Housewrap for Air Barrier Under Tongue-and-Groove Ceiling
cynshaaron
| Posted in General Questions on
In the past I have put a layer of taped sheetrock under my T&G cathedral ceilings, but I am wondering if I can accomplish the goal of creating an air barrier with a housewrap and forgo the cost of the sheetrock installation. The ceiling is a 12:12 cathedral, 2×12 stick framed, with R30 insulation. We are in the marine 3c zone. Any input is appreciated.
And to clarify, it is a vented roof assembly with soffit and ridge vents and a 2″ continuous airspace.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Hi Cynthia,
I would suggest using a variable permeance membrane such as Siga Majrex, ProClima Intello or Certainteed Membrane. These will work as an air barrier and also respond to different relative humidities in different seasons. During colder, dryer conditions, when condensation risk is high, this material allows less vapor to pass. During warmer, more humid conditions, when drying is the goal, the material allows more vapor to pass.
I would connect the air barrier to the walls to make it continuous. If you are using the sheetrock on the walls as your air barrier I would lap the membrane down a few inches onto the top of the wall and then use a sill seal at the top plate, corners and bottom plate as a easy and affordable option for a decent air barrier. Another option would be to take a strip of membrane over the top of the top plates prior to setting the rafters and then connecting this to the WRB on the outside.
I would also suggest using a vapor diffusion port at the top of this vaulted assembly. https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports This works well in your climate zone and with the smart membrane, air barrier and diffusion port this should be a durable assembly.
If this is a VENTED roof assembly, a vapor barrier would probably actually be better than a vapor retarder here, since a vapor barrier would help to limit the amount of moisture the vent gap would need to remove under all conditions.
The usual reason to go with something like drywall here is that it's more durable than the membranes are. I've used polyiso with taped seams as an air barrier here before which works well, and provides a bit of extra R value too -- including over the rafters where you have thermal bridges. A sheet of 1/2" polyiso costs more than a sheet of drywall, but you don't have any of the labor of even rough finishing the drywall -- foil tape goes up quick and easy.
Bill
The first part of Bill's reply puts me in mind of a Martin Holladay article that Cynthia (or anyone) might appreciate: Vapor Retarders and Vapor Barriers.
Zepher,
Good point. Clarification on if the cathedral assembly is vented or not is impactful...
The assembly is vented, with a continuous ridge vent and soffit vents and a 2" airspace.
For climate zone 3c with a vented roof assembly and T&G ceiling, I would go for a permeable membrane to allow drying in both directions. Drywall is probably the easiest and cheapest. A poly membrane would run the risk of condensation on the back side of the membrane when the A/C is running. 1/2" polyiso might stay warm enough on the backside to avoid this, but not in super humid weather. 1" would certainly do it, but that's more expensive than drywall with rough finished seams. It would give the assembly an extra R7 of continuous insulation though. Check out the smart membranes. This could be what you're looking for.
Cynthia,
If you decide to go down the smart membrane road, take a look at this article: Smart Vapor Retarders for Walls and Roofs.
Thanks so much for all the replies! I will do more research on the smart vapor retarders.