pressure difference, so the next step in making the results easy to understand is trying to figure out how much the house will leak at more normal pressures. There are two ways to get there – the easy, inaccurate method and the harder, better method.
The easy way is to divide the ACH50 by a number that’s close to 20 and call that the ACH natural, written ACHnat. It’s a bit more complex than that, but the upshot is that it’s a very subjective, inaccurate way of trying to figure how much leakage the house has when you turn off the Blower Door. A lot of people and some programs use it, unfortunately, so it’s worth knowing. The range of ACHnat results is from less than 0.1 for a super-tight house to 2 or higher for a sieve of a house.
The harder, better method is to do what’s called a multi-point Blower Door test. Here, the analyst measures the cfm of air moving through the fan at a range of pressures, typically 15 to 60 Pascals, instead of the single pressure of 50 Pascals. Plotting the data allows the analyst to extrapolate and find the actual leakage for any pressure difference between the house and outdoors.