Air Changes Per Hour: What it Means and How Many Do You Need

By: Eric Garcia on March 11th, 2020

You may have heard the term “air exchanges per hour” or of a house being “too tight.”

All of these center around air changes per hour, or ACH.

This is an important thing to know because it lets you know if your house is losing too much air that you're conditioning. It also lets you know if you're not replacing your air fast enough and it's becoming stagnant.

This is particularly important to whoever is installing or replacing any HVAC equipment or building envelope materials in your home. It’s also important because it can lead to several issues you might not be aware of.

As the Professor of Foam at Foam University on our YouTube channel, it’s my job to help educate homeowners on all things home insulation. So sit back, relax, and let’s dive right into how to calculate air changes per hour and exactly what it means.

What are Air Changes Per Hour?

Air changes per hour is the number of times, per hour, that enough air enters or exits your house to fill the volume of it.

When this happens, it does not replace all the air in your house, some will stay and linger, and some will be replaced.

It may take several cycles for all the air to be completely replaced.

Why is Air Changes Per Hour Important in Your Home?

Building code will call for your house to not have more than a certain number of ACH, depending on what climate zone you live in, according to the U.S. Department of Energy.

That number of ACH is between three to five depending on the climate zone, so a state like Michigan can have no more than four.

For an existing home, this will let you know if your house is losing too much of the inside air that you are paying to heat or cool.

Knowing your ACH will also let you know if you need to address any ventilation issues that may be causing moisture issues in the home. These moisture issues can lead to the formation of mold or mildew in the home, which can be a huge headache.

If your home is getting too few ACH, you may need to introduce mechanical ventilation. This mechanical ventilation system will circulate fresh air using ducts and fans, rather than rely on airflow through small holes or crack in the home, roof, or windows, according to Energy Star.

How to Calculate Air Changes Per Hour (Formula)

Now it’s time to talk about math.

Here is the air changes per hour formula:

ACH= CFM x 60 (minutes)
--------------------------------
Volume of House (ft³)

First, you run a blower door test to get the cubic feet per minute (CFM). Then you multiply that by 60, which gives you the cubic feet per hour.

Next, you need the volume of the house (length x width x height).

Finally, you divide the cubic feet per hour into the volume of the house. Basically, all this math will tell you how many times, per hour, your house’s capacity could be filled and emptied with air.

Knowing Your Home’s Air Changes Per Hour

Knowing your home’s ACH can give you a good insight on knowing if your building envelope needs to be addressed.

This will tell you if you're getting adequate air changes in and out of the home to keep a constant supply of fresh air inside. Having stale or stagnant air inside your house can lead to a laundry list of problems, including mold and mildew.

The remedy to the house that is too tight or doesn’t get enough ACH can be as simple as turning on a bathroom fan or kitchen hood once daily. This will help bring in the fresh air, so your home doesn’t become stagnant, and help to keep it a healthy environment.

If you want to learn more about building science and all things home insulation, head over to our Learning Center, where you’ll find a ton of helpful resources. We also have a bunch of awesome videos on our YouTube channel, mostly starring me, that will answer all of your questions.

How to Perform a Blower Door Test

Airtight House: Can Spray Foam Make My House Too Tight?

What is a Blower Door Test? (How it Works/Measurements/Cost)

About Eric Garcia

Eric brings his knowledge and training in building science, training in spray and injection foams from the manufacturers, more than 8 years installing foam insulation, as well as selling and managing in the foam insulation industry. He is also BPI and Dale Carnegie certified and has taken several building science courses including air sealing and building envelope. Eric’s responsibilities include overseeing and giving support to all of the branches of the RetroFoam of Michigan company, office, estimates, and installs. He is also the Professor of Foam on our educational YouTube series Foam University. Even when Eric is off he is usually still “working” or thinking about work, but when he can get away he enjoys camping, hiking, hunting, and woodwork.