Unvented Attic with Trusses (New Build)
I’m trying to figure out the best way to do my attic insulation. I’m in northern Alabama, climate zone 3a. I want an unvented attic because my HVAC air handler and ducting will be in that space. So my roof top to bottom will be: shingles, underlayment, 7/16” decking, 1” furring, 3” polyiso rigid foam, 5/8” zip sheathing, and an unknown interior insulation against the underside of the decking. With the 3” of continuous rigid insulation on top of the roof sheathing, that gives me roughly R16-R18 before starting on the interior side of the deck. I’d like to try to get to R49 if possible, code is R38, we are still under IECC 2015 in Alabama. I understand I can’t do closed cell foam on the underside because that would sandwich my decking between two water impermeable layers. I assume my options would be open cell foam, fiber batt, or another permeable material, to allow drying to the inside, also knowing that I may have to use a vapor diffusion port. The engineered truss plans only call for the use of 2×4’s (some are 2×6) on the top that supports the roof decking and that doesn’t leave me with a lot to work with. On top of that, the truss webbing would make it hard to get any sort of seal. I feel like that leaves me only with open cell foam, but are the 2×4 bays going to support the almost 10” of foam it will take to get to R49? What are my options? We have not ordered the trusses yet. Would it be better, or even possible, to just do rafters? Thank you for your help, I’ve got a lot to learn!
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Replies
The way I see it putting equipment and duct work in the attic is like shooting yourself in the foot on purpose. Moving the insulation to the roof line and gables will have a much larger surface area 50-100% larger generally. In the very unlikely event that you apply the same R value to the roof line it will lose 50 -100% more energy forever. Putting the insulation at the roof line all but forces you to use foam that costing much more for ever R and every extra square foot. For a song and a dance, you could cover the attic floor with R60 of cheap fluffy insulation.
Being new construction seems like with some planning one could avoid such silliness.
Consider plenum trusses or counseled ducted mini split.
Walta
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That stack up you are suggesting is likely to fail. For it to work the rigid would have to go above the sheathing.
The one that does work in your climate is all fluffy insulation in your trusses with a diffusion vent at the ridge:
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
This is a code compliant assembly in your climate, simple to build and works.
If the top chord of your trusses is not deep enough for the fluffy you need, you can use thicker batts held in place with webbing. You can also strap out the top chord with 2x lumber and plywood gussets to add more depth.
You want to make an unvented roof so you need to air seal the attic space. This means sealing up the soffit space. A bit of spray foam is usually the simplest for this but can be done with sheet goods (plywood or rigid insulation) and caned foam. Also remember to seal off the attic space from the garage if it is attached to the house.
Thank you for the reply Akos. Looking back at my post I guess I wasn’t clear where the polyiso is in the layers. The 3” of polyiso would be on top of the 5/8” zip sheathing that would be sitting on top of the trusses. Does that make more sense?
The article you linked is one that I referenced when trying to research this set up. My main concern is that with the 2x4 trusses my underside insulation bay wouldn’t be deep enough to accommodate the additional ~10” of insulation it will take to get to R49. The second half of your response helps me visualize ways that it can be done.
Something I also didn’t mention in my original post that you hit on is sealing the soffit. My plan is to connect the zip roof decking to the zip wall sheathing through the soffit to form the continuous envelope.
Thank you again for your response.
Since the rigid is above, what you propose will work. It is a pretty expensive roof. There is no ROI for building high R value roofs with expensive materials (rigid, spray foam etc) in my way colder climate, in zone 3 the ROI is never. I know it feels wrong but build to code min.
I would talk to your truss supplier to see what is the largest top chord they can provide. Might be a bit more money for the truss and overkill for the load but you can now do a simple diffusion vented roof. Rafters only make sense if you have a simple roof shape, even there trusses will be faster to build.
Akos,
Living in the climate I do, I haven't paid much attention to roofs that rely on vapour diffusion ports, but wouldn't a roof that used permeable insulation and relied on them be very dependent on a good interior air-barrier to limit the moisture the port had to deal with? That would seem to preclude using it on a trussed roof.
If you’re willing to put 3 inches on top, why not 6 and you’re basically to code? Your house, your money but if I was building a new house I could find way more places to find value for my money than getting to R-49 in zone 3a, and for sure I wouldn’t use spray foam if I could avoid it.
This. Just put more polyiso on top of the roof. What is the practical difference between driving a screw through 3” of rigid foam versus 6”? Use a drill guide like Rockler makes.