All buildings are subject to something called the "Stack effect". When a building has openings, cracks, or ventilation at the top of the building, and similar ventilation at the bottom, there can be air movement either up or down, through the building. When the outside air is colder (denser) than the inside air, the flow goes out at the top, and in at the bottom. The reverse is true when the outside air is warmer than the inside air.
Outside air drawn in through basement leaks is exacerbated by the stack effect created by leaks in the attic. As hot air generated by the furnace rises up through the house and into the attic through leaks, cold outside air gets drawn in through basement leaks to replace the displaced air. This makes a home feel drafty and contributes to higher energy bills. After sealing attic air leaks, complete the job by sealing basement leaks, to stop the house stack effect.
One common area for air to come in, low on the house, is in the basement where your first floor meets the basement foundation walls. When you are in the basement looking up, you can see floor joists running across the basement ceiling. Capping those off around the perimeter of your house are rim joists. The sill plate is what those two pieces sit on.
Though you may not be able to see cracks in the rim joist cavities, it is best to seal up the top and bottom of the inside of the cavity. Also, rim joist air sealing is especially important at bump out areas such as bay windows and cantilevers that hang off the foundation. These areas provide greater opportunities for air leakage and heating and cooling loss.
BASEMENT AND CANTILEVER SEALING
In the Summer months the dynamics change allowing warm air to push the home's cooling out those same windows and rim joists areas
Winter months allow cold air to enter the home from lower levels in areas like unsealed rim joists and leaky old windows.
The lower level of your home including the basement, rim joists and cantilevers must be sealed to stop the stack effect.
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