Distance between air barrier and insulation
I am building a home in zone 2. The roof has a flat pitch (1:12) with long overhangs (6′). Given the flat roof pitch, I understand that the attic cannot be vented so I am planning to insulate below and above the roof deck. I am considering making the air barrier continuous between the walls and ceiling and placing a service cavity below the ceiling air barrier to run HVAC, water and electrical lines. This would make air sealing easier as I would not have to seal around all of the overhangs. I do not, however, think that it makes sense to separate the insulation and air barrier by that much distance – is this correct? If so, would it be best with my flat roof design, to simply try and seal around all the eaves? I have reviewed the idea of “stuck on” eaves but I do not think this is structurally an option in my case given their length. Thank you.
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I'm not sure where the problem is.
In zone2, your plan definitely makes a lot of sense as it takes very little insulation above the roof deck for condensation control.
Having your HVAC be in a service cavity inside the air barrier also a good idea.
Generally, you have gaps between the air barrier and the insulation if the insulated cavity is well sealed on six sides. Air sealing overhangs is pretty straight forward. Cut a bunch of foam block and seal them in place with canned foam.
I guess my question is, can the insulation be at the top chord of the truss and the air border at the bottom chord? Or should they both be in the same location?
This is an interesting question. Normally, in colder climate it is fine to have a gap between the insulation and warm side air barrier, but we are taking something on the order of an inch or two.
If you are taking trusses, the gap would be much bigger, closer to an attic space. I think it should still work.
If you do want your hvac up there, I would go with open cell SPF under the deck which also serves as your air barrier. You can also try to get your air barrier further up, but it is hard to air seal around trusses.