Air flowing in and out of a building can cause lots of problems; in fact, air leakage can account for 30 percent to 50 percent of the heat loss in some homes. But air flowing through a building can help solve lots of problems too — as long as it’s the result of a blower-door test. With a blower door, builders can quantify airflow and the resulting heat (or cooling) loss, pinpoint specific leaks, and determine when a home needs additional mechanical ventilation.
First developed in the 1970s as a research tool, a typical blower door consists of a powerful variable-speed fan mounted in an adjustable panel temporarily set up in a doorway. The fan moves air through the building in a controlled fashion, while a pressure gauge — connected to the fan and to the outdoors by small-diameter pressure tubes — measures the rate of airflow required to maintain the building at a certain pressure. The blower creates exaggerated air leaks, which can then be found with the help of tools like smoke puffers or infrared cameras, or even just by feeling with the face or the back of the hand.
Blower doors for residential work now weigh less than 50 pounds and can be easily carried in a small trunk. A basic kit costs between $2,500 and $3,500 and can be set up, used, and repacked in a half-hour. (For more information on blower-door kits, see “Blower-Door Manufacturers.”)
Pressure, Airflow, and Holes
The amount of air that flows through a hole depends on the characteristics of the hole and the pressure driving the flow. Since the three variables — hole, pressure, and flow — interact, a change in any one also changes at least one other. This behavior can be measured fairly reliably, so given any two of these variables, we can calculate the third.
If we know the size and shape of a hole and the force pushing the air, we can figure out how much air must be going through.
If we measure the amount of air going through a known hole, we can calculate what pressure must exist in order to push that much air.
If we know nothing about the hole, but can measure the pressure and the flow, we can figure out what the hole must be like. That’s what a blower door does: It generates and measures airflow and pressure. We then use that information to describe the size and shape of the hole.
About natural infiltration. Once we have used flow and pressure to determine what the leaks are like, we can use that hole description, along with weather and site data (the test pressure), to estimate the airflow that can be expected under normal conditions. But estimates of “natural airflow” are inherently inaccurate, because it’s difficult to know how the wind blows on a particular site, or what the occupant behavior is like, or how the mechanical equipment interacts with the building. So it’s important to know whether airflow descriptions are measurements of leakage under specified conditions or estimates of airflow under normal conditions.
To measure airflow, a closed-up house is depressurized with the blower-door fan to a constant pressure differential as compared with outside conditions, typically 50 pascals (Pa). A pressure gauge attached to the blower-door assembly measures the rate of airflow required to maintain that pressure differential in cfm (cubic feet per minute).
Sometimes several readings are taken at different pressures, then averaged and adjusted for temperature using a simple computer program. This provides the most accurate picture of airflow, including leakage ratios, correlation coefficients, and effective leakage area.
Most of the time, though, this detailed output isn’t needed, and all we want to know is how much the building leaks at the specified reference pressure of 50 Pa. So-called single-point testing is popular with crews who do retrofit work, because once the door is set up, it takes only about a minute to measure the effectiveness of their air-sealing strategies.
The pressures exerted on a building are quite small (50 Pa is the suction pressure required to lift a column of water up a soda straw less than a quarter inch), so test results can be affected by wind gusts. There are some tricks for moderating wind effects and increasing accuracy: For example, multiple tubes protected with wind dampers can be run outdoors to sample air pressure on different sides of the building, and several measurements can be taken and averaged. Using these techniques, blower-door testing can be done in all but the windiest weather. An experienced operator can tell whether or not reasonable measurements are possible by the behavior of the gauges. Computer analysis of the data — if it’s done — also includes a check for accuracy.
Cfm and ACH. While airflow can be measured in cfm, it can also be expressed as airflow compared with volume, or air changes per hour (ACH). ACH50 indicates air changes per hour at a 50 Pa pressure difference (not to be confused with natural ACH). Generally speaking, houses with less than 5 to 6 ACH50 are considered tight, and those over 20 are quite leaky, though these numbers can be misleading without considering other variables such as climate, house size, and old vs. new construction.
While the airtightness and ventilation requirements of a space have traditionally been expressed in ACH, many blower-door professionals routinely use cfm as their primary unit of measure. Cfm is easier to obtain, because it doesn’t require calculations of volume. More important, it’s a more direct expression of the main variable with which we are concerned — namely, air leakage.
If we’re considering ventilation levels, we can more easily deal with cfm than ACH, and are probably more concerned with absolute flow than the flow as compared with volume. If we’re dealing with large spaces with few occupants — or small, heavily occupied spaces like trailers and apartments — ACH can be misleading because it can make a large space look tighter and a small space look leakier. For these and other reasons, cfm is being used more often and ACH less. Because cfm50 (the cubic-feet-per-minute airflow with a 50 Pascal indoor-outdoor pressure difference) is easily obtained with single-point tests — and is low enough to be consistently reached yet high enough to be resistant to the effects of wind — it has become the main unit of measure for the description of airtightness. Tight houses tend to measure less than 1,200 cfm50, and moderately leaky homes measure between 1,500 and 2,500 cfm50. Homes that measure over 3,000 cfm50 are considered leaky.
Testing a Home
Blower-door tests are performed with doors and windows closed, and often decisions have to be made concerning doors to semiconditioned spaces. The rule of thumb for basements and similar spaces is to include any area that is at least semiheated (even if unintentionally, as in an unfinished basement with a furnace). Often, it makes sense to test both ways, which is simple once the blower door is set up.
Whether or not intentional openings like ventilation ports are temporarily sealed depends on the test being performed. For a description of how an existing house normally behaves, such openings are usually left uncovered. On the other hand, if a new house is being tested for sufficiently tight construction, it may make sense to seal intentional openings, removing them from the measurement.
Since the test depressurizes the house, sucking air in through all the openings (including flues), combustion devices must be disabled. Heating systems and gas water heaters must be shut off. All wood-burning appliances in the house need to be out, which requires prior notification for occupied houses during the heating season.
Checking for backdrafting. An analysis of a house’s airflow should include a check of all combustion equipment. Any device that uses indoor air for combustion must have an adequate air supply. The greatest occupant safety hazard — backdrafting — tends to be the result of excessive negative pressure caused by air-moving appliances. This works the same way as the blower door: A fan moves air out of a space, which produces a pressure difference relative to the outside. This fan can be one that is intended to remove air from the building — like a bathroom exhaust fan, range hood, clothes dryer, or central vacuum system — or it can be a fan that moves air within the building, such as a furnace fan. It can also be a combination of several fans or an exhaust force other than a fan, such as the heat-driven force of a chimney. If the negative pressure in a combustion appliance’s space is greater than the chimney draft (often only 3 to 5 Pa), the airflow in the flue will be reversed and flue gases will be dumped inside.
Although backdrafting tends to be more common in tight houses, it is also affected by the specific appliances involved and where they are located. Compartmentalization created by interior doors can contribute to the problem as well.
To check for the likelihood of backdrafting, I place the house in a worst-case condition, turn on the air-moving equipment, and either measure the resulting indoor-outdoor pressure or fire up the combustion device. Many testing protocols (such as the Building Performance Institute’s) specify a maximum allowed depressurization. If this maximum is exceeded, or if the appliance does not establish draft under the worst-case condition, some action must be taken to either improve the draft or reduce the depressurization so that flue gases are reliably exhausted outside.