Building Science-ACH50/ACHnat

I’ve written several times on this blog about the importance of blower door testing in new construction or during some remodels.  In this blog posting I would like to dive deeper into air changes per hour at 50 Pascals and air changes per hour natural.

My blower door testing equipment.

First, we need to understand how we measure pressure. What is a Pascal?  We need to induce a pressure on a home to test air leakage.  (There is a method using sound waves, but I have yet to see the equipment needed for this type of air leakage testing).  We measure this pressure in Pascals, but what is a Pascal?  One Pascal is equal to one Newton per square meter or one kilogram per square centimeter or 1 Joule per cubic meter.  Not much help there!  How about 50 Pascals is equal to roughly .2 inches of water column.  Take a straw and place it in a glass of water, suck the water up the straw about a quarter inch, that is how much pressure we are using during a blower door test.  Now take a six-inch pipe that is 8 feet long and place it in a swimming pool and try sucking the water up the pipe one quarter inches, I’ll bet you can’t do it.  Now change the pipe so it has the same volume of a house, most houses I test have around 14,000 cubic feet.  You have to be able to suck a lot of air to create .2 inches of water column or 50 Pascals.  It turns out that roughly a 20 mile an hour wind blowing on all sides of a home at the same time will create around 50 Pascals of pressure.  A little clearer, right?

Now that we have a rough idea of the pressures needed to test a home, how is the pressure induced?  The pressure is induced by moving air into or out of the structure.  My single blower door fan is capable of moving 6000 cubic feet of air per minute (CFM).  Most new homes I test move around 500 cubic feet per minute of air across the blower door fan.  A leaky home might move 2,000 CFM or more.  I’ve only tested a couple structures where I was unable to achieve the 50 Pascals of pressure.  Moving more air out or into the home faster than it can be replaced is what creates the pressure, measured in Pascals.  

What is ACH?  ACH stands for air changes per hour, or the total volume of air inside a home being exchanged with air outside a home a number of times per hour.  Let’s look at a code minimum new home built with 3 air changes per hour at 50 Pascals.  While the blower door is operating, we are changing inside air with outside air 3 times per hour.  We are testing the home at a pressure that is not normal.  As it turns out, 3 ACH50 is equal to roughly .2 air changes per hour natural, (without the blower door running). The house built to the code minimum has the potential, on average, to change its entire volume of air with outside air once every five hours.  The .2 ACH is not a reliable number, the home can be at a higher or sometimes lower air change rate.  The amount of air changes naturally occurring in a home is affected by the wind, the temperature difference between inside and outside the home and any mechanical air moving equipment being used inside the home.  On a day with no wind and an outdoor temperature of 65 degrees with an inside temperature of 68 degrees with no bath or kitchen fans in operation, no heating or cooling equipment being use, and no clothes drying in the dryer, the home will most likely leak very little, if any air.  Now take that same home on a windy day with an outside temperature of -10 degrees, inside temperature of 68 while doing laundry and using a bath fan and the air changes per hour will be substantially higher.  Double, triple or possibly more than the .2 ACH.  I wouldn’t be surprised if the air change rate was one every two hours for a code minimum house under those conditions.  

Thermal Imaging of cold air infiltration under poorly sealed bottom plate.

The math needed to calculate the air change rate.  For the calculation, you need to know how much air flow is moving across the blower door fan and the volume of the home.  Let’s say we are testing a home that is 25 x 40 with 10-foot ceilings.  25 x 40 = 1000 x 10 = 10,000 cubic feet.  During the blower door test, the home is moving 500 cubic feet per minute of air across the blower door fan.  The formula used to calculate air changes is cubic feet per minute at 50 Pascals times 60 (this changes the cubic feet per minute to cubic feet per hour) divided by the volume of the home.  In our equation: 500CFM x 60/10,000 cubic feet = 3 air changes per hour.  This home would pass a code required blower door test for a new home.  Again, the air changes per hour naturally would be around .2 air changes.  Want to know how to calculate the natural air leakage rate?  I cover that here.

How many natural air changes per hour would you like your home to have?  Having fresh air in a home is critical to control humidity and keep occupants healthy.  Air changes can also effect heating costs and building durability.  I would rather have control of how many air changes my home has.  Right now, the 1952 Cape I live in has too many natural air exchanges, around 1 per hour.  Heating bills haven’t been outrageous, but the home is uncomfortable.  I will be making an effort to make this home tighter.  So, how much fresh air does a home need?  This is the topic for next week.

4 Replies to “Building Science-ACH50/ACHnat”

  1. Your explanation implies that 1 newton is equal to 1 kilogram. I am old but remember some physics. Do you mean the force of one kilogram at (earth) sea level?

    1. Hi Dave, I wasn’t trying to say 1 newton is equal to 1 kilogram, sorry if it came out that way. What was meant is 1 Pascal is equal to 1 Newton per square meter, or one Pascal is equal to 1 kilogram per square centimeter, or 1 pascal is equal to 1 Joule per cubic meter. My manometers do not measure any of those other units nor do I ever work with them. I can measure in Pascals or inches of water column, which is only a mathematical conversion happening in the manometer. Thanks for the comment!

  2. The statement “On a day with no wind and an outdoor temperature of 65 degrees with an inside temperature of 68 degrees with no bath or kitchen fans in operation, no heating or cooling equipment being use, and no clothes drying in the dryer, the home is at roughly .2 air changes per hour. ”

    Assuming the house is completely air tight, like a balloon, how would the air change just based on temperature difference?

    1. Hi Dave,
      That statement you are referring to is a little misleading, I’ll re-write it in the post.

      If the home tested at 3 air changes per hour with the blower door operating, the house would have a potential natural air leakage rate of roughly .2 air changes per hour. The rate can go much higher based on wind, stack effect or mechanical ventilation rates which put higher pressures than normal on the home. But, if the conditions are like as they are written, there will be less natural air leakage. The conditions are constantly changing.

      To answer your question, if the house is completely air tight, there would be no air exchange potential, but there is always some leakage, even in very tight houses. The mechanism that I’m referring to is Stack Effect. Air that is heated becomes buoyant and rises to the top of the building enclosure putting pressure on the ceiling and upper walls. Any holes present in the ceiling will allow the air to leak out. If air leaks out, an equal amount of air will leak in to balance the pressure, usually at the bottom of the home. This air is heated and the cycle continues. Stack effect is based on the temperature difference between inside and out and the height of the building. The taller the building, the more pressure at the top. Interestingly, this is the reason revolving doors were invented, stack effect on very tall buildings prevented doors that opened outward on the lower level of the structure from easily opening. Vestibules and revolving doors help lessen those issues.

      Good question, thanks,

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