This article first appeared in issue 304 of Fine Homebuilding Magazine.
I bought my first blower door in 2009, back when new construction was in a downturn and energy auditing and weatherization projects were on the rise. I took a 40-hour energy auditing training course at a local college which included hands-on training on how to use a blower door. It took many tests before I became comfortable in its operation and understood the information it was providing. Though one of the more expensive tools I own, I’ve been able to keep it busy and add this specialized testing to my business’s income stream.
What does a blower door do? The main purpose of a blower door is to test the integrity and continuity of the air control layer or air barrier. The test is conducted by either pressurizing or depressurizing a building to a specific pressure, typically 50 Pascals, and measuring the cubic feet per minute (CFM) of air moving across the fan. The CFM number can then be added to a formula to calculate the air change rate at 50 Pascals (ACH50) or calculate the cubic feet per minute of air flow per square foot of building surface area (CFM/ft² of surface area).
There are a few reasons to perform a blower door test on a building.
Code compliant testing or home certification. Current codes, which have been adopted in many areas, require new homes to achieve a certain level of air tightness. Depending on your location, a test result of 3 or 5 ACH50 or less may be needed. HERS Ratings, Energy Star Rated and Zero Energy Ready Homes all require a blower door test for certification.
Pre and post remodeling. It’s a good idea to perform a blower door test before and after a renovation or addition to an existing home. There may be opportunities to improve the performance of the existing home during the remodeling which may be shown with the test. There is also a chance a change to the building may adversely affect indoor air quality or the combustion process in fossil fuel burning equipment. Blower door testing may also provide information indicating a problem.
As part of an energy audit. An energy audit is an inspection and analysis of energy flows in a building. Outside air moving through a home will affect heating and cooling costs, testing can determine how big an affect air leaks have on these costs.
Testing during new construction. When building a new home, a certain air tightness level might be needed to achieve the goals of the design. Interval testing will confirm if the home is on track to meet these goals.
Blower door directed air sealing. Using a blower door to find and seal air leaks is another use of the equipment. Blower doors can be set on a “cruise control” where they move a steady amount of air through the holes in the home. These holes can then be found and sealed.
The parts and pieces. The two big manufacturers making blower door test equipment in North America are The Energy Conservatory (TEC) (upper photo) and Retrotec (lower photo), both companies have similar designs. An adjustable frame that will fit in most door and some window openings, a nylon panel that fits over the frame, a powerful fan and a manometer that measures pressure and air flow. One difference is TEC has a separate fan speed control and Retrotec’s speed control is part of the fan housing. The latest versions of both company’s manometers are very versatile and intuitive, basically small computers designed specifically for blower door testing. Each offer several different ways to control the equipment, simply with the manometer and speed control or by connecting to a computer, tablet, or smart phone. When using a computer, tablet or smart phone, software supplied by the two companies will not only control the test, but also provide a report showing the test results.
Conducting the test. I try to follow the same procedure when preparing for a blower door test. I start by walking the exterior of the home. This gives me an idea of the shape of the structure which may not always be evident from the interior. While on the outside, I look for any penetrations through the air barrier. Exhaust and dryer vents, plumbing vents, electrical chases and any parts of the heating and cooling system may be potential air leaks.
After moving to the interior, I follow the same process of inspecting the areas of the home taking note of potential problem areas and checking all windows and exterior doors to see if they are closed and locked. All interior doors that are within the conditioned space of the home must be open. I also turn off any heating and cooling equipment, fossil fuel burning equipment, exhaust fans and dryers. If I turn off any equipment, I leave my car keys near the controls of the equipment so I can’t leave without turning them back on. I also make sure any fireplace or woodstove is cold and either sealed or all ash removed. A blower door test cannot be conducted when a fireplace or woodstove is in operation.
I then move to measuring the home. The Residential Energy Services Network or RESNET, who helps design a set of standards for conducting a blower door test, suggests all measurements be taken to the exterior dimensions. Personally, I prefer to take all measurements from the interior. My reasoning is usually the interior finish material is part, if not the main air control layer. In my area of climate zone 7, polyethylene sheeting is widely used on the warm in winter side of the wall assembly and is typically installed as the main air barrier. Measuring from the interior also simplifies the measuring process. I use a laser measuring device which is much easier to use indoors. Another reason I measure on the interior, during the winter months, snow around the building may make it difficult to move about. I start the measuring by figuring the square footage of floor area, and then multiply that by the height of the building. It’s seldom that simple though, convoluted designs, varying ceiling heights and cathedral ceilings all complicate the process of figuring the volume. I often also calculate the surface area which requires areas of all sides of the structure, including the floor and ceiling, above and below grade. Again, this can be complex to figure. Often, measuring the home is the most time-consuming portion of the test.
