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What is ACH and Why Is It Important?

by | May 7, 2018

Leaky Homes

Last December, I stayed up late one night, working while a storm dropped a couple of feet of snow. My office is downstairs, with a large panel window along one wall, and I could feel the cold the whole time–some seeping in through the window and the rest coming from all the other, unknown gaps in my house. After just a few minutes I had to get a blanket and some gloves–with the fingertips cut off of them so that I could type.

My house was built in 1955, and is far from airtight. The same problem rears its head in the summertime, when I have to blast the air conditioning because the house leaks so much air.

Air Changes per Hour, or ACH, is a term used in the building industry to talk about air tightness. ACH is a number that helps explain how much air leaks out of your house.

In other words, ACH tackles the question, “how long does it take for all of the air to leak out of my house.”

My house is so leaky that, in the middle of winter, it is so cold that it feels like my home is buried in snow.

Photo by Medena Rosa on Unsplash.

What is an ACH of 1?

Let’s start with the number 1. If you have 1 air change per hour, that means that ALL of the air in your house leaks out of it ONCE every hour. All of the air leaves and is replaced with all fresh air from outside (or, if you live in Salt Lake City in January, it’s replaced with all smoggy, pollutant-riden air). It’s like a full line change in hockey, if that’s your thing.

Okay, this isn’t a full line change, but it is hilarious.

Now let’s see what an ACH of 2 means: If you have 2 ACH, then ALL of the air in your house leaks out of it TWICE every hour. That means that every 30 minutes you lose all of your carefully conditioned air.

How Do You Calculate ACH?

Measuring a home’s ACH involves a “blower door test.” If you’re not familiar with what that is, a blower door is a contraption that can be installed into an existing door frame in a few seconds, and fitted with a super-powered fan, or blower. Here’s a picture of a blower door in action:
Once all of the windows and doors have been shut, and any intentional openings have been taped off (things like dryer vents), you crank up the fan and blow air out of the house.

Blowing the air out puts negative pressure on the house, and the tech measures how much force it takes to get the house up to 50 pascals of pressure, which is the equivalent of a 20 mph wind. If it takes very little force to get to pressure, then your home is really air-tight. If it takes a lot of force, then it means your house leaks lots of air.

Once they know the force, the tech can plug that number into a formula and calculate the air changes.

Here’s a quick table to help you see what different ACH scores mean:

Now, remember that these air change rates are based on when the house is being pressurized by the blower door, so the actual air change rate of your house is different.

Actual air change rates are actually relative, because they depend on outside temperature, pressure, and wind speed. That’s why the blower door test is used, in order to standardize the air leakage measurement.

What is a Decent ACH Score?

Comfortable homes are air tight, so lower ACH scores are considered better.

Most homes built in 2018 have an ACH score around 3-6 ACH. In Utah, where we operate, code only requires a score of 6 ACH or better, but that is changing next year and will be reduced to 3 ACH.

The very best homes have an ACH score below 1 ACH, and ultra-efficient homes are in the range of 0.2-0.6 ACH.

If you are looking into buying a home, and want one that is air-tight, then ask your builder about the ACH score and what their homes typically score. If they make homes that have a 2 ACH or lower then they are building pretty tight homes. If they are over 2 ACH then they probably aren’t taking air tightness seriously, and do as little as possible to mitigate air loss.

Traditional construction methods use materials and processes that leave a lot of gaps in the building. Many of those gaps are small, but when there are small gaps all over the house, they add up to significant air leakage.

How Does AeroBarrier Help ACH?

As builders try to make a house air-tight, they traditionally have to spend a lot of time and money on sealing as many gaps as possible. They use materials like caulking and spray foam to fill gaps, and the cost adds up.

Even then, there are smaller gaps that they miss, simply because they can’t find them, so ultra-efficient home builders often spend weeks on air tightness, over-compensating so that they can cover everything.

AeroBarrier is a groudbreaking solution to air tightness because is cuts out all of the extra caulking and spray foaming, and automates the sealant process. We do this by using a non-toxic aerosol sealant that self-seals all of the air leaks in the entire building, down to the microscopic leaks–AeroBarrier can find leaks that are as thin as a human hair.

What’s more, we can dial-in your desired air-tightness and guarantee results. If you want a 0.5 ACH score then we can deliver it. What usually takes builders a week or more to do can be done in an afternoon.