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Community and Q&A

Insulating over cedar siding with 1-inch rigid foam underneath

SolarBilly | Posted in Energy Efficiency and Durability on

Hi All, Just purchased a home in northern Michigan. Built in 1988 with 1″ of TUFF R(?) foam under vertically applied cedar 3″ T&G. After inspection there are 2×4 stud walls, 3 1/2″ fiberglass insulation,plywood sheathing, 1″ TUFF R foam, 5/4 cedar siding, and no drainage opening. I was trying to add insulation to the exterior. Thinking about ROXUL board directly over the cedar, furring strips, and different siding. Any thoughts on this plan.


  1. GBA Editor
    Martin Holladay | | #1

    Your plan will work, as long as you are willing to tackle the "mineral wool is squishy so it can be tricky to make the furring strips co-planar" problem.

    If you wanted, you could install a layer of rigid foam (instead of mineral wool) on the exterior side of the cedar boards, followed by furring strips and new siding.

    In either case, you need to come up with a good flashing plan for the window openings.

  2. SolarBilly | | #2

    Thank you Martin for your quick response.Although there are small vertical openings where the cedar T&G intersect I was concerned that moisture may be trapped between the cedar siding and rigid foam.This would result in having the cedar sandwiched between 2 layers of foam board with little air flow.Should I be concerned with this foam , cedar , foam application?
    I believe I will be checking all the flashings around doors and windows. The previous owner replaced 4 windows on the west side which were improperly flashed.

  3. GBA Editor
    Martin Holladay | | #3

    Although it's true that sandwiching materials made of wood between two layers of rigid foam is usually not a good idea, in this case the risk of problems is low, especially if the work is performed when weather conditions are good (ideally, sunny and dry weather -- at a minimum, on a day without driving rain).

  4. heidner | | #4

    Martin, if he is using the Roxul Comfort Board IS - it's pretty rigid. Much of that "squishy-ness" of the bats is gone. Or did I miss something?

  5. GBA Editor
    Martin Holladay | | #5

    You're right that Roxul Comfort Board IS is much stiffer than a fiberglass batt, but it is still squishier than (for example) XPS or EPS foam.

    Whether a builder finds it easy or tricky to ensure that the furring strips installed over the mineral wool are co-planar depends on the skill of the builder. Some builders say it's a snap; others have a hard time getting the hang of it.

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