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Q&A Spotlight

Heat Losses Are Way More than Planned

An owner/builder in Ontario feels deflated after heat-loss calculations come in much worse than he expected — but numbers are not always what they seem

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Bob Holodinsky is planning to build this house in Ontario but he's discouraged that the home's design heat loss is higher than he'd hoped for. Since this drawing was made, the area of the first-floor windows has shrunk by about 10%.
Bob Holodinsky is planning to build this house in Ontario but he's discouraged that the home's design heat loss is higher than he'd hoped for. Since this drawing was made, the area of the first-floor windows has shrunk by about 10%. The floor plan for Bob Holodinsky's planned house.

Bob Holodinsky was hoping for a better outcome from the heat loss calculations he received for his new Peterborough, Ontario, home — calculations that appear to have upset his plans for heating with a ductless minisplit. “I thought I was on the right track,” he writes in a Q&A post at GreenBuildingAdvisor, “but now I am not so sure.”

The 1 1/2-story, slab-on-grade house will be built with 2×6 exterior walls, insulated with blown-in fiberglass and 2 1/2 inches of expanded polystyrene insulation (EPS). Conditioned space totals about 1,900 square feet.

There are lots of triple-pane windows on the northeast side of the house, which faces a lake. The living-dining-kitchen area alone is responsible for a heat load of 17,000 Btu/hour, and the house as a whole shows a heat loss of 42,000 Btu/hour.

Holodinsky’s builder says that insulation in the roof will equal R-75. Exterior walls will have R-24 insulation between the studs, with an additional R-10 in the exterior foam sheathing. The slab is insulated with 3 inches of EPS.

“I was hoping to heat it with a Fujitsu minisplit on each level,” he writes. “Is that still a viable option?”

Holodinsky’s misgivings are the topic for this Q&A Spotlight.

Heat pumps are not the right option

Although he can lower his energy demands by air-sealing and increasing the amount of insulation in the house, minisplits may not be the best fit for this Climate Zone 6 location, writes a GBA reader who calls himself Flitch Plate.

“I would be concerned with minisplits as the sole heat source in Peterborough, no matter what the BTUs are,” Plate says. “Heat pumps are the poorest performers and least efficient at the times you need them most: very low temperatures. I would design in a backup heating solution, such as a small wood stove centrally located on the main floor or an electric heat source in each room.”

Jason Hyde says he’s not aware of any Fujitsu units suited to Peterborough’s winter design temperatures. Fujitsu says that its XLTH series works down to 15 degrees below zero, Hyde says, but these units produce only 18,000 Btu/hour at 47 degrees F. “I can’t find anything telling me what they will put out at lower temperatures,” he adds. “In any case, two units is still only 36,000 Btu/hour.”

Given these numbers, and without seeing plans or heat-loss details, Hyde doesn’t see any single upgrade that will put him within Fujitsu range.

Too much glass, not enough foam

One problem, says Matthew Nolette, is that 2 1/2 inches of EPS aren’t enough exterior foam for that climate zone. He points to an article by GBA senior editor Martin Holladay on calculating the minimum thickness of exterior foam. However, extruded polystyrene (XPS), with a higher insulating value than EPS, would work at that thickness.

Another problem is all that glass on the northeast side of the house, which Nolette says “would be indicative of design desires conflicting with energy goals.”

Passive solar building guidelines suggest minimal glass on the north side of the house because windows contribute no direct solar gain but plenty of heat losses. Windows on the east and west sides of the house also should be limited.

“You’re in a tough site with northeast views in a cold climate; there will be some performance compromises if you cannot determine how much northerly glass is a lot and how much is enough,” he writes. “Has your builder completed a house with these design goals before? Are they designing it? Do you have access to the designer and are you using a third-party energy modeler?”

Interpreting those pesky heat loss calculations

Yet another complication could be the energy loss calculations themselves, which were produced with a program called Wrightsoft. The report is complex and “more than a bit confusing for me,” Holodinsky says, but it seems to predict 0.48 air changes per hour.

“The assumption is way high and suggests a very leaky house that would have a blower door test of 6+ [air changes per hour at 50 pascals of pressure],” Jerry Liebler replies. “With minimal attention to air sealing, you should be closer to 1.5 ACH at 50 Pascals. Ask your builder to do a blower door test and ask for a guarantee that it’s below 2 ACH @ 50 pascals. Then ask that the infiltration rate for the heat loss calculations be 2/12 = 0.1666 ACH .”

