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Will I get condensation if my roof insulation isn’t thick enough?

hamiltonworks | Posted in General Questions on

Hello GBA,

Climate Zone 5B, Eastern Sierras.
I’m converting a detached studio/shed building, (200 sq.ft.) to be an office space, material storage, and do the occasional glue-ups (woodworking glue-ups) when my shop is too full.
That being said, the space will be “somewhat conditioned”, i.e. a little space heater in winter, a window A/C unit in summer, only when need be. I am working on air sealing and insulating. 
It’s a monoslope 3:12 roof with 2×6 rafters, and a vaulted interior ceiling. I have planned a 3.5″ air gap/vent channel under the roof sheathing that is connected to continuous soffit and ridge vents. Since my rafters aren’t very deep, and I don’t want to get too carried away with this project because it’s just a “shed/office”, I was thinking of using 2″ polyiso (R-13) in the remaining 2″ space in my rafters. I can attach them to cleats inside the rafter bays, and they will be flush with the bottom edges of my rafters, making one single plane. I will air seal any gaps.
-My question is, can I just put my finished ceiling material (1/2″drywall, or 1/2″ ply) over this, assuming I do airtight drywall or airtight ply? Is this an issue because my thermal insulation layer isn’t thick enough and will result in condensation between the polyiso and the drywall/ply ceiling?  
I understand that I can fur out my rafters to get thicker insulation to meet code req., and perhaps do a layer over the rafters, then install strapping, but again, I’m trying to keep this simple. Any advice/insight would be greatly appreciated! 
Thanks to everyone!

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  1. brendanalbano | | #1

    Is there a particular reason you want to do a 3.5" air gap? 2" is pretty standard. That would give you room for a little more insulation.

    In a vented assembly like you've described, too little insulation isn't a condensation risk, so you don't have to worry about that.

    The "not enough insulation condensation risk" you're thinking of is probably from advice about when you are using a mixture of impermeable (typ. rigid) + permeable (typ. fluffy) insulation in an unvented roof, and it is all about the ratio of rigid to fluffy. Again, in a vented roof assembly like you are describing, you don't have to worry about this.

    In a vented assembly, it is critical that your ceiling, whether it be drywall or plywood or something else, be detailed in an airtight manner, which it sounds like you are already aware of.

    Just to get into the weeds a little, I guess if your insulation was exceptionally thin, so thin that it's R-value was less than the r-value of your ceiling gypsum, you could potentially end up with a condensation risk between the two layers. You're not anywhere close to this scenario though.

    1. hamiltonworks | | #2

      Hi Brendanalbano,
      Thanks for the heads up on this. I stand corrected on the roof pitch, it's 2:12, not 3:12. Anyhow, the thought behind the deeper air gap is simply from various articles and publications by Martin H. and Joe L. indicating that venting a low-slope to nearly flat roof is difficult, and the bigger the vent channel it seems the better...? I suppose I wanted to error on more venting space instead of inadequate venting.

      The thought about condensation concern was that; since my thermal layer isn't thick enough to keep the finished ceiling material warm enough to be above dew point, I would essentially have a "warmish surface" (the ceiling) in the winter, meeting a cold surface (the polyiso) and thus condensing would happen in between these two layers??
      Thank you for clarifying though. I appreciate your input.

      1. brendanalbano | | #3

        I see. Extra ventilation space certainly doesn't hurt, hopefully someone else will chime in with some experience regarding if 2:12 is low enough of a slope to need more than 2" of ventilation gap!

        Regarding the worry about the thickness of the thermal layer regarding dew points, it's all about relative thickness, not absolute thickness.

        The surface temperature on the inside surface of the gyp is going to be pretty near room temperature. The surface on the outside of the insulation is going to be pretty near the exterior air temperature. The temperature within the assembly is between those two values, and what matters is what proportion of the total R-value is on each side of the surface in question.

        The interface between the gyp and the roof insulation still has something like 95% of the total r-value outboard of that surface, even in your scenario where you only have 2" of foam, so that surface should still be pretty warm.

        1. hamiltonworks | | #4

          Awesome, thanks so much for the clarification! Really appreciate it.

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