Location of air barrier in roof assembly
I have been mentally working on details for a future renovation and have a question about locating an air control layer in the roof assembly.
My house is a 1920s semi-detached 2 storey located in Montreal Quebec, climate zone 6. The upper floor is about 750 sqft.
The existing ceiling/roof assembly is as follows, from the interior:
-Plaster and wood lathe
-1″ wood strapping
-2×6 wood ceiling joists @ 24″ o/c
-1″ wood plank decking, with big gaps between the boards
-wood pony walls built to create a low slope to the central roof drain, from a few inches high near the drain to about 12″ high at the roof perimeter
-wood roof decking
-TPO roofing, light beige in colour. The roof has 3 goose-neck roof vents that are about 12″x12″
This is a very typical assembly here in this part of the city. there is no insulation and no air or vapor control other than the painted plaster ceiling.
Some important criteria for me renovation:
-improve insulation and air sealing
-maintain as much as possible to ceiling height, we love the 9ft ceilings
-the assemblies must be DIY friendly, using locally available materials as I plan on chipping away at them over several years, room by room.
The exterior walls will be straightforward assemblies so I am not worried about how to do them. The roof assembly however is more challenging to me.
My current thinking is to build, from the interior:
-1/2″ drywall, painted with vapour retarder paint or I could use a smart vapour retarder behind the drywall.
-new 2×4 @ 24″ o/c on edge, perpendicular to the joists, fill the void with R-15 rockwool (rockwool chosen for easy DIY installation and availability)
-Tyvek as air control layer, nailed to joists and taped seams and nail heads
-existing wood 2×6 ceiling joists, fill the void with R-23 rockwool
-existing wood plank decking
-existing pony walls
-existing roof decking
-existing roof membrane
My thoughts are that putting the Tyvek between the R-15 and R-23 rockwool will be easier to detail than airtight drywall and easier to knit together as I renovate adjacent rooms.
I know the total R-38 of this assembly doesn’t meet the local code minimum, but it is much better than the nothing currently there and it doesn’t drop the ceiling height much and it is easy to install in readily available formats of batts. And it does limit thermal bridging to the intersection of the 2x4s with the 2×6 joists.
My concern is that the Tyvek might be a condensation risk. I will have 62% of my R-value outboard of the Tyvek however, so I think I should be ok as this is more than the recommended 51% outboard insulation recommendation I have seen in tables posted here and elsewhere on various building science forums for unvented cathedral ceilings, plus I have a vented roof space on top of it all.
So am I thinking incorrectly that this assembly make sense and ought to be safe?
Sorry for the rambling information and for your thoughts
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Replies
Your proposed assembly is safe from a moisture control standpoint. You could replace the Tyvek with a smart (or even dumb) vapor retarder, save the effort at the interior drywall, and still be safe from a moisture standpoint. You could also use loose-fill insulation in the attic, as deep as you want it. In fact, an easier and cheaper solution would be to apply the air and vapor control membrane to the underside of the rafters and just fill the entire "attic space above with loose fill of your choice. Yes, the total R-value would change based on how thick the insulation is, but you would still have at least 8"-9" of insulation at the low point of the roof and a foot or more at the higher sections. Your R-value would range from about R-35 to R-60. This approach would be harder to do room-by-room, as the roof assembly would be somewhat prone to moisture damage from the leaks in untreated ceilings. All of these approaches are sensitive to installing a good air and vapor retarder. Because the roof will now be much colder, bypasses could create significant condensation and damage.
Thanks for the reply Peter.
You're suggesting moving the vapour control layer to the same plane as the air control layer where I placed the Tyvek, between the R-15 and R-23 rockwool batts? Smart vapour retarders are not readily available here and need to be ordered online, hence the choice of Tyvek, which is available everywhere and is robust stuff and easy to work with. The possible use of vapour retarder paint makes the vapour control layer not challenging at the drywall ceiling.