An owner-builder planning a new home in southern Ontario isn’t looking for a net-zero house, just one that’s well insulated and protected from moisture problems. The question is whether his proposed wall system is his best option.
“I’m unwilling to put foam board on the outside and vapor barrier on the inside,” writes User 6782048, whom we’ll just call Ontario, in a question posted on our Q&A page. “Just seems wrong.”
He does plan on some exterior foam — a 1-inch-thick layer over the studs, followed by plywood and then Tyvek housewrap. After rough wiring, the 2×6 wall cavities would get a 2-inch layer of closed-cell foam and then batt insulation.
The technique of combining spray foam with batt insulation is called “flash and batt”, and Ontario believes the total of 3 inches of foam would keep the exterior sheathing above the dew point in winter, thus reducing the chance of moisture condensation inside the walls.
“Thoughts, please!” Ontario writes. That’s the topic for this Q&A Spotlight.
(For more on how to change a user name from a number to a real name, a quirk in GBA’s registration process, follow this link.)
The downside of flash-and-batt
Filling a stud cavity with closed-cell foam provides an excellent air seal, but it’s expensive — and that’s why some builders advocate the flash-and-batt approach. But, writes Andy Chappell-Dick, it’s still costly, “it relies on the least ‘green’ insulation, and it’s interrupted (and thus substantially degraded) every 16 inches by a stud.”
The technique may be useful in tightening up an old house in an efficiency-inspired renovation, but for new construction it would be better to put all of the foam on the exterior of the house. Further, Chappell-Dick adds, it would be wise to substitute a smart permeable vapor retarder for a polyethylene vapor barrier on the warm side of the wall.
Another problem with Ontario’s proposed wall is that 1 inch of exterior foam doesn’t offer much protection against thermal bridging, while the assembly would require unnecessary time and money.
“Seems like a lot of work (wind bracing) and money (spray foam) for a slightly above average wall,” writes Rick Evans, suggesting the proposed wall would have an R-value of just over 28. “Using spray foam to gain an extra R-3.5 or so probably has a long payback period from an environmental standpoint as well. (How many BTUs are you really saving with that?)
“I suspect you could add 3 inches of exterior rigid foam (Type II or higher EPS) over the 2×6 studs and sheathing for the same cost as the enclosure that you described,” Evans continues. “This would give you an R-33 wall with better protection from thermal bridging. From a moisture standpoint, this wall is at least as safe as the one you are considering, so long as you lose the interior vapor barrier. (Obviously, be sure to add an air gap under the siding in both enclosures.) If code requires a vapor barrier, then slice open the poly once the inspector leaves or opt for a smart vapor barrier like MemBrain or Intello Plus.”
Resistance on exterior foam
Three inches of foam on the exterior of the building no doubt would be better from a thermal point of view, replies Ontario, but that’s not going to happen.
“I agree totally that the 3 inches of foam on the outside would give better insulation results,” Ontario writes. “It’s the added time and costs that I’m trying to avoid, such as 6-inch screws to attach strapping, boxes to house windows, installation of the foam itself, self-supporting decks. By no means am I trying to go net-zero or near. Just a well insulated house without moisture problems.”
The thickest layer of foam Ontario would consider is 1 1/2 inches, although the 1-inch layer he has proposed is more attractive.
And as to the cost of the spray foam, Ontario will be getting that at cost, courtesy of a friend for whom he’s done work in the past.
An option suggested by Anthony Mach is to use Roxul ComfortBoard insulated sheathing, available in several thicknesses with corresponding R-values up to an R-12 panel 3 inches thick. “It’s permeable, fireproof and soundproof,” he says. “I’m in Ontario and it’s code-compliant.”
Or, adds Dana Dorsett, use rigid polyisocyanurate foam instead. “At the same wall thickness you can get higher performance at a lower cost out of 2 inches of exterior polyiso held in place by 1×4 furring through-screwed to 2×4 framing with 5-inch pancake head timber screws 24 inches on-center, R-13 or R-15 batts in the stud bays, with the siding mounted to the furring,” Dorsett writes.
“The plywood layer can be between the foam and studs, and fully structural,” he continues. “Your proposed stackup has the plywood out in the cold, where it it is subjected to exterior moisture drives (and would need to be rain-screened, not just housewrapped) to be reasonably well protected. The 2×4/R-13 + 2 inches polyiso solution comes in at slightly higher thermal performance, and in a southern Ontario climate it would not accumulate moisture over the winter, even using just standard latex paint on gyprock as the vapor retarder (though you may need to install a 2-mil nylon ‘smart’ vapor retarder such as MemBrain to convince the inspectors.)”
