Heating: 1.0 clo, Top 20–23.5°C | Cooling: 0.5 clo, Top 22.5–26°C
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ASHRAE 55 comfort zone: Canada's thermal comfort standard
ASHRAE Standard 55-2023, Thermal Environmental Conditions for Human Occupancy, defines the combinations of indoor temperature and humidity that at least 80% of building occupants find thermally acceptable. Canada's National Building Code 2020 and ASHRAE 90.1 both reference ASHRAE 55 for occupied space comfort requirements in commercial and institutional buildings. Most Canadian jurisdictions follow the NBC, making ASHRAE 55 effectively the design standard for any HVAC system serving people.
The standard uses a two-season model based on typical clothing insulation. In the heating season, occupants wear heavier clothing (1.0 clo), so they tolerate cooler operative temperatures. The acceptable range is 20 to 23.5°C. In the cooling season, lighter clothing (0.5 clo) shifts the acceptable range to 22.5 to 26°C. Both ranges apply at typical sedentary activity (1.0 met) and assume relative humidity stays at or below 60% and humidity ratio doesn't exceed 12 g/kg.
Operative temperature: more than just dry-bulb
ASHRAE 55 uses operative temperature rather than dry-bulb temperature. Operative temperature (Top) is the uniform temperature of an enclosure that would exchange the same amount of heat by radiation and convection as the actual non-uniform environment. For well-insulated interior spaces with no significant radiant heat sources or cold glass, Top closely equals dry-bulb temperature. Near large windows, exterior walls in winter, or radiant panel systems, the mean radiant temperature (Tmrt) diverges from Tdb. When air speed is below 0.2 m/s, Top = 0.5 × (Tdb + Tmrt). Use the MRT offset field above for spaces where radiant conditions differ from air temperature. For most Canadian open-plan offices and classrooms, the offset is small.
Humidity limits and why they matter in Canadian buildings
ASHRAE 55 caps humidity at two levels: relative humidity must stay at or below 60%, and humidity ratio must not exceed 12 g/kg. The 12 g/kg absolute limit prevents oppressive conditions at the upper end of the summer temperature range, where 60% RH at 26°C already represents about 12.7 g/kg. In Canadian winter, the real challenge runs the other direction. Outdoor air at -20°C and 80% RH contains less than 0.6 g/kg of moisture. Heating that air without humidification drops RH below 20%, causing dry skin, eye irritation, and increased susceptibility to respiratory illness, which is why a correctly sized humidifier is standard equipment in Canadian heating systems. The humidity ratio calculator helps you find the minimum outdoor humidity for any mixing scenario. Target indoor W above 3-4 g/kg during Canadian heating season to keep occupants comfortable and healthy.
Higher air speed and summer cooling in Canadian offices
ASHRAE 55 allows the upper temperature limit to be raised when occupants control their own air speed. Ceiling fans or operable windows providing 0.6 m/s of air movement permit the upper temperature limit to increase by about 1.2°C, extending the summer comfort zone to approximately 27.2°C. At 1.2 m/s, the extension reaches 2.8°C. This is increasingly used in Canadian commercial buildings pursuing LEED or WELL certification to reduce cooling energy while maintaining comfort. The trade-off is that occupants must have personal control of the air speed device. Central HVAC systems delivering higher air speed without occupant control don't qualify for the full extension under the standard. Use the psychrometric chart calculator to visualize the resulting state point at higher temperature limits.
Frequently Asked Questions
ASHRAE 55-2023 defines two operative temperature ranges for sedentary occupants (1.0 met). For the heating season with winter clothing (1.0 clo), the acceptable range is 20.0 to 23.5°C. For the cooling season with summer clothing (0.5 clo), the range is 22.5 to 26.0°C. Both ranges require RH at or below 60% and humidity ratio at or below 12 g/kg. Canada's National Building Code 2020 references ASHRAE 55 for occupied commercial and institutional spaces. Higher air speed above 0.1 m/s (when occupant-controlled) extends the upper temperature limit by up to 3.5°C. Use the dew point calculator to check whether surface temperatures in the space are above the dew point to avoid condensation.
Humidity ratio directly measures the mass of water vapour in the air regardless of temperature, making it a better indicator of the total moisture load on the body. A relative humidity of 60% at 20°C equals about 8.8 g/kg, while 60% at 26°C equals about 12.7 g/kg. Without the 12 g/kg cap, high summer temperatures combined with 60% RH would create oppressively humid conditions that exceed occupant tolerance. The 12 g/kg limit ensures the absolute moisture load stays acceptable across the full summer temperature range. In Canadian HVAC practice, the 12 g/kg limit rarely constrains winter heating design but becomes critical for summer dehumidification sizing in humid Canadian cities like Toronto, Ottawa, and Windsor. Use the grains of moisture calculator to convert humidity ratio to grains per pound for equipment specifications.