Ask Focus on Energy

Finding out how much air leakage your home or business has.

To calculate air leakage, most competent energy professionals use “benchmarks” related to blower door test results when a home is pressurized or depressurized to 50 Pascals (pressure difference between the inside and outside of a home). Typically, the volume of air that is drawn through the blower door fan and measured by a pressure gauge is expressed as CFM50 (cubic feet per minute at 50 Pascals). The CFM50 number can be compared to the volume of the home to be used to calculate an air change per hour rate and is expressed as ACH50. Homes that have ACH50 rates of 5 – 7 are considered to be fairly tight and would need mechanical ventilation. Another ratio that is used compares the CFM50 to the square footage of living (floor) space. Another common ratio used by professionals is to compare the CFM50 to the square footage of the entire shell (conditioned envelope) of the home. The shell includes the combined areas of basement slab, foundation walls, rim and band joist areas, above grade walls, knee walls, windows & doors, and ceilings. Yet, another way that is used to express efficiency is to calculate and express the number of BTU’s per square foot of living space per heating degree day (in northern climates).

In reality, there is no real benchmark for existing homes in terms of air leakage rates like there is for new construction, but it is safe to assume that the tighter the home, the better, in terms of energy loss and comfort. Keep in mind that moisture and other pollutants are typically generated daily within the home and the need for operating mechanical ventilation devices increases with building tightness in order to maintain indoor air quality.

To address your question about insulation levels in Wisconsin, check out ENERGY STAR’s table of Recommended Levels of Insulation throughout the United States.
 

Published Wednesday, April 27, 2011