Building pressurization and infiltration control

Doors that whistle or will not close, lobbies that are freezing in January, condensation on the upper floors of a tower, and kitchen odours drifting into the dining room are all symptoms of the same problem: building pressure that is out of control. In Canadian climates, where the indoor-outdoor temperature difference can exceed 40 °C (72 °F) in winter, uncontrolled air movement through the envelope wastes energy and can damage the building itself.

This guide explains what drives infiltration and exfiltration, how HVAC systems influence building pressure, and how to design, balance and verify a building that behaves.

What drives air through the envelope

Air moves through cracks and openings whenever there is a pressure difference across them. Three forces create that difference.

Stack effect

Warm indoor air is less dense than cold outdoor air. In winter, this makes a building act like a chimney: air leaks in at the lower floors, rises through shafts, stairwells and elevator hoistways, and leaks out at the upper floors. The neutral pressure plane is the height where indoor and outdoor pressures are equal. Stack pressure grows with building height and temperature difference, so a 20 storey tower in Edmonton in January can see pressure differences of tens of pascals at the top and bottom floors. In summer, with air conditioning, the effect reverses but is much weaker.

Wind

Wind creates positive pressure on the windward face and negative pressure on the leeward face and roof. Wind effects fluctuate constantly and are hard to control mechanically, which is why airtight construction matters.

Mechanical systems

Supply and exhaust fans can push a building positive or pull it negative. An imbalance of only a few percent of supply airflow can overwhelm natural forces in a tight building. Exhaust-heavy spaces such as kitchens, labs and washrooms, and economizer operation without proper relief, are common culprits.

Why pressure matters

In winter, warm humid indoor air exfiltrating through the envelope can reach a surface below its dew point and condense or freeze inside the wall assembly. This is a leading cause of hidden moisture damage in Canadian buildings and a key reason not to over-pressurize. See condensation and mould prevention.

The design approach

Start with an airtight envelope

Mechanical systems cannot compensate for a leaky envelope. The NBC requires an air barrier system in assemblies separating conditioned space from the exterior, and NECB 2020 adds airtightness provisions that can be met through compliance with prescriptive detailing or by testing. Some provinces and municipalities, such as those adopting the BC Energy Step Code or the Toronto Green Standard, require whole-building airtightness testing for certain building types. Requirements vary, so confirm with the authority having jurisdiction. Whole-building testing is typically done to ASTM E779, ASTM E3158 or CAN/CGSB-149.15.

Set a pressurization target

Most commercial buildings are designed to run slightly positive relative to outdoors, typically in the range of about 2.5 to 12 Pa (0.01 to 0.05 in. w.c.) measured at the ground floor. Taller buildings and buildings in very cold climates usually target the lower end to limit moisture-driven exfiltration at upper levels. The target should be stated in the design documents and sequence of operation.

Build an airflow balance

The mechanical designer should prepare an airflow balance for each system and for the building as a whole:

1. List all supply airflows, including outdoor air at minimum and at full economizer.
2. List all exhaust airflows: washrooms, kitchens, labs, garages, process exhaust and relief.
3. Assign a net positive surplus of outdoor air for pressurization, often expressed as a percentage of outdoor air or a fixed airflow per floor.
4. Check every operating mode: occupied, unoccupied, economizer, smoke control and kitchen or lab exhaust on and off.
5. Provide make-up air for large exhaust systems so they do not draw air through the envelope.

Manage the stack effect

Compartmentalization helps. Vestibules or revolving doors at main entrances, sealed shafts, weatherstripped stair doors and floor-by-floor air barrier continuity all reduce stack-driven flows. Heating at entrances, such as cabinet heaters or air curtains, deals with what remains. Stairwell and elevator shaft pressurization for smoke control is a separate life-safety system with its own design and testing requirements under the NBC.

Controlling pressure in operation

Several control strategies are used, often in combination:

• Return or relief fan tracking. The return or relief fan follows the supply fan, either by fixed airflow offset (airflow tracking using flow stations) or by speed ratio. Airflow tracking is more accurate but depends on well-calibrated flow stations.
• Direct building pressure control. A relief fan or relief damper modulates to hold a measured differential between a representative indoor location and a shielded outdoor reference. This works well with economizers, provided the outdoor sensor is protected from wind.
• Outdoor air flow measurement. Measuring and controlling minimum outdoor air keeps ventilation and pressurization stable as the supply fan modulates.

Common problems found in the field include outdoor pressure references blowing in the wind, relief dampers stuck closed, economizers opening without adequate relief, and exhaust fans added in renovations without matching make-up air. Economizers are a frequent source of over-pressurization when relief capacity is overlooked.

Testing and verification

Pressurization should be confirmed during balancing and commissioning, not assumed from design airflows.

• Measure outdoor, supply, return and exhaust airflows for each system, using duct traverses rather than relying only on terminal readings.
• Measure building differential pressure at the ground floor and upper floors in winter conditions, with all normal exhaust systems operating.
• Check door opening forces at entrances, stairs and corridors. The NBC limits opening force on certain doors, and excessive pressure can make doors difficult for people with disabilities.
• Verify pressure control sequences across modes, including economizer and unoccupied operation.
• Use infrared thermography in winter to locate envelope leakage paths and confirm air barrier performance.

Frequently asked questions

Should a building be positive or negative?

For most commercial buildings, slightly positive. Exceptions include buildings with dominant contaminated spaces, and spaces within buildings such as kitchens, washrooms and labs, which are kept negative relative to their surroundings.

Why is my lobby cold even though the heating works?

Usually stack effect or building depressurization pulls cold air through the entrance doors. Checking the airflow balance and building pressure often solves the problem more effectively than adding heat.

Key takeaways

• Stack effect, wind and fan imbalance all drive air through the envelope, and stack effect dominates tall Canadian buildings in winter.
• A slight, controlled positive pressure is the usual target; too much drives moisture into walls.
• An airtight envelope and a building-wide airflow balance are the foundation of good pressure control.
• Economizer relief, make-up air for exhaust and protected outdoor pressure references are frequent problem areas.
• Verify pressure in the field, in winter, with all exhaust systems running.