We see a lot of houses. We have a lot of data on houses. For a while now, I have been wondering whether or not I’d be able to find some meaningful relationships with this data. I am not even sure what I would use it for, but I have taken some time to compile the data and pull some numbers that I thought were interesting. More than anything, the data just shows typical values for ‘regular houses’ in the Chicagoland area.

DISCLAIMER: This isn’t a scientific research study in any way, shape, or form – there are many things that you may point out that would make this little ‘research project’ invalid. I randomly selected 98 properties that we have done some sort of analysis for over the years. These properties were a mixture of single-family homes, condos, and townhomes. All of them were in the general Chicagoland area and were not built/designed to be high performance.

Here is a quick glimpse at some of the home size statistics from our sample:

Metric Largest  Smallest Average
Size (SF) 9058 1155 3963
Volume (CF) 86866 11459 34305
Shell Area 21723 2079 7565
SA/VOL 0.35 0.15 0.23

The SA/VOL is a ratio of the surface area of a building to its volume. Generally, ‘efficient’ shapes have large volumes with low surface areas, therefore making this ratio as small as possible. In European countries, this metric is often a key design consideration. Most homes in Chicago will have a SA/VOL ratio of 0.2-0.23 because of long, slender lots (typically 20’ wide and 55’ long).

Now lets talk about some consumption statistics and how they relate to home size:









SF/kBtu Heating




% AC Size Discrepancy




% Furnace Size Discrepancy




Annual kWh/SF




SF/Ton AC: The number of square feet of the home divided by installed capacity of the cooling equipment. 1 Ton is approximately equally to 12,000 Btu/hr.

SF/kBtu Heating: The number of square feet of the home divided by installed OUTPUT capacity of the heating equipment.

% AC Size Discrepancy: The % difference between the capacity of the installed cooling equipment and the calculated amount needed from an energy modeling software.

% Heating Size Discrepancy: The % difference between the OUTPUT capacity of the installed heating equipment and the calculated amount needed from an energy modeling software.

Annual kWh/SF: The actual electrical consumption over a year long period divided by the area of the home.

 It should be noted that square footage, installed equipment capacities, and annual kWh are actual measured values, or determined through inspection. The % discrepancy between installed equipment and calculated loads is based off an energy model, so these values have inherent errors in them because of this. Regardless, the results are pretty typical with standard construction – heating and cooling equipment is usually oversized for the needs of a given home (positive % indicates oversized, and negative % indicates undersized).

Lastly, I’d like to point out the variability in annual electrical consumption in relation to home size.

Electrical consumption is largely based on occupant behavior, installed gadgetry, and family size. Typically, the phone calls we’d receive regarding large electrical bills were pretty simply explained by a few big consumers like pool/hot tub heaters, pumps, electric radiant heat, and entertainment systems.

In closing, the information provided here gives you an idea of standard homes in Chicago. I am not a fan of ‘rules of thumb,’ so the above metrics aren’t intended for any form of comparison against new homes. The overall idea is always to engineer a perfect home, install all components to specification, and test them once they are in place. Or as Corbett Lunsford would say it, ‘Control is the goal.

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Corbett walks you around Venice, Italy on a recent trip, and teaches the implications of building science on this beautiful city that’s being torn apart by seawater. Incredible case study of a lovable place where you wouldn’t want to own a home:

Capillary action is the phenomenon of water soaking upwards, like in the roots of a tree. The same happens with buildings that are founded in the sea- the moisture damage to brick, wood, and plaster can be easily seen here:

And once the moisture damage has torn these homes apart, they have surgery to keep them held together for the next hundred years:

HIGHLY RECOMMENDED as a vacation spot, and the buildings are beautiful to see- but be careful what you invest in while you’re there!

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To keep most people in first world countries comfortable, you need at least two things: an enclosure to hang onto heated or cooled air, and an engine to make the air that way. ALWAYS FIRST: create an enclosure at is relatively airtight and insulated. Now for the engine: the HVAC- air conditioner, furnace, heat pump, etc.
Chris McTaggart with a 20 SEER 4 Ton A/C
The question is: how much heating or cooling should I install in this home so it’s both very comfortable and cost-effective?

Great question! There are very precise calculations for this (spelled out in ASHRAE Fundamentals and in Manual J from the Air Conditioning Contractors of America. You take detailed information about weaknesses in the home’s enclosure (window size and orientation, how much air leakage exists, etc) and put it into a computer.

Doing an HVAC load calculation by hand isn’t good enough in most homes because you need an hour-by-hour look at the effect of the sun and weather on the home. This calculation gives you a very specific amount of heating and cooling that a home needs. You don’t need to add anything to this, because the calculations were written by engineers, who are very cautious and conservative people- they’ve added all the safety margins already to make sure you’ll be comfortable.

Next question: how are we going to deliver the warm or cool air evenly everywhere in my home? Wow, that’s a great question! Guess what? There’s a calculation for that!

Again, ASHRAE Fundamentals or Manual D will tell you exactly how the ducts should be designed and installed so that every single room feels comfortable and refreshing, which is possible for every home, in every place, new and old alike.

Now that we know exactly how much heating and cooling each home needs, we have to buy an engine that can make it. Here’s where most people make a big mistake: they buy an engine that’s too big. These people may think that they need a little extra power for when the weather is really crazy. In fact, many of my clients have twice as much machinery as their home actually needs. That’s like building an escalator and installing a V8 engine to run it- it takes you where you need to be super fast, but it’s also uncomfortable, really loud, and slightly nauseating. So you pick an air conditioner, furnace, or heat pump that fits the home like a tailored suit.
Duct testing with Corbett Lunsford and John Bergman
Last step: we test the tightness of the duct system. A duct system is plumbing for air- you do not want it leaking. You might think if the ducts are all inside the envelope and they leak air here or there, it’s not such a big deal. It IS a big deal, and here’s why:

If we spent time and energy calculating exactly how much heating and cooling this home needs, exactly how the duct system needs to be designed and installed, and picked out the perfect heat pump or furnace, all of that gets flushed down the toilet if the conditioned air doesn’t actually go to the rooms where you need it!

Proof Is Possible, and careful planning and performance testing are the keys. Your home won’t need rescuing if you do it right the first time.

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Building Performance Podcast with Corbett LunsfordToday we talk with Kristof “Handsome” Irwin, founder of Positive Energy in Austin Texas, about why your air conditioner is probably an idiot (bless its heart) and HVAC heaven that you might not have heard about.

Hear all the podcast episodes at BuildingPerformancePodcast.com

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This week’s minute means Corbett is squeezed into a client’s attic, testing the unspeakably awful forced air furnace and air conditioner there. If you haven’t been inside your own attic, PLEASE GO THERE RIGHT NOW and see if it looks like this one!

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