So, your local professional has come in and performed the blower door test on your home. He or she has calculated all the information and turned it into numbers that reference your home's air leakage. But wait, slow down, what do these numbers mean? It isn't too difficult to understand. Let's start from the beginning.
What is a blower door?
A blower door is simply a diagnostic tool used to measure how much air filters out of your house (your home's "airtightness"). The blower door allows testers to apply a consistent and measurable pressure to the house so that houses can be compared accurately.
You undoubtedly have called this professional because you have perceived issues in your home. This apparatus will allow your energy auditor to quantify some of those issues.
The auditor will set up his blower door in an outer door facing outward, to depressurize your home (in most cases). He will systematically move about your home setting up the house for the test. The test requires a few parameters be set to yield accurate results. Once he is done with his walk through, your auditor will begin the test. Depending on the size of your home, the test takes roughly 2-4 hours.
What do the numbers mean?
Your test is done and now the auditor wishes to inform you of his findings. The first couple numbers you will hear are:
- ACH at 50: This term refers to “Air Changes per Hour” at 50 pascals. In layman's terms, it means, at a test pressure of 50 pascals, the total volume of air in your home will be replaced 5 times in one hour. Relatively speaking, ACH5 @ 50 is a semi-tight home, ACH5 – ACH9 is a moderately leaky home, and anything over that is a very loose and leaky home. Remember, the lower the number the better!
- CFM at 50: This term refers to “Cubic Feet per Minute” of air at 50 pascals. Basically, it's telling you in its most basic form, how much air is moving from your home, through the fan, and to the outside. This number is used to calculate the above mentioned ACH at 50. Typically, tight homes will be under 1250 CFM at 50, moderately leaky homes will be between 1300-3000 CFM at 50, and any number over that would indicate a very leaky home, as seen in most older built structures.
- ACHnat or ENIR (Estimated Natural Infiltration Rate): You may hear this term, but it isn't considered by many to be very accurate. It is simply a measure of the “Natural Air Change Rate.” But, don't get overly bothered by this number.
For reference, one square inch of air leakage is equal to about 10 CFM. So, if your home had, let's say, 2200 CFM at 50, you would have roughly 220 square inches of leakage. Remember, there are 144 square inches in 1 square foot. That's not that bad, considering most homes I've checked.
The bottom line here is: Do not get lost in the numbers. Think of them as a reference point to use as you begin to seal and tighten your house. Choose a professional who specializes in blower door testing and check their credentials. Use the results of the blower door test to get an understanding of where your home is leaking and why. For example, leaking from around your front door might not be as bad as leaks from under your home in the crawl space. It all depends on where and how much air is entering (or leaving) your home.
Armed with your blower door report, and with your auditor's recommendations, take the next step and fix the problem areas in your home. Once you do the energy upgrades, ask your auditor to re-test your home to check that your blower door numbers have improved.
Final note: Don’t forget to discuss mechanical ventilation with your professional when the house has been sealed tight.