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Moisture risk in climate zone 1 unvented attics

albertoarriaga33 | Posted in Energy Efficiency and Durability on

Hey all,

My company is developing a common construction system for a hot, humid climate in the Caribbean. We are worried about the interior moisture load presenting a risk to our plywood deck. However, we dont have access to closed cell foam, and installing an interior vapor retarder does not make sense to us in this climate zone. Currently this is the setup that makes the most sense to us:

From the top down: Metal roof -> rain screen -> solitex mento 3000 (38perms) -> 2 inch polyiso rigid insulation (1 in staggered) -> 1/2 in plywood -> fiberglass batts in metal framing cavities. 

The roof is low slope, with a small attic space that is supplied with air via an ERV. The plywood is the air barrier as it will be taped at the seams and roofline. From a water vapor perspective, the plywood is essentially the vapor retarder. Assuming an adequate amount of ACH in the attic space, do you reckon the plywood is at risk of moisture accumulation? This would be monitored post construction with a thermo-hygrometer. 

Any tips and opinions would be great!

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  1. Jon_R | | #1

    An ERV isn't 100% efficient and the output will have a high dew point. Supplying interior air or AC air will keep the attic drier.

    In that climate, worry about exterior moisture. I'd say your polyiso rigid insulation is your exterior side vapor retarder.

    1. albertoarriaga33 | | #2

      Do you reckon an ERV return with passive intake vents in the 2nd story ceiling could work better? That way were sucking up conditioned interior air.

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