Too much of a hassle
Building exterior walls with 2x4s, one option suggested by Dorsett, would make it harder to place roof trusses, Ontario says, because the top plate is that much narrower. And as for a rainscreen, well, that’s probably not going to happen.
“I have resided houses 30+ years old with no housewrap and the plywood has shown no sign of moisture damage,” Ontario writes. “If you side correctly, no rainscreen is required. As for the environmental issue [of using spray foam insulation], I’ll not comment unless you live in a straw bale home. We are all just as guilty. It’s ridiculous [that] people bring it up, like rigid foam is a natural resource and not produced in a factory. LOL.”
The exchange prompts this reply from Andrew C: “If you come to a website with the name Green Building Advisor and ask for input, you shouldn’t be surprised if some of the advice is about how to make a house ‘greener,’ in addition to more comfortable, efficient, durable, etc. If you have different views, you’re free to ignore comments, but you’re not likely to encourage people to share their valuable experience and knowledge if you slam the door in their faces.
“Your personal experience in working on old houses may be pertinent to a new build with different materials and different levels of air-sealing and insulation than were common in the past,” Andrew C continues. “Maybe not. If the majority of experts now recommend rainscreens for most siding applications, perhaps you should consider it.”
Not at all, says Ontario.
“I came to a website and got some good information, but never really got my question about moisture answered!” Ontario says. “I don’t feel I slammed the door, but got the door slammed on me. It seems most people want to plug their own ideas without any regard for other ideas. The exterior Roxul batts was the only idea that even remotely took my thoughts into context. Sorry not everyone agrees with yours or others thoughts, but some of us look at it from a building standpoint. Time and pressure. Maybe something you fail to comprehend. Lucky you!”
In response, GBA senior editor Martin Holladay writes, “Here is the answer: Your house is located in Climate Zone 6. If you intend to install rigid foam on the exterior side of your wall sheathing, and if you want to avoid moisture problems in your 2×6 walls, then the minimum R-value of the rigid foam is R-11.25 (assuming, that is, that the stud bays are filled with a fluffy insulation product like fiberglass).
If you install only 1 inch of rigid foam (about R-4 to R-6, depending on the foam type selected), you risk moisture accumulation in your stud bays. To learn more, read this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.”
Our expert’s opinion
Peter Yost, GBA’s technical director, adds these thoughts:
If you specify a vapor-permeable water-resistive barrier (WRB), putting your structural sheathing to the exterior of your rigid insulation does give the sheathing good drying potential. While you are using plywood, the assembly you describe is a lot like Zip System R sheathing. Of course, the type of sheathing fasteners and fastening schedule are important when you place rigid insulation between the sheathing and your framing. With one inch of rigid insulation, that’s pretty straightforward; it gets increasingly difficult as you move to greater thicknesses of rigid foam.
The moisture performance of your proposed flash-and-batt system depends on the R-value of the components, the indoor relative humidity, and your climate. Here are the assumptions I am making about your wall assembly and environment:
- 1 inch of rigid insulation: R-5.
- 2 inches of closed-cell spray foam: R-12.
- 3 1/2 inches of fiberglass batt insulation: R-13.
- Outdoor average wintertime temperature for the three coldest months in Ontario: 23°F.
- Indoor temperature average in your home for winter: 68°F.
- Indoor relative humidity for the same period: 40%.
A quick dew point calculation indicates that your first condensing surface temperature (the inside face of your spray foam) would average 49°F, well above the dew point for your interior air of 43°F.
This suggests that only a Class III vapor retarder (1 to 10 perms) would be needed for your assembly to maintain good moisture performance of your wall assembly. Would a smart vapor retarder be a good idea? Yes. It would further improve the moisture performance of your wall assembly. Would polyethylene sheeting (a Class I vapor retarder) be a good idea? No, it would reduce the drying potential to the interior unnecessarily.
Finally, the most important moisture performance of your assembly — given that you are already aware of and committed to a good WRB and a good flashing system to manage bulk water — is your continuous air barrier. Despite the great air seal of your cavity spray foam, I would be detailing the housewrap, the plywood sheathing, or the rigid insulation to act as a continuous exterior air barrier.