J Pritzen’s single-story Illinois house was built in the 1950s. It’s heated with a gas furnace fully capable of meeting the heating load, but somehow it isn’t getting the job done.
The single-story brick house has a mostly insulated, but unheated, basement. Warm air is distributed on the main floor by a series of floor registers set near exterior walls, and an energy audit tells Pritzen the furnace is cranking out 10,000 Btu more per hour than is lost through the walls and roof.
“Yet it’s not comfortable when the heat isn’t blowing,” he writes in a Q&A post at GreenBuildingAdvisor. “For example, my fingers and feet get pretty cold sitting typing on this computer even though the thermostat is happy.”
Pritzen has taken some temperature readings and discovered warm air is collecting at ceiling height, where it can be 13 F° warmer than it is at floor level. If the thermostat is set at 68°F, Pritzen’s feet will be in 59°F territory. If he could reach up and touch the ceiling, it would be 72°F degrees.
“My baseboard registers simply blow the air up, and most [of the registers] are right up under a window. (Makes me wonder if the heat is just going out the window instead of into the room),” he says. “I was wondering if there are any resources which describe what today’s guidelines are for maximizing forced-air system efficiency by register placement and diffuser selection, in case my home needs an update. For example, is there any reason for a home to really have 48-inch baseboard diffusers? Is that hurting or helping air mixing?”
Pritzen’s heating problems are the topic for this Q&A Spotlight.
Blame the stack effect
To GBA managing editor Martin Holladay, it’s open-and-shut case.
“The reason for the stratification you describe is almost certainly air leakage driven by the stack effect,” he tells Pritzen. “The solution is to perform air-sealing work in your basement and attic.”
The stack effect is the movement of air in a building upward, from basement to roof, caused by air leaks. Warm air rises to the top of the house and is replaced by cooler air coming in through leaks in the bottom of the house. When air leaks are sealed, air doesn’t tend to stratify, as it is in Pritzen’s house.
But Pritzen says that his house is fairly well sealed. The energy auditor told him that the leakage rate was 0.2 ach(natural) — that is, air changes per hour without any mechanical intervention. That translates to about 4 air changes per hour at a mechanically induced pressure difference of 50 pascals (ach50), the standard units used to report the results of a blower-door test. The auditor did find a few trouble spots, such as poorly sealed windows and a drafty mail slot, and recommended better insulation in the walls.
That level of airtightness is “OK, but not great,” Holladay tells him. “Tracking down the remaining air leaks — the ones that are causing you to be uncomfortable — is detective work,” he says. “With persistence and patience, you should be able to seal some of your leaks. Or you can hire a contractor familiar with blower-door-directed air sealing to help you.”
Diffusers won’t help here
A different register configuration or diffusers to change the direction of air flow won’t really do much to fix this problem, writes Dana Dorsett. In fact, the location of registers under windows made sense at the time the house was built.
“Registers blowing straight up under a window counteracts the natural convection of cold air cascading down the face of the cold glass, and was a common (and reasonable) thing to do when windows were all ~U-1 (or worse),” Dorsett says. “Yes, it raises the heat load by putting 120°F air next to the point of high loss (the window), but it improves average comfort levels.”
What would help, he continues, is investing $50 in a infrared thermometer and using it along with a window fan to track down the source of air leaks and missing insulation.
“Concentrate on the leaks in the basement and at the ceiling plane of the first floor,” he suggest. “Be sure to investigate the airtightness of all plumbing stacks, flues, and electrical chases, too. Air could be convecting straight through to the attic from the basement, bypassing the first floor but chilling off the basement with the infiltration.
“Sometimes cold first floors can be caused by band-joist leakage into the joist bays of a finished basement ceiling,” Dorsett continues. “If the basement is warmer than the measured 59°F at the floor level, that’s a real possibility. But sections of first floor wall with missing insulation can also cause cool air to cascade and pool down at floor level, with a similar sort of symptom. This is also pretty easy to spot with a in IR thermometer.”
Inadequate insulation and air leaks both contribute to stratification, adds Charlie Sullivan, and in Pritzen’s case both may be to blame. A lack of insulation also means a lack of comfort, because the human body can sense the temperature of walls and flooring through radiation, as well as feeling the air temperature, he says.
The details on basement and first-floor insulation
The basement, it turns out, is mostly rather than fully insulated. The rear of the basement, Pritzen says, is not insulated at all. Most of the ductwork is hidden from view, between the drywall ceiling in the basement and the floor above. Where they are visible, ducts have been sealed with mastic but are not insulated.
The above-grade first-floor masonry walls are insulated with 1 inch of fiberglass; these walls were described in the energy audit report as having a U-factor of 0.13 (R-7.7).
One completely uninsulated wall in the basement is making a major contribution to the cold-floor problem, Dorsett says. “When it’s 20°F outside and 55°F in the basement, every square foot of above-grade foundation is losing 35 Btu/h, and the below-grade portion is losing something like half that,” he writes. “If there’s 2 feet of above-grade foundation x 30 feet wide that’s 60 square feet, and 2,100 Btu/hr of heat loss, and you’re probably losing another 2,500-3,000 Btu/hr out the below-grade section. Even one uninsulated basement wall in a 55°F is the heat loss equivalent of a decent-sized insulated first-floor room.”
