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Community and Q&A

8″ of exterior mineral wool (Comfortboard 80)

David Bainbridge | Posted in Energy Efficiency and Durability on

I am in zone 7 (Northern Minnesota). Looking at building timber frame structure without SIPs. I love the idea of using Roxul Comfortboard 80 for exterior insulation. I want to avoid foam as much as reasonably possible even though I will most likely need to use it for the roof. Wall assembly would be 2x4s sistered onto outside of timber frame, exterior plywood, WRB (Delta Vent SA), 8″ of Roxul Comfortboard 80, vertical furring strips and then siding.

I have not seen example of anyone using 8″ of mineral wool on exterior wall. This would be 2 layers of 3″ and 1 of 2″. 8″ would give me approximately R34. Adding mineral wool between 2x4s would give at least R40. The city of Vancouver in British Columbia has an excellent document that details how to attach 8″ of exterior mineral wool.

Besides worrying about hitting a stud through the furring strips, is there any reason why this wall would not be feasible or performant? Anyone used 8″ of Comfortboard 80 on exterior?

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  1. User avatar GBA Editor
    Martin Holladay | | #1

    Roxul has published a design guide with advice on continuous exterior mineral wool panels up to 12 inches thick (see table reproduced below).

    Here is a link to the design guide: Residential Insulated Sheathing – Design Guide.

    For more information, contact a technical help representative at Roxul. (The company now has a new name: Rockwool.)

    For information on a project that included 6 inches of exterior mineral wool, see Wrapping an Older House with Rock Wool Insulation.

  2. David Bainbridge | | #2

    Thank you for that link! I find it somewhat difficult searching on their website.

    Edit: After reviewing that doc I see they then reference another doc for fastening that I had to google:

  3. David Bainbridge | | #3

    Found another excellent reference from the 15th Canadian Conference on Building Science and Technology in a paper titled "STRUCTURAL TESTING OF SCREWS THROUGH THICK EXTERIOR INSULATION"

    The primary objective of the study was to evaluate the vertical load resistance of long screws through insulation as a cladding attachment technique with specific focus on relatively thick insulation layers of mineral wool (equivalent of Comfortboard 80 and 110).

    They found that with a typical cladding load of 25 lb per fastener the deflection was less then 0.025" which is likely insignificant considering a one story wood-frame structure is expected to potentially have moisture shrinkage of 3/8".

    Some key findings I noticed:
    - deflection difference between 6" of XPS and 6" of mineral wool was 0.005". Basically no difference.
    - the difference in deflection between using 6" of mineral wool and 12" of mineral wool was 0.007"
    - when fasteners were installed into 3/4" plywood sheathing only it provided similar load deflection response to fasteners installed minimum 1" into stud framing.

    This gives me confidence that even 9" of Comfortboard 80 will not be an issue, especially when using a Heco Topix screw.

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

    Thanks for the second link.

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