Energy Audit-Blower Door Testing-A Deep Dive

I’ve talked about blower door testing several times on Green Building Advisor and on this blog.  This discussion will dive deeper into blower door testing, when it should be completed, the different tests done with the blower door, and interpreting the information.

There are four testing levels that I typically perform.
1. Code compliance testing                                                                                        2. Testing during construction
3. Contractor diagnostic testing
4. Homeowner diagnostic testing.

There is a fifth reason to use a blower door, blower door directed air sealing, but that is less about testing and more about air sealing a structure.

Let’s first start with code compliance testing, which is the most common type of blower door testing I conduct.  The building code, from the 2018 IRC which applies only to new construction states:

” The building or dwelling unit shall be tested and verified as having an air leakage rate of not exceeding five air changes per hour in climate zones 1 and 2, and three air changes per hour in climate zones 3 through 8.  Testing shall be conducted in accordance with RESNET/ICC 380, ASTM E779 or ASTM E1827 and reported at a pressure of 0.2-inch w.g. (50 Pascals.)  Where required by the building official, testing shall be conducted by an approved third party.  A written report of the results of the test shall be signed by the party conducting the test and provided to the building official.  Testing shall be performed at any time after creation of all penetrations of the building thermal envelope.” 

This code requires all new residential construction to pass an air leakage test of less than 3 or 5 air changes per hour (depending on your climate zone) at 50 Pascals.  This test is a pass/fail test to meet code requirements and is typically performed at the end of construction after all HVAC equipment and plumbing fixtures have been installed (plumbing traps should be filled with water).  This is the least expensive of all the blower door tests I conduct, I’m not looking for the air leakage locations, performing zonal or pressure pan testing, or taking thermal imaging photographs, just a number to satisfy the code requirement.

A second type of testing I sometimes perform is blower door testing during the build.  The timing of this testing depends on the goals of the contractor or homeowner, complexity of the structure, and if the building is trying to achieve a certification, such as Passive House.  Testing during the build will require not only the air change per hour number and/or surface area leakage rate, but also hunting down air leaks.  Sealing air leaks is almost always easier before finishes are installed.  This type of testing can take some time, costs are higher than the simple code compliant test.

The third type of testing is the contractor diagnostic testing.  I recommend performing a blower door test before any remodeling work that effects the building shell.  The dynamics of a home can easily be changed during remodeling, some of the changes can adversely affect the home’s performance.  Knowing how “tight” a home is before remodeling may provide additional opportunities for thermal or air leakage improvements.  Unseen problems that may unexpectedly add to the cost of the project can also be identified before work begins.  A second test should also be performed at the end of the project to see how the home was changed.  Indoor air quality and fossil fuel burning appliances can be affected.

The forth type of testing is similar to the contractor diagnostic testing. The homeowner diagnostic testing is typically performed as part of an energy audit.  A full energy audit is conducted when a homeowner is concerned about home performance, energy costs, or comfort within the home.  Often, a blower door test will identify problem areas.  The software I use will provide homeowner with an estimate cost of the air leakage.  The homeowner can then make decisions based on the recommendations provided by the energy auditor.

How a blower door works.  There are four main components of a blower door. A metal expandable frame designed to fit tightly in an exterior door or larger window opening.  A nylon panel which attaches to the frame and makes the assembly airtight.  A calibrated fan installed in the nylon panel and used to push air out or into the structure.  Lastly, a manometer or pressure gauge used to measure the pressure in Pascals and the flow in CFM’s.  I use a laptop computer to control the test and record the results.  The software will quickly provide me with additional valuable information.

The test is conducted by either pressurizing or depressurizing the structure to a specific pressure, typically 50 Pascals.  A pascal 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.

