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Community and Q&A

Horizontal furring over rockwool insulated sheathing: Are vented strips necessary?

jberks | Posted in Energy Efficiency and Durability on

I was watching some of Big Joe’s talks on youtube the other day on rainscreens. Specifically discussing allowing a vertical air patch behind the siding for adequate drying. It got me thinking about my siding plan as I’ll need horizontal furring strips.

My wall assembly is as follows:
2×4 studs
7/16 osb sheathing
2″ roxul comfortboard 80
furring strips***
1.5′ x 4′ JH Fiber cement panels with 1/2″ reveals

Climate zone 6 (Toronto)

My question is:
Will the roxul sheets behind wood furring strips provide adequate draining/drying capability, or should I splurge on corrugated plastic vented furring strips (cor-a-vent is what we can get here in Toronto) for 5.5 times the cost?



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  1. GBA Editor
    Martin Holladay | | #1

    Wood furring strips are fine. The mineral wool insulation is air-permeable and free-draining, so you don't really need to worry about trapping moisture or a slow drying rate.

  2. RMaglad | | #2

    curious how you plan to finish your windows? nailing flange to the sheathing/wrb, all under the roxul? are you boxing out your windows? innie windows or outtie?

    While i am planning a much thicker wall, i have a similar exterior (wood siding, furring strips, 2" comfortboard 80, WRB, sheathing, 2x6 stud bay, smart vapour barrier, and insulated 2x4 service cavity)

  3. jberks | | #3

    Hi Ryan,

    I had the window guys do a 2" outie box of 2x4 and plywood stacked flat which create a total depth of 2" to accept the 2" roxul. they nailed the window flange on top of this wood Buck, which leaves about an inch and an eighth of the window sticking out. The furring strips will be around 3/4" and the fiber cement boards will be 5/16". so the total depth of the siding will be flush with the depth of the windows. I'll terminate the fiber cement flush to the window with j-trim.

    This is not an ideal scenario, but between how the windows come and the installers comfort level, this was the best I could get.

    After my windows got put in, matt risinger timingly released a video review of thermal buck. I'd post the link to he video but it got blocked on the forum. Next house I do I'll try to get my hands on these and pre-prep all the windows myself.

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