Nits & grits
SPECIFICATIONS:
Climate zone 3A (Atlanta, GA)
Bonfiglioli wall
2×6 studs
Batt insulation in 7.25″ cavities, type TBD
Plywood sheathing
Seams taped, type TBD
Housewrap lapped, type TBD
No exterior insulation
Air gap to majority brick veneer
Rainscreen to minority fiberboard veneer
Site-built roof frame
Spray foam roof insulation, type TBD
Plywood sheathing
Peel & stick lapped, type TBD
No battens
Clay roof tiles
Target <2 ACH 50 CONTEXT: New construction single family home in historic district. Architectural requirements for exterior aesthetics, skill sets of local trades, and desire for high robustness and low uncertainty have informed selection of the above specifications. We do not want to make walls measurably thicker. AIR SEALING AT WALL: We are considering an interior air barrier in addition to the exterior air barrier for redundancy against long-term failure of tape on plywood. Collectively exhaustive options we are considering [along with concerns in brackets] are as follows: (1) Smart vapor retarder [it eventually gets stupid, and fails in closed position] (2) Open cell spray foam flash coat inside stud cavities [off gas to indoor environment, uncertainty regarding my understanding that such will not yield condensation in wall] (3) Taped and caulked drywall [performance uncertainty, lots of effort, and easily compromised] Am I missing any alternatives? Am I missing any concerns? Suggestions? ROOF INSULATION: The low slope hips and HVAC in attic make venting infeasible. Accommodating exterior insulation is also infeasible given other project constraints. If I have done my homework correctly, only remaining functional option is spray foam between rafters, which is a concern for indoor air quality (as well as environmentally unfriendly). Mutually exclusive choices we are considering [along with concerns in brackets] are as follows: (A) Open cell foam, which is local building standard [interior moisture rots sheathing] versus Closed cell foam [roof leak rots sheathing] (B) Foam filled to rafters, which is local building standard [exposed rafters diminish effective insulation] versus foam filled past rafters [installation and performance uncertainty] (C) Foam board taped below spray foam for continuous interior insulation as well as air barrier to isolate spray foam from indoor air [access in low hipped roof] (D) Membrane air barrier to isolate spray foam from indoor air [unknown problems from sealing spray foam after initial cure but prior to occupancy] Am I missing any alternatives? Am I missing any concerns? Suggestions? Thanks for your help, and please don’t mind my feelings if you have relevant advice. I much rather be called stupid now when it’s free than find out later that I’ve spent money on something stupid.
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Replies
P.J.,
I know what "brick veneer" is. It's a type of cladding (siding).
What is "fiberboard veneer"?
Your peel-and-stick roofing underlayment should probably be one of the new vapor-permeable underlayments, since clay roofing tiles offer the ability to dry to the exterior.
Smart vapor retarders don't turn stupid.
If you are worried about spray foam odors, don't design a roof system that needs spray foam. Instead, design a house with a vented unconditioned attic, and don't put any ducts or air handlers in the attic.
Thank you for the response, and sorry for any confusion.
By brick veneer, I mean single layer of non-structural masonry. By fiberboard veneer, I mean veneer again as non-structural exterior surface but mistyped fiberboard instead of fiber cement, probably a flat joint lock Hardie product.
Is your suggestion of vapor permeable roof underlayment irrespective of whether spray foam below is closed or open cell?
This project is unfortunately constrained away from having a vented unconditioned attic, and we are accepting the risks associates with spray foam both to the global environment as well as to our indoor air quality. However, I am still trying to manage those risks as much as possible so would like to consider ways to seal the spray foam away from the indoor air, probably only as a mitigation option in the event of problems. My working hypothesis is that since closed cell foam would already be an air barrier, an additional air barrier layer (membrane, taped foam sheets, or taped drywall) properly installed right up against it would not introduce new hazards. I understand that it would be unusual and expensive and might not even work, but do you have an opinion as to if it would cause problems? Know of anyone who tested this hypothesis?
My alternate hypothesis is that is that open cell foam covered by air barrier would allow easier identification of roof leaks and temporary removal of air barrier for drying, but I am more concerned then about humid air from interior getting into the roof sheathing. Is this a rational concern? With plywood roof deck, permeable underpayment, and clay tiles that are mostly lifted slightly above underlayment due to how they are lapped, perhaps it would sufficiently dry to exterior.
For reference, I have asked several HVAC contractors what they do if customer is having air quality problems blamed on spray foam insulation, and I only received 3 responses:
1) Tear out all of the spray foam. This was only in walls, though, as attic was vented. It seems less feasible solution for conditioned attic in our case with low hips difficult to vent and mechanicals with no convenient space for relocation.
2) Tune the HVAC to positively pressurize the house, including conditioned attic. He suggested that this worked to reduce the odor. I do not know if this could be discontinued after some period of remediation, but in any case, it is unclear where the air was pushed to.
3) Install drywall ceiling and walls in the attic. This one was only a suggestion and had never been tried, but it lead my thinking to current state of options.
P.J.,
If your spray foam contractor makes errors resulting in lingering odors, you aren't going to be able to fix the problem with drywall, air barriers, or fans. You'll be fixing the problem by moving your family out of the building as fast as possible, into a hotel, and hiring a lawyer. For more information, see Spray Foam Jobs With Lingering Odor Problems.
In short, these problems are very rare -- but when they happen, you have a major headache with a price tag of tens of thousands of dollars.
