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Northern Michigan un vented, un air conditioned Cabin roof design

user-6676471 | Posted in GBA Pro Help on

Northern Michigan un vented, un air conditioned Cabin roof design.
After reading several articles, opinions and Blogs I have a better understanding of the moisture vapors that exist and the problems they can create in walls and especially the roof. But I’m a little unsure of the proper design and construction for my great lakes, shore line cabin roof design. I have detailed my current plan below. Could please you give input as to where my design plan is OK or deficient in proper vapor management for this environment and situation.
I’m building a Building a 12×28 cabin with a 1 ½ story loft on one end, in upper peninsula of Michigan on lake Huron shoreline.
It is typically a humid environment as the cabin is taking the west wind blowing off lake Huron.
The cabin will see intermittent vacation use mostly in the spring, summer, fall and some winter use. Heat (if needed) will be turned on and off at each vacation use. Electric heat, No air conditioning.

Starting on the exterior of roof, I plan to use hidden fastener steel Roof. A layer of standard felt paper under steel. Plan is to use 1/2” OSB or plywood for roof decking. Steel, Felt paper, OSB or plywood, will rest on 2”x10” trusses. Between the 2”x10” trusses I will have 10” of craft faced toward interior fiberglass insulation. Next 1” Dow foam board as a air barrier, followed by T&G wood for interior of ceiling.
Thank you for your advice,

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  1. GBA Editor
    Martin Holladay | | #1

    You can't install a fluffy insulation like fiberglass between rafters unless you include a vented air channel between the top of the fluffy insulation and the underside of the roof sheathing.

    If you want to build an unvented roof assembly, you need either (a) rigid foam above the roof sheathing, or (b) closed-cell spray foam on the underside of the roof sheathing.

    Using rigid foam as an air barrier is possible, but may not work long-term (due to possible shrinkage of the rigid foam). A better interior air barrier is taped drywall. After the drywall has been taped, you can then install boards as your finish ceiling if you want.

    For more information, see this article: How to Build an Insulated Cathedral Ceiling.

  2. MichiganNick | | #2

    I am sure this is too late for you, however, most steel roof manufacturers require a high-temperature synthetic underlayment under their metal roofing. Secondarily you may consider an air gap (like a cool roof install) by lifting your metal off of the roof deck and providing a cooling air gap eve to ridge which can make a huge difference in thermal bridging in the summer months when you are most likely to be occupied.

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