If you are a builder or you have built a home in the last couple years there is a chance you know what a blower door test is. If you plan to build a home in the future you will definitely know what one is. Here in Michigan, we are scheduled to have a new energy code adopted by mid 2016. Blower door testing will be required in this code change. Fortunately, many builders are already doing these on their homes right now.
What is a Blower Door Test?
Basically it’s a fan that measures how air tight a home is. That is a very basic description. Here is a more in depth look at the test. A fan gets inserted into an exterior door. That fan is hooked up to a manometer (pressure gauge). We close the house up tight. Turn off the HVAC system. Take a baseline pressure differential reading between the inside and outside of the home. Turn the fan on to depressurize the home to -50 Pascal’s (you can also pressurize the home). Mark down a CFM50 and ACH50 reading (CFM50 is cubic feet per minute at 50 Pascal’s and ACH50 is air changes per hour at 50 Pascal’s). 50 Pascal’s is the equivalent of a 20 MPH wind on a home. The test results are good for comparing one home to another with regard to air tightness.
What are its limitations?
The blower door test does have limitations. When I first got into the business I thought this test will tell us everything we need to know about how airtight the home is, potential indoor air quality issues, how much ventilation a home needs and many other assumptions. The truth of the matter is a blower door test is a great tool, but we do have to understand what it means and what it doesn’t mean.
It is not a good indicator of how much mechanical ventilation a home needs. It’s not a good indicator of how many air changes a home is getting each hour or day. It can give us an idea that a home may need mechanical ventilation, but not how much. Why is this? Well, every home has a natural ventilation rate, but that depends on a lot of factors. For example, wind direction and speed are not the same day to day. Occupant behavior changes day to day and house to house. Air leakage only occurs when there is a path and a pressure difference on each side of that path. Knowing what that pressure differential is becomes difficult and that difference can change with changing conditions. There are just too many variables at any given time.
Is a blower door test a waste of time?
No, it is not a waste of time. The test is great for comparing one home to another. Also building codes and building practices are requiring builders to tighten up the home and without this test it would be hard to determine how well those practices are working. There is a significant difference when testing a home that was insulated with spray foam verses a home that is insulated with fiberglass batts. There is also a significant difference between houses where the builder was very careful air sealing compared to a builder who didn’t. When we run a blower door we are creating a simulated condition in which home is under. That condition is a constant variable when we go from house to house, which makes a great baseline for comparison.
What about the need for ventilation?
A Blower door test can tell you that a home should have mechanical ventilation but not how much. Most new homes, at least here in Michigan, are testing out at below 3.50 ACH50 (air changes per hour with the blower door running). Homes that are air sealed well and/or have a good percentage insulated with spray foam are getting under 2.0 ACH50. That is tight! There is a good chance that the home will not be getting enough air changes naturally to keep a healthy indoor air quality. The best bet for the builder is to have mechanical ventilation in the home that can be adjusted to fit how the home ends up performing. Many builders are already doing this.
How much ventilation does a home need?
This has been an industry debate for a while, but currently there are two national standards. ASHRAE 62.2 and BSC-01 (Building Science Corporation). ASHRAE’s standard has been around for a while and BSC is more recent. Building Science Corporation has argued that ASHRAE’s rate is too high, not based on science and does not take in consideration what type of mechanical ventilation system is in place. Either way the best thing a builder can do is provide mechanical ventilation and have that system be able to provide a ventilation rate that meets one or both of the national standards.
Blower door tests are here and seem to be here to stay. They are extremely useful as long as you know the limitations. Builders that are not using them should start now and be prepared for the new code. The current energy code (2009 Uniform Michigan Energy Code) requires a home to have 7 ACH50 or less and allows the building inspector to do a visual inspection or have a blower door test performed. The new code (2012 Uniform Michigan Energy Code) that is coming next year will bring that number down to 4 ACH50 and requires a blower door test to be done. No more visual inspection. Most builders will make this number no problem. However, there are some builders out there that will have some problems.