ADU/Office Vaulted Ceiling
Hi everyone,
Bit of a long two-part question.
We have an old summer kitchen that we are turning into a conditioned office / ADU. The building is ~200 sf and has a mix of true 2×4 & 4×4 rafters (I know). The rafters are not perfectly in plane which is the second part of my question. We are planning to fur down the rafters with 1 inch of poly iso and rips rips of 2x4s (we had a bunch of these from the project for a total depth of 6.5 inches. We are painfully aware we still are short on depth but I think this is where we are at for this building. It will be heated and cooled with a mini split. We plan to use drywall as a air barrier and then shiplap the ceiling.
My question is given 6.5 inches what do you all view as the best option to insulate (and vent … eeek) the roof deck. I want to avoid spray foam for both cost and the likelihood of having to do something to the roof in the not so distant future. We live in the Louisville, KY area so we do have winter but its pretty moderate by comparison. The roof is 12/12 and has shakes covered with ag panels right now.
The second part of the question has to do with the rafters being out of plane. Do you all have any suggests on how to bring them closer so the ceiling isn’t wavy? We plan to run a number of string lines and shim or plane the furring strips as best we can but hope someone has a firmer understanding or idea!
Thanks in advance,
Jay
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Replies
Have you read these articles?
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
https://www.greenbuildingadvisor.com/cad/detail/unvented-roof-for-cold-climate-cathedral-ceiling-asphalt-roofing
Walta
Jay,
I would use R-20 fiberglass batts separated from the 1" vent channels by Tyvek baffles.
Roofs typically get more insulation that walls for two reason: There is usually more space, and the stack effect makes them prone to more heat loss. You don't have that extra space, and with good air-sealing you can limit much of the losses, so I'd do the R-20 and be done.
If you want more R-value I would substitute a continuous layer of foam board on the underside of the rafters for the strips and furring. Tape the foam as an air-barrier, strap the foam, omit the drywall and go straight to the wood finished ceiling.
Unfortunately there is no magic solution to planing the rafters beyond what you are suggesting.
You can attach the rafter furring with plywood gusset plates to join the sides of the rafters to the sides of the furring. That will effectively give you the ability to float the furring as needed to level out the rafter plane or add a straight furring to a curved rafter, etc.
There's nothing that says a single method needs to be used; you can change methods to suit certain problem areas.
Jay,
Chris makes a good point. If you have the headroom, you can make the rafters as deep as you want with plywood gaskets and strapping. The gap also gives you an effective thermal break.
Thanks very much Chris and Malcom, this is super helpful. Depending on how uneven it turns out to be I may very well go that route!