I almost always depressurize the home for the test.  I have only three reasons I would perform a pressurized blower door test, first, if I feel depressurizing will damage a component of the structure.  An example is if I were testing with only polyethylene sheeting or some other fragile product acting as the air barrier attached to the interior side of the framing, a depressurization test may pull the material from the framing.  The second reason to pressurize is to pinpoint a leakage area using a theatrical fog machine.  This method is used to pinpoint holes in the air barrier that is not evident by other means.  Lastly, I will use a pressurization test to check air tightness of windows or the seal of an exhausting damper, such as a dryer or bath fan vent.  Pressurizing will force those dampers open.  Testing a home using both negative and positive pressures can provide a more accurate result.  I do not test homes where Vermiculite insulation is present, testing may cause the asbestos fibers to become airborne.  A risk I’m not willing to take.

The actual test will only take about 10 minutes, (it takes longer to assemble the equipment).  I have performed a few different test standards, the Canadian General Standard Board or CGSB, the RESNET Multi-point Test as well as the RESNET repeated single point test.  All the testing I have conducted are automated through a laptop computer.  Automation can also be achieved by one of the smart monometers or pressure gauges that both the manufacturers in North America have developed.  The CGSB is a multi-point test that starts at 50 pascals and reducing to 15 pascals in 5 pascal increments recording 100 data points per pascal increment.  I find this to be a very accurate and repeatable test, but it is not recognized by the latest building code as a test standard.  The RESNET Multi-Point Test starts at 60 Pascals and reduces to 18 Pascals in 8 different building pressures.  Both tests start and finish with a building baseline pressure reading.  The single point test only tests at 50 Pascal but conducts the test several times with a baseline reading between each test.  There will be a future posting all about test standards, stay tuned.

A couple notes on testing.  Testing in windy conditions complicates the testing procedure.  The wind can “bounce” the interior pressures or have an effect on the outside reference pressure tube making it harder for the software to stabilize to take the pressure readings.  Sometimes this influences the accuracy.  Testing in very cold weather can also be difficult.  The software askes the outdoor and indoor temperatures to account for their effect on the test results.  An additional consideration, moving cold air into the building can quickly reduce the indoor temperature.  Performing the test quickly in these conditions is important.  Lastly, performing the RESNET tests, the software will ask your elevation above sea level.  High elevations can also influence the test results.

Information needed.  To provide a customer with a precise blower door test, I need accurate information about the home.  The volume, floor area, surface area, climate zone location, wind shielding, and indoor and outdoor temperatures and sometimes elevation are required to conduct the test.  Additional information I collect is the number of bedrooms and number of occupants which is used to calculate ventilation requirements.  I also find out the type and efficiency of the heating and cooling equipment along with cost of the energy which will estimate the cost of the air leakage.

Setting up the test.  The home needs to be set up a certain way to perform the testing.  All windows and exterior doors need to be closed and locked.  I lock the doors to keep people from entering or exiting the building during the test.  I’ve had a few occasions where someone unexpectedly enter a home during a test.  The sudden pressure change pulled the blower door assembly out of the door frame with the fan operating at a high speed.  All interior doors need to be opened, including closet and basement doors (if the basement is inside the building envelope, I have yet to test a home in my area where the basement is not conditioned).  Heating, cooling, and ventilation fans are required to be off.  I need to be sure no gas burning appliances fire during the test.   This can backdraft carbon monoxide into the home.  If I turn off a heating or cooling system, I leave my truck keys next to the thermostat or switch used to control the unit, this way I cannot leave without turning it back on.  Most importantly, there can be no fires in any wood burning appliances, sealed or not.  I also require all ashes to be cold and if possible, the ash to be removed before testing.

Other testing.  After I complete the basic test, which supplies me with the cubic feet per minute at 50 Pascals number, I have the option to conduct additional testing.  The blower door can be set to maintain the 50 Pascals pressure within the home.  If there is at least a 10-degree temperature difference between the inside temperature and outside temperature, a thermal imaging camera can assist in finding the locations of the air leaks.  I can also conduct zonal pressure measurements.  This test measures pressure differences in rooms or spaces within the home.  For instance, I can close the door to a bedroom and measure the difference in pressure between the bedroom and the area outside the bedroom or the difference in pressure between the attic and home.  This information will suggest if I need to look for an air leak source within the room or area being tested.  A similar zonal pressure test uses what is called a pressure pan.  This sealed box can detect leakage around duct boots used for heating and cooling, switch, electrical outlets, small light fixtures, or other small penetrations in walls, ceilings, or floors.

Interpreting the results.  There is a lot of information supplied by the blower door test.

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/sq ft.  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 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 home, the ACH50 number indicates the number of times all the air inside the home is exchanged with outside air.  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.  I use this number as a metric for the next step.  Of course, new houses being tested need to be under 3 or 5 ACH50 depending on location to meet code.  I like to see older homes 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.

CFM50/sq ft number.  Air leakage happens through surfaces, yet we are testing and reporting the findings 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/sq ft.  A test of 3ACH50 will roughly be the equivalent of .25CFM50/sq ft.  This information is included with the test report if the surface area of the building is calculated.  Green Building Advisor has several very good articles on CFM50/sq ft. (Martin Holiday’s Is it Time to Move Away From ACH@50 Pascals” from September 14, 2018)

Let’s use a fictional blower door test as an example of how much information can be supplied by CFM50.  I’ve calculated a home’s volume at 15,000 cubic feet, 4,600 square feet of surface area, and the blower door has supplied us with 1,000 CFM at the test pressure of 50 Pascals.  We will use these values for calculating ACH50 and CFM50/sq ft.

First, the ACH50 number.  The formula is CFM50 x 60 minutes per hour / volume of the home.  In our example 1000 CFM x 60 minutes per hour/15,000 cubic feet give us 4 air changes per hour at 50 Pascals or 4 ACH50.  Does this home pass a code compliant blower door test?  Depends on where in the country you are located.  In my area, climate zone 7, this home has failed.

Now let’s calculate the cubic feet per minute at 50 Pascals per square foot of surface area of the home CFM50/sq ft.  The home’s surface area is 4600 square feet.  This includes all six sides of the structure, roof, walls and floor, there is some discussion on whether the floor should be counted in all situations.  I feel to keep the numbers consistent; it should be.  CFM50 / surface area of the home.  Again, using the fictional test, 1000 CFM50/4600 square feet of surface area = .217 CFM50/sq ft.  Most of us are familiar with the ACH50 number, it will take some time to become familiar with the CFM50/sq ft calculation.

Estimated annual infiltration.  The air changes per hour natural, ACHnat, is an estimate of how much air leakage is occurring naturally within the structure. Testing a building at 50 Pascals is subjecting the building to pressures it typically doesn’t see.  Some of the leakage that is present at the elevated pressures will not leak at normal conditions.  The software I use to conduct blower door testing, Tectite from the Energy Conservatory, calculates an estimated natural infiltration rate, which can be unreliable. A home that tests to the code minimum of 3ACH50 will have a rough natural air leakage rate of .2 air changes per hour naturally or .2ACHnat.  This home will exchange its entire volume of air every 5 hours, or roughly 5 times per day.  That is considered “tight”.

Estimated design infiltration.  The software also calculates design infiltration which is used to calculate heating, cooling, and ventilation equipment for the building.

Leakage area.  The software I use for blower door testing estimates the cumulative size of all the air leaks within the building.  Helpful for visualization the total size of all the air leaks.

Cost estimates.  The final report also includes how much the leakage is costing in energy usage.  Information needs to be supplied to the software about the heating and cooling system, such as type and cost of fuel and efficiency of the system.  This information is helpful for the return on investment for any improvements made.

Blower door testing is just one metric of a home’s performance.  There are many other variables that effect the cost of operating and comfort along with indoor air quality.  The best advice I have to builders and homeowners building a new home is to set an air tightness goal and test to assure the goal is reached.  Make sure the HVAC contractor knows the planned performance of the home.  Those planning on remodeling or upgrading their home, I recommend at least a pre- and post-test of the home.  Changes in building tightness can impact the home, sometimes the changes have undesired consequences.

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