Roof Assembly in zone 6
Hello,
I’m looking at building a 32×60 single story ICF house in climate zone 6, upstate NY. I’m trying to create a simple, bulletproof R60 ceiling by using engineered open web ceiling/floor trusses to clear span between the ICF walls. On the bottom of the trusses, I’d run a vapor barrier like intello, then run strapping across the bottom of the ceiling/floor trusses to hang the drywall on as a monolithic air barrier and provide space to run wires. I’d dense pack the trusses from above with cellulose to R60, then cover the top of the ICF walls and trusses with a single layer of Advantec flooring. Over the Advantec I’d put standard pitched trusses that would hang 3′ past the ICF walls, put purlins over the trusses and skin with metal. My thought would be that I’m creating a cold vented attic over an airtight box. Does this sound like a viable system or am I missing something like vapor diffusion at the Advantec layer?
Thanks for the feedback and look forward to hearing from the community.
Bill (Owner builder with just enough knowledge to get myself way over my head)
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Bill,
That you are suggesting yields an excellent roof system, the only downside is the additional cost of the ceiling trusses and sub-floor, which is not insignificant.
There are a few differences between your system and the way it is commonly done:
- The sub-floor is usually detailed as your primary air-barrier, not the drywall, or a separate membrane.
- TJs or solid lumber are often used over open-web trusses to form the ceiling cavity, as it isn't carrying much load once the roof trusses are on, and you only need enough depth for services.
- Rather than dense-pack the ceiling cavities, the attic floor above is usually loose-filled with cellulose. This keeps the ceiling cavity in the conditioned space, and inside the air-barrier, meaning you can use it for services, and don't need to worry about strapping the ceiling. It also means you have as much depth as you want for insulation.
- Purlins may or may not fly depending on whether your roof needs a sheathed diaphragm for structural purposes.
Malcolm,
thanks for the reply- my concern on the ceiling trusses vs TJ's or solid lumber was the clear span over 32 feet- I haven't priced it yet, but I thought the parallel chord open web trusses would be cheaper than TJ's as well as offer less thermal bridging, and the cost of beams or solid lumber that could clear span that distance I really don't want to think about in this area.
The strapping and intello also lets me separate the drywall from the cellulose so if I ever do have a problem or trying to install a new light/fan etc. I'm not dealing with cellulose falling through the hole (hopefully).
How do you deal with vapor diffusion if you use the subfloor in my example as the primary air barrier? I've seen a lot of issues with old sips that rotted because they accumluated moisture at the seams and that's one issue i'm trying to avoid!
Bill,
- If you leave it as a dedicated service cavity with the insulation above, you don't have to worry about thermal bridging.
- The only time the ceiling structure carries any substantial load is when it acts a scaffolding while the roof trusses above are being installed. I suspect you could do that span with quite shallow TJs, but yes definitely price out trusses.
- No insulation in the cavity, means no cellulose to disturb when you work on the services there.
- Another advantage of moving the insulation is the sequencing of the construction. You don't need to worry about the weather, or pause the framing to install the cellulose, before laying the sub-floor and finishing the roof.
- The sub-floor is there to stop air movement, not vapour. You still need a vapour-retarder on the interior, which can be your painted drywall, or if you prefer the intello. It doesn't need to be sealed.
https://www.greenbuildingadvisor.com/article/zip-system-panels-as-primary-air-barrier
Malcolm, I've used blown in cellulose if roof trusses before and we had issues with wind wash redistributing the cellulose away from the edges, even with the baffles we put in. Any suggestions on how to avoid that issue if I put the blown cellulose over the advantec deck?
Bill,
If you use permeable material for the baffles - like house-wrap - you can run them up the trusses as far as you need to to avoid wind-washing without worrying about condensation.
Before you get on the train consider ICF is no longer a new idea like it was 20-30 years ago. The fact that ICF has not become the standard wall tells us the market has spoken for all of ICFs perceived advantages the market has decided the costs are just too great. That large a sample simply cannot be wrong.
Before you pull the trigger gat bids for both and look at the bottom line.
The roof you have described sound great but very expensive. Seems unlikely to save enough energy to ever recover the costs. When the next owner remodels they will be very surprised at what you did.
Walta
We were planning on doing exactly that framing system on a current project of mine, though with double-stud walls, not ICFs. We were planning on using Triforce open-web joists but they discontinued their 9 1/2" height just as we were wrapping up detailing, and due to height restrictions we didn't want to go with their 11 7/8" height. Local truss companies wouldn't make 9 1/2" joists either. We considered T-studs (now Thermal Studs) but they were limited in length and had open areas too small to run ductwork. In the end, we went with a membrane directly below the roof trusses and a site-built suspended ceiling of 2X lumber with vertical gussets.
I agree with Walta regarding ICFs. If you're set on using them, perhaps consider ICCFs instead: https://www.greenbuildingadvisor.com/article/the-perfect-block-foundation.
Michael, I'm planning on using the AMVIC R-30 blocks, they're manufactured relatively close by across the border in Canada, have fewer issues with blowouts and a higher R value than most of the ICF's. The ICCFs look interesting and I'll look into them more but for now, I'm pretty set on the AMVICs. As with most building decisions it comes down to what's available in the local market and comfort level and from what I can see it looks like a relatively cost effective and simple way to get an airtight, near passive house structure. I'll definetly be posting more as the build progresses though so we'll see!
This seems like a good system, though one benefit to the floor truss/tji is not worrying about truss movement. You could still use truss clips for this, or maybe those site built furring allow enough movement to negate any issues at interior partitions.