Brenton is building a new house in southern Maine (Climate Zone 6) that he currently plans to insulate with a combination of open- and closed-cell spray foam plus a continuous layer of exterior insulation in the form of R-6 Zip R-sheathing.
He’s willing to take suggestions on that approach, as he explains in this Q&A post, but a more pressing concern is how to heat and cool the house.
“I have been planning on installing a hydronic radiant system throughout the whole house with a wall hung Viessmann Vitodens 100 propane boiler,” Brenton writes. “My wife would like ac for the humid summer months here in Maine so I’ve been considering adding a 5 zone Fujitsu air sourced heat pump.”
“My question is this,” he continues, “would I be better off going solely with heat pumps for my heat and ac and scratch the radiant and put that money into solar? Or should I install the radiant and have the heat pumps for ac and auxiliary heat?”
That’s where we begin this Q&A Spotlight.
First, review your insulation plan
Before discussing heating and cooling, GBA Editor Brian Pontolilo suggests Brenton take a second look at his plans for spray-foam insulation, particularly in the roof where Brenton is planning to use 11 inches of open-cell foam.
“Because you weren’t specific about the roof and venting, I want to point out that when using open cell spray foam in the roof, the assembly should be vented,” Pontolilo says. “Closed-cell spray foam is the only reliable option for beneath the roof deck in an unvented assembly.”
A second option for an unvented roof would be to install rigid foam over the roof deck, which would allow Brenton to use a number of insulation types below the sheathing.
As to the exterior walls, Pontolilo points out that Brenton won’t need to rely on spray foam for air sealing because the ZIP R-sheathing will take care of that. But he has another concern.
“Also, keep in mind that R-6 Zip R in your climate is not enough R-value for condensation control on the inside face of the sheathing’s insulation layer,” he writes. “That would require you to use the R-12 panels on 2×6 walls. So you should probably have some interior vapor control, perhaps a smart vapor retarder.”
Akos advises Brenton to skip the spray foam altogether, suggesting he think of it only as a “last resort.”
“There are many ways of designing a house without spray foam,” he says, “even without any rigid insulation, that are more energy efficient and cheaper.”
He echoes Pontolilo’s advice on not using open-cell spray foam in the roof because it’s vapor permeable. A better option, he adds, would be a vented roof with blown-in or batt insulation, or an unvented roof with exterior rigid insulation.
Consider I-joists for the roof framing
Akos has another suggestion for the roof: frame it with I-joists and fill the cavities with fluffy insulation. If he were to use a 14-inch deep I-joist, he could staple a 1-inch polyiso vent baffle to the bottom edge of the top flange and get 11 1/2 inches of cavity insulation. If we wanted a lower profile, he could use a 11 7/8-inch I-joist with a layer of rigid insulation on the inside.
For specifics, he suggests Brenton have a look at this GBA article.
Because the web of an I-joist is typically just 3/8 inch thick, thermal bridging potential is much less of an issue than it would be with framing lumber, reducing the framing factor from 15% to 4%.
“Keep it simple, use TJIs and build a standard vented roof assembly with a vent baffle,” Akos adds. “This is low labor cost and pretty high performance. If you want super insulated roof, bump up the TJI to 16″ but you might need to fur out the bottom for the ceiling to support the additional insulation wight. No need for any vapor open special membranes and foams.”
Now, about your heating system
The idea of installing two heating systems seems like a bad plan to Trevor Lambert.
“If you’re installing a heat pump for a/c, then a hydronic heating system is a colossal waste of money,” he writes.
Akos would start the heating discussion with a look at local energy costs.
“In my area of expensive electricity and very expensive propane, a cold climate heat pump with a COP [Coefficient of Performance] above 2 (good ones will do better than this even bellow 10F) has much lower operating cost,” Akos says.
Propane—Brenton’s planned fuel for his boiler—might be a good option for a backup generator if the power grid is unreliable, he says, adding, “Even there it is cheaper to run a heat pump for bulk heat and a through the wall propane heater for backup.”
Akos recommends a wall- or floor-mounted minisplit head on the main level, provided the space is open, and a ducted unit for the bedrooms upstairs.
“Hydronic heat is something that will add $25k to $30k to your build,” he says. “There are much better places to spend that money. Most well insulated houses have such low load and temperature difference (most surfaces are the same temperature) that there is little to no benefit for radiant heat.”
Cheaper doesn’t mean free
Brenton says a close friend is a master plumber and a licensed tech who installs boilers and heat pumps on a daily basis—and that means “massive savings.”
“He could install the full radiant system and heat pump parts and labor for me for a little under 20k,” Brenton says.
Fine, replies, Akos, but “even cheap hydronic is not free, you are still essentially installing two complete heating systems.”
If Brenton is paying anything less than $0.20 per kilowatt hour of electricity, propane heat would cost more, to say nothing of the extra $20,000 for the installation of a boiler and radiant system.
As to any concerns that a heat pump wouldn’t have enough capacity in the winter, Brian P adds this:
“We heat and cool our 1300+ sq ft house (2 story) in the White Mountains [New Hampshire and part of Maine] with a single heat pump on the first floor,” he says. “A properly designed/installed heat pump system would be great for the Maine coast.”
What our expert recommends
Peter Yost, GBA’s technical director and the owner of Building-Wright, a consulting firm in Brattleboro, Vermont, added this:
Great discussion of wall and roof insulation/air sealing systems, including vented or unvented roof systems. I like the idea of using the I-joist top chord as the stop for rigid sheet goods to create the soffit-to-ridge vent chute. I looked around, and regardless of I-joist manufacturer, the chords are at least 1 inch deep. As the result of my Wingnut roof venting work, I think a 1 1/2-inch depth for roof vent chutes may be the sweet spot.
There are a couple of things to consider on the radiant floor system.
I used to dismiss radiant cooling systems out of hand, but we shouldn’t. Check out this series on the topic by Robert Bean of Healthy Heating. Robert, a brilliant mechanical engineer, argues that radiant cooling requires moisture control but moisture control is required for any space cooling approach.
If I were going to install a radiant system (whether it is a floor system or not), I would work with an HVAC engineer or contractor to design the system for both space heating and cooling.
My second concern with high-mass radiant systems is losing the energy savings of nighttime setback. This is a complex topic. GBA already has an interesting and thorough discussion of radiant floor heating systems and nighttime setback here.
There will come a day when we can manage thermal comfort the same way we do lighting: background ambient combined with task coverage. We should not be conditioning entire spaces for thermal comfort. Rather, we should have baseline ambient overall heating and cooling and personal task-oriented and individualized people heating and cooling.
Check out the very cool (no pun intended) research on Personal Comfort Systems by the Center for the Built Environment. Sure, right now it’s for commercial offices and high-end vehicles, but with PSC you can achieve setback savings and increased individual thermal comfort.
-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine. “Our Expert” is Peter Yost, GBA’s technical director and founder of a consulting company in Brattleboro, Vt. called Building-Wright.