Keeping Cool in a Two-Story House
If your second floor gets hot during the summer, there’s a good chance that your house is leaky and poorly insulated
I was recently a weekend guest at the house of some friends who live in Climate Zone 5 (a zone which includes Nebraska and Massachusetts). Since I have no interest in embarrassing anyone, I won’t mention any names or the home’s precise location. The story, however, is true.
The weather was hot. The two-story house was built in the 1980s; it included a single-zone split-system air conditioner that delivered cool air through ductwork to every room in the house.
My hosts complained that the second floor was always hot, even when the air conditioner was running full blast. Curious, I poked around a bit. There was an access door to a kneewall area upstairs, and I saw that the house had a crazy, chopped-up roofline with several distinct attic areas. The second floor had varying ceiling heights, and the first floor included a “great room” with a double-height ceiling. Most of the second-floor ceilings were sloping ceilings that followed the roof plane.
Houses with these features have complicated thermal boundaries, and rarely have an uninterrupted air barrier. Snooping around in the attic, I saw blown-in fiberglass installed above horizontal ceilings, blown-in fiberglass installed above sloping ceilings, and fiberglass batts installed in vertical kneewalls. Although the insulation job was neat and the insulation was undisturbed, none of the sloping or vertical insulation was protected by an attic-side air barrier.
What else? The second-floor ceiling was peppered with recessed can lights. And there were two skylights upstairs.
Typical 1980s construction
Needless to say, I wasn’t traveling with a Duct BlasterCalibrated air-flow measurement system developed to test the airtightness of forced-air duct systems. All outlets for the duct system, except for the one attached to the duct blaster, are sealed off and the system is either pressurized or depressurized; the work needed by the fan to maintain a given pressure difference provides a measure of duct leakage. or a blower door, so I didn’t have a chance to test the home’s duct system or envelope leakage. But anyone familiar with homes built in the 1980s probably has a pretty good idea of what was wrong with this house: the fiberglass insulation is thin; air is leaking in and out of the building shell at a fast clip; the air leaks degrade the performance of the insulation; and the ducts are probably undersized and leaky.
With a home like this, it’s very hard to design a single-zone forced-air distribution system that works as well during the summer as it does in the winter. During the winter, the second floor is usually comfortable. The forced-air system keeps the upstairs warm, but the first floor may feel cold and drafty. During the summer, however, it's the first floor which is comfortable. While the AC unit may keep the downstairs cool, the upstairs will be uncomfortably hot.
You can try to solve the winter problem by oversizing the ducts and registers serving the first floor. But that just makes the summer problem worse. On the other hand, you can solve the summer problem by oversizing the ducts and registers serving the second floor. Oops! Now the house won’t be comfortable during the winter.
Of course, the right way to address the comfort problems in a leaky house like this is to perform air sealing work and insulation upgrades. Once the building envelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. is tight and well insulated, there won’t be any temptation to tweak the specs of the forced-air system to address oddities caused by massive air leaks.
The AC is cooling the first floor while the upstairs windows are open
So now we come to the sad part of the story. How did my hosts handle the overheating problem? First of all, they turned on the air conditioner to make sure that the first floor rooms were comfortable. The air conditioner appeared to work well, and since the unit was almost certainly oversized — most residential air conditioners are twice as big as necessary — the downstairs rooms stayed cool.
Since they knew from experience that their second floor was hot, they kept comfortable upstairs with natural ventilation and fans. They opened the two operable skylights. They also opened both windows in the upstairs bedroom. Then they placed big rectangular fans in the open windows to keep the air moving. (I forget whether the fans were pushing air out or pulling air in, but that detail hardly matters.)
There are no villains in this story — just ordinary homeowners trying their best to stay comfortable in hot weather. The result is an energy nerd’s nightmare. The building envelope had several huge holes — the open windows upstairs — with fans accelerating the rate of air leakage through the holes. And the air conditioner was running around the clock, doing its best to cool the first floor. (Actually, the air conditioner was doing its best to cool the whole neighborhood.)
When homeowners like this eventually get fed up with their comfort problem, who are they likely to call? They will probably call an air conditioner repair person. And what are the chances that Bob's HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. will correctly diagnose the ceiling leaks or air-barrier-continuity problems?
Living with the legacy of sloppy construction details
This 1980s house is typical in many ways. It was slapped together quickly by builders who had no idea what an air barrier is. Its defects were covered up by finish materials that look attractive. And it was probably sold at a fat markup to eager buyers.
The problems in this house are fixable, but the fixes aren't obvious to the typical homeowner (or even, for that matter, to the typical handyman or carpenter). Intuitive solutions aren't likely to help, and solutions suggested by a local contractor are just as likely to aggravate the problems as they are to solve them.
Our entire country is living with the consequences of decades of shoddy construction. The tragic twist to this story is that when this home was built, progressive builders and building scientists had already perfected the airtight construction methods and superinsulation details needed to build high-quality homes. What the country lacked in the mid ’80s wasn't building science knowledge; it was the will to implement building code changes that would encourage quality construction.
Fortunately, our building codes are now ratcheting up. While our country is now committed (I hope) to building code improvements, we seem to lack the will to enforce today's increasingly stringent codes.
We will all have plenty of work in the years ahead.
Martin Holladay’s previous blog: “How Much Fresh Air Does Your Home Need?”
- Martin Holladay
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