Choosing open-cell spray foam over closed-cell spray foam will not make an appreciable difference in how soon you notice a roof leak. All roofs leak eventually, and roof leaks are hard to pinpoint, regardless of what type of insulation you install. If you want to be able to pinpoint roof leak locations, by far the best strategy is to build a vented unconditioned attic.
Move the HVAC system out of the attic. IMO every other option is a compromise.
Personally I would contact Carl Seville. He's a GBA contributor and is located in Decatur area and just built a new house which appears to have been subjected to the same government design constraints.
https://www.greenbuildingadvisor.com/blogs/dept/green-building-curmudgeon/new-house-year-one-update
I'm seeing a lot of Grace Ice/Water shield going up underneath tiled roofs in Buckhead. It must be the "in thing".
Thank you both for being clear, and I understand: A vented unconditioned attic is elegantly simple, and spray foam carries an unavoidable small risk of a major problem that is catastrophic.
Not what I wanted to hear but glad I did.
"If you are worried about spray foam odors, don't design a roof system that needs spray foam. Instead, design a house with a vented unconditioned attic, and don't put any ducts or air handlers in the attic."
Martin, I appreciate your advice. My name is James. I've tried to update my account to reflect this.
For a flat roof in a warm climate, is exterior foam alone adequate and leave the rest of the flat roof space empty? Would you vent or go ventless?
James,
Q. "For a flat roof in a warm climate, is exterior foam alone adequate and leave the rest of the flat roof space empty?"
A. Yes, exterior rigid foam alone is adequate, as long as you install rigid foam with sufficient R-value to (at least) meet minimum code requirements. This is the standard approach for commercial buildings with a flat roof. For more information, see "Insulating Low-Slope Residential Roofs."
Q. "Would you vent or go ventless?"
A. I would choose an unvented assembly with an adequately thick layer of rigid foam on the exterior side of the roof sheathing.
This is the proposed design. It's not yet final. Please feel free to critique.
Martin, thank you. There is something unsettling about an unvented attic. My mind tells me that attics need fresh air. I'll read the article again.
James,
I'm not sure whether the decks will be framed with wood or they are concrete slabs.
If they are cantilevered slabs, they might present serious thermal bridging problems.
In any case, these decks need to be insulated assemblies (because there is conditioned space below the decks). Since there is conditioned space below the decks, the decks need to be very carefully detailed for water management (roofing and flashing) issues. It's also essential that your designer and builder pay attention to the continuity of the thermal envelope at these decks -- meaning that you need continuity of the air barrier and continuity of the insulation layer, without thermal breaks.
Thank you Martin. They are wood decks. How would you insulate? It's just like a flat roof except you can't put rigid foam on top. There will be tile.
@ user-6872033
Calm your fears by remembering that commercial buildings have had unvented roofs for decades. They just have to be built correctly. The problems occur when people try to find an alternative to exterior rigid foam.
Thank you John. If you use exterior rigid foam, what is the harm in putting in some vents in the soffits and roof to allow a bit of air flow and an escape for the heat? I'm in Southern California.
James,
Q. "If you use exterior rigid foam, what is the harm in putting in some vents in the soffits and roof to allow a bit of air flow and an escape for the heat?"
A. You never want to ventilate a space on the interior side of the insulation. If you invite exterior air (cold in winter, hot in summer) indoors, why bother to insulate? Just install your insulation on a clothesline in your backyard -- it will be doing just as much good as your roof insulation, if you ventilate between your insulation and your living space.
James,
Q. "They are wood decks. How would you insulate? It's just like a flat roof except you can't put rigid foam on top. There will be tile."
A. Are you the designer? Or is there an architect?
If you want to install a tile deck over a wood-framed flat roof, with living space under the deck, everything needs to be carefully detailed: the air barrier; the insulation; the roofing and flashing; the dimple mat and coverboard above the roofing; and the tile deck.
If all this is new to your architect, you're in trouble. If your architect knows what he or she is doing, and is drawing the details, relax.
We can't give you a full lesson in all of the potential ways to screw these details up in a Q&A column.
Thank you Martin. Do you have a link to a diagram showing the layers for a flat roof (ie rigid foam, sheathing, moisture barrier, etc...all the way down to the interior drywall? I don't have a builder retained on the project yet. I'm trying to get educated for myself. Appreciate the help.
Thanks Martin. Yes I have an architect but I'd like to know these things for myself. Often times architects and builders go with the cheapest not necessarily best methods.
James,
We can't give you a full lesson in construction in this column.
I already provided a link to an article that provides advice on insulating low-slope roofs. Here is the link again: Insulating Low-Slope Residential Roofs.
If you are a GBA Prime member, you have access to the GBA detail library. Click the green "Strategies and Details" tab at the top of any GBA page.
There are a thousand ways to screw up a "roof you can walk on" -- especially if you are designing a patio with tile flooring above a wood-framed flat roof. This article explains some, but not all, of the ways that you can screw things up: Decks—Roofs You Can Walk On.
Martin, if this was your house, would you go with a low slope hip roof over the flat roof? They wouldn't look much different from street level looking up. Thank you.
James,
I would never build a house with a low-slope roof. I would build a house with a nice steep roof with a 12-in-12 slope.
But this is your house, not my house. It's possible to build a low-slope roof. Just get the details right.
Thanks for the help Martin.