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Q&A Spotlight

Insulating a New House

An owner-builder looks for advice on the best way to insulate exterior walls in a new build in Ontario

A homeowner is considering the “flash-and-batt” method of insulating his exterior walls — a method that calls for spray foam to be combined with batt insulation. But he wants advice on alternatives.
Image Credit: Dunktanktechnician / CC BY 2.0 / Flickr

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.


  1. User avater
    Stephen Sheehy | | #1

    No rain screen required if you side correctly?
    Isn't a major benefit to a rain screen the ability of siding to dry out, significantly prolonging paint life?

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to Stephen Sheehy
    Although the person referred to as "Ontario" doesn't feel that a rainscreen gap is needed, many building experts disagree. For a full discussion of rainscreens, see this article: All About Rainscreens.

    It's certainly true that it's possible to install siding without a rainscreen gap (at least in parts of Canada where the building codes don't require builders to include the gap). That said, the many advantages of including a gap make the small investment worthwhile, in my opinion.

  3. User avater
    David Goodyear | | #3

    Within the past 50 -100 years (and before) clapboard siding was the mainstay for cladding in Newfoundland (I'm in Zone 6) until the introduction of vinyl. In Newfoundland, most if not all, houses were built without rainscreens using spruce clapboard and there are still many still standing with the original siding. People painted their siding with alkyd based oil paints and they always bubbled, chipped and had to be scraped and painted. It is likely that some (if not all) of this was due to the lack of a rainscreen cavity. I assume that moisture migration through the back of wet cladding caused the finish to release from the cladding surface. I would think that eventually this would lead to rotting clapboard and exterior sheathing. This being said, from my discussion with local people, rotting didn't happen that often...maybe because temperature extremes here didn't allow long enough for mould and rot to take over....or maybe a couple of wood stoves firing for 9 months a year in a house with empty wall cavities increased the drying potential so the exterior sheathing and clapboard remained fine. This being said, those houses always have a unique smell to them! likely due to moisture, mould, and mildew.

    Rainscreens are a cheap addition....and a cheap insurance policy....there's isn't really any reason not to include it knowing what we know now about building envelopes.

  4. John Burk | | #4

    Exterior insulation
    Just finished building 2x6 with plywood exterior, 4" of reclaimed T&G roofing foam and 2" sprayed interior closed cell foam, roof 6" foam and sprayed too. Built in Ontario just South of Sudbury almost on Georgian Bay, zone 5+. Foam is strapped with 1" pine to provide rain screen and hold foam in place. Still doing interior and have to finish siding but haven't spent a full winter yet. Amazed at the comfortable feeling and stable temperature, heating easily with a mini split and fireplace usually the fire. All in all really happy with the build so far.

  5. Steve Grinwis | | #5

    Drying to the outside?
    This question is similar to mine, though hopefully without the hostility...

    I'm having a home built in Ontario. I want to improve the thermal performance, and one of the easy ways to do this is to simply fill the existing 2*6 wall cavity with spray foam.

    I think that would give me an R-35 wall assembly. 5" of closed cell, + 1" EPS rigid insulation in the outside. I'm worried about this assembly drying though. The inspector will almost certainly demand a polyethylene vapor barrier.

    Would an assembly like this sufficiently dry to the outside? I guess through the osb and EPS? seems like if any water at all gets in your done. Thoughts?

    This isn't a custom build, but they're willing to entertain my requests.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Response to User-6930437
    Care to tell us your name?

    I really don't recommend the wall assembly you describe. (And putting polyethylene on the interior is especially nuts. Don't building inspectors in Canada know that 5 inches of closed-cell spray foam is a vapor barrier?)

    The wall you describe would probably work (from a building science perspective), as long as the wall had good flashing. Needless to say, a rainscreen gap between the siding and the WRB would help.

    The problems with this wall have to do with its high cost (that's a lot of closed-cell spray foam), its environmental impact (bad, because of the blowing agents in the foam, unless you choose one of the new closed-cell spray foam with a more benign blowing agent), and the fact that the 1 inch of EPS is insufficient to do much to address thermal bridging through the studs.

    In Southern Ontario (Climate Zone 6), the best approach for a 2x6 wall would be to include thicker rigid foam (a minimum of R-11.25 of rigid foam, which means about 3 inches of EPS) and to install fluffy insulation (fiberglass, mineral wool, or cellulose) between the studs. This type of wall doesn't need an interior vapor retarder, but if you have an old-fashioned building inspector who is hard to educate, you can install interior MemBrain, a "smart" retarder, instead of polyethylene.

    For more information, I suggest that you read these two articles:

    Installing Closed-Cell Spray Foam Between Studs is a Waste

    How to Design a Wall

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