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GBlist: 2x4studs,eps, vapour diffusion
I have a question on vapour permeance on a wall assembly that consists
of a continuous eps (type2) layer over 2x4 (flat). Full assembly
therefore is 1-2/2 int. air space, 3-1/4 eps type 2, 3/4 strapping,
fibre building wrap, cedar channel siding.
Permeance on type 2 is 200ng/Pa.s.m.sqM (code req.are 230), construction
will allow seams to be either foamed or taped from either back or front
and plates can have a poly overlay which can be taped to the foam.
Generally I feel that given the continuous nature of the foam wrap and
with no exterior sheathing the permeance rating is probably sufficient
but I'm concerned about the proximity of the foam barrier to ext. air
temp and the potential in colder weather for moisture to condense within
and diffuse through the material as the outer inch or so of material
will be below the interior dew point . Also as EPS degrades (as
everything does) is there a potential for further moisture diffusion
through the material
I'm also concerned about moist interior air movement within the
concealed 1-1/2" interior air space and the inevitable cold spots. Is
there any advantage to this concealed space other than as an electrical
chase and a slight insulation gain.
I am trying to eliminate any unecessary materials from this assembly so
would like to do without an added poly vapour barrier but am starting to
think it might be necessary. Any thoughts or experience with eps as a
low permeance material. (Please don't suggest using foam panels in this
situation as I would if they were available)
Thanks
John
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