The Energy Audit-Blower Door Test

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

There are three testing levels that I typically perform.
1. Code compliance testing
2. Contractor diagnostic testing
3. Homeowner diagnostic testing.

Lets first start with code compliance testing.  The building code, from the 2012 IRC which was adopted by the state of Minnesota in 2015 and only applies only to new construction states:

” The building thermal envelope shall be constructed to limit air leakage…the building or dwelling unit shall be tested and verified as having an air leakage rate of not exceeding…3 air changes per hour in zones 3 through 8. Testing shall be conducted with a blower door at at pressure of 0.2 inches w.g. (50 Pascals).” 

This code requires all new residential construction to pass an air leakage test of less than 3 air changes per hour at 50 Pascals or 3 ACH50.  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).  Additional blower door testing may be required on high performance home or structures with complex designs.  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.

The second type of testing is the contractor diagnostic testing.  I recommend performing a blower door test before any remodeling work is performed.  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 third type of testing is very 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 the problem areas and provide an estimate cost of the air leakage.  The homeowner can then make decisions based on the recommendations provided by the energy auditor.  I strongly recommend a second blower door test be performed after any improvements are made to a home.

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.  The fan senses how much air is moving through the fan’s shroud and displays this volume of air as cubic feet per minute, or CFM.  Lastly, we have the monometer used to measure the pressure in Pascals within the building.  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.  If there is asbestos or vermiculite present, or if I feel depressurizing will damage a component of the structure.  An example is if I were testing with only polyethylene sheeting acting as the air/vapor barrier and retaining wall or roof insulation, a depressurization test may pull the polyethylene from the framing.  The last 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.

The actual test will only take about 10 minutes, (it takes longer to assemble the equipment).  I perform a test called the Canadian General Standards Board or CGSB test.  This multi-point test 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.  A single point test is also an option, this test requires achieving the 50 Pascal pressure difference between inside and outside the building and recording the cubic feet per minute moving across the fan.  You can then convert the CFM to an air changes per hour number.

A couple notes on testing.  Testing in windy conditions complicates the testing procedure.  The wind will “bounce” the interior pressures making it harder for the software to stabilize for pressure readings.  Sometimes this affects the accuracy.  Testing in very cold weather can also be difficult.  Moving cold air through the building can quickly reduce the indoor temperature.  Performing the test quickly in these conditions is important.

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 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 door to keep people from entering or exiting the building during the test.  I once had 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.  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.

Other testing.  After I complete the basic test, which supplies me with the air changes per hour 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.

The ACH50 number.  I use this number as a metric for the next step.  Of course, new houses being tested need to be under 3 ACH50 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.

CFM50/FT2 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 cubic feet per minute per square foot of surface area of the home, or CFM50/FT2.  A test of 3ACH50 will be about the equivalent of .25CFM50/Ft2.  This information is included with the test report if the surface area of the building is calculated.

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 won’t see.  Some of the leakage that is present at the elevated pressures does not leak at normal conditions.  The software calculates an estimated natural infiltration rate, which can be unreliable.

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 the air leaks.

Cost estimates.  The final report also incudes 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 sometimes maintaining a home.  The best advice I have to builders and homeowners building a new is to set an air tightness goal.  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 affect the home, sometimes the changes have undesired consequences.

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