The principle of the stack effect
The stack effect, also called thermal draft or natural convection, is a phenomenon in which warm air — less dense than cold air — rises toward the upper parts of a space and creates negative pressure in the lower parts. This pressure difference draws fresh air in through low openings and exhausts warm air through high openings.
This mechanism is the same one that allows household chimneys to function: warm air from combustion rises up the flue and pulls fresh air from the room. Applied to an entire building, this principle becomes a tool for passive ventilation.
Physical mechanism in detail
Air density and temperature
Warm air is less dense than cold air at equal pressure. This density difference creates an upward buoyant force. In an enclosed space heated by the sun or by occupants, air warms, rises, and exerts positive pressure on the upper parts of the building envelope.
In turn, negative pressure develops in the lower parts. If a low opening exists, cooler outdoor air is drawn inside to compensate. This continuous cycle is the stack effect in natural ventilation.
Factors affecting draft intensity
The intensity of the stack effect depends on two main parameters:
- The temperature difference between indoor and outdoor air: the greater the difference, the stronger the draft. In a Canadian winter, this gap can be very large, but it comes with heat loss that must be managed.
- The vertical distance between low and high openings: the farther apart the openings are vertically, the taller the air column and the more effective the draft. A two-storey home with low openings on the main floor and high openings under the roof has stronger natural draft than a bungalow.
Technical note
The stack effect works independently of wind. It is therefore useful even in calm weather, unlike cross-ventilation, which requires outdoor air movement. The two mechanisms often work together for the best result.
Applications in Canadian homes
Designing vertical openings
To take advantage of the stack effect, the placement of openings is critical. Air intakes should be low (main-floor windows, foundation vents, ventilation slots in interior doors) and air outlets high (roof lanterns, clerestory windows, attic ventilation).
In common Canadian pitched-roof homes, ridge vents paired with soffit vents provide passive roof ventilation based on exactly this principle. Air enters through the soffits (lower edge of the roof) and exits at the ridge (peak).
Stairwells and open volumes
A stairwell in a multi-level home acts as a natural conduit for the stack effect. Warm air rises from the lower level to upper floors and exits through high windows. Leaving the basement door open in summer, if the basement is cooler, can amplify this effect and support passive cooling throughout the house.
Roofs and skylights
Operable skylights placed at the upper part of a building are a direct application of the stack effect. In summer, opening them at night allows warm air accumulated during the day to exhaust quickly. Combined with open low windows, they create an effective natural vertical flow.
In Canada, skylights must be designed with effective seals to withstand snow accumulation and freezing rain in winter, while remaining operable for summer ventilation. Electric or motorized models allow controlled opening without physical access.
Seasonal limits in Canada
Winter: risk of excessive heat loss
In winter, the stack effect runs very strongly because of large temperature differences between indoors (around 20 °C) and outdoors (which can drop to -20 °C or lower in several regions). This creates cold air infiltration through joints and unintended openings, contributing to heat loss.
Well-sealed homes control this phenomenon by limiting unwanted infiltration. In winter, ventilation should be deliberate and brief to renew air without excessively cooling living spaces. Heat recovery ventilators (HRVs) are a complementary solution to natural ventilation in this context.
Shoulder seasons and adaptation
Spring and fall offer ideal conditions for using the stack effect: outdoor temperatures are moderate, and indoor-outdoor thermal differences allow sufficient draft without excessive heat loss. These periods are well suited to intensive natural ventilation to refresh indoor air after winter months.
External reference
The Natural Resources Canada EnerGuide program provides information on home ventilation, including recommendations on HRVs and passive ventilation suited to the Canadian climate.
Integration in architectural design
The stack effect is a principle that architects specializing in sustainable construction integrate from the design phase. Ceiling height, opening placement, and the presence of a central atrium or open stairwell all determine how effective natural ventilation through thermal convection will be in a new home.
For renovations, targeted changes — adding a skylight, installing roof vents, opening a volume between two levels — can noticeably improve air circulation without major construction work.
