Thermal comfort is one of the main drivers for us to spend time in buildings. How the building is shaped and what materials make it affect the passive thermal comfort and how much energy we must use to keep it comfortable. Our prior blog post on the subject introduced the technical concept of thermal comfort, how it goes beyond air temperature. The physical, measurable elements that affect thermal comfort are:
- Air Temperature
- Humidity
- Radiant Environment
- Air Speed
- Occupant Clothing
- Occupant Activity Level
In Standard No. 55, ASHRAE has distilled the research into a workable set of relationships for sedentary occupants. Berkeