Hello All,

Location: Seattle
Climate Zone: 4C

I am in the process of fabricating two large (approx. 4’x7′) out-swinging carriage doors for a conditioned garage workshop in Seattle, WA.  The build will be assembled as follows (from inside to outside):

-3/8″ marine plywood (primed and painted)
-1 1/2″ rigid foam insulation (Polyiso or EPS) within 2×3 lsl frame
-1/2″ marine plywood
-1/2″ marine plywood furring strips, with Coravent strip at bottom for ventilation
-3/4″ horizontal T&G cedar siding (no finish, left to weather)

These doors will be partially exposed to weather.  My question is how I should treat the outer layer of plywood, particularly as it relates to moisture accumulation and longevity.

My first thought is to treat the plywood like sheathing on an exterior wall assembly.  The easiest way to do so would be to use a fluid-applied product that can be rolled on, such as Prosoco’s Cat5 (with furring strips attached after the application).  However, this product is expensive and I’m loathe to purchase a 5-gallon container for such a small project.  The alternative would be to apply a sheet material (Tyvek or similar) and tape the edges.  I have leftover WRB rolls and zip tape from other projects.

My second thought is that perhaps a WRB is unnecessary in this instance.  The rigid foam sandwiched in the door is air-impermeable, and with the addition of the ventilation channel the marine plywood should be able to dry to the exterior.  Definitely not confident in this solution without input from folks here.

Finally, I should mention that the materials for this build have already been purchased and I have started to fabricate the doors.  In hindsight, I probably would have used 1/2″ Zip on the exterior to simplify this issue; unfortunately not an option now.

I know there are other methods I have not listed here and I’m curious how the GBA community thinks I should proceed.  Open to any/all suggestions.  Thanks in advance for your time.

Cheers,

-Adam