Nik Fiorito is grappling with the same issues every owner/builder eventually confronts: What’s the best way of insulating a new house? Only in Fiorito’s case, it gets a little more complicated.
First, he’s building in Climate Zone 7, forty minutes north of the U.S.-Canadian border, on a hilltop where the temperature averaged 3 below zero F (-19.6 degrees C.) this past February. He’s also considering a fully off-grid photovoltaic (PV) system plus a ground-source heat pump for both heat and domestic hot water.
Throw those variables in the mix and Fiorito’s situation is a little more complicated than building, say, a nice little ranch in suburban Cleveland.
“The home has large, south-facing windows with a walkout basement and as Thunder Bay is quite sunny (no city east of us in Canada gets more sunshine), I think we’ll have a nice solar gain into the home during the winter,” he writes in a Q&A post at Green Building Advisor.
What, he asks, should the wall assemblies look like? He’s leaning toward using Roxul mineral wool insulation in the 2×6 stud cavities, with Roxul Comfortboard mineral wool panels on the outside to lessen thermal bridging and a polyethylene vapor barrier to keep water vapor out of the walls.
A second option would include the use of 1 1/2-inch tongue-and-groove rigid foam on the interior over mineral wool batts. “This would allow me to avoid a poly vapor barrier and flash the windows against the exterior sheathing (OSB, most likely),” he writes. “I know this method (interior rigid foam) is less favored here, but I’m thinking it might work.”
Insulating his planned wood foundation is an entirely different issue.
Those are the topics for this Q&A Spotlight.
First, the question of the heat pump
If Fiorito is really going off-grid, he should reexamine his choice of a ground-source heat pump, writes GBA senior editor Martin Holladay, because it won’t work.
“You can’t operate a ground-source heat pump with an off-grid electrical system,” he says. “When you get a week of cloudy weather in December, there is no way that your battery system can power your heating system.”
He offers three suggestions: a wood stove, a propane space heater vented through the wall, or an oil-fueled parlor stove gravity-fed from an outdoor, elevated fuel tank.
Nonsense, says Richard McGrath: a heat pump, possibly with a propane or oil backup, will work just fine.
“Solar thermal storage has moved into the 21st century,” McGrath writes. “You could easily enjoy [coefficient of performance] over 5.5 during heat pump operation. Possibly a diesel generator as electric backup just in case the sun doesn’t shine.”
No, Holladay replies, “it’s nuts to run a heat pump with a fossil-fuel powered generator.”
“Just as nuts, McGrath says, “to build a large enough battery for PV during those times that that bright thing is not up in the sky. Hydronic solutions are readily available to store plenty of water at a temp that would heat a well built home for quite a long time with properly sized storage and controls. It is a shame that you are unaware of these things or just discount them.”
Holladay, who has lived off-grid for 40 years and has been using a generator for 22 years, would advise Fiorito not to build off-grid at all, but to connect his PV system with the grid.
“I stand by my advice that for an off-grid house, the only heating systems worth considering are heating systems that don’t require electricity,” he says. “When off-grid homeowners ignore this advice, they usually abandon the expensive heating equipment that requires electricity within one or two years.”
Maybe a Honda cogeneration system is the answer
AJ Builder has another suggestion — a cogeneration system built by Honda that uses propane or natural gas to create electricity, domestic hot water and heat. “No noise,” AJ Builder writes, and a long life. “Perfect.”
Holladay thinks the U.S. distributor went out of business, but Dana Dorsett writes that FreeWatt actually sold the business to another company, which eventually dropped the unit because of slow sales. “Honda still makes ~1 kW natural gas and propane generators, which saw a huge uptick in domestic sales in Japan in the wake of the Fukushima disaster,” Dorsett says. More than 100,000 of the devices are running in Japan, and one version can be used in off-grid systems.
