Leaky house? Bring in the blower door

With winter upon us, a favorite gadget of CNET's Martin LaMonica is a blower door, a green building tool for measuring air flow and finding leaks.

Martin LaMonica Former Staff writer, CNET News
Martin LaMonica is a senior writer covering green tech and cutting-edge technologies. He joined CNET in 2002 to cover enterprise IT and Web development and was previously executive editor of IT publication InfoWorld.
Martin LaMonica
4 min read

The next time someone tells you that you need all sorts of expensive equipment for energy-efficient buildings, make sure to mention air sealing.

Most people know that adding more insulation to an attic is a good way to cut your energy bills and environmental footprint, but restricting the flow of air into a building is also very valuable, say green building pros.

On a fittingly chilly day last week, I was reminded how important air sealing is when I had a blower door test done at my house. The test, which measures how leaky a building is, showed me that some of my efforts have actually improved the situation and it helped identify other spots where the cold is coming in.

A blower door, along with an infrared camera, are standard tools for building-performance professionals. It's not the sort of thing that homeowners will use on their own but a test done by a professional will give you a better grasp on how quickly your home is losing conditioned air.

Almost two years ago, I had an energy audit when I had my first blower door test. Where I live, a utility-funded organization offers free audits, but one with a blower door test, a little bit of air sealing, and a set of recommendations cost me between $500 and $600.

A blower door is just a cloth door equipped with a fan that fits over a doorway. With the fan on, the house reaches a certain air pressure and a sensor can measure the rate of air flow.

The first test was no surprise: I have an old New England home with lots of air flowing through it. Through my work as a weatherization volunteer, I've learned that many cracks and crevices in many homes can add up to a hole almost as big as a small pizza box.

But using the blower door as a diagnostic tool is more interesting.

With the blower door on, I tailed the energy auditors around the house to find where the big gaps are. A lot were in the basement where windows and penetrations to the outdoors create conduits for outdoor air to flow into a house and upward through the building.

The pros used spray foam for the bigger holes in the basement and caulk around windows, installed some weather-stripping, and filled the holes in the attic where the bathroom fan created a big gap--that sort of thing. Then I followed up with similar work of my own in spots they missed.

I had the chance for a follow-up blower door test last week and the results were a significant decrease in the rate of air flow. It's cut down on my heating (and likely cooling) bills and made things more comfortable inside.

The second test also gave me fodder for future improvements--there's always a hole that you missed somewhere. And all this can be done using hardware-store type materials--weather-stripping, tape, etc.--with techniques you could call weatherizing 101. (Two online stores specialized in home energy efficiency I'm familiar with are Energy Federation Incorporated (EFI) and Energy Circle.)

Air tight
Can you make a house too tight? Yes, it's possible but, particularly for older buildings, it's very hard to, say green building professionals. Once again, a blower door test will let you know if your house is so tight that it needs mechanical ventilation.

"The reality is that most people cannot make houses so tight that it's a problem. You're just going from a really old leaky house to a plain old leaky house," said Matt Golden, the president of Recurve, an energy efficiency company based in San Francisco.

Martin LaMonica/CNET

There's a cottage industry developing around deep energy retrofits, where contractors seal the building envelope very tight and add lots of insulation on the exterior of a building, known as superinsulation. A more specific standard for super-energy efficiency buildings is called Passive House, now getting a foothold in the U.S., which also calls for air-tight buildings.

When a house is sealed that tight, it needs a machine for taking in outdoor air and circulating it through the house. Typically, an air-tight building will use a heat-recovery ventilator, which preheats incoming air with outgoing indoor air.

Recurve's Golden said there are real concerns over air quality if a building is super air-tight and not ventilated. (There are also important concerns about carbon monoxide buildup.) But a leaky house doesn't necessarily mean better air quality.

Air tends to flow from the bottom up and out through the attic of a building, a phenomenon called the stack effect. So a very leaky house would pull a lot of air from underneath a house--basement and crawl spaces--where the air quality isn't necessarily good, Golden noted.

Air sealing isn't a replacement for insulation but it should be done before insulation, say pros. That's simply because once insulation is added, it's often tougher to find and seal air cracks, Golden said. Sealing the attic is typically a good place to start, he added.

A lot of air sealing is do-it-yourself type work but it's not hard to make mistakes. For instance, I caulked the spot where a deck meets the foundation, which a friend pointed out didn't allow for proper drainage.

That's one reason why getting a professional who knows about building science and weatherizing homes is a good idea. Plus, every home is different so finding someone capable of diagnosing where you'll get the biggest bang for your efficiency buck is important.

I realize most people would rather spend their time planning a kitchen renovation than tightening up their home (though renovations are a great opportunity to improve efficiency). But with the colder months, I find the winter chill always motivates me to chip away at my efficiency to-do list.