The adaptive thermal comfort model, explained

Why an adaptive model?

Heat-balance models such as the predicted mean vote (PMV) estimate comfort from air and radiant temperature, humidity, air speed, clothing and activity. Field studies showed that occupants of naturally ventilated spaces respond differently: their comfort depends partly on the outdoor climate, because of their thermal experience, changes in clothing, the control they have and shifts in expectations.

The ASHRAE 55 adaptive method is derived from a global database of 21,000 measurements taken mainly in office buildings. It relates acceptable indoor operative temperatures to the outdoor climate, so clothing values do not need to be estimated, and no humidity or air-speed limits apply.

The ASHRAE 55 adaptive method

Comfort temperature (°C) = 0.31 × prevailing mean outdoor air temperature + 17.8. The 80% acceptability band, ±3.5 °C, is for typical applications; the 90% band, ±2.5 °C, can be used when a higher standard of comfort is wanted. At a prevailing mean outdoor temperature of 20 °C, for example, the comfort temperature is 24.0 °C, the 80% band runs from 20.5 to 27.5 °C and the 90% band from 21.5 to 26.5 °C. The method applies only when all of these hold:

• The space is occupant-controlled and naturally conditioned, with operable windows that open to the outdoors and that occupants can readily adjust.
• There is no mechanical cooling system and no heating system in operation.
• Occupants have metabolic rates of 1.0 to 1.5 met in the current wording (earlier editions said 1.0 to 1.3 met).
• Occupants are free to adapt their clothing within a range at least as wide as 0.5 to 1.0 clo.
• The prevailing mean outdoor temperature is above 10 °C and below 33.5 °C; outside that range the method may not be used.

The running mean outdoor temperature

Both standards compare indoor temperatures with a mean of the daily mean outdoor temperatures of the preceding days. ASHRAE 55 has allowed a simple average of the 7 to 30 days before the day in question, or a weighted average in which each day counts 0.6 to 0.9 times as much as the day after it, and monthly means from published climate data when daily data are not available.

EN 16798-1 uses an exponentially weighted running mean. Where it is not available, the standard gives an approximation from the previous seven daily means: (T−1 + 0.8 T−2 + 0.6 T−3 + 0.5 T−4 + 0.4 T−5 + 0.3 T−6 + 0.2 T−7) / 3.8, where T−1 is yesterday's daily mean.

The EN 16798-1 adaptive categories

The European standard sets comfort temperature = 0.33 × running mean outdoor temperature + 18.8 °C. At a running mean of 20 °C that gives 25.4 °C. The bands around it are asymmetric, wider on the cool side:

• Category I, high expectation, recommended for occupants with special needs such as children, the elderly and the sick: +2 °C / −3 °C.
• Category II, normal expectation, recommended for new buildings: +3 °C / −4 °C.
• Category III, moderate expectation, recommended for existing buildings: +4 °C / −5 °C.
• Scope: running mean outdoor temperatures of 10 to 30 °C, buildings without mechanical cooling, mainly sedentary occupants with easy access to operable windows and freedom to adapt their clothing. In the heating season, the limits for mechanically cooled buildings apply instead.
• Air movement: at operative temperatures above 25 °C, higher air speeds from fans or personal systems under the occupants' control can raise the upper limit.

Using adaptive comfort in design

• Screen the climate: compare hourly outdoor temperatures with the adaptive band to see how much of the year a well-shaded, naturally ventilated building could stay within it. This is a first check, not a compliance calculation, because the standards define the band for indoor operative temperature.
• Test the design: simulate the free-running building's indoor operative temperature and count the hours above the upper limit.
• Provide the conditions the method assumes: windows that occupants can open, no strict dress code, and fans under their control if you rely on the air-speed extension.
• Check future climate: a warmer running mean raises the comfort temperature, but hotter peaks can still exceed the upper limit.

How C4B shows adaptive comfort

C4B's hourly temperature and humidity analysis compares the site's hours with an ASHRAE 55 adaptive comfort band at 80% acceptability, on the Free plan with weather-station data. In projects with site-specific data, it adds a temperature trend (the latest 30 years, extended back to 1941) projected to 2030, 2050, 2070 and 2090 under five SSPs. The band uses the ASHRAE 55 equation with each month's mean outdoor temperature, so it is constant within each month, and it is drawn in every month, including those outside the 10 to 33.5 °C range the standard sets. The passive design strategies on the psychrometric chart use fixed comfort limits (16 to 27 °C, 20 to 80% relative humidity), not the adaptive model.