I have talked many times about blower door testing, air sealing and air leaks in this blog, on the Green Building Advisor’s website, and more recently, in the pages of Fine Homebuilding Magazine. Most of what I’ve written deals with testing a home or techniques used in air sealing a home. This time I’m going to discuss the mechanisms that cause air to leak, there are only three of them, but first a little science.
For air to leak into or out of a home there needs to be two holes and a pressure differential. The building will always want to be at an equal pressure with relation to the outside, if air leaves the building for some reason, an equal amount of air will enter, this is the second law of thermodynamics. Temperature, moisture and pressure all want to be in equilibrium.
The first two ways a building will leak air are caused by mother nature, wind and temperature. We will start with wind. Wind produces a positive pressure in the home on the windward side, or area of the home that the wind is pushing against. Any openings or holes in the air control layer, the wind will blow through. Common sense, right? Wind rarely blows directly on a flat surface of a building, often it will be blowing on a corner or at some angle to the home. It’s constantly changing direction and speed. If you have another building or barrier close to your home, the wind may increase in velocity as it’s directed through the narrowing between the two structures. Roofs also affect how the wind moves. Once over the roof or around the building, the wind may swirl. All this effects the pressures on the leeward side of the home. The pressures on the side opposite of where the wind is blowing will have a negative pressure. Air will leave the building on the leeward side.
I hate to say it, but this is my home during a windy day with outside temperatures around 30°F. I have a Cape Cod style home with attic knee walls on the second level. The ceiling on the main level is partially connected to both the inside and outside through those knee walls. Something I will be fixing when I get to the upper-level renovation.
I’ve set up a little experiment in my home, a wing nut test if you will. I have a two-channel manometer stationed in my basement. I ran a tube (the two blue tubes) to the upper level of the home within a few inches of the ceiling, approximately 16 feet from the location of the manometer. I also have installed a reference tube that goes outside. (The red tube.) The left side or channel A of the manometer is measuring the upper pressure with reference to the outside. The right side or channel B is measuring the pressure of the upper level with reference to the basement, which is where the manometer is located. (This is showing a little stack effect which we will discuss in a bit.) I took this video on a windy day. You can see where the wind is having an effect on building pressures in the upper level of the home with reference to the outside but the basement area remains fairly constant with reference to the upper level, the pressures caused by the wind are fluctuating equally in the upper level and basement. If there was a fluctuating difference in pressure between the upper level and basement during the windy days, the reading on the right would vary like the reading on the left.
The second way mother nature affects how buildings leak air is through temperature and building height. By now you know I live in a very cold climate which requires everyone in my climate to use heat during the winter months. The temperature difference inside and outside a home are not equal. This causes a phenomenon called the stack effect. As air is heated inside, it becomes more buoyant and rises to the top of the building, hot air rises. If there are any holes in the ceiling or upper walls, the air will leak out of the building. Remember, any air that leaks out, an equal amount will leak in. This happens in the lower area of the home, in a basement, crawl space or near the floor of the lowest level. This incoming air will be very close to the same temperature as the outside air. If it’s -20°F outside, that’s close to the temperature of the incoming air which then needs to be warmed and the cycle continues. The larger the delta tee, which is the difference between inside and outside temperatures, and the taller the building, the bigger the stack effect and more pressure at the top of the structure. This process reverses during cooling. To overcome stack effect, we want to do a really good job air sealing both the upper and lower areas of the home. My home in the video was showing a positive pressure on the upper level, 1.4 Pascals. There was a delta tee, difference between inside and outside temperatures of around 40°F. I am expecting the pressure on the upper level to rise as the delta tee becomes greater.
Somewhere near the middle of the home is the neutral pressure plane, this area is at an equal pressure with relation to the outside. Air will not naturally leak out or in a home at this location due to the stack effect, but if the wind is blowing, air can find a way in or out, we still want to do a good job air sealing the middle of the structure. All this is a simplistic description of the stack effect, the location of the neutral pressure plane might be higher or lower in the home than directly in the middle based on if the upper or lower level is better air sealed than the other, mechanical ventilation will also change the location of the neutral pressure plane. That gets us to the third way a home can leak air, by mechanical ventilation.
Think of a kitchen exhaust fan, bath fan or dryer. These devices are designed to move air from a inside the envelope to the outside. Again, pressures inside the home want to be equal with relation to the outside and removing any air from inside the home by way of an exhausting appliance or fan will result in an equal amount of air leaking in. The locations where air leaks back into a home to equalize the pressure caused by exhausting ventilation may be a designed make-up air system or through unintentional holes anywhere in the building’s envelope.
The photo above is of a bathroom exhaust fan, which is poorly installed, any guesses as to why it is poorly installed?
In some cases, there may be a supply ventilation system forcing air into a home which causes a positive pressure. To equalize the pressure with the outside, air will leak out any hole it can, anywhere in the home. Supply only ventilation is not common in a heating climate for obvious reasons, -20°F air entering our homes during the heating season would not be good. The supply ventilation systems installed in my climate that I am aware of typically have some sort of heater tied in-line with the supply ductwork. Usually, these systems are installed as powered make-up air for very large kitchen exhaust hoods or woodburning fireplaces.
Three ways a home will leak air, by wind, stack effect and mechanical ventilation. Depending on the temperature differential, wind speed or direction and the use of exhausting fans will all have an influence on just how much air is leaking through a structure at a given time. Air sealing is the best way to control infiltration and exfiltration of air through our homes.