Ventilation and Air Leakage

Denver heating and air companies define air leakage is the passage of air in and out of various cracks or openings in buildings. It is also sometimes referred to as infiltration. Air leaking into a building may be caused by wind pressure or by differences in temperature inside and outside of the building. In the former case the wind builds up a pressure on one or two sides of a building, causing air to leak into the building. The action of the wind on the opposite side or sides produces a vacuum that draws air out of the building.

When the wind hits the front side of the building, its momentum builds up a pressure higher than inside the building, which causes the air to leak through any cracks present. AS the wind traverses the length of the building the air currents as the continue past the other side converge and produce a vacuum along this side by induction. Because the pressure on the outside of that side is lower than the inside of the building, air leaks out. Air leakage due to temperature difference or thermal effect is usually refereed to as stack or chimney effect. Air leakage due to cold air outside and warm air inside takes place when the building contains cracks or openings at different levels. This results in the cold and heavy air entering at low level and pushing the warm and light air out at high levels, the same as draft taking place in a chimney. This should be considered when designing for optimal HVAC and furnace efficiency.

When the temperature inside a two story building is higher than outside, the heavy cold air from outside will enter the building through window on the first floor of the house and push the warm, light air through upper level windows of the house. As it cools it will increase in weight and circulate downward. Although not appreciable in low buildings, air leakage is actually beneficial to health. Any attempt to seal a building to an air tight level will cause the indoor air to become stale and putrid. Emphasis should be placed on the reduction of heat transformation rather than the absolute elimination of air leakage so that you can obtain optimal furnace performance.

The application of storm sash to poorly filtered windows will generally result in a reduction of air leakage of up to 50%. An equal effect can be obtained by properly installed weather stripping.