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Interior insulation for a backyard cabin with an unvented roof

Jeff_CP | Posted in General Questions on

I’m building a small 10×10 ft 3 season cedar cabin in my backyard.  The framing and exterior plywood sheathing is complete and I need to figure out how to insulate it.

The roof is a shed style roof (lean to).  I’ve installed blocking between the 2×6 rafters at the ridge and eave for strength.  I plan to have an unvented roof.  Due to height restrictions, I will be insulating the roof from the interior.  The roof consists of 5/8” plywood, synthetic underlayment, and a sheet metal roof.

I’ve read Martin’s response to the question in this thread, which partially answered my question (

For interior insulation, my plan is to put Rockwool batts between the 2×4 studs and between the 2×6 rafters.  I’m going to skip the 2 inch layer of continuous foam to maximize interior space.  The next layer would be a product called Thermoquiet (, which would be installed over the entire interior surface.  This would act as the vapor barrier. 

The interior of the cabin will be finished with cedar planks.  Do I need the layer of drywall or can I attach the cedar planks directly to the studs?  I should also mention that the space will be heated with a baseboard heater and cooled with either a portable or window mounted air conditioner. 

The exterior is 1/2” plywood, Tyvek Housewrap, and cedar siding attached to furring strips.

Any tips or considerations would be greatly appreciated!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Jeff.

    Where is the cabin located?

  2. Expert Member
    Akos | | #2

    I have a similar bunkie by the cottage in zone 6. Generally you can pretty much do anything since the place is not occupied full time and there is no interior moisture source. I would make sure the interior air barrier is tight, the underlayment you are suggesting is not really designed for this and it might be hard to tape. The unerlayment is thin enough that is not adding any real R value to the assembly, I doubt it is worth the additional cost.

    If you take a bit of care and use air tight device boxes, standard 6 mil poly can be detailed as your interior air barrier. Drywall or a thin layer of taped OSB is more robust.

    I would figure out how to add a bit of drying to the ceiling, in the case of my bunkie, this is a mini attic near the peak with ridge vents. An unconditioned space still gets a bit of night time condensation on the underside of the roof deck, you want to provide a means for this to dry to the outside.

  3. Jeff_CP | | #3

    Thanks for your replies.

    Brian - The cabin is located in my backyard, in Ottawa, Ontario, Canada. It gets very cold here in the winter, sometimes down to -40 F.

    Akos - If I do need to vent the roof (which it sounds like I do based on what you said), I can ensure the Rockwool batts are 2 inches from the plywood roof. I can drill 2 or 3 holes in the blocking at the ridge and eave end which would vent out through a perforated soffit. Would this suffice? Given that the framing and sheathing is already done, if I could go back in time, I probably would have used blocking that was 4 inches high, which would have left a ~ 1 inch gap at the top for venting at the ridge and eave.

    Also, if you think the Thermoquiet isn't worth it, then I won't use it. If I do decide to go with 1/4 drywall or OSB, do I still need to use a vapour barrier between the Rockwool batts and the drywall/OSB layer?

    1. Expert Member
      Akos | | #4

      I would strap out your rafters with either 1x2 or 1/2" strips of foam and then install the batts. As long as you keep the batts flush with the interior surface, this would give you a bit of a vent channel without having to downsize your batts. It is not full size per code, but it will provide more than enough drying if you get the ceiling air tight. Couple of holes at the eaves as you suggested and a ridge vent, you'll have a solid roof.

      The underlayment is around 1/8", at best it is R0.6. The reflective facer is not facing an air gap, you'll get no thermal benefit from it. Overall, I don't think it is worth the cost.

      If you do want higher performance walls, your best bet is continuous layer of interior rigid insulation over the studs. Taped and caulked around the edge, this could also be your interior air barrier.

      1" foam would bring the wall up to similar assembly R value as a 2x6 wall and it is thin enough that you can still nail the lumber through it with 2.5" finish nails. 1" foam is a bit more than drywall but it gives you a real R value boost. You would have to space out your device boxes a bit so they'll be flush with the finished surface.

      1/4" drywall is usually not worth the cost as it is the same price as 1/2". If you want a cheap air barrier, 7/16 OSB is your best bet.

      1. Jeff_CP | | #5

        Thanks for the info. If I do decide to go with 1/2'' drywall (or OSB), do I still need a vapor barrier between the Rockwool batts and the drywall (or OSB)?

        1. Expert Member
          Akos | | #6

          I always install one (plus it is code), also makes for a decent secondary air barrier. You can skip it as well since the building won't be occupied full time. A coat of latex paint is all you really need for a vapor retarder.

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