Thermal Comfort: ASHRAE 55 + 20-26C Office Rules

What You Need to Know

Thermal comfort is how warm or cool people feel in a building. Two standards set the rules: ASHRAE 55 and CIBSE Guide A. Both say the same thing in different ways. For a typical office, keep the operative temperature between 20°C and 26°C, and most people will be comfortable. Get this wrong, and you get complaints, lost productivity, and failed Green Star submissions.

• Office operative temperature should sit between 20°C and 24°C in winter and 23°C and 26°C in summer (ASHRAE 55-2023)
• The PMV (Predicted Mean Vote) must stay between -0.5 and +0.5 for standard comfort. This means fewer than 10% of people will feel too hot or too cold (ISO 7730:2005, Category B)
• CIBSE Guide A says offices should not exceed 28°C for more than 1% of occupied hours (CIBSE Guide A, Table 1.5)
• Humidity should stay between 30% and 60% RH for comfort (general industry guideline). ASHRAE 55-2023 sets an upper limit at a humidity ratio of 0.012 kg/kg but does not specify a lower RH limit. CIBSE Guide A recommends 40% to 70% RH
• Air speed in the occupied zone should be 0.05 to 0.2 m/s for sedentary work (CIBSE Guide A, Section 1.6)
• HVAC systems need a control dead band of at least 2°C between heating and cooling setpoints (NCC 2025, J6D3(1)(h))
• For NCC Performance Solutions, thermal comfort must achieve PMV ±1 for 98% of occupied hours across 95% of occupied zones (NCC 2025, Section J)

Take a typical 500 m² open-plan office in Sydney. In summer, the air conditioning should keep the operative temperature between 23°C and 26°C. In winter, the heating system should hold it between 20°C and 24°C. Operative temperature is not just air temperature. It factors in radiant heat from sun-facing glass, warm ceilings, and cool walls.

A west-facing glass wall will push the radiant temperature up in the afternoon. People sitting within 2 metres of that glass will feel hotter than the thermostat shows. The air might read 24°C, but the operative temperature at the desk could hit 27°C or more. That is outside the comfort zone.

The fix is good glass (low solar heat gain coefficient), external shading, or moving desks away from the glazing. The mechanical system alone cannot solve a radiant comfort problem.

Humidity matters too. In coastal cities like Brisbane and Sydney, outdoor humidity is often above 70% RH. The HVAC system needs enough cooling coil capacity to pull moisture from the air, not just lower the temperature. If humidity stays above 60% indoors, people feel sticky and warm even at 23°C.