Condensation and mould prevention

Mould needs three things: spores, which are everywhere, food such as drywall paper, wood or dust, and moisture. In buildings, moisture is the only one we can practically control. Condensation, the moisture that forms when humid air meets a cold surface, is one of the most common sources, and in Canada it shows up in both seasons: on cold windows and inside walls in winter, and on chilled water pipes, ducts and diffusers in summer.

This guide explains why condensation forms, where it typically appears in commercial buildings, and how HVAC design and operation prevent it.

The dew point principle

Air holds water vapour. The dew point is the temperature at which that vapour begins to condense. Any surface colder than the dew point of the air touching it will collect condensation.

At 21 °C (70 °F) and 40 percent RH, the dew point is roughly 7 °C (45 °F). At 21 °C and 60 percent RH, it rises to about 13 °C (55 °F). This is why a window frame that stays dry at modest humidity can stream with water when humidity rises. The psychrometrics guide explains these relationships in more detail.

Mould does not need liquid water. Sustained surface relative humidity above roughly 80 percent, which can occur on surfaces a few degrees above the dew point, can support growth on many materials. Preventing mould means keeping surfaces comfortably warm relative to the air's dew point, not just above it.

Winter condensation

Windows and frames

In winter, the coldest interior surfaces are usually window glass edges, frames and mullions, especially with older double glazing or non-thermally-broken aluminium frames. Condensation forms when indoor humidity is high relative to those surface temperatures. Humidification targets must account for this: buildings in Edmonton or Winnipeg often cannot sustain 40 percent RH in January without frost on windows. See humidification in cold climates.

Thermal bridges

Slab edges, balcony connections, steel studs, uninsulated lintels and penetrations create cold spots on interior surfaces. Mould often appears in patterns that trace these elements.

Inside the envelope

The more damaging problem is hidden. When a building is over-pressurized in winter, warm humid air exfiltrates through gaps in the air barrier and can condense or freeze inside the wall or roof assembly. The damage appears months later as stained ceilings, rusted fasteners, rotted sheathing or ice in roof assemblies. An effective air barrier and controlled building pressure prevent it. See building pressurization and infiltration control.

Summer condensation

Ducts, pipes and equipment

Chilled water piping, refrigerant suction lines and cold supply ducts run below the dew point of the surrounding air whenever that air is humid. Without continuous, vapour-sealed insulation, condensation drips onto ceilings and grows mould inside the insulation. Gaps at hangers, valves and fittings are the usual failure points.

Diffusers and grilles

Supply diffusers delivering cold air into a humid space, or into a space that is warmer and more humid than design, can sweat. This often signals high space humidity or infiltration, not a diffuser problem.

Ceiling plenums and crawlspaces

Plenums open to humid outdoor air, or unconditioned crawlspaces, can carry humid air past cold ducts and pipes.

Cooling coils and drain pans

Coils condense moisture by design. Problems arise when drain pans do not drain fully, traps are incorrectly sized for fan static pressure, or water is carried off the coil at high face velocity. Standing water in a drain pan quickly grows biofilm.

Common problem areas and remedies

HVAC measures for prevention

1. Control indoor humidity in both seasons. Keep summer RH at or below about 60 percent; ASHRAE Standard 62.1 requires systems that dehumidify to limit space RH to 65 percent at design conditions. In winter, set humidification targets based on envelope performance and outdoor temperature, resetting downward as it gets colder. See dehumidification strategies.
2. Avoid over-pressurizing the building in winter. Aim for a slight positive pressure, and verify it on upper floors of tall buildings.
3. Deliver supply air across cold surfaces. Perimeter heating and air distribution that washes glazing keeps glass surfaces warmer.
4. Insulate cold surfaces correctly. Specify continuous insulation with sealed vapour retarders on chilled water and refrigerant piping, and on supply ducts in unconditioned or humid spaces.
5. Design drain pans and traps properly. Slope pans to drain, size traps for fan static pressure, and keep coil face velocities within manufacturer limits to avoid carryover.
6. Manage moisture sources. Exhaust showers, kitchens and pools directly; keep pools and other wet spaces at negative pressure relative to surroundings.
7. Limit unoccupied humidity drift. Night setback and summer shutdowns can allow humidity to rise; many buildings need humidity control even when unoccupied.

Finding problems early

Infrared thermography, done in winter with a meaningful indoor-outdoor temperature difference, reveals cold spots, missing insulation and air leakage paths before visible damage appears. Moisture meters confirm whether materials are wet. BAS trends of space humidity, supply air dew point and building pressure help identify systems that drift out of range. Our infrared thermography service is often used to locate these issues.

When mould is found, Health Canada advises that the moisture source be fixed and mould cleaned up or removed, with the approach depending on the extent of growth. Large areas generally call for professional remediation with containment.

Key takeaways

• Condensation forms on any surface colder than the air's dew point; mould can grow at high surface humidity even without liquid water.
• Winter risks include windows, thermal bridges and hidden condensation from air exfiltration.
• Summer risks include poorly insulated chilled water piping, cold ducts, sweating diffusers and drain pans.
• Control humidity in both seasons, avoid winter over-pressurization and insulate cold surfaces continuously.
• Thermography, moisture checks and BAS trends find problems before visible damage appears.