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How to Look at a House like a Building Scientist (Part 1: Air) Kohta Ueno | Published on 8/15/2023
PA-1901: How to Look at a House like a Building Scientist (Part 1: Air)
Kohta Ueno
Effective Date
September 18, 2019
Using diagnostic tools to find air leaks that make homes inefficient, uncomfortable, and are often the cause of moisture-related problems.


The work I do for Building Science Corporation (Joe Lstiburek’s company, for those who don’t know) involves forensic investigations of moisture-related (or similar) building failures—i.e., sleuthing out problem buildings.  Despite the fancy name, this typically involves crawling around the bowels of commercial and residential buildings to look at the problem areas and figure them out. They range from sleuthing out strange odors that seem to emanate out of nowhere, windows that leak water during rainstorms, indoor swimming pools with rotting walls, freezer warehouse buildings with icicles growing out of the ceiling, mega-mansions with out-of-control humidity levels that are damaging the art collection, and moldy and wet crawl spaces.

To solve these problems, a set of eyes and an understanding of building physics are the most important tools.  But there are many clues that are not visible—thus the use of building diagnostic tools: the focus of this column. This series will cover a selection of my go-to tools for this type of building science diagnostic work.  I’m not pretending this is a comprehensive survey of the available tools—but they are my go-to items in day-to-day investigations.  Also, I am deeply indebted to all of the other practitioners who have shared their knowledge, instruments, and tips/tricks—I can only hope I’m doing a little bit to pay it forward here.

The structure for these columns will be broken down roughly into the following topics:

Part 1: Air. Devices that measure air leakage (such as Blower Doors™ and Duct Blasters®), differential pressures, and airflow.
Part 2: Heat. Infrared cameras, temperature meters, thermal bridges… and when combined with airflow tools, finding air leaks
Part 3: Water. Moisture meters, water testing windows, demonstrating drip edges and slope with squirt bottles
Why Air Measurements are Important

Controlling the air inside the building—by limiting air leakage—is critical for conditioning the air, and therefore affects comfort and energy use in buildings.  But in addition to carrying heat, air leakage often carries moisture with it: this can lead to a variety of durability problems.  Examples of air leakage problems include growing mold on roof or wall sheathing due to outward air leakage in cold climates, or condensation on cold ducts or pipes due to inward air leakage in hot-humid climates.  In addition, I’m seeing more and more indoor humidity problems up and down the East Coast, and air leaks are a part of them. When I hear folks complaining about “new buildings being too airtight,” I let them know that I’ve investigated far more problem buildings that were too air leaky rather than too tight.

Of course, air is invisible—although you can feel it moving from one place to another, instrumentation can tell you a lot more.

Air Leakage Testing

A blower door (Minneapolis Blower Door™) is a calibrated fan used to measure air leakage in buildings; it is typically installed in a fabric shroud mounted in a doorway (Figure 1).  The fan is used to depressurize the building to a known test pressure (typically 50 Pascals), and the airflow (cubic feet per minute or CFM) required to reach that pressure is a measurement of the building’s total air leakage (reported as “CFM at 50 Pascals” or “CFM 50”).  Current codes require airtightness testing; this equipment is used for both houses and large commercial buildings (although more fans are needed).

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