Once the measuring is complete and inputted into the software, I then proceed to assembling the blower door and connecting the manometer to my Microsoft Surface Pro, which has become my preferred way to conduct most tests. I choose the door to conduct the test based on wind speed and direction. Wind can have a large effect on the test result, and it’s recommended to set the fan on the leeward side of the building, if possible, when winds are above 5 MPH. After the equipment is setup, I start the test which will require some additional inputs into the software. The indoor and outdoor temperatures (this becomes important when there is a 30°f difference between inside and outside temperatures) and elevation (above 5000 feet is when the elevation input becomes important) will be needed, then the test can begin by taking a baseline pressure. The baseline pressure, which can be positive or negative, is the difference between inside and outside the home and will be accounted for with an automatic adjustment in the software control of the equipment. After the baseline adjustment is complete, the test will begin by prompting me to remove the appropriate ring or range plug based on what I guess is the estimated leakage of the home. If the fan cannot reach pressure, another ring or more range plugs will need to be removed, or if the fan is moving at too slow of a speed to register flow, the size of the ring will need to be reduced or more or a smaller range plug will need to be installed.
Pressurizing vs depressurizing during a blower door test. Why choose one over the other? I almost always depressurize, mainly because a depressurization test is easier to conduct and locate air leaks. Negative pressures tend to pull doors and most windows into their weather-stripping and exhaust dampers are pulled closed. The test results are usually lower but negative pressure alone might not be a good representation of how the home actually leaks air. During the heating season, positive pressures might be present high in the building with negative pressures low due to the stack effect. The opposite may also be true during the cooling season. Conducting both a positive and negative test and averaging the two can be a more accurate representation to what pressures a home may naturally see.
A good reason to perform a positive pressure test is to find air leaks from the outside. Filling a home with a theatrical fog, and then adding pressure will force the fog through the holes to the outside where it can be seen. Sometimes the “visual” is the best way to find the air leaks.
Occasionally, a home may be so leaky, a single blower door fan cannot reach the desired test pressure. When this happens, there are two choices. The first is to use a second (or more) fans to have the ability to move more air. In a residential setting, this option is rarely used. A second choice is to extrapolate the test pressure. This can be done mathematically, but it is easier to have software calculate the “can’t reach 50” number along with the estimated CFM rate.
I usually chose to conduct multi-point testing on nearly every blower door test I perform. The multi-point test I most often use is the RESNET/ICC 380 standard which starts the test samples at 60 Pascals of pressure and reduces the pressure to 10 Pascals. The standard requires a minimum of 5 test pressures at equally spaced intervals. Each of the test pressures samples will be recorded and averaged over a ten second interval. I feel this test is accurate, repeatable and supplies me with the most accurate effective leakage area (ELA) data. ELA is the cumulative area of air leaks in the shape of a smooth hole, similar in shape to the range plugs or rings of the blower door if the pressure inside the home was 4 Pascals. Basically, this gives us an idea of how big a hole is in the building’s shell at a natural building pressure. This is a handy visual for homeowners. I also have the option to conduct a single point test which also requires a baseline, but then only conducts one test sample at 50 Pascals with at least a ten second average. Both test results are supplied in cubic feet per minute of air flow across the blower door fan. If using software to control the test, the software will calculate the ACH50 and CFM/Ft² of surface area if the building size information is supplied. If not using software, calculations will need to be done manually.
What does it all mean? The blower door is supplying us with data as to how air-tight a home is. This can be communicated in several different metrics.
CFM50 number. This number represents the cubic feet of air moving across the fan per minute at the test pressure of 50 Pascals. This is the most important information supplied by the blower door. We need this number to calculate the ACH50 and CFM50/Ft². I have seen CFM numbers as low as 100CFM (a very tight home) and as high as over 5,000 (very leaky). The CFM number alone does not quantify good or bad, we also need to consider the size in both surface area and volume of the home.
