Thermal Comfort Requirements in Regularly Occupied Spaces: A Guide Based on ASHRAE 55
Thermal comfort, or the sensation of feeling neither too hot nor too cold, is a critical aspect of indoor environmental quality (IEQ) in buildings. Achieving thermal comfort is important for the well-being, productivity, and satisfaction of building occupants. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has developed guidelines and standards to define thermal comfort and provide guidance for designing and operating buildings to meet these requirements. In this article, we will explore the thermal comfort requirements in regularly occupied spaces as per ASHRAE 55, a widely recognized standard for thermal comfort in buildings.
What is ASHRAE 55?
ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy, commonly referred to as ASHRAE 55, provides guidance for determining the acceptable indoor thermal environmental conditions for human occupancy in buildings. It considers various factors such as air temperature, radiant temperature, humidity, air speed, and clothing insulation to establish thermal comfort criteria.
Thermal Comfort Criteria
ASHRAE 55 provides two methods for determining thermal comfort: the graphical method and the analytical method. The graphical method uses a psychrometric chart to plot the various environmental factors and determine the range of conditions that are thermally comfortable for most people. The analytical method uses mathematical equations to calculate the predicted mean vote (PMV) and predicted percentage of dissatisfied (PPD) based on the environmental conditions and human factors.
ASHRAE 55 classifies thermal comfort into three categories based on the PMV values:
- Thermal Comfort Zone: This is the range of conditions where most people feel comfortable. ASHRAE 55 defines the thermal comfort zone as PMV values between -0.5 and +0.5, with PPD values less than 10%.
- Marginal Comfort Zone: This is the range of conditions where some people may feel slightly uncomfortable, but the majority of people are still comfortable. ASHRAE 55 defines the marginal comfort zone as PMV values between -1.0 and -0.5 or between +0.5 and +1.0, with PPD values between 10% and 20%.
- Outside Comfort Zone: This is the range of conditions where a significant number of people may feel uncomfortable. ASHRAE 55 defines the outside comfort zone as PMV values less than -1.0 or greater than +1.0, with PPD values greater than 20%.
Thermal comfort requirements in regularly occupied spaces
ASHRAE 55 provides specific thermal comfort requirements for different types of regularly occupied spaces in buildings, including conditioned spaces, naturally conditioned spaces, and mechanically ventilated spaces.
Conditioned Spaces: Conditioned spaces are spaces where the temperature and humidity are controlled by mechanical systems such as air conditioning and heating. ASHRAE 55 recommends the following thermal comfort criteria for conditioned spaces:
- Air temperature: The recommended range for air temperature in cooling mode is 23.5°C to 25.5°C (74.3°F to 77.9°F), and in heating mode, it is 20.5°C to 24.5°C (68.9°F to 76.1°F).
- Radiant temperature: The recommended range for radiant temperature is 21°C to 24°C (69.8°F to 75.2°F).
- Relative humidity: The recommended range for relative humidity is 30% to 60%.
- Air speed: The recommended range for air speed is 0.15 m/s to 0.25 m/s (30 ft/min to 50 ft/min).
- Clothing insulation:
Here’s a table summarizing the thermal comfort requirements for air-conditioned spaces, non-air-conditioned spaces, and mixed-mode spaces based on ASHRAE 55:
| Space Type | Air Temperature (°C) | Radiant Temperature (°C) | Relative Humidity (%) | Air Speed (m/s) | Clothing Insulation (clo) |
|---|---|---|---|---|---|
| Air-Conditioned Spaces | 23.5 – 25.5 (Cooling) | 21 – 24 | 30 – 60 | 0.15 – 0.25 | 0.5 – 1.0 |
| 20.5 – 24.5 (Heating) | |||||
| Non-Air-Conditioned | 20 – 27 (Summer) | Varies | 30 – 60 | 0.15 – 0.25 | 0.5 – 1.0 |
| Spaces | 20 – 27 (Winter) | ||||
| Mixed-Mode Spaces | 20 – 27 (Summer) | Varies | 30 – 60 | 0.15 – 0.25 | 0.5 – 1.0 |
| 20 – 27 (Winter) |
Note: The values provided in the table are general guidelines and may vary depending on specific building design, climate, and other factors. It is always recommended to consult the latest version of ASHRAE 55 for detailed and up-to-date thermal comfort requirements.
In air-conditioned spaces, the air temperature should typically be maintained within the range of 23.5°C to 25.5°C (74.3°F to 77.9°F) in cooling mode and 20.5°C to 24.5°C (68.9°F to 76.1°F) in heating mode. The radiant temperature should be within the range of 21°C to 24°C (69.8°F to 75.2°F), relative humidity should be between 30% and 60%, and air speed should be between 0.15 m/s to 0.25 m/s (30 ft/min to 50 ft/min). Clothing insulation is typically recommended to be between 0.5 and 1.0 clo.
For non-air-conditioned spaces, the air temperature may vary depending on the season, with a wider range of 20°C to 27°C (68°F to 81°F) in summer and winter. Radiant temperature, relative humidity, air speed, and clothing insulation requirements remain similar to air-conditioned spaces.
Mixed-mode spaces, which combine natural ventilation with mechanical ventilation, may have similar thermal comfort requirements as non-air-conditioned spaces, with variations based on outdoor conditions and ventilation strategies being used.
It’s important to note that achieving thermal comfort in a building requires careful consideration of various factors such as insulation, building orientation, glazing, shading, and HVAC system design. Consulting with qualified professionals and following relevant standards and guidelines, such as ASHRAE 55, is crucial to ensure that occupants in a building are comfortable and productive.