Irene lives in a 1901 house in the Pacific Northwest with an unfinished basement that’s slowly been improved but is still unheated. In previous remodeling, Irene removed fiberglass insulation that had been installed on the upper sections of the basement walls and improved drainage to solve a water infiltration problem.
Now, the object is to air seal the basement, but Irene has been thrown a curve by an HVAC contractor, as she explains in a post in the Q&A forum.
“I realize the basement is considered part of the conditioned space, but I was a little startled at an HVAC guy proposing to cut heat runs into the basement (which we do not plan ever to finish)…,” she says. “We are just getting around to properly air sealing it. I am embarrassed it took us so long, but natural gas prices have been low enough that I hadn’t worried about the bills and didn’t realize our usage was as high as it was. (The furnace is on its way out as well).”
Irene had always assumed a basement should be “cellar temperature of thereabouts” — that is, not too warm, not too cold.
“It’s been convenient for us to keep beer down there, for instance,” she writes. “Plus I don’t want to overwork the heating system by adding square footage to the officially heated area. But is it inefficient to have the basement much cooler than the rest of the house?”
In addition to the heating question, Irene also wonders how to finish a framed and sided section of the basement that’s beneath a porch (see the photo above). “Some of it doesn’t even have siding,” she writes, “At that point the wall of the house is one board thick … Should something be added on the outside?”
Those are the topics for this Q&A Spotlight.
Energy efficiency won’t be lower
No, replies GBA editor Martin Holladay, keeping the basement cool will not result in higher energy bills. Keeping the basement cool will actually lower energy bills compared with heating the basement.
But, adds Dana Dorsett, insulating basement walls will significantly lower heat loss through the floor between the basement and the house.
“The heat loss through the floor to an uninsulated basement is usually a double-digit percentage of a home’s heat loss, even without parasitic losses from ducted hot air heat,” Dorsett says.
In January, with a mean outdoor temperature in Seattle of 40°F and a basement temperature of 50°F, above-grade basement walls could be losing 2800 BTU per hour (assuming 200 feet of basement perimeter 2 feet above grade). Losses would be even higher if the outdoor temperature were a little lower and indoor temperature a little higher.
“And that’s just the above-grade portion,” Dorsett says. “There is still heat loss to the soil — it adds up. That’s why current code spells out either R-30 insulation between the joists when the floor is over unheated uninsulated space, or R-15 continuous foundation insulation. If you insulate the foundation walls to R-15, the basement temperatures will rise (even though you’re not actively heating it), but the heat loss numbers for the basement as a whole will drop by about 90%, despite the fact that it’s idling at temperatures of 55-60°F (sometimes higher) during midwinter rather than at 50-55°F.”
As to the unfinished wall, Holladay says that it can be insulated on the interior, just like any other above-grade wall. “The first step is to take steps to seal air leaks,” he writes. “The second step is to install a layer of insulation. And the third step (in most cases) is to install a layer of 1/2-inch drywall as a thermal barrier (for fire safety).”
An opportunity to right-size the heating equipment
If Irene will be replacing the furnace, it’s a rare opportunity to choose heating equipment that’s sized correctly for her house — and that probably will be a heat pump rather than a new furnace, Dorsett says.
“Even with cheap gas, at cheap electric rates and temperate outdoor temperatures of the [Pacific Northwest] a right-sized heat pump can beat it on operating cost,” he says. Unbalanced, unsealed duct systems can lead to “significant parasitic losses,” but it’s also important not to install a furnace that’s way too big for the house.
“Getting a handle on the actual heat load and not oversizing by too large a fraction is really important for comfort, and sometimes efficiency as well,” Dorsett says. “Oversized furnaces will often satisfy the thermostat quickly, before the rooms at the farthest end of the duct runs are fully heated. They are prone to temperature overshoots. too. The ideal sized furnace would run very long, with almost continuous runs, during the coldest hours, giving you a steady, warm summer breeze effect rather than the hot-flash followed by the chill.”
He adds that ASHRAE’s recommendation is oversizing no more than 1.4 times the load at the 99% outside design temperature. That should be enough to cover even the coldest cold snaps while providing reasonable recovery times from overnight temperature setbacks.
“If going with a heat pump solution, sizing the heat pump for exactly the 99% heat load and a constant room temperature (no setbacks) would yield higher efficiency,” he adds. “There are lots of trade-offs to be made and details to work out, but without a good handle on the heat load numbers, you’re doomed. The last thing you want to do is blindly replace like-for-like.”
Calculating heat losses
Dorsett suggests running a fuel-use heat load calculation, a process that uses the existing furnace as the measuring instrument, as he explained in this GBA article. The premise in this 2016 post was that most new furnaces installed in the U.S. are too big.
If Irene runs through the numbers and arrives at an estimated heating load, she could knock off at least 10% of that, possibly as much as 25%, if she were to insulate the basement walls, and still have room for a margin of error.
With the estimates Irene has provided, Dorsett says the heating load appears to be in the 27,000 BTU/hour range, and will probably be less than 25,000 BTU/hour after insulating the foundation and carrying out some air sealing.
“And don’t forget that’s really an upper bound,” he says. “There are parasitic losses from air handler-driven air infiltration, and it’s possible or likely that the burner isn’t really still hitting its 80% efficiency. (If abandoning the ducts in favor of a ductless approach, the heat load could easily be 10-15% lower than the fuel use calculation indicates, because the parasitic load of air-handler-driven infiltration goes away.)”
If Irene decides to install a heat pump rather than a new furnace, she should not be using the 1.4x multiplier for sizing.
“That oversize factor is appropriate when overnight setback strategies are being used,” says Dorsett. “Using overnight setbacks with heat pumps (particularly modulating heat pumps like minisplits) ends up using more electricity than a ‘set and forget’ approach … Both modulating and non-modulating heat pumps will do worse when using overnight setback strategies, and worse still if oversized to the point where the recovery ramps from setbacks are fairly quick.”
Our expert’s opinion
Peter Yost, GBA’s technical director, added this:
There are no code requirements for a supply and return in an unfinished basement in an existing home. This is according to Pam Cole, research scientist at the Pacific Northwest National Lab. Cole writes all of the Building America Solution Center Code Briefs — really handy and worth a look if you’re not familiar with them.
Basement insulation requirements can get tricky. Once a supply and return are added, code requirements for basement insulation apply. If the unfinished basement HVAC replacement does not add a supply and return to the basement, then you are not required to update the basement insulation. “Existing buildings are claused in,” Cole says.
Of course, regardless of what the code may require, insulating the rim joist and first four feet of the basement exterior walls is worth every penny.
In new homes, the code is not completely clear on just what conditioning is required in unfinished basements. My code expert and friend Peter Kulczyk tells me that in Minnesota, new homes must have one supply and one return in an unfinished basement. I could not determine what the national codes require.
The major conditioning issue not addressed in either this Q&A exchange or the codes is moisture management in an unfinished basement. If an unfinished basement is kept cooler, it can become more problematic in terms of mold and mildew. If upgrading the HVAC means pulling a furnace from the unfinished basement, then the incidental heat loss from this unit and ducts in the unfinished basement is gone and the space will become that much cooler during the heating season. And as temperatures drop in the basement, standard dehumidifiers can struggle without some sort of reheat to keep coils from icing up.