Sloped roof air barrier/strapping/mechanical cavity
I am building shed roof design in zone 6 maine with trusses, double studd design (2×4) blown in cellulose, two story house.
-There will be a standing seam metal roof
-high temp synthetic roofing underlayment
-5/8″ APA sheathing, engineered roof truss system
-create a 1.5″+ air gap at top for venting with Accu-vent product
-22″ deep blown in cellulose for R74
-Siga Swiss Majrex 200 at truss bottoms – taped as ceiling air barrier and installed prior to erecting second floor interior partitions
-2 5/8″ (half ripped 2×6) strapping installed on edge at 16″OC installed perpendicular to trusses supporting Majrex200, strapping creates mechanical chase possible Rzr style lighting fixtures
-1/2″ GWB ceilings throughout.
Question:
1. Does this system look good overall? Suggestions
2. Air barrier: Thoughts on Majrex 200 vs OSB as air barrier?
3. Air Barrier/Insulation Order: Is builder constructing all walls/partitions/air barrier and then insulators cutting/drilling holes in Siga or OSB in ceiling in order to blow in cellulose? Or is builder holding off completing partitions for entire second floor including air barrier for ceiling, then insulators do entire house, then builder returns to complete ceiling air barrier and construct all partitions?
4. With sloped ceilings and an air barrier, in order to run wires for lighting between air barrier and drywall ceiling, what are people using for strapping to provide the needed cavity? Also the depth of the wire cavity specifying? Was thinking of slim LED recessed but concerned of lots of holes in drywall and integrity of the air barrier so now leaning toward surface mount lights with low profile electrical boxes as needed…try to light with more sconces. I prefer to gain as much space so I can be just adding 8″ of space or something for a cavity.
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Replies
Your roof assembly is similar to several I have been involved with over the past few years. I'm a big fan of strapping the ceiling before interior partition walls are constructed. I like the Majrex, but it is expensive. Usually, we get the air/vapor material installed, then strap the ceiling. (You'll need to coordinate with any trades that have to get any needed penetrations through the roof, like a plumbing or radon vent pipe, or an electrical chase for PV panels. I'd do that work before the Majrex is in place). After the air/vapor retarder is in place and the ceiling strapped, I would have the insulator come in to blow the ceiling. They will be poking holes through the Majrex, but they are easy to patch using Siga's Rissan tape. After the interior walls are built, the mechanical/electrical/plumbing contractors can finish doing their thing. Because the ceiling is strapped, you have a bunch of choices for lighting. Be sure to have those lighting choices made before the assembly is built, you could change the depth of the ceiling cavity if needed to accommodate some lighting fixtures that require a thicker cavity. If you are using only box-mounted fixtures, a 2x4 flat to the ceiling is enough of a cavity for most electrical ceiling boxes, but the 2 5/8" cavity would be better.
Good luck with your project,
Randy
Thank you...Question, why insulate ceiling before the interior wall partitions are installed? Why not strap and do partitions and then the insulators come to blow the ceiling.
Often, we are trying to condition the space as we build out and finish the interior. If this isn't required, then insulation can be completed anytime.
Randy
The accuvent are not permeable. They are meant for soffit vents in a loose fill vented attic and won't work in a cathedral ceiling.
You want to use something permeable for your baffles. Insulweb, house wrap or thin OSB/CDX/MDF stapled to 2x2 on the side of the truss all work.
Low slope roof like this doesn't need to be dense packed as the insulation won't slide as it settles. A semi dense pack is good enough as it keeps the weight and pressure down a bit.