Blower door testing explained
When I do a home energy audit for my customers in Montana, it's usually the first time they've had a blower door test, so I often get asked to explain the process.
A blower door test is a key part of a good energy audit and can help in three main ways with making your house more energy efficient:
1. A blower door test measures how leaky your house is, giving an objective number for how much potential improvement can be made through air-sealing.
2. A blower door test can be used to measure the relative leakiness of different areas of the house, such as the attic, to help prioritize air-sealing.
3. A blower door test can help pinpoint air leakage pathways, especially when paired with a thermal camera.
Before we dig into each of these in more detail, let's back up and talk about why air leaks matter. In an area like Montana that has cold winters, the majority of our home energy use is from heating (and to a lesser extent air conditioning during our hot summers). So we spend a lot of energy and money conditioning the air inside our houses, and we don't want that air to just leak out. But, especially in houses built before modern energy codes, a lot of air does leak. And the worst leaks usually aren't in places you'd expect.
A blower door test is all about getting a handle on air leakage and coming up with a plan for air-sealing to make your home more energy efficient and comfortable.
Measuring overall house leakiness
At its most basic, a blower door test provides an objective measurement of how leaky a house is. It does that by slightly depressurizing the inside of the house to a standard of 50 Pascals and measuring how much air is being sucked out of the house.
Here's how it works: When I show up for an energy audit, I do some quick measurements to calculate the interior volume of the house. Then I set up the blower door kit. We choose an exterior door that's out of the way, and a frame stretched with fabric is pressed non-invasively into the open doorway. A fan fits into a hole in the fabric, and some small tubes connected to a computer controller measure the pressure inside and outside the house. When we run the test, the fan revs up, sucking air out of the house. This usually lasts only a minute, and if you were standing in another part of the house you wouldn't really know anything is happening. The 50 Pa pressure is equivalent to a 20 mph wind hitting all sides of the house, so you don't feel you ears popping or anything like that.
By knowing the volume of the house and the amount of air passing through the fan to achieve the 50 Pa pressure, the computer calculates a number called ACH50, or air changes per hour at 50 Pa. This is a standardized number used in the building industry. Builders within city code jurisdiction in Montana have to hit ACH50 less than 4. Some new high-performance homes are as tight as 2 or even 1 ACH50. But older homes often have an ACH50 of 8 or more, meaning they're twice as leaky as the worst new home.
The ACH50 number is useful in a home energy audit because it gives you a general sense for how much improvement you can make with air sealing. If we get an ACH50 of 8, we know that air-sealing can make a big difference. If it's 4, we might work on tracking down a few leaks, but would focus on insulation or moving ahead with something like replacing the furnace with a heat pump.
Sometimes, a second blower door test is done after the air-sealing work, to confirm that it was effective before moving ahead with insulation and other upgrades. It can be very satisfying to get an objective before-and-after measurement like this.
Identifying leaky areas of the house
The blower door can also be used to find the general areas of the house that are the most leaky. In the weatherization world this is called zone pressure diagnostics. It basically involves measuring some relative pressures between different areas of the house while running the blower door.
Here's one example of how it works. A common place for air leakage is through the attic. To confirm this, we can do a blower door test while measuring the pressure inside the attic with the attic hatch closed, then do the same test with the attic hatch open. By plugging the numbers into a calculator tool, we can get an estimate of the percentage of total house leakage that's occuring through the attic. That can give you confidence with moving ahead with air sealing in that zone.
Pinpointing air leaks
Zooming in further, the blower door can be used to find some of the specific air leaks to seal. Because the blower door depressurizes the house, air from the outside is sucked through every air leak. This is basically reversing and exagerrating the home's natural air leakage, making it easier to detect.
When it's cold or hot outside, there's a temperature difference with the air inside the house. By using an infrared camera, we can see where this hot or cold outside air is being pulled into the house by the blower door. (The thermal camera can also see where the hot or cold outside is conducted through the building envelope due to lack of insulation.)
With the blower door running, we can often feel air rushing in from the air leaks. We can actually see the air moving by using a fog pencil that puffs some of the fake "fog" used for dramatic effect at concerts.
Conclusion
These three testing methods make a blower door a valuable part of a home energy audit. If you're in the Livingston/Bozeman area and are interested in a home energy audit with a blower door test, give Denizen LLC a call today. I also do blower doro testing for new construction.
