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Cold roof design for existing structure

Mbhoss | Posted in General Questions on

I have an early post and beam house renovation underway. All exterior walls are the finish interior. I am installing Blums skin then skinning walls with 1” ridged insulation, then layer of ½” zip wall then clapboards. Roof will be layered using ½” ply, taped, vapor / air barrier, 1” ridged insulation taped, roof underlayment, strapping then zip roof, underlayment and slate roof. Obviously not going for high r value due to numerous other constraints. This is a guest house used primarily for 3 seasons. I would appreciate any constructive thoughts or advice.

Southern Vermont, Climate Zone 4b

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  1. Mbhoss | | #1

    Thank you for that suggestion, I’m new to this.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Mark.

    I don't see anything wrong with your wall assembly, but I am not sure why you need the self-adhering WRB on the existing sheathing. The ZIP sheathing will provide a WRB and you can tape and detail both the insulation and the ZIP sheathing for air sealing. Now that I think about it, why not just install ZIP R-sheathing over the existing sheathing? Are you familiar with the product. If not, read this:

    And, include a vented rainscreen detail in your wall assembly: All About Rainscreens

    Your roof details can be simplified too. I'm not sure that you need a ventilation channel above the roof deck in this case. Plywood with taped seems is a good idea for your air barrier. You don't need an additional vapor barrier. You do need enough rigid foam R-value to keep the sheathing warm--R-20 or more in your area, or you could have moisture issues when you heat the building. Then an additional layer of sheathing, underlayement, and roofing. This may be helpful: How to Build an Insulated Cathedral Ceiling.

  3. Mbhoss | | #3

    Hi Brian,
    Thanks for your reply. The reason for the WRB was to have a vapor barrier to the inside of the wall assembly. The existing sheathing is the finish interior wall. The same reasoning for the roof assembly. I can not add any more insulation to the roof system and we are not attempting to get an r20 but felt something is better than nothing. As with the wall, the thought is to stop the vapor migration. The vent system within the roof system is to allow for drying and was recommended by the slate roofer.

    What’s unique to this is that there is no wall or roof cavity to allow for added insulation. And roof build up is restricted.

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