8.4 Thermal Comfort
Key Takeaways
- Thermal Comfort (1 point) requires designing heating, cooling, ventilation, and humidity controls to meet ASHRAE Standard 55-2017 for at least 90% of occupants.
- ASHRAE 55-2017 governs six factors: metabolic rate, clothing insulation, air temperature, radiant temperature, air speed, and humidity.
- Individual thermal comfort controls must be provided for at least 50% of individual occupant spaces, allowing each person to adjust at least one of: operative temperature, air speed, or radiant temperature.
- Group thermal comfort controls are required for all shared multi-occupant spaces, such as classrooms, conference rooms, and patient rooms.
- Projects must commit to a thermal comfort survey within 6-18 months after occupancy and a plan to correct conditions for any space where less than 80% of respondents express satisfaction.
EQ Credit: Thermal Comfort (1 Point)
Intent: Promote occupants' productivity, comfort, and well-being by providing quality thermal comfort.
Design Compliance with ASHRAE 55-2017
Design the heating, ventilating, air-conditioning, and humidity control systems to meet ASHRAE Standard 55-2017, Thermal Environmental Conditions for Human Occupancy, for at least 90% of occupants.
ASHRAE 55 evaluates six factors that combine to produce a Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD):
| Personal Factors | Environmental Factors |
|---|---|
| Metabolic rate (activity level, met) | Air temperature |
| Clothing insulation (clo) | Mean radiant temperature |
| Air speed | |
| Humidity (relative or dew point) |
Design targets:
- PPD ≤7% corresponds to roughly 93% comfortable - exceeds the 90% threshold.
- Operative temperature typically falls in 68-78 deg F depending on clothing, season, and activity.
- Relative humidity between 30% and 60%, with dew point ≤62 deg F to limit microbial growth.
- Air speed ≤0.20 m/s (40 fpm) at occupant level unless local air movement is desired (elevated air-speed comfort zone).
Individual Thermal Comfort Controls
Provide individual thermal comfort controls for at least 50% of individual occupant spaces. An individual occupant space is a workstation or office assigned to one person. Acceptable controls allow the occupant to adjust at least one of:
- Operative temperature (via thermostat, fan coil, or VAV)
- Air speed (personal fan)
- Radiant temperature (under-desk radiant panel)
Group Multi-occupant Controls
For every shared multi-occupant space - classrooms, conference rooms, patient exam rooms - provide a group control that adjusts thermal conditions for the whole space. A central building automation system that can only be adjusted by facilities staff does not satisfy the requirement.
Post-Occupancy Thermal Comfort Survey
The project must commit to conducting an anonymous thermal comfort survey between 6 and 18 months after occupancy. The survey collects responses on a 7-point scale.
Failure trigger: If more than 20% of respondents in any space report dissatisfaction, the project owner must develop and implement a corrective action plan.
Common Exam Traps
- Confusing ASHRAE 55 (thermal comfort) with ASHRAE 62.1 (ventilation) or ASHRAE 90.1 (energy). On the exam, thermal comfort = 55, ventilation = 62.1, energy efficiency = 90.1.
- Forgetting that the 50% threshold applies to individual occupant spaces only; multi-occupant spaces are evaluated separately with group controls.
- Assuming a smart thermostat counts as an individual control when it serves an entire open office. Each individual space must have its own adjustable parameter.
Designing for the Adaptive Comfort Model
ASHRAE 55-2017 introduced an adaptive comfort model for naturally conditioned spaces (operable windows, no mechanical cooling). Acceptable operative temperature is defined as a sliding function of the prevailing mean outdoor air temperature, with the 80% acceptability band extending roughly ±3.5 deg C from the comfort optimum. Projects using natural ventilation should document compliance through the adaptive model rather than the PMV/PPD method, which assumes mechanical conditioning.
The elevated air-speed comfort zone lets designers offset higher operative temperatures by increasing local air speed (e.g., ceiling fans). For each degree Celsius above the upper comfort limit, an additional 0.10-0.20 m/s of air movement can keep the PPD below 20%, useful in warm climates that want to soften peak setpoints during cooling-tower upgrades or VAV resets.
Post-Occupancy Corrective Action
When the post-occupancy survey identifies dissatisfaction above 20% in any space, the corrective action plan typically targets one of three root causes: control imbalance (oversized VAV minimums, wrong zone grouping), envelope issues (radiant temperature asymmetry from cold glass or hot west walls), or operational drift (BAS overrides, locked setpoints). The credit does not require a specific remediation - it requires a documented plan that is actually executed.
Which ASHRAE standard governs the Thermal Comfort credit in LEED v4.1 BD+C, and what design percentage of occupants must be comfortable?
A 200,000 sf office includes 300 private offices (each an individual occupant space) and 25 conference rooms (multi-occupant). To earn Thermal Comfort, how many private offices must have individual controls, and what is required for the conference rooms?