Larsen truss questions
Hello. I was hoping to get some more feedback from the GBA folk.
We have just framed a small house in an area with a -32 design temperature. our plan is to attach a Larsen truss to a conventionally framed wall. the wall detail is as such: 2x4 stud wall, sheathed with 5/8' t+g OSB, 8"-ish Larsen truss, cross-strapped with 1x, rainscreen, cedar siding. we have to use batt insulation because dense-pack or wet-pack cellulose is not available (we will use blown cellulose for the attic).
The OSB has acoustic sealant in the t+g joint and has been acoustically sealed to every framing member it touches effectively isolating every stud bay. all the joints were then siga-taped on the exterior side of the panel.
My interpretation of the british columbia building code (bcbc) combined with consulting various people including building scientists and the plywood council of bc, suggests the osb can serve as both the air barrier and vapour retarder. if i insulate the stud wall with r12 and the larsen truss with r28, what and where should i put the vapour retarder?
Have i commited myself to the osb as the vapour retarder and therefore must omit the insulation in the stud cavity due to the 1/3 rule?
What sort of weather barrier should i use and where should i put it? on the exterior of the truss or on the exterior of the 1x cross-strapping?
Is it a horrible idea to skip the 1x cross-strap and rainscreen combination and instead, place the weather barrier directly on the larsen truss and then cross-strap with 1x on a 45 degree angle? this would brace the larsen truss, support the weather barrier, create airflow behind the cedar cladding, and allow moisture to escape via gravity.
Posted Oct 3, 2012 4:27 PM ET
Edited Oct 4, 2012 6:24 AM ET
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