The ACH50 number. Air Changes per Hour at 50 Pascals. Think of all the volume inside a structure, the ACH50 number indicates the number of times all that inside air is exchanged with outside air per hour. A home testing 3 ACH50 will exchange all its inside air with outside air 3 times per hour at the test pressure of 50 Pascals. New houses being tested need to meet the code requirement of 3 or 5 ACH50 or less depending on location in the country. When testing existing buildings, I like test results under 5 ACH50 for my climate. Results over 5 have an opportunity for improvement which will increase comfort and reduce energy costs. The best test I have personally conducted is .33ACH50 and the worse was just over 15ACH50. The formula for figuring the ACH50 number is: CFM50 x 60 minutes ÷ volume of the structure = ACH50
CFM50/Ft² number. Air leakage happens through surfaces, yet building codes require testing to be reported as a volume calculation. Many testing professionals and building scientists prefer the information shown as feet per minute per square foot of surface area of the home, or CFM50/Ft². A test of 3ACH50 will roughly be the equivalent of .25CFM50/Ft². There isn’t an exact relationship between volume and surface area, it’s dependent on the size and shape of each home. CFM50 ÷ Ft² of surface area of the structure = CFM50/Ft²
What is a Pascal? A Pascal is a measurement of pressure and can be hard to visualize. One pascal is defined as one newton per square meter or one kilogram per square meter or one joule per cubic meter. Not much help there! The best description for 50 pascals is it’s about the equivalent of a 20 mile per hour wind blowing on all sides of a structure at the same time. 50 Pascals is also the equivalent of .2 inches of water column. Think of sucking a liquid through a straw and moving the liquid up less than ¼ inch. That is 50 Pascals of pressure. Now, take a large diameter pipe and stick it in the ocean, see if you can draw the water up .2 inches, probably not because now you have a volume to overcome.
50 Pascals of pressure is used during a blower door test because wind and stack effect become less of a factor and you are less likely to damage the structure being tested, though I have seen polyethylene sheeting stapled to a ceiling get pulled off during a blower door test.
Mistakes can be made. The key to having an accurate test is by inputting accurate data into the formulas or software. Measuring a home for its volume, and to a lesser extent, surface area, can be a challenge. An error in the volume calculation will have an effect on code required air changes per hour test results.
Temperature, elevation and wind can also affect the precision of a blower door test. Using software will improve accuracy, a manual test will require these conditions be taken into account.
Another mistake I’ve caught myself making is having the wrong setting in the software or manometer for the ring size or number of range plugs. This setting changes a calculation in the manometer as to how many CFM’s are moving through the fan. I now double check those settings in the software or manometer so that they match the equipment settings.
How does a house normally leak air? Houses naturally leak air in only three ways, by wind, stack effect, or mechanical ventilation. Wind is self-explanatory, a positive pressure is exerted on the home on the windward side with a negative pressure on the leeward side. More wind velocity will result is a greater amount of air moving through the home.
Stack effect is a function of building height and temperature difference between outside and inside, called the delta tee. During the winter, air inside the home is heated and becomes more buoyant. It moves higher in the structure and if there are holes high in the air control layer, this air leaks out. The loss of air causes a negative pressure low in the building which draws outside air in. The taller the building, or the bigger temperature difference between inside and out, the more the air moves.
The last way air leaks through a home is by mechanical ventilation. Exhaust ventilation strategies will naturally create a negative pressure inside the home. The air that leaves the building will be replaced with air from outside the building envelope. Unsealed ducts of a forced air system that are outside the building envelope can also create artificial pressures inside the home. Depending on the location of the leakage, either in the supply or return duct, the pressures can be positive or negative on the home. This is one of the reasons codes now require ductwork to be tested for tightness. This duct tightness test is very similar to a blower door test.
Back when I purchased my blower door, the tool was mainly being used by energy auditor, raters, weatherization specialists along with a few insulating contractors. Today, more and more builders are choosing to purchase their own blower door to test the projects they are involved in. In my opinion, the current 3 or 5 air changes per hour code requirement for new construction will eventually begin to ratchet down, making passing a test more difficult. We are already seeing this “tightening” of buildings in a few areas around the country and Canada. Understanding where buildings leak air can be learned through testing, and the best way to test is by using a blower door.