Wind Pressure on a House and What It Moves

Why this matters

The faults wind causes are the ones that are never there when you arrive. The fireplace smokes "only when it's blowing," the back bedroom is freezing "on some nights," the attic sheathing is stained on one slope and dry on the other. A tech who shows up on a calm afternoon finds nothing, closes the ticket, and gets the callback the next windy night. Wind is the one pressure driver that changes direction and strength by the minute, so the diagnosis is mostly about asking the right questions and matching the complaint to the weather.

What wind does to the skin of a house

Wind blowing against a house slows down as it hits the windward wall, and the slowed air builds pressure on that face. It speeds up as it goes over the roof and around the corners, and the fast-moving air drops in pressure. Behind the house, in the wake, the pressure is low as well. So with wind blowing:

• The windward wall runs positive. Outdoor air is pushed in through every leak on that side.
• The side walls and the leeward wall run negative. Indoor air is pulled out through leaks there.
• Most roofs run negative, strongest near the windward eave and ridge on a low-slope roof. A steep roof can see some positive pressure on its windward slope.

The interior of the house settles at whatever pressure balances the air coming in on the windward side against the air leaving on the others. Where that lands depends on where the holes are. Big leaks on the windward side push the whole interior positive; big leaks on the leeward side, such as a leaky attached garage wall or a row of old windows, pull it negative.

See related: The House as One Pressure System, for how this adds to stack and fans.

Wind pressure goes with the square of the speed

The pressure the wind can produce is set by its dynamic pressure, one half times air density times wind speed squared. With air at about 1.2 kg per cubic meter:

Check the scaling: doubling from 10 to 20 mph multiplies the pressure by four, 12 to 48. That squaring is why a house that is fine in a breeze can misbehave badly in a gust: a gust at one and a half times the average speed carries more than twice the pressure.

The wall does not feel the full dynamic pressure. A pressure coefficient for each face says what fraction it feels. Published coefficients, such as those in the ASHRAE Handbook, Fundamentals, for an exposed, isolated low-rise building put the windward wall at roughly half to most of the dynamic pressure, positive, and the leeward wall at a smaller fraction, negative. Those are for open exposure. In a neighborhood, trees, fences and other houses cut the wind at eave height and scramble its direction, and the coefficients drop sharply. Weather-station wind is measured in the open at about 10 meters up, so the speed at a suburban house's eaves is usually well below the forecast figure.

Worked example: the back bedroom on a north wind

A one-story ranch with the back bedroom on the north wall. The customer says that room is cold and drafty "when the wind is out of the north," fine otherwise.

Exposed estimate. Say a 15 mph north wind hits the house in the open: 27 Pa dynamic pressure. Using illustrative coefficients of +0.6 for the windward north wall and -0.3 for the leeward south wall, the north wall sees about +16 Pa and the south wall about -8 Pa relative to the undisturbed outdoor pressure.

Where the interior lands. If the north and south walls have about the same leakage area and the holes behave like simple orifices, the interior settles halfway between, at about +4 Pa: (16 plus -8) divided by 2. That leaves about 12 Pa pushing cold air in through the north wall's leaks and about 12 Pa pushing warm air out through the south wall's.

Compare with stack. The same house's winter stack, a one-story over a crawlspace, might be only a few pascals bottom to top. On a windy night the wind term at the north wall is several times larger, and it is concentrated on one face. That is the bedroom draft.

Sheltered estimate. Now say the house sits in a subdivision with a row of houses and a tree line to the north, so the wind at its eaves is half the forecast, about 7.5 mph. Half the speed is a quarter of the pressure, so every figure above drops to a quarter: about 3 Pa pushing in at the north wall. Still a real draft through an unsealed outlet box or a leaky window frame, but a much smaller one.

What changes the answer. The leakage on the north wall, more than the wind. If that bedroom's exterior wall has an unsealed rim, an outlet on an exterior wall with no gasket and a window frame with no air seal behind the trim, the room takes a large share of the inflow. Sealing those holes cuts the draft at any wind speed. Swap the wind to the south and the same holes leak warm air out instead, so the room is fine, which is exactly the "only on a north wind" pattern.

            WIND from north
              |   |   |
              v   v   v
   +----------------------------+
   |  north wall   + 16 Pa      |  windward: air in
   |  back bedroom   <- cold in |
   |                            |
   |  side walls run negative   |  air out at sides
   |                            |
   |  interior about + 4 Pa     |
   |                            |
   |  south wall   - 8 Pa       |  leeward: air out
   +----------------------------+
          wake behind house, low pressure
   plan view, exposed site, illustrative values

Three faults that come and go with the wind

The chimney that spills only on windy days. A chimney termination that sits in a zone of positive pressure, below the ridge of a taller roof, beside a taller wing, or in the lee of a big tree or a neighboring building, can have wind pushed down it. A termination on the windward face of a steep roof can see the same. And if the room the appliance sits in is on the leeward side of the house, the room is being pulled negative at the same time the chimney top is being pushed positive. Either one weakens draft; together they can reverse it. That case has its own procedure: see Smoke Spillage Only on Windy Days Decision Tree, and for why the draft is weak in the first place, How Draft Actually Works. If smoke or flue gas is in the room, or a CO alarm is sounding, turn the appliance off at its own control only if you can reach it without lingering in the room, get everyone outside, and call the fire department from outdoors. Nobody goes back in until responders clear the house.

The draft that one room feels. As in the worked example, the windward rooms take in outdoor air and the leeward rooms lose indoor air. The windward room is cold and the leeward room can be fine or even slightly warm. A complaint tied to one side of the house and one wind direction is almost always leakage on that wall, not the duct system. Ducts get blamed because the supply register is the thing in the room you can see.

Attic moisture or frost on one slope. Wind across a vented attic pushes air in at the windward soffit and pulls it out at the leeward soffit, ridge and roof vents. The ventilation is strongest along that path and weak in corners and hip ends the path does not reach. Frost, staining or mold that sits on one slope, or in one dead corner, often marks a spot the wind-driven ventilation misses, combined with a ceiling bypass below it that feeds moist house air. The windward soffit can also blow air across the top of the insulation at the eaves, which is wind washing. See related: Insulation Contact and Wind Washing, and Attic Moisture + Ventilation Diagnosis Reference.

How to work a wind-tied complaint

Ask about the weather first. Which direction was the wind on the bad days, and how hard? Customers rarely know the direction offhand, but the bad dates are often in a text thread or a calendar. Match them to the recorded wind for your area from a public weather history. A complaint that lines up with one direction is a wind problem until proven otherwise.

Find the face that matters. Draw the wind on a plan of the house. The windward face is where the cold air comes in. The leeward face, and the roof, are where house air goes out, carrying moisture. The chimney terminal and its surroundings matter for any vented appliance.

Look for the holes on that face. Outlet and switch boxes on exterior walls, window and door frames behind the trim, the rim joist on that side, the soffit and attic hatch, any wall penetration for a line set, dryer vent or hose bib.

Test on the right day, or simulate it. A calm-day test proves nothing about a windy-night fault. Either return when the wind is blowing from the complaint direction, or use a blower door to depressurize the house and find the leaks on the suspect face, which does not depend on the weather. See How to Find Air Leaks While the House Is Depressurized.

Route what is not yours. Chimney and vent terminations go to a chimney professional or a qualified gas tech. Drafts through walls and windows go to whoever seals and trims. A repeated vent fault on windy days is also a reason to make sure working CO alarms are installed outside sleeping areas.

What wind is not

Wind does not explain a steady, every-night winter complaint at the top or bottom of the house; that pattern belongs to stack effect. It does not explain a problem that starts the moment a hood or dryer switches on; that is fans. If the complaint does not move with the weather, look at the other two drivers before chasing the wind. See related: Stack Effect and the Neutral Pressure Plane.

References

• See related: Smoke Spillage Only on Windy Days Decision Tree; How Draft Actually Works
• See related: The House as One Pressure System; Insulation Contact and Wind Washing
• See related: Attic Moisture + Ventilation Diagnosis Reference
• ASHRAE Handbook, Fundamentals, chapter on airflow around buildings (wind pressure coefficients and shelter)