The fact that there’s no access to the rim joist area means there’s no way to air-seal and insulate it. That’s another problem.
Pritzen’s description convinces Sullivan that the “barely insulated walls” are the primary source of cold air in the house. “The air near the walls is cooled by the cold walls,” Sullivan says. “That cool air is more dense than the rest of the air in the room and it falls to the floor. Hence, you have cool air on the floor. You could get some benefit by insulating the not-yet-insulated basement wall, or from finding and sealing additional air leaks, but the stratification and comfort issues would be improved the most if you upgraded your wall insulation.”
Is the potential fix worth the expense?
The time and trouble of insulating exterior walls, however, looks overwhelming to the homeowner. The auditor has recommended he do exactly that, but the prospect of removing interior finishes and windows, insulating the walls, and then reinstalling finishes looks like a mountain too high to climb.
He’s also read that insulating brick walls should only be undertaken if the brick is in good condition and water drainage issues have been taken care of.
Exterior Insulation and Finishing System (EIFS) has been used successfully on thousands of older masonry buildings in Europe and North America, Holladay tells him, but the expense is beyond what Pritzen is willing to pay. “At a cost of at least $15,000,” he says, “my comfort will just have to suffer.”
All of this is one reason Dorsett advocates tackling other jobs first. “Start by fixing the cheap and easy stuff, like the uninsulated part of the basement and chasing down the remaining air leaks, both of which are very good from a bang-for-your-buck perspective,” Dorsett says. “You don’t need IR imaging and a blower door to find the bigger holes — a window fan, a wet finger, and a $50 infrared thermometer can do a lot. Insulating the remainder of the basement and getting serious about chasing down air leaks will probably cut the whole-house heat load by a solid double-digit percentage, and raise the temperature of the basement (and first floor floors) by 3 to 5 F°, possibly more.”
Our expert’s opinion
GBA technical director Peter Yost added this:
Thermal comfort is a tricky thing that Ole Fanger spent years trying to understand and quantify. The simplest expression of thermal comfort is the operative temperature, most easily approximated by adding the air temperature and the mean radiant temperature (MRT) together and dividing by two.
This means we are ignoring the impact of air speeds, relative humidity, your metabolic rate, and the type and total amount of clothing you might be wearing. It also means that the two main, dominant, and roughly equal determinants of your comfort are the air temperature and the surface temperatures all around you.
Interestingly, an ach50 of 4 is pretty darn close to the air tightness of my 100-year-old, southern Vermont, deeply energy retrofitted home. But back when our first floor walls were just split-faced architectural block (ungrouted), wood lathing, and plaster — and we had a forced-air furnace heating the home, with that air supplied to floor registers on exterior walls — my wife would come downstairs on a cold winter morning and say, “OK, Mr. Building Scientist, explain to me why you have been up for two hours, you turned the thermostat up to 68°F and I come down and the furnace is off, the t-stat still reads 68°F and I am freezing standing in the middle of the dining room (where the first floor thermostat is on — of course — an interior wall)?”
And as in most building science discussions with my unimpressed wife, I respond: “I can tell you why you are cold, but it won’t improve your thermal comfort…” Not a particularly gratifying answer.
And of course, since the thermostat is reading the air temperature, as soon as the air gets up to 68°F, the furnace and blower turn off, regardless of how cold those exterior, uninsulated concrete block walls are. So, if the air temperature is 68°F and the dominating exterior wall surface temperature is, say, 55°F, the operative temperature is going to be far less than 68°F, far less than comfortable, and stay that way until the interior air convects enough heat (if ever, depending on what direction the wall is facing and just how cold it stays that day) to the interior surface of the exterior dining room wall.
As an example, I’ll use our former unconditioned front porch — now a home office (see Image #2, below). This room is 9 feet by 13 feet with four 2’6″ x 5’0″ windows. Retrofitting that porch and keeping the window number and sizes for the space was a big building science mistake on my part, because of the windows’ impact on the operative temperature of that tiny space. The mean radiant temperature is dominated by the exterior walls and the R-4 or R-5 windows, and that has a huge and hard-to-overcome impact on thermal comfort.
We largely solved this by adding a 2’0″ x 6’0″ radiant ceiling panel, which has a huge impact on the MRT in the office. (The panel when operating has a surface temperature between 165° and 175°F.)
The side-track insulated cellular window shades also help. The window shades contribute (nominally) about an additional R-4, but only when fully closed.
But sure enough, when my wife’s hands are hidden from direct view of the ceiling panel, and she asks me, “OK, Mr. Building Scientist, why are my hands cold despite all the time and effort you have spent on this ‘high performance’ office?” I humbly apologize and say, “I can explain why your hands are cold, but it won’t improve your thermal comfort…”
You should purchase that digital infrared thermometer or rent the Home Depot IR camera. I bet you will find that the surface temperature of those nearly uninsulated above-grade exterior walls are dominating your MRT, your operative temperature, and hence your thermal comfort.
If you are unwilling or unable to insulate those walls — to significantly pull up your MRT and hence operative temperature — I suggest you consider adding a ceiling radiant heating panel and to keep those hands in full view!