In fact, Liebler adds, actual air infiltration is only a fraction of the test number because the test “artificially creates a greater pressure difference than actually will occur.”

“With the 2 ACH test data the heat loss due to infiltration should be about one-third of what was calculated,” Liebler continued, “By the way, the proverb ‘garbage in = garbage out’ applies. I’d be very suspicious of all the heat loss calculations and assumptions. For example, were the U-factors of the windows you are paying for actually used, or did they use something else? Triple-pane windows should be U=0.2 or less.”

We’re smack dab in the middle of the Information Age, adds Sonny Chatum, so Holodinsky should consider going online and getting a copy of the BEopt software from the National Renewable Energy Laboratory and running his own calculations. Chatum used the software himself to predict the heating and cooling loads at his own house after an energy retrofit (losses dropped from 40,000 Btu/h to 14,000 Btu/h, which Chatum handles with two ductless minisplits).

“It is useful to note that sometimes experts and professionals get hung up in their traditional ways,” Chatum added. “Sometimes, and in some ways, the Information Age can possibly make you as much or more of an expert than ‘the expert’ … All the traditional contractors were astonished with what I had done, and did not believe that the minis would be adequate. They were wrong.”

Sharpen your pencil, indeed

When Holodinsky tracked down the energy modeler who worked on the report, he found that a series of miscalculations, sloppy inputs, or erroneous assumptions skewed the numbers significantly.

The overworked technician, working on four computer screens at once, listened to Holodinsky’s concerns and made some adjustments to keep program inputs much closer to what the house would actually look like when complete. By raising the R-value of exterior walls, adjusting specs for window performance, and changing a program setting for airtightness, heating loads dropped by 10,000 Btu/h, to a total of 32,000 Btu/h.

“Much better,” Holodinsky said.

But maybe not good enough.

“The heat loss was high and the designer was sloppy from the details imparted here,” writes Morgan Audetat. “DIY heat loads for modern construction with ERV, triple-glazing, and rigid foam in cold climates is not for the amateur or the harried professional.

“If you are building your home around your heat source, you need a proper ACCA Manual J. If your builder will agree to a blower door and a specified ACH — good luck with that — then that number can be plugged into the Wrightsoft program and all your worries are over…”

Our expert’s opinion

Here’s what GBA technical director Peter Yost said:

There is not much left for me to deal with here, except to echo these points:

Additional or “backup” heat source. Taking a look at some historical and recent winter weather data for this neck of the woods, it’s clear you need to have another heat source in addition to the cold climate minisplit heat pump you have cited.

From the WeatherSpark website: “The coldest day of the last 12 months was January 22, with a low temperature of -26°F. For reference, on that day the average low temperature is 8°F and the low temperature drops below -11°F only one day in 10. The coldest month of the last 12 months was February with an average daily low temperature of 1°F.”

And for a “down Maine” confirming testimonial, take a look at the info here from Efficiency Maine.

Heat load calculations. These are very dependent on the assumptions regarding data you might have on airtightness. Simple programs like Manual J may have a three-level category for airtightness — leaky, average, and better — that is wholly inadequate for higher performing building enclosures when 25% to 40% of the heating load in a climate like this may be due to air leakage. The long and the short of it: get a blower door test to nail down this portion of your heating load.

Undersizing any space conditioning equipment isn’t good. But oversizing is less of an issue for heating equipment than it is for cooling equipment. Heating appliances that are a little bigger than necessary provide a quick recovery from deep overnight setbacks, and that’s not such a bad thing.


  1. Vince Caruso | | #1

    An old fashioned option is
    An old fashioned option is insulated window covers for night use. We use them to great effect. We have insulated roman shades and cellular room darkening shades. Need to have them seal on the edges to be effective. Dropping them on a cold night completely changes the feel of the room.

  2. Kyle Anders | | #2

    Updated Heating/Cooling Load Calcs for CSA F280
    Canada's go-to heating/cooling load sizing calculations, CSA F280, is currently in a transition phase. After 22 years, the update was officially released in April of 2012, but hasn't actually been put in force yet, i.e. most people are still using the older standard. This new update includes allowances for inputting specific air leakage numbers, takes into account load reductions from HRV/ERVs, and has improved calculations for basement heat loss. To HVAC trades: find the next HRAI course and get up to speed on this new standard, so that we can stop the oversizing madness (especially for new housing!). Now if we can just get HVAC manufacturers to in turn make smaller equipment...

