Wednesday, September 02, 2009

Discovering Where our Energy Dollars Go

We've taken the summer off of blogging, but things are starting to pick up again here at Humphrey House. Sorry for the delay, but life happens sometimes. Anyway, I recently borrowed my friend Jim's blower door as a sort of DIY energy audit, looking at the overall "tightness" of our home.

Blower Door Test www.humphrey-house.comFor those unfamiliar, a blower door is basically a high-powered fan that sits in a tarped-ff doorway. You close all the windows in the building and turn this sucker on High. This will suck the air out of the house, and then start pulling air in from outside the home through any cracks and crevices. The fan is attached to an air pressure gauge monitoring both indoor and outdoor pressure, to ensure the home is depressurized to around 50 Pascals (Pa). At this level, noticeable drafts occur. [More info]

Once I had this setup and running in our kitchen doorway, I went around the home looking for obvious air drafts. Our house is now littered with pieces of blue masking tap indicating all the leaks and drafty spots in the house that should be sealed up before winter comes along. Most interesting finding: the older windows did not leak around the window sashes so much as they leaked around the frame. For example, more air moved from a gap underneath the window stool (interior window sill) between that and the piece of trim below the window (the window apron).

The really cool techie thing about the blower door test is the fact that with the gauge, you can tell just how leaky a building is. Despite consciously remodeling with energy efficiency in mind, I knew that I'd be in for some surprises. After all, we're living in a home built 100-years ago, when there was likely no insulation, and air barriers were really more like "hair barriers".

So, the air pressure gauge measures the volume of air moved out of the home. As you may recall from high school, volume is usually measured in cubic terms. In this case, we're looking at cubic feet of air, but we want to know that over time. So the gauge tells at a certain air pressure (50 pascals) how much air is moved in a minute (CFM)? Well, in our 1.5-story home, it's around 4400 CFM.

What does this number mean in real terms of how leaky the house is? Well there's a rough back-of-the-envelope calculation that can make this more meaningful. (Math Warning!)

We need the volume of air in the house. Given the average 8-foot ceiling height and 3500 sq ft of living area (2135 above ground + 1365 finished basement), we have about 28,000 cubic feet of air in the home. (Humphrey House air volume = 3500sf x 8ft = 28,000 cubic feet).

We can find out how many times the air in the house turns over per hour by taking the air pressure gauge measurement, 4400 CFM, multiply by 60 minutes to convert to hours, and then divide by the air volume in the home: (4400 CFM x 60 M) / 28,000 cubic feet = 9.4 air changes each hour (ACH).

You might be tempted to say, "That's one leaky house!" but remember, this is at a high pressure of 50 pascals. We can "normalize" that number by using a factor for a well-shielded home in an urban area to find the natural air changes (ACHn). For simplification purposes, we'll use a factor of 16.7 (this is derived from LBL empirical data). So we divide our 9.4 ACH by 16.7 to arrive at a normalized air exchange of 0.56 ACHn.

This means roughly 1/2 of the air in the house is changed over every hour. For comparison, the 1989 standard for air exchanges recommends 0.35 ACHn. So, in other words, despite soy foam in part of the house and other energy efficiency efforts, we're still pretty leaky.

We have a lot to do to air seal the home, but thankfully the blower door helped us find all the little holes where our energy dollars were flying out the house. Next step is to work on fixing them. If you're interested in getting a detailed analysis like this of your home, follow the directions on, or find a professional energy auditor at or If you're in Chicago, look at any of these.