Although Dorsett’s business partner has one “humming away quietly in his house,” a cogeneration system may not be the best fit in Fiorito’s area. “In the much more temperate climate of Japan they are usually mounted on the exterior of the house, which would have serious freeze-up potential in that configuration,” Dorsett writes. “(It’s probably able to handle coastal British Columbia type climates, though.) But it’s probably adaptable for mounting inside of conditioned space.”
Is poly in the walls a good idea?
Using polyethylene sheeting in the walls as a vapor barrier was routine not too many years ago, but it’s use has fallen out of favor as builders recognize it can trap moisture in wall cavities. Increasingly, designers emphasize the importance of air barriers to reduce the flow of warm, moist air into cold wall cavities.
“Shouldn’t we just be doing air barriers?” asks Lucy Foxworth. “There are a number of air barrier materials that allow drying to either side that would work better, I think. Some examples are MemBrain from CertainTeed, Intello from Pro Clima (distributed by 475 Building Supply in the U.S.) and Siga Majpell from Small Planet workshop.”
Foxworth adds, however, that Fiorito’s plans for above-grade wall insulation seem a little skimpy. With insulated 2×6 walls and 2 inches of mineral wool panels on the exterior, she suggests, would add to up R-28 to R-30, which “won’t get you very far in terms of insulation in a zone 7 climate.”
If the walls can dry in both directions with a more vapor-permeable air barrier and mineral wool insulation, she adds, the threat of condensation inside wall cavities may not be as much of an issue as Fiorito thinks.
“Poly is the rule around here, not the exception it seems.” Fiorito adds. “That being said, I am meeting with the building inspector on Monday and could see how high his eyebrows rise at the mention of omitting it.”
Insulating wood foundation walls
Fiorito’s plans also call for a wood foundation, and that raises its own insulation issues.
He thinks he’ll insulate the stud cavities of the foundation with mineral wool, and then add between 2 and 4 inches of rigid foam (expanded polystyrene). That would be capped with drywall and paint.
“I don’t think it ever makes sense to install an air-permeable insulation like mineral wool on the interior side of the cold plywood of a wood foundation,” Holladay says. “If you get condensation against the cold plywood, water can trickle down the plywood and form puddles at your bottom plate.
“The only interior insulation I can recommend for this type of foundation is closed-cell spray polyurethane foam. Of course, you can always insulate the plywood on the exterior (with mineral wool, EPS, XPS, or closed-cell spray foam).”
Yes, insulate the foundation from the exterior, says Dorsett, it will keep the wood warmer and drier.
Our expert’s opinion
Peter Yost, GBA’s technical director, added these thoughts:
If I were designing and building a home off-the-grid in “chilly Thunder Bay,” I would beef up the enclosure performance, minimize and simplify my heating system, and plan on keeping cool mostly with ceiling fans and natural ventilation.
We are about 250 miles south of you and cool passively on all but a handful of summer days (Brattleboro, Vermont, has about 390 cooling degree days, base temperature 65°F). Not a scenario into which a ground -ource heat pump fits very well.
Thunder Bay also seems like a good fit for use of a smart vapor retarder (SVR) system, which can be — and typically is — detailed as an air barrier, and more easily and effectively than polyethylene plastic.
In terms of continuous exterior rigid insulation on either the above-grade or the below-grade wood walls: warm the wood by putting the insulation on the exterior. The wood-frame walls will be more hygrothermally stable over time, and this approach will reduce thermal bridging. Sure, this will complicate both the planar location and the flashing of your window installations, but GBA is chock-full of all sorts of resources on just these issues.
Finally, if you are pursuing net-zero energy for an off-the-grid home in Climate Zone 7, it’s worth more than a look at Thorsten Chlupp’s work on off-the-grid high performance homes in Alaska.
- Using Sand to Store Solar Energy
- A Passive House Design for Alaska’s Frigid Climate
- Natural Resources Canada Net Zero Energy Housing
- Net-Zero Energy Coalition. We’re going to lob this one over to them. Stay tuned.