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Community and Q&A

Where to Put the Vapor Retarder

laura_v | Posted in General Questions on

Zone 6 – Dry – Montana

I am finishing my roof assembly design and no matter how many times I read the code I am still not sure where the vapor retarder needs to go.

I have two different roof conditions that I need to plane out with each other; structural timber with 2×6 overbuild and conventional trusses with 2×6 top chord.  We are respecting the insulation ratios to avoid condensation on the roof deck.  We are not putting an airspace under the metal roof because of wildfire risk and the desire to not make more places for embers to catch)
Our walls are 2×6 w/air permeable batt insulation (R20), zip sheathing taped, 3″ Rockwool Comfortboard 80 (12.6), furring and metal siding

Proposed Roof Assembly, from inside out:
1 – 2×6 rafter over timber  or truss top chord with 5.5″ Rockwool comfortbatt between rafters (R24)
2 – Zip roof sheathing, taped
3 – 5″ poly-iso (R32ish, but downgrading to R25 for cold temps and time)
4 – Fire resistant plywood (wildfire area)
5 – Roof Underlayment
6 – Standing seam metal roof

Where does my vapor retarder go???  My understanding is that I need a Class II somewhere.  With the timber section it would be a little tedious, but not impossible to put it between the timber and the rafters.  We could put a layer of drywall behind the t&g if necessary or a smart membrane?  But where we are using conventional trusses we are also using a drop ceiling (not a vapor retarder), and putting mechanical in the conditioned attic space.
How can you attach a vapor retarder to the bottom of the top chord of a truss?
Do I even need a vapor retarder by code?
If the wall vapor retarder is the painted drywall, how do you make it continuous to the roof vapor retarder?

Thank you in advance for any help!

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  1. Expert Member
    Michael Maines | | #1

    Laura, drawings or photos would help, but I can say that closer to the interior you have the vapor retarder, the better. I might place it directly over the existing sheathing, before you set the 2x6 rafters.

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