  3. Jin Kazama | | #3

    How much is worth your view of the lake ?
    The north windows facing the lake might be the best architec. feature of the house,
    how much is it worth to you ??

    I'd say keep the windows and plan for exterior insulated shutters.

    And he will need backup heating,
    believe me, when it's -30c outside, the minis don't shoot anything else than warm air,
    so don't count on them to keep you warm during the night time, unless you double your planned insulation.
    Baseboards backup and a small efficient wood stove ( only if you have access to free wood or very cheap one ) will be required.

    with 25 to 40% of heat load due to air leaks as a "random" factor here,
    i'd plan for the worse.

    Put some $$$$$$ on air sealing , if it trully equates to 25%of heatload,
    it should be worth it to invest alot of money into it ( that is higher % than your window heat loss )

  4. Peter L | | #4

    House Orientation
    As mentioned, I believe the orientation of the home is the biggest detriment. I know it's after the fact but for future vacant lot buyers, I would recommend that they always check on the orientation of the lot to the way the home will face for the views. If you want passive solar heating, always orientate the house facing south with your majority of glazing facing south. That way you get the best of both worlds; the views and free heat during winter. If you can't get that, I suggest finding a lot that does have these attributes.

    I'm not scolding you but on your design you have the majority of the glazing on the NorthEast elevation and that is the complete opposite of what a passive solar home would necessitate. "Free heat" via window glazing during winter would come from the south elevation. "Heat loss" through the windows will always be on the north, east and west sides of the home.

    Another thing to consider is that if you are facing a lake and your NE facing has all those windows, you will most likely get blasted by Zone6 winter winds coming off of the flat unobstructed surface of the lake and hitting that NE side. What is your Exposure Rating? Exposure "D" I assume? If so, that is important to take into consideration as these high winds can play a role in air infiltration and subsequent heat loss.

    I believe you can still do the ductless mini's. Mitsubishi has heat pumps that go down to -20F below zero. I would also recommend that you do NOT get an open wood stove for supplemental heat. You will FAIL the blower door test as that fireplace will leak like a hole in your wall or roof, because that is what it basically is. Get an electric resistance fireplace for supplemental heat. No cutting holes and venting to worry about. Plus the newer ones look very realistic and can put out around 3k-6k BTU's.

    What type of triple pane windows are you getting? (SHGC, AI Ratings, U-Values, etc)

  5. Bob Holodinsky | | #5

    Thanks for the inputs
    Hello everyone..lots of advice to consider. First of all I have reduced the bottom front windows by about 10%...the outside foam will R20....this gets me below 30,000 btu heat loss ...the mini splits will have a propane direct vent heater as a backup...I chose propane as it will power my back up generator and I have no time or inclination to use wood ....The lot was a family cottage so the choice of the site was never presented and the view of the lake is why we want to be here .The builder is forward thinking and has had experience with mini splits and tight houses and is ready for the challenge...the building inspector so far wants a heat source in every room ,so baseboard heaters will have to be installed unless we can change his mind....All the testimonials I have read from mini split users in New Brunswick and Quebec suggest the mini splits will work with a little back up as suggested. The report I have from the CCHT Experimental House in Ottawa that suggested the mini splits supplied all but 2% of the heat necessary for an Ottawa winter in a typical subdivision house built very well 15 years ago.The back up was a gas fireplace.the energy savings were considerable in that report ,which is now a few years old .Mini splits have gotten more efficient since then.All things considered I believe the project will work and I will keep everyone in the loop as I proceed...I hear the naysayers but the science and research win out...regards,Bob

  6. Malcolm Taylor | | #6

    Reply to Peter L.
    Except for a very small percentage of builds on large lots it is pretty rare to have the luxury of orienting a house to maximize solar gain, and in most circumstances I don't think it should be the determining factor when other concerns, like the relationship to other buildings or the surrounding landscape, are in play.
    If we limited ourselves to only building on sites where the views aligned with south facing windows large parts of cities like Vancouver wouldn't exist.
    Being interested in energy efficient construction it is tempting to make it the chief determinant for all sorts of building decisions, especially window size and placement, but I think it's useful to remember we are creating human artifacts that fulfill a multitude of needs - including sometimes our joy in looking out into our north-facing surroundings.

  7. M F | | #7

    Fujitsu output higher than that, according to my literature. The 15k unit puts out 15k @ -15; about 24k @ +47(?)

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