More Building Science
If you’ve been studying building science concepts, the four control layers are not new. Thinking about each layer —water, air, vapor, and thermal—individually, and how they interact together helps in the planning and execution of building a better home. This concept of separating the layers of a building’s assemblies can also be applied to a different topic, namely the air inside a home.
Most of us probably take air for granted—we breathe it, it’s always around us, but inside a home airflow needs to be managed. Recently, I caught a presentation by Pat Huelman, a building science educator and researcher from the University of Minnesota. Part of his discussion focused on the need for projects to include an “air manager,” meaning someone who pays attention to how air interacts with a building and its mechanical equipment.
Huelman identified four different types of air in our homes—combustion, makeup, ventilation, and circulation—and how each contributes to the quality of the indoor environment, energy conservation, and a building’s durability. Here, I will discuss the four types of air individually, starting with combustion air and makeup air, which I consider the most important because they can jeopardize occupants’ health if not addressed correctly.
Air composition
Air is 78.1% nitrogen (N₂), 20.9% oxygen (O₂), 0.93% argon (Ar), 0.04% carbon dioxide (CO₂), and a varying ratio of water vapor, along with several other gases of lesser percentages. Even though the percentages of the different gases in air are usually the same for each type of air on Huelman’s list, it’s important to think about how air functions in each case.
Combustion air
Combustion air required to support the combustion of fuel used to operate a home, which might include space and water heating, cooking, and even clothes drying. For combustion…
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6 Comments
Great article, Randy
Do we still think of adequate fresh air ventilation as 15 cfm per bedroom plus 15 cfm continuous. This always sounded good to me, so the average 3 bedroom home would be 60 cfm of continuous ventilation. I too will go along with whatever Pat Huelman is saying, he is a serious expert.
Doug
Thanks Doug!
Minnesota's ventilation calculation is (.02 x square foot of conditioned floor area) + (15 cfm x (the number of bedrooms + 1)). ASHRAE 62.2 is similar, but there are several versions. The main difference between ASHRAE and Minnesota's ventilation rates, ASHRAE will allow you to use the estimated natural air leakage rate (determined by a blower door test) of the home to reduce the overall ventilation rate requirement. Minnesota does not allow this adjustment.
Pat is also one of my favorites.
I forgot about the .02 per sf of floor area. So 2,000 sf with 3 bedrooms needs 100 cfm. At 7,500 hdd we use about 5,700 kWh for ventilation with no heat recovery. With electric resistance heat, 5,700 kWh @ .12 is $684.00 per year.
Thank you, Randy, for this clear explanation. I'll be using this framework to refine our air quality discussion with our clients.
Thank you, Monica! I enjoyed your energy audit article in FHB, and recently on GBA. Hope we get to more content from you in the future.
Excellent Randy. This is a really helpful way to look at it, and your fleshing out of each is very helpful.
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