Wednesday, August 25, 2010

Unvented Roof Space/Evening Fun with the Building Code

As per our previous post, one of the challenges with the City granting our permit was that we did not want to vent the joist space between our 3rd floor ceiling and the roof deck.  We are trying to achieve R85, which we can do with 16” of Roxul in the stud space and 3” of re-used/salvaged XPS outboard of the roof membrane.  The outboard XPS not only adds thermal performance but will also significantly increase the longevity of the roof membrane.  The challenge with this section is that if you put a vented space between the top of the Roxul and the roofdeck/XPS – then the XPS no longer contributes to  keeping the house warm, and we are left with an R60 roof.

Ontario Building Code (OBC) requires a vented space between the insulation and sheathing except where it can be shown to be unnecessary. OBC Appendix A- explains that the venting is required to limit moisture induced deterioration that can arise from imperfections that normally exist in vapour and air barrier systems.  The Appendix further explains that the exception to venting accommodates specialized ceiling roof assemblies that are sufficiently tight to prevent excessive moisture accumulation. 

Typically we think venting roof space is a good idea as we've seen too many moldy attics and too much shoddy construction to think unvented roof spaces should normally be standard practice. But this is no standard house, with a much better then average design, and a commitment to details by everyone involved.

So – we called on our good friends at Halsall Associates to help us develop a ceiling detail that would be sufficiently tight to achieve this.  The following details describe the assembly.  The City has accepted this – so we are good to go.  (Note – this assembly is specifically for our house – and may or may not be right for another project, you really need a good Building Scientist for this kind of detail)

Specialized Ceiling-Roof Assembly

The following specialized assembly is engineered specifically for 125 Third Avenue and does not require a vented cavity below the roof sheathing to control moisture.

The design consists of the following, from top to bottom:

·      Surface Finish - Wood roof deck or aggregate (varies over the roof area)

·      Permeable water resistive fabric (Tyvek or equal)

·      3” extruded polystyrene (R15)

·      2 ply modified  bitumen membrane system

·      Nail applied cover board (compatible with, and part of the membrane system)

·      3/4” tongue and groove plywood roof sheathing

·      16” I-joists, cavity filled with mineral wool insulation (Roxul) (R60)

·      Tyvek (or equivalent) vapour permeable air barrier over top of wall header prior to installing I-joists. Seal to exterior sheathing and to ceiling polyethylene to protect against wall cavity air rising into the ceiling space. Lap and seal all joints.

·      15 mil ceiling polyethylene vapour barrier with all joints lapped and sealed with acoustic caulking, and acoustic caulking applied at every joist to seal staple and screw penetrations.  Lap and seal the ceiling polyethylene to the wall polyethylene vapour barrier.

·      Foil faced drywall with foil facing down; joints are to be caulked and then taped with foil tape. This is also to be sealed and taped to the wall polyethylene.

·      2” x 2” furring along the bottom chord of every joist, bedded in acoustic caulk prior to nailing

·      3/4” strapping perpendicular to joists, screws to penetrate furring only

The above assembly is to be air tested with a blower door and smoke to confirm the quality of the air barrier assembly as a quality control measure. We understand the City Building Inspector will be invited to witness this test.

·      1/2” drywall finish

·      Finally, the cladding materials at the ends of the roof I-joists are to be vapour permeable; mineral wool insulation with Tyvek is to be provided instead of foam insulation.

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