Necessity of smart vapor retarder in Tiny House on Wheels in Zone 4A?
We’re in the final stages of a 24’ DIY “tiny house on wheels” build (photo attached) and struggling with the ever-complex vapor retarder question. We’re in the mid-Atlantic (Climate Zone 4A, mixed-humid but just coming out of a colder-than-normal winter), so my understanding of code and best practices is that we only require a Class III retarder. However, one ‘counterpoint’ that has me struggling is the fact that even with more than adequate mechanical exhaust ventilation, moisture produced by daily activities in 200 square feet can potentially be an issue for walls with fibrous insulation. Another factor/variable is our use of thin interior paneling, with unknown vapor permeability.
Here’s our wall assembly, from exterior to interior:
-Stained T&G 1×6 cedar siding
-Cor-a-Vent 3/8” rainscreen gap, vented at top and bottom
-Tyvek housewrap, fully taped at all edges and seams as an air barrier
-1/2” CDX plywood sheathing, glued/nailed and taped with 3M 8067 at all seams
-2×4 stud walls, 24”o.c. insulated with Roxul at R-15
–Possibly Certainteed MemBrain detailed as an air barrier/smart vapor retarder layer
-5mm (1/4” nominal) birch PureBond formaldehyde-free 3-ply plywood* as interior paneling, primed and painted with two coats of latex paint
*Possibly not thick enough to serve as an air barrier?
Vented cathedral ceiling assembly, from exterior to interior:
-Galvalume standing-seam metal roof
-Grace Ice and Water Shield, High-Temp version covering entire roof deck
-5/8” CDX plywood sheathing
-1” continuous, uninterrupted vented airspace between insulation and sheathing in each rafter bay, from eave to monoslope ridge
-2×6 rafters, 24”o.c. insulated with Roxul at R-23 (furred out 1” on their underside with strips of 1” XPS foam to allow for both the full Roxul batts and vent gap)
-Same PureBond plywood as the walls, noted above
Our mechanical ventilation/HVAC plan:
-Panasonic Whisperwall in the bathroom hooked to an automatic humidity sensor
-Broan range hood in the kitchen above the 2-burner cooktop
-Pioneer 9000BTU mini-split for heating/cooling, with dehumidification capability
As noted, I have been weighing the inclusion (or not) of Certainteed’s MemBrain smart vapor retarder product as a compromise to address my above points/counterpoints (it’s an internal struggle!), to balance what I’ve read/studied about vapor retarding in ‘normal’-sized houses with what I know about tiny house living – all with the additional variables of living in a mixed-humid climate and using a thin, unknown-permeability interior plywood paneling. Needless to say, I would appreciate any/all input, feedback, criticism, etc.!
Thank you in advance!
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