Wind Can Push Drafts Through a House

Wind pressure is highest at corners. On a gusty winter day, the pattern of wind pressure is clear: the wind is stronger at corners and blows snow off the hips and ridges of this roof.

Air Barriers Push Back

Many houses built to code and a lot of green commercial buildings have significant rates of air leakage. In cold climates as well as hot, humid climates, air leakage can be responsible for a third or more of the energy transfer across the building enclosure, says Straube. “There are also sound transmission issues. If you’ve got a hole even the size of my finger in a wall—a partition wall or an exterior wall—you’ll reduce your sound transmission class rating by five to ten points,” he says.

So how exactly does air move through a house? Three ways: wind, fans, and stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season.. Wind is somewhat predictable — at least average speed and direction. Fans include kitchen and bath exhaust fans, HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment fans, and clothes dryers. Stack effectAlso referred to as the chimney effect, this is one of three primary forces that drives air leakage in buildings. When warm air is in a column (such as a building), its buoyancy pulls colder air in low in buildings as the buoyant air exerts pressure to escape out the top. The pressure of stack effect is proportional to the height of the column of air and the temperature difference between the air in the column and ambient air. Stack effect is much stronger in cold climates during the heating season than in hot climates during the cooling season. is an air pressure phenomenon; in winter, indoor air pressure increases with height.

The wind is predictable (to a point)
Peak wind loads listed in the model building codes are fairly high. On average, however, local wind pressure is quite a bit lower. “In a low-rise building, 10 Pascals would be considered pretty high, but 5 Pascals is more likely,” says Straube. “And in a high-rise building, maybe 40, 50, or 60 Pascals.” A pressure of 240 Pascals is equivalent to 5 lbs. per square foot. The pressure exerted by a blower door — 50 Pascals — equals about 1 lb. per square foot (about equal to the pressure of a 16-mph wind).

A low-slope-roof (under 3-in-12 pitch) is usually under negative pressure. Air is sucked up through the roof because the aerodynamics of the wind passing over the roof’s leading edge causes negative pressure. On a house with a sloped roof, the pressure is positive on the windward side — provided the slope is over 3-in-12 or 4-in-12 — and negative on the leeward side.

But wind is highly complicated. When wind tries to flow around buildings, the highest pressure is on the middle “sweet spot.” As the wind goes around the corners, it creates large swirls and negative pressures. According to Straube, wind-related structural damage often occurs at these spots — the high-pressure "sweet spot" or the areas of low-pressure swirling.

Wind exerts positive pressure on the windward walls of a building, causing air leaks on the side of the building facing the wind. On the leeward side, negative pressure causes suction that pulls air through walls and windows.

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