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Why Is It So Cold In Here?

The furnace can put out plenty of heat, but the house feels cold unless the furnace is running

Posted on Feb 1 2016 by Scott Gibson

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 loadRate at which heat must be added to a space to maintain a desired temperature. See cooling 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 auditEnergy audit that also includes inspections and tests to assess moisture flow, combustion safety, thermal comfort, indoor air quality, and durability. tells Pritzen the furnace is cranking out 10,000 BtuBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. 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 effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.," 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 testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas.. 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 thermometerA digital thermometer capable of measuring the temperature of a surface from a distance ranging from a few inches to a few feet. Most hand-held infrared thermometers include a laser to help aim the device; the laser plays no role in temperature measurement. Used as an inexpensive substitute for a thermal imaging camera, an infrared thermometer can detect hot or cold spots on walls, ceilings, and duct systems. ."

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-factorMeasure of the heat conducted through a given product or material—the number of British thermal units (Btus) of heat that move through a square foot of the material in one hour for every 1 degree Fahrenheit difference in temperature across the material (Btu/ft2°F hr). U-factor is the inverse of R-value. 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 temperatureIn determining thermal comfort, operative temperature is roughly the average of the air and mean radiant temperature (MRT) a person is experiencing., most easily approximated by adding the air temperature and the mean radiant temperatureMean radiant temperature (MRT) is roughly the average temperature of all the objects or surfaces that a person "sees" inside a building, with the surface temperatures being weighted by their area. A surface or object's contribution to MRT is also based on its temperature in comparison to the person (temperature difference or differential) and the viewing angle between the person and the surface. (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 cameraA camera that provides an image showing radiation in the infrared range of the electromagnetic spectrum. Since the amount of infrared radiation emitted from a surface varies with temperature, a thermal imaging camera is a useful tool for detecting hot or cold areas on walls, ceilings, roofs, and duct systems. When used to scan a building envelope, a thermal imaging camera can detect missing insulation or locations with high levels of infiltration. Thermal imaging cameras can provide useful information when the difference in temperature (delta T) between the indoors and the outdoors is as low as 18F°; however, the higher the delta T, the easier it is to see building defects.. 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!

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Image Credits:

  1. Image #1: Whiskers /
  2. Image #2: Peter Yost

Feb 1, 2016 11:11 AM ET

Edited Feb 1, 2016 11:16 AM ET.

I've thought about this problem
by Alan B

I have found that as long as all 6 sides are at room temp or have R5 (which keeps them at close enough to room temp) you will be comfortable. This is a rule of thumb, not an exact number. Heat vents near windows help keep them from feeling cold at a distance.
As an aside i am curious at what point you no longer need the heat vent near the window, would R2-4 be sufficient?

The house in question has cold floors, which will make quite the difference, determining the cause is a big step because i have a similar problem, i have 2/3 basement and 1/3 crawlspace and a single floor house. When i moved in all rooms were comfortable temp. However i later discovered that the crawlspace was getting lots of heat from a disconnected vent. When i plugged it the floor above the crawlspace became very cold, making that third of the house quite chilly, sounding similar to the OPs problem

In my case the cause is simple but hard to remedy (cause is likely different from the OPs, but explained here for interest), there are gaps to the outside i can't reach because of the shallowness of the crawlspace, and the house is balloon framed and the crawlspace portion of the house is an addition, hence cold air comes in, gets "chimneyed" to the attic and out the attic vents cooling everything it touches, When it was getting loads of heat it kept the crawl and connected surfaces warm (at high energy wastage), but now the floors are very cold till i can fix the air leakage problem someday.

An additional note, many furnace techs will tell you the "solution" is to run the furnace fan 24/7. They don't care about the energy cost, and will often start a huge argument if you even mention it, but it is a band aid solution that does often work because the always moving air helps warm cold surfaces, but at high energy cost.

Feb 1, 2016 12:31 PM ET

"I write a blog, therefore I'm an expert..."
by Ted Kidd

"my fingers and feet get pretty cold sitting typing on this computer even though the thermostat is happy."

How is the "thermostat happy" if the occupant isn't?

"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."

I wish I could physically slap you every time you say something stupid so you wouldn't say so many stupid things! You haven't even been to this house! Yet like Kreskin you know exactly what it needs?

And when they've air Sealed, further reducing load and increasing short cycle time and temperature imbalances?

Doing this correctly is work. Designing solutions that anticipate systemic interactions takes time. Jumping to design conclusions and making off the cuff recommendations fails on so many levels. One big one is you are dismissing a whole diagnostics and design profession as unnecessary. Another is you have no accountability for outcome failure of your recommendations.

Stop. You are commuting malpractice AND insulting the building science profession.

Ask yourself; "do I have any understanding of integrity or ethics?" If the answer is yes you must recognize the dissonance of prescription without diagnostics.

Feb 1, 2016 6:57 PM ET

Edited Feb 1, 2016 7:00 PM ET.

Response to Ted Kidd
by Martin Holladay

If it makes you feel better, feel free to slap me next time you see me.

I never said this is an open-and-shut case. Scott Gibson said that. I suggested a cause that I called "almost certain." Remember, I wrote "almost."

It goes without saying that we are doing our best here at GBA to provide our best guesses without visiting the site. Our free advice is generally worth what you pay for it. If you find it irksome that we offer free advice even though we have no access to the job site, there is no reason to read our Q&A column.

Feb 1, 2016 7:12 PM ET

Agree with Martin
by Alan B

My first thought on most of the questions is i wish i could see it first hand

Feb 1, 2016 11:04 PM ET

Edited Feb 1, 2016 11:05 PM ET.

Forget it
by Malcolm Taylor


Feb 3, 2016 12:25 PM ET

Edited Feb 3, 2016 12:26 PM ET.

What I Did
by David Eakin

In closely reading the description, it seems to me that the house in question is a block frame with brick facing construction (typically with 1x strapping on the interior to hang gypsum board/plaster - which would allow for the 1" insulation). I'm not sure that an EIFS system would be the preferred solution here due to the rain screen space between the brick facing and the block. In any case, what I decided to do for my home (double-wythe brick) was to create a second, inner framed/insulated/drywalled wall against the original cement-covered brick wall - creating extension jambs for windows and exterior doors. In our dining room, where there were too many complications to permit doing this on both exterior walls, I only created 1 "new wall". But it was on the North side of the house and was the largest exterior wall of the room. Even with this partial solution, we have experienced significant improvement in comfort. Yes, you loose some interior floor space, but you gain much in comfort.

Feb 3, 2016 10:27 PM ET

how do you do that?
by Jerry Wright

"a window fan, a wet finger, and a $50 infrared thermometer can do a lot."

Please tell us how we would do this. Thanks.

Feb 4, 2016 1:06 PM ET

Mixing and stratification
by Mark Heizer

From the look of the baseboard grilles and the likely age of the system, there's more to this than just leakage. My guess is that the furnace is putting out very hot air through grilles that aren't providing mixing. There's not enough velocity out of the grilles to mix thoroughly (from back in the day when most installers relied on natural buoyancy and old octopus furnaces were still common). Overhead heating delivery should never be more than 10-20F warmer than the space. For floor delivery, if you're putting out 120F air, it's shooting to the ceiling without mixing at all. Thus the stratification. You can put out as much heat as you want, but at thermostat level, you'll only get "warm" after the air at ceiling level is hot. And yes, this drives faster exfiltration, which is the reason the home is inefficient. But if you want comfort: Better mixing and lowered delivered supply temp. = increase fan speed if possible, get a grille that mixes the air, one time where ceiling fan might help. Get the supply air temp where the delta is reasonable.

Feb 8, 2016 2:37 PM ET

Cold near floor
by Ed Henderson

I live in a 1100 sq ft home built in 1973-4. It is what we call up here in Calgary a raised bungalow. The original Natural gas furnace is still heating my home.
We leave the furnace fan on from Sept to about mid May. When we developed the basement a very senior carpenter who helped and directed the job insisted that we install cold air returns at basement floor level in the basement.
Our home has additional insulation in the top ceiling,,about R 40 up there. Our home is cozy and warm all winter long. Our TV room is in the basement and we can lie on the floor down there when it's 25 below outside.

