Whole home remodel – spray foam & IWS issues?
Hi all,
In looking through the Q&A forum, I see a number of questions with very similar details to mine, but given the stakes I’d like to make sure I get this right for my client and have the right knowledge going forward.
I’m doing an extensive whole-home remodel of a dated 60’s era ranch in climate zone 3A (coastal NC). We are doing a couple of additions, a complete gut of the rest of the home, and eventually further improvements to the exterior. With it also being in a high-wind zone, as part of the remodel, we are replacing the roof under the Fortified Roof program which stipulates full IWS over the entire deck.
The existing roof assembly was vented, though I’m not sure how effective it was as the vented vinyl soffit was placed directly over the original (no vent) plywood soffit which we have removed. My client would like to insulate the underside of the roof deck with spray foam – the air handler and duct work are in the attic (a ridiculous detail that I observe far too often in this region, but I digress).
I want to make sure that we aren’t running into a building science SNAFU by entombing the plywood roof deck with close cell on the underside, and IWS above. I would imagine that there is not considerable vapor transmission upwards through a standard shingle roof anyway, but the extreme heat/humidity that we experience in this zone leaves me wary of doing so. In reading some of the other forum topics, it seems that as long as the attic is conditioned, we should be in good shape. Is this the case?
Thanks in advance for some insight on this matter – been a long time FHB enthusiast, and GHB lurker, but nice to finally have something substantial to bring to the forum.
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Replies
Seaside,
From this article: https://www.greenbuildingadvisor.com/article/sandwiching-roof-sheathing-two-impermeable-layers
"You can install the closed-cell spray foam. Just make sure that the roof sheathing is dry — that is, with a moisture content below 18% — on the day that the insulation contractor installs the spray foam. Remember: There is no reason for roof sheathing to need to dry out if it never gets damp in the first place. So you need good details — details that prevent warm, moist air from contacting cold sheathing. If you’ve got good details, then your roof sheathing doesn’t necessarily need a drying path.
We have decades of experience with this type of roof assembly. These roofs work.
Eventually, of course, the roof will leak and get wet; that happens to all roofs. When the roof leaks, it’s time for a roof repair or new roofing — and in some cases, some sheathing repair. But these types of repairs are needed in all types of buildings when roofing fails.
I recently telephoned Joseph Lstiburek, a principal at Building Science Corporation, and asked his opinion on the matter. “We have double vapor barriers in commercial roofs all the time,” Lstiburek told me. “When people say, ‘The roof sheathing has to be able to dry in at least one direction,’ I say, ‘That’s ridiculous.’ It doesn’t have to be able to dry out. We have lots of historic experience with commercial roofs that can’t dry out in either direction. In commercial construction, we rarely have a situation where the sheathing can dry inward.”
All that said however, in climate zone 3 you can use vapour diffusion ports and permeable insulation to crate an unvented attic. I'd consider that too:
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
Malcolm,
Thank you for your thoughtful reply! I have been digging into each of the various articles / posts linked in your comment, as well as the rabbit holes each of those respective articles lead to.. I feel like I'm getting somewhere but want to keep the dialogue going.
I don't know that vapor diffusion ports are right in this case, as I'd still like to put some insulation on the attic floor. The 2x4 top chord of the trusses won't be getting us enough R value to meet code, and though I think I'm leaning towards the idea of conditioning the attic and simply sealing the roof deck, but I don't know what stipulations this creates in terms of doing both.
Additionally, to complicate matters, there is a small addition going on the back of the home with a vaulted ceiling with 2x10 rafters. I wonder what kind of implications there will be to an unvented, insulated cathedral ceiling over-framed on an unvented, conditioned attic.
Thanks! - Sal / SC
Once you apply the spray foam to the rafters and block the vent you need to start calling it a conditioned attic.
All too often people are looking for the free lunch and are unwilling to size the equipment to cover the heat load for the attic. They refuse to install supply and return registers in the attic. They don’t care what the temp and humidity in the attic is.
If you fail to condition the attic the wall surfaces may fall below the dew point of the air in the attic. Things may get wet and rot.
Note the other problem is that spray foam is so very expensive that almost nobody installs the code min R value simply because it will never save enough fuel to recover it’s cost.
If this project is still in the planning phase make sure to run the numbers to bulldoze the site and rebuild all new. My guess is it will cost 15% less and you will have a better building.
Walta
Hey Walta,
Thanks for the info. I like the idea of conditioning the attic where the equipment is enclosed. We are going to be doing an extensive upfit of the HVAC system so I will insist we integrate the consideration of sufficiently sized equipment to handle the additional load. Do you think we would run into issues if we conditioned the roof deck (enough to enclose the attic) as well as the attic floor (enough to meet code minimum)?
Appreciate it
Sal / SC