When this isn't an option, I like the AccuVent cathedral vent system--it's sturdy and can be purchased with starter pieces to provide a good transition from top plate to roof deck.

  4. Rocky12 | | #4

    Hi Martin. I read your article. You clearly reccomend to install the rafter vents in an airtight manner. It seems this would mostly be to protect rockwool from wind washing. Do you believe this is still crucial if I will be in a warm dry climate? I would imagine the vents alone would do a pretty solid job of protecting the insulation alone.

    I really would like to use a (very) vapor permeable material. I imagine fiberboard would be the best option? I've never worked with fiberboard. Do you reccomend a certain fiberboard that is vapor open but also good with moisture should any get up there?

    My other idea is to buy the accuvent product, cut out small circles every few feet along the vents, and tape (vapor open) over the holes with house Wrap. This would accomplish the goal of letting vapor pass through.

    Hey Jon, do you have any guides or pictures of this?

  5. GBA Editor
    Martin Holladay | | #5

    Rocky,
    Q. "You clearly recommend to install the rafter vents in an airtight manner. It seems this would mostly be to protect rockwool from wind washing. Do you believe this is still crucial if I will be in a warm dry climate? I would imagine the vents alone would do a pretty solid job of protecting the insulation alone."

    A. There is no need to protect the insulation from moisture. (Water doesn't hurt mineral wool insulation.) When I wrote that an airtight baffle reduces windwashing in air-permeable insulation like fiberglass or mineral wool, I was talking about avoiding a reduction in the insulation's thermal performance. I wasn't talking about protecting the insulation from moisture.

    The need to protect insulation from windwashing has nothing to do with whether your climate is warm and dry or cold and wet.

    When wind blows through fibrous insulation, it strips away the pockets of air hiding between the fibers, thereby robbing the insulation of its ability to reduce heat flow through the insulation.

    So you want your baffle to be airtight, regardless of your climate.

    I don't think that you need to obsess about the vapor-permeance of the baffle. Real-world field experience shows that vapor-impermeable baffles work fine. (I explained why in my article.)

    If you want to use fiberboard, however, you can. Here is a list of fiberboard manufacturers:

    Building Products of Canada
    Lasalle, Quebec, Canada

    MSL Fiberboard
    161 St-Paul Street
    C.P. 38
    Louiseville, Quebec J5V 2G9, Canada
    Manufacturer of SonoClimat Eco4
    http://www.mslfibre.com/Produits/Fiche/13/SONOclimatECO4

    International Bildrite
    101 Fourth Street East
    International Falls, MN 56649
    218-283-3900
    http://www.bildrite.net/sheathing.htm

    Georgia-Pacific
    Manufacturer of Stedi-R structural sheathing and QuietBrace structural sheathing
    http://www.gp.com/build/product.aspx?pid=6056

    Homasote Company
    PO Box 7240
    West Trenton, NJ 08628-0240
    http://www.homasote.com/

    Blue Ridge Fiberboard
    Danville Plant
    250 Celotex Dr.
    Danville, VA 24541
    866-850-8834
    434-797-1321
    http://www.blueridgefiberboard.com
    Manufacturer of SturdyBrace structural fiberboard sheathing

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