The small holes and cracks in the floors, walls, and ceilings of a house are where all the air leakage occurs. Well, duh, of course. All houses have them—some more than others—and they provide a continual air exchange route that reduces indoor air pollution. 

    The U.S. Department of Energy (EPA) even has a definition for it: "Natural ventilation is the uncontrolled air movement in and out of the cracks and small holes in a home.” So what’s wrong with something that’s “natural” and has been the standard, if unintentional, way of freshening the air in house for hundreds of years?

    Pressure differentials between the indoor and outdoor surfaces of a home are constantly changing and are usually different at each exterior surface of the house. Your home may be sucking air from the outside through one wall and pushing it out on the opposite side of the house, or pulling air from under the floor and sucking it up through the ceiling, or the opposite of both. When you don’t control where the air is coming from or going to, it’s possible that the “natural” ventilation is coming from your garage with carbon monoxide or chemical fumes, an attic that may have rodent droppings in it, or underground with radon gas. Also, the rate of air exhange may be higher than necessary for good quality indoor air and therefore wasting heating/cooling energy.

    There are three causes of the pressure differentials: 

1. The stack effect - Houses suck. In other words, a house acts like a chimney, in cool weather the rising warm air wants to exit at the top and pull air in from the bottom. The stack effect can also be reversed on hot summer days when hot outside air pushes cooler inside air downward.
   
2. Wind - Creates a positive pressure on the windward side of a house and negative pressure on the leeward side. Wind swirling around a building creates complicated pressure highs and lows, and wind sweeping over a low-slope roof (less than about 3/12 pitch) tends to cause negative pressure because the roof acts like a wing and creates the equivalent of lift. Higher slope roofs act more like walls, with positive pressure on the windward side and negative on the leeward.
3. Fans in the home - Usually exhaust air, which creates negative pressure and sucks outdoor air in throught any small openings to replace the volume exiting. Conversely, an air conditioning supply duct serving a room with no return air or jump duct and a closed door will pressurize the room, pushing air out. 

    And, of course, these three forces can act to enhance or negate each other. Here’s a pie chart from the EPA showing how much of the leakage in an average house comes from what part of the building envelope. Notice that what would appear to be the worst offender, doors and windows, only account for about 1/5 of the air leakage.

     If you have an older house, caulking and sealing can reduce the amount of energy loss. You can do it yourself and you can read more about it at our blog How can I improve the energy efficiency of my older home?  

    It is also possible for a home, especially a newer one, to be too well sealed and have an inadequate air exchange. There are several solutions, including exhaust, supply or balanced ventilation systems, or an energy recovery ventilation system. The last option is the most sophisticated, and provides controlled ventilation while minimizing energy loss. It does this by transferring the heating or cooling of the indoor air being exhausted to the incoming outdoor air. So you get air exchange with minimal energy loss. An example of one is shown below and can installed by your HVAC contractor.

      Also, see our blog post How did homes stay cool in Florida before air conditioning?

     Visit our ENERGY EFFICIENCY page for other related blog posts on this subject, or go to the INDEX for a complete listing of all our articles.