In the Canadian maritime province of Nova Scotia, a GBA reader named Janet is building a new weekend home whose heating system will include both a wood stove and radiant-floor heat powered by a ground-source heat pump.
The two-level, 2,000-square-foot house will consist of a walkout basement with two bedrooms, a bathroom, and a laundry room, plus an upper floor with a kitchen and living room and another bedroom and bath. The house will be well insulated: R-20 under the slab, R-34 basement walls, R-24 second-floor walls, and R-60 in the attic.
The first floor slab will be concrete, which Janet plans to heat with in-floor tubing. On the second floor, Janet will install a wood stove that she plans on using regularly when she’s there.
“We are unsure of what other heat, if any, we need to install on this level,” Janet writes in a Q&A post. “How much heat can we expect on the 2nd floor from the 1st level in-floor heat?”
Installing hydronic heat on the second floor — either in-floor tubing or baseboard radiators — seems like overkill because it won’t be used when the wood stove is running, she says.
Would a few baseboard electric units do the trick?
That’s the question for this Q&A Spotlight.
Ground-source heat is expensive
A ground-source heat pump (sometimes called a geothermal heating system) is the “most capital-intensive heating system imaginable,” Walter Ahlgrim tells Janet, and it will do little to make the second floor of the house comfortable.
“How many hours a day will you spend in the basement enjoying the warm floors?” he asks. “Or is that money better spent upstairs where you would benefit from it every day?”
Ahlgrim says that wood stoves and high-performance houses are rarely a good combination.
That said, he adds, without a Manual J calculation to estimate heating and cooling loads, “everyone is just guessing how the system will work.”
Peter Engle thinks that the radiant-floor system on the first floor will be enough to keep the second level from freezing, but it may seem a little chilly. “Even a very small wood stove may overheat the space,” Engle says.
He also points to the high cost of a ground-source heat pump given the relatively small amount of heat Janet will need in a well-insulated building.
The system will cost $20,000
Although Janet is still waiting for a final number, the preliminary quote for a ground-source heating system is C$20,000, which the installer says will pay for itself in between 5 and 7 years. The alternative is a wall-hung electric boiler.
One reason that ground-source heat pumps are so expensive is that heat exchange tubing is typically installed in trenches or in wells, either one of which can add considerably to excavation costs.
But in Janet’s case, the property will be dug up anyway for the driveway, septic system, and yard.
That may help reduce the costs, but the system itself is flawed, writes Andrew C.
“Knowing that one should never say ‘never,’ you should never put geothermal into a well-built house with reasonable air-sealing and insulation,” Andrew says. “It costs too much, requires one-off engineering design and installation, and likely (by all accounts) to significantly under-perform calculations. Other than one or two die-hard advocates, few that haunt the GBA website promote the use of geothermal. It can be done correctly, but it’s rare, and still more expensive.”
Further, the concrete slab won’t feel warm under foot until the temperature rises to about 80°F, says Dick Russell.
“The heat loss from the building will be very low (if it’s properly built), so with little way for the heat to escape, making the entire floor that warm would raise the room temperature to the upper 70s, which would feel stifling,” he says. “Modulating the water temperature to keep the floor in the low 70s won’t make the floor feel warm to bare feet, so why go to the expense of installing all that plumbing in the slab?”
Windows could make an important contribution
Robert Opaluch raises a new issue: the potential for solar gain through any south-facing windows on the first floor.
“If your walkout level faces southward, the south side is longer than east and west sides, and you have more or larger windows facing south, and are not obstructed much by trees or other buildings, or some combination of these … your wintertime solar heat gains could be substantial,” Opaluch writes. “If facing north, very little solar heat gain.”
Although windows and doors account for a small proportion of the building envelope, he adds, they often lose as much or more heat than the walls and ceiling. Argon-filled insulated glazing units, or the addition of a suspended-film Heat Mirror insulated unit, is usually a minor upcharge that can improve performance substantially, Opaluch says.
“You didn’t mention it, but I hope you are budgeting for good quality windows,” he adds.
The mini ductless option
Russell installed a ground-source heat pump in his own home in New Hampshire, and has been satisfied with the results. He minimized installation expenses by using a well for the heat exchange, and distributes heat throughout his house with forced air rather than a hydronic tubing in the floor.
Even so, Russell wonders whether Janet would be better off with air-source heat pumps instead of the arrangement she’s proposed.
“Even with these circumstances, a couple of minisplit air-source heat pumps provide a low-cost and efficient way to heat a superinsulated house and ought to be considered,” he says.
This is exactly the plan GBA Editor Martin Holladay would endorse. “There are three things wrong with your plan,” he writes. “One, you don’t want in-floor hydronic heat. Two, you don’t want a ground-source heat pump. Three, you don’t want a wood stove.
“Except for that,” Holladay adds, “your plan is fine.”
He suggests, as other GBA readers did, that air-source minisplit heat pumps — either ducted or ductless — would heat the house “at a cost that is thousands of dollars less than the equipment you describe, without any of the combustion-air problems associated with wood stoves.”
Fine tuning the building envelope
Janet plans on insulating the foundation walls with rigid foam, but makes no mention of adding a layer of rigid insulation to the outside of the second-floor walls. The “old school” builder will use housewrap and, Janet assumes, a rainscreen beneath the wood clapboard siding.
“Housewrap is a good step,” Opaluch says. “I’d worry that an ‘old school’ builder may not pay attention to the many details that can result in a fairly leaky home.”
He points to the importance of air-sealing details for windows and doors, mud sills, vents, attic openings, and other trouble spots.
“Two more things to point out,” Yupster adds. “You said R-24 walls but didn’t mention any exterior board insulation to eliminate thermal bridging. Please be aware that if you are just installing R-24 fiberglass batts, which is very common in Canada, then the actual performance of R-24 insulation in framing 16 inches on-center is only R-15.”
Don’t use the word “eliminate” when discussing the value of continuous insulation on the outside of framed walls, suggests Dana Dorsett. The proper word would be “mitigate.”
That aside, insulating concrete forms might be a good alternative for basement walls. He says a number of ICF foundations can hit whole-wall values of R-34, including one from Logix.
Our expert’s opinion
Peter Yost, GBA’s technical director, had this to say:
I decided to send Janet an email to find out a bit more about her project. She told me that building started this week, that they had an architect help them with the design, and that they do have a builder. Still, I’m afraid there are just too many unknowns here to guarantee the performance of Janet’s home will meet her expectations.
How well any heating system services the first and second floor spaces in this home can’t be determined without a lot more information on building enclosure performance, particularly concerning airtightness. With air leakage comprising between 25% to 40% of total heat loss in any home in a cold climate, knowing more about air sealing details, air control layer continuity, and plans for blower door testing is a must.
I found this document on the building code for Nova Scotia, indicating that there are three options for meeting the airtightness requirement, one prescriptive (“Clause a”) and two performance-based (“Clauses b and c”) under Section A-126.96.36.199 (1) “Controlling Air Leakage.” While I could not find a required building airtightness number, it sure seems as though Janet and her builder need to get a real handle on this issue.
In terms of rainscreen claddings, I found this document on Nova Scotia building code requirements for vented air space behind all wall claddings. Just as in all Canadian coastal areas, you need a 10 mm (3/8-inch) ventilated rainscreen regardless of what type of cladding you use.