The Blower Door

In the past, heat loss and condensation were studied in terms of conduction and diffusion. As time goes on, building scientists have linked building performance more closely to air movement. Today we know that conduction and diffusion are relatively weak processes and seldom cause failures in the thermal envelope. The blower door was a key analytical instrument in the development of this knowledge.

The BD is a fan that can be sealed into a door. The original devices were huge and required great effort to set up and calibrate. Now they can fit in the trunk of a car, carried by one person, and set up in 3 minutes. There is an adjustable frame that will fit snugly into most door jambs. A nylon panel fits over the frame to seal the opening tightly. The fan fits through the panel. It is reversible and has a variable speed controller. Manometers (pressure gages) measure the flow through the fan in cubic feet per minute (CFM) and the pressure across the panel in Pascals (Pa).

The flow of any fluid through an orifice can be measured by determining the pressure needed to drive the fluid, and the area of the hole .

Flow = f(Pressure, Area)

If the area of the hole is expressed in square inches, the equation is

Q = 1.07 X A X (P) !/2

In real buildings, we can use the BD to impose a known flow through the building and then measure the pressure. Or we can induce a known pressure on the building and measure how much flow it takes to produce that pressure. In either case, the remaining variable, hole size, can be calculated from these measurements. Now we can learn some practical things about the pressure boundary of the building without ever seeing or putting a hand on the defects. This is important because the key holes in a thermal envelope are often buried behind finished walls that we don't want to disturb.

Look what happens when we turn the fan up to 5000 CFM and generate just a few Pa of pressure. The leakage area in the shell amounts to 30 or 40 square feet! This really happens in badly built commercial buildings where the roof and attic vents become the pressure boundary.

We can now say with complete certainty that buildings with larger holes in their air barriers tend to use more heating fuel. Weatherization contractors depend on the BD to guide their air sealing efforts to make the holes smaller. Aside from the quantitative analysis, the BD can be used to simply provide some pressure on the thermal envelope. With the infiltration artificially enhanced, we can survey the house, room by room, to look for leaky areas. By cracking open a bedroom door and felling for a draft, we tell right away if there is a large bypass in that room. If the air is still, we will go elsewhere to find better opportunities for air sealing.

Wherever there is a bypass to the outdoors, a draft results that we can feel. If there is a hole or crack that appears to lead outside, but no air is blowing through it, the weatherizer will not waste time and materials on fixing it.

It takes some training and experience to interpret BD results. Like any other exhaust fan, it applies a vacuum evenly to the entire shell. This is generally not how the building behaves naturally. Particularly in ceilings, air will normally flow upward into the attic, but a depressurization test will pull air down and toward the BD. The BD treats all holes in the same regard, even if nature does not. The actual pressure on any particular hole (and the resulting flow of air) with vary greatly depending on it's position in the shell. So even knowing the size of the holes, we must be able to tell which of them are benign and which are the real trouble makers by understanding the adjacent forces that create air pressure.

Typically, most of the air sealing work that a weatherization crew performs will be invisible or barely noticeable. It's important that the crew gets feedback on their efforts and the client can see some real improvement. By taking before, during, and after pressure measurements, the tightness of the air barrier can be documented and the work verified as effective. When the measured CFM50 is cut in half, everyone knows that some real changes in the building behavior will result. Conversely, if a crew spends 4 man-days caulking and the tightness of the shell decreases from 3500 CFM50 to 3350 CFM50, we can tell that much of their effort was wasted.

Before pressure testing, the building has to prepared. We turn off all the combustion applicances or set gas burners to pilot. All the windows and doors are shut just as they would be under Winter heating or Summer cooling conditions. The exhaust systems are shut off including the fireplace dampers. If there are ashes in the fireplace, we cover them with newpaper to keep the dust down. Expecting the unexpected, it's best to start the fan slowly and run through the house looking for trouble before applying 50 Pa of pressure.

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