I need help! The roof assembly for my attic on my personal new construction build has caused many sleepless nights. I’m hoping a few of you will provide ideas on how you would handle this. My goals are to create an insulation and venting assembly that will meet code, allow me to have a continuous air barrier, not rot the roof sheathing, and not cause hours of labor and time…sounds simple enough.

Facts:

1.       The home is in North Carolina, climate zone 3A per building code

2.       Rafters were stick built framing versus trusses (2×8 – I realize not ideal and would change if I could)

3.       All Sheathing is ZIP, roof and walls (It is regular ZIP, not R)

4.       The home is already sided (hardiboard) and painted

5.       The soffits are vented on the non-gable ends (hardi as well)

6.       HVAC is already installed (conventional heat pump)

7.       Attic space has HVAC vents and is designed to be a conditioned space used for storage and mechanicals

8.       Standard gable roof with an 8/12 pitch (see attached pic prior to the roof being installed)

9.       Architectural asphalt shingle roof. It was installed with ridge vents and with synthetic underlayment (not peel and stick, except one small valley and on a few low-pitched, 3/12 areas). The roof is installed now, even though not shown in the pic.

10.   The home has a sealed and air-conditioned crawlspace foundation (closed cell foam underneath a thin concrete slab within the crawlspace) – completing the air envelope

11.   Due to the conditioned attic space, I’m approaching this the same as insulating a cathedral or vaulted ceiling.

12.   I also expect on almost any assembly I will need to use gussets of some sort to extend the depth of the rafters to obtain R38, as required per my zone.

I kick myself for not having the framers complete a wrap of the rafters with Tyvec commercial wrap, prior to furring and sheathing, similar to what was provided in this article: https://www.finehomebuilding.com/2021/05/27/building-a-vaulted-high-performance-and-foam-free-roof-assembly

Options I’ve thought about, including limitations:

·       Option 1 – Cutting EPS foam sheets @14.5” and creating a baffle by blocking with foam or furring strips above the foam. Spray foam or caulking the edges of the foam sheets, then batt insulation below…seems like a lot of failure points, poor air sealing potential, and still a lot of taping with a vapor diffusing membrane of some sort at the ridge (ie, Tyvec commercial wrap, which I have readily available)

·       Option 2 – I have considered baffles, then batts, then polyiso or EPS foam on the inside/bottom of the assembly. That could solve a few things, including maybe not having to gusset additional depth (or as much) on the rafter, and it would give a pretty good air sealing continuous layer. Again, being in the South, I worry about moisture trying to move to the interior in the Summer, but I know enough about dewpoint to determine that here? I would also think I would still want a vapor diffusion port at the peak of the attic. This assembly is #2 in the following article: https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work.

·       Option 3 – Using a product like Tyvec Commercial Wrap WRB, which has good vapor diffusion, but I think is still an air barrier? For this I’ve thought about two options:

1.       One would be to cut the wrap into 24” rolls making it easier to manage, and using furring strips to effective create baffles in each rafter bay all the way to the ridge. Then, I would seal to the top plate, each side where it hits the rafter, and wrap around the ridge beam and down the other side the same way. I would use caulk, tape, or foam to seal the joints.

2.       I’ve also considered trying to use a 6’ roll of WRB and essentially wrap the bottom of each rafter and up into the cavity within 1-2” of the roof sheathing and use a finish nailer or stapler to put furring strips on the side of the rafter to hold the WRB at that depth. I would cut the strips into 4’ sections to try to make this more manageable, leaving 1’ to overlap at the seams.

·       Option 4 – I could also use a WRB or potentially a smart membrane product on the inside of the insulation. I would install baffles, and then have the insulator use regular batt insulation to the baffles, and then I would put a continuous barrier via WRB or mart membrane with taped joints flat on the bottom of the gusseted extended rafters. This option seems like the best complete air barrier. Being in the South, it seems like vapor retarders on the inside of the assembly is not ideal, but it also sounds like some of the smart membrane products or maybe even a simple WRB for diffusion could mitigate that?

My insulator seems very method agnostic. The only method he was really opposed to was spraying closed cell foam under the roof deck. I would only entertain doing that in a non-vented assembly, which I guess I could still accomplish. However, it’s super expensive, and I don’t see it as ideal either.

My insulator also said in many vaulted ceilings in our area he does a lot of open cell foam applications straight to the underside of the roof deck. However, in my opinion, that seems like a bad idea, even with a vapor diffusion port at the top.

The insulator can do fiberglass or dense pack cellulose. However, his supplier cannot get mineral wool batts in anything other than r15, 3.5”, at the moment.

Some of the movement limitations in cutting around obstacles are kicker braces every 4’ and where the rafters and top plates meet, the ceiling joists are there too, meaning a cut-down in every bay from 14.5” to 13”. I’ll try to grab some pics this afternoon or weekend.

Please! I need your help. Let me know your thoughts about how you would proceed. In hindsight, I would definitely do some things differently, but this is where it’s at.