Feb 14, 2016 10:45 AM ET

right up my alley
by kathleen sisco

I wish I had my Henderson's elderly carpenter here. I think that my house was originally done by an elderly carpenter as the original heat was a metal wood furnace with built in hot water tank. Replaced by gas Goodman 46,000 btu Hi Eff 92% using same ductwork only the heat now is blown from the cold air returns as the wall ducts are a foot off the floor and perhaps stripped the heat. The wood probably heated the entire basement up. Not so with that cold-to-the-touch gas furnace.
I agree with this discussions point of the walls being a source of leakage. Especially under the windows. Why infra red scans are not required I do not know.
Following is post: I moved into 1,000 sq for one level ranch that is 65 years old or more. Looks modern. Originally had alum siding and maybe the insulated windows are original. For some reason, I suspect elder abuse, the siding was removed and replaced with vinyl. This is my second winter here; my inspector told me it needed more insulation but I looked at the attic and iffy. But here at Feb 2016 I see the frost on the inside bottoms of the windows and feel the cold sills I realize that when they pulled the alum off they were careless.

To explain why I feel this way I lived in Duluth in the subsidized towers and while there the agency 'upgraded' the slide bys to crank outs. While it was supposed to save money I myself witnessed the unnecessary removal of a fiberglass insulating strip --4x 36--being pulled out and tossed from the bottom of the window in the space between the window frame and the wall stud. The company had rented a crane --we were an 8 story building--and maybe didnt want a breeze to blow this out in their way or compromise the timely removal and insertion of the new windows. I saw first hand that there is no understanding of the need to consider all facets. The agency would have bigger light bills, we residents would be hotter and colder all because of this attitude. If I had not seen it I wouldn't have believed it.

Here the black mold on the aforementioned windows is now understandable. There is air leakage not from the house inside but from underneath the windows where the insulation was removed or just settled. The sill is very cold. In order to verify this lack of insulation I would have to pull off the siding and backing board and black paper wrap AND maybe the original wood siding. There is no obvious way to do this otherwise. Guessing by drilling holes and pumping in foam is not acceptable after all the work of removal.

The issue of moisture barrier is also complicated. Originally the plastic wrap should have been ON THE INSIDE WALL BEFORE DRYWALLING as putting up a moisture barrier ON THE OUTSIDE OF THE STUDS TRAPS MOISTURE IN.

Which seems to be what I have. I have yet to see a forum that makes it clear that putting up a moisture barrier on the outside of the studs is pointless and even damaging. Because it seems that no one wants to pull off all that wood. And then how would you put in a plastic wrap? Well, you would have to use fiberglass batts that are plastic wrapped wouldn't you? That would at least reduce the moisture getting to the outside but not completely as it could pass into the wood studs.

What do contractors say about this?

My issue is the electric bill, $250 and gas for heat $68. So I am blaming the fan running constantly.

And there is the claim that heat loss is over 80% thru the ceiling into the attic so why go to the cost of redoing the entire siding.

I feel that Rural Development should long ago have required infra red scanning of the winter high use month in their loans. Why constantly discover a problem when it can be noted and included in the loan papers? This is yet another hidden issue that is being allowed to be invisible like easements covered by the wording 'any and all' or non-disclosure of fill.

Ms Kathleen Sisco
1126 N 9th Ave
Iron River MI 40035

Feb 14, 2016 11:08 AM ET

by kathleen sisco

Just so you know, there are other problems I have dealt with a few, like drafty doors. My first winter here was 70 degree humidity in uninsulated basement, this winter 50. Upstairs with this below zero (20 below) now 30 RH. So the humidity is not from the house. Here there is nearly always high humidity outside. Last year heating was a nightmare constantly cold even with liv/din blocked off. Thermo set at 58-62. Heated one small room with mica panel. Basically lived in it. Almost no reduction in elec cost.
This winter tried blocking vents, nearly died from CO. Used two elec heaters--no fan--for month of mild December with all room vents blocked except for kit/hall/bath--one oil filled heater in kit--and my little room--one mica heater. Furnace off at circuit breaker. Result $350 light bill. Gave up turned furnace on, unblocked all vents except my little room, set thermo at 66. Light bill for warm house $250. But Feb elec bill might be the reason the house was foreclosed.
Social security precluded second mortgage for boiler/hot water system which is my choice.
I have you tube house videos under katesisco.

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