Exterior rigid insulation – semi-permeable EPS with poly vapour barrier
We are currently working on two residential projects where we have specified exterior rigid insulation over a 2×6″ wood framed wall. Both projects occur within Climate Zone 5 but they also have very different amounts of annual rainfall (260mm in the interior of British Columbia vs 1650mm closer to the coast). The wall assembly looks like the following in both cases with the minor difference of the insulation in the stud cavity.
Exterior Wall Assembly
– Cement Board or Cedar Cladding
– 1/2″ Plywood Strapping – Rainscreen
– 2″ EPS Rigid Insulation – Type 1 (10 psi, 5.0 perms water vapour permeance per 1 inch) – seams are not taped or staggered
– Tyvek Weather Resistant Barrier with lapped and taped seams
– 1/2″ OSB sheathing
– 2×6″ Wood Studs at 16″ o.c.
– R22 Fibreglass Batt Insulation (coastal) or R20 Densepack Cellulose (interior)
– 6 mil poly vapour barrier with taped seams
– 1/2″ Gypsum Wallboard
Technically we could get away without the vapour barrier on the interior but after researching this endlessly we believe that by using the semi-permeable, not taped, not staggered seam EPS, the wall assembly will dry to the exterior through the foam and the seams rather than being encouraged to dry to the interior where the mechanical system has to deal with the excess moisture.
Are we incorrect in our analysis of the permeability of the EPS or should we be taping or staggering the EPS seams and eliminating the interior vapour barrier? We are open to having a WUFI analysis on the different climates (rainfall/moisture) but would appreciate any input.
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