Enclosed attic detail review – zone 2
Zone 2 – Phoenix, AZ
Single level residential home
Hello. I’m hoping to get some feedback on my plan to create a conditioned attic on my flat (0.5:12) roof. I’d prefer to stay away from foam if possible … except for the few penetrations into the attic.
Questions,
1. Overall, will this suffice? – Only flag I see is the bridging between truss and roof decking. (Rockwool will go between trusses). I’m just not sure how much it matters.
2. In zone 2, do I need a vapor barrier? If so, will the 5/8 Zip sheathing suffice?
–link to full size image detail if the uploaded is too blurry: https://photos.app.goo.gl/HAwzR94QMhFzpQcn9
Any feedback would be greatly appreciated, thank you.
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I'm also in Phoenix. A vapour barrier is not required by code. Generally, folks who use one in a cooling-dominated climate will put it on the outside of the wall assembly, allowing drying to the inside. Since the AC runs for months, that's usually how things go in Phoenix. But of course we are so dry most of the year that condensation on sheathing or in the wall cavity is most often a non-issue.
I've seen Zip, and other sheathing+membrane products use on the exterior as a combination air barrier and weather barrier. Just take care to tape and seal the entire building, otherwise you're creating pathways for both water and air. More energy loss and moisture infiltration will occur via the cracks than through the material itself (360x, if memory of my CPHT training holds.)
But that is more of a case for walls than roofs, especially flat roofs. In this case, I would consult someone who is expert at flat roofs. They are, I'm told, a beast to maintain even in a relatively rain-free place like Phoenix.
Great insight, thank you.
"Just take care to tape and seal the entire building, otherwise you're creating pathways for both water and air. More energy loss and moisture infiltration will occur via the cracks than through the material itself (360x, if memory of my CPHT training holds.)"
Makes me question my choice of a CMU block wall (Omni Block in this case). I think it's relying on thermal mass for efficiency vs being "tight". https://www.omniblock.com/thermal-mass/
Hope I'm not creating a different problem using Omni Block and a conditioned attic for the design. //
CMU walls are tight as far as building shell types go. Poured concrete might be tighter but I don't know that much else would be.
Why have a parapet on the low side? If you don't need it that is a collection point for water and you would be better off without it if possible.
How will you hold the mineral wool insulation in place below the roof? Will the bottom half of the roof trusses be exposed to the living space below or will there be a ceiling below the trusses? How will you be conditioning the attic?
“Why have a parapet on the low side? If you don't need it that is a collection point for water and you would be better off without it if possible.”
Good Q. I guess my head was that when it rains hard, I’d like to collect that water and daylight somewhere away from the home. I’ll give it a second consideration.
“How will you hold the mineral wool insulation in place below the roof?”
I’ll probably have to get creative and run some sort of strap.
"How will you be conditioning the attic?"
I won’t be directly conditioning it … maybe I used the wrong term. My intent is to simply have an unvented attic that’s thermally connected to the building's thermal envelope.
I appreciate the input. My first time doing this. ))
The detail works. Air sealing the truss end to the block will be hard, make sure to work out the detail properly. I would treat it like a rim joist on a platform framed floor.
If the trusses are not built yet and the cost up charge is not too much, I would bump up the upper chord to match the thickness of the batts you need. If not, you can use webbing or staple the facer to the truss members to hold the batts in place.
Arizona is a place with large diurnal temperature swings so heat capacity does help but you still need insulation. I would look at some rigid outside the wall or filling the block with insulation like peralite. No need to go overboard with insulation, but you do want some.
Block wall on its own is not air tight. To make it air tight, it needs to be parged, coated with a fluid applied WRB or drywall applied on the inside surface. If drywall is your air barrier, it should be continuous around the structure so it should tie to the plate under the trusses and be carried up to the roof deck air barrier.
Great food for thought, thanks Akos.