Hot Up Top – Cool Down Below
Everyone knows that hot air rises and that cool air sinks. Convection: It’s a basic law of thermodynamics aptly demonstrated by a hot air balloon that ascends as it’s filled with heated air. When the air inside the balloon cools down and becomes denser, the balloon begins its journey back to earth.
Exfiltration + Infiltration = the Stack Effect
The connection between convection and home energy performance is known as the Stack Effect. In a house, this air movement phenomenon is most noticeable during cold winter months. Here’s how it works: Indoor air that’s been warmed by the heating system rises by convection to the upper areas of the house. Unfortunately, this warm air doesn’t stop moving when it gets to the top floor’s ceiling. In a typical house, hundreds of gaps and cracks provide passageways for warm air to escape into the attic. Holes for recessed ceiling lights, gaps around a drop-down attic stairway and wall cavities used as electrical or plumbing chases are common leak locations. Interior air that leaks out of the house is called “exfiltration.”
Losing the air you’ve paid to heat is bad enough, but this escaping air creates negative pressure inside the house –a vacuum that causes air infiltration. Cold outside air is sucked into your living space through other cracks, gaps, and openings. In the winter, cold air infiltration usually occurs in lower parts of the house –like the basement and/or crawl space. But air infiltration can also occur around window and door framing and around electrical outlets located in exterior walls.