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Natural ventilation for Canadian shoulder-season design
Natural ventilation moves outdoor air through a building with pressure differences instead of fans. The Whole Building Design Guide describes the two drivers: wind, which creates positive pressure on the windward face and negative pressure on the leeward face, and buoyancy, where warm indoor air rises and leaves through high openings while cooler air enters low. Wind-driven cross ventilation and stack ventilation each have their own formula, and this calculator has one tab for each.
In Canada the strategy fits spring, summer and fall, when outdoor temperature and humidity sit in a comfortable range. Many institutional and office buildings use mixed-mode design, with operable windows or louvres in good weather and a switch to mechanical ventilation when conditions are too cold, hot or humid.
Wind-driven cross ventilation
The standard form is Q = Cv × A × V. Cv is an effectiveness coefficient, about 0.5 to 0.6 when wind meets the openings squarely and about 0.25 to 0.35 when it arrives diagonally. A is the smaller of the inlet and outlet free areas and V is the wind speed at the building. With 2 m² openings, 3 m/s of wind and Cv 0.55, Q = 0.55 × 2 × 3 = 3.3 m³/s, which is 3,300 L/s or about 6,992 CFM. Use a reduced design wind speed, because terrain, neighbouring buildings and window restrictors that limit how far a sash opens all cut the real flow below a fully open aperture. The presets here default to diagonal wind (Cv 0.3), since wind rarely hits a facade squarely.
Stack effect for atriums and stairwells
Stack flow depends on the vertical distance between the low inlet and the high outlet and on the indoor-outdoor temperature difference. The formula is Q = Cd × Aeff × sqrt(2 × g × H × (Ti − To) / Ti), with Cd about 0.65, temperatures in kelvin, and the neutral pressure level taken midway between equal openings. Two equal openings act in series, so the effective area is A / sqrt(2), which is what the tool uses. For 1.5 m² openings, 6 m apart, with 24 °C indoors and 18 °C outdoors, the effective area is 1.06 m² and the flow is about 1,060 L/s (2,250 CFM). Tall spaces heated by the sun do better still, and the passive solar calculator helps judge how much heat a solar chimney or glazed atrium collects to drive the stack.
Where natural ventilation stops working
Open windows do nothing about humidity, so a humid day can bring in air that feels worse than the room it replaces. Check the outdoor dew point against your comfort limit with the dew point calculator before counting on it. Stack flow also reverses when the outdoors is warmer than the inside, and it all but vanishes at temperature differences under a couple of kelvin, which is why a stack with no wind help is unreliable on mild days. In winter, large temperature differences drive strong stack flow, but that shows up as infiltration through the envelope rather than controlled ventilation.
Combining with mechanical systems
Keep mechanical ventilation for winter and for days when the openings cannot be used. Use the ACH calculator to compare the natural flow with your target air changes, and the ventilation rate calculator to size the backup. Natural ventilation can also cool a building overnight. The U.S. Department of Energy notes that night flushing with cool night air can cool a building and its thermal mass, so it works best where nights are cool and breezes are regular, and it should not be used when outdoor air quality is poor.
Frequently Asked Questions
Use Q = Cv × A × V. Cv is an effectiveness coefficient, about 0.5 to 0.6 for wind hitting the openings squarely and about 0.25 to 0.35 for diagonal wind. A is the smaller of the inlet and outlet free areas, and V is the design wind speed at the building. For 2 m² openings, 3 m/s and Cv 0.55, Q = 0.55 × 2 × 3 = 3.3 m³/s, which is 3,300 L/s or about 6,992 CFM. Use a reduced design wind speed instead of peak weather station readings, because terrain, neighbouring buildings and partly open windows all cut the real flow.
It works best in the spring, summer and fall shoulder seasons, and especially in milder coastal climates such as Vancouver. In winter, outdoor air at -10 °C to -30 °C makes open windows a comfort and energy problem. Many Canadian buildings use mixed-mode design, with operable windows in good weather and mechanical ventilation and heating the rest of the year. Use the ventilation rate calculator to size the mechanical backup.
Stack flow follows Q = Cd × A × sqrt(2 × g × H × (Ti − To) / Ti), with Cd about 0.65 and temperatures in kelvin. This tool applies it to equal inlet and outlet openings, which act in series and have an effective area of A divided by the square root of 2. With 1.5 m² openings, 6 m between them, 24 °C indoors and 18 °C outdoors, the flow is about 1,060 L/s (2,250 CFM). Flow rises with the square root of both height and temperature difference, so it is small on mild days and for short stacks.
ASHRAE 62.1 treats a space as naturally ventilated when its openable area is at least 4 percent of the net occupiable floor area and the space lies within about 8 m (25 ft) of those openings. A cross-ventilated room is often designed with a total opening near 2 percent of its floor area, split between the two walls, and a room depth of no more than about 5 times its height. These are starting points. Check them and the backup ventilation requirement against the code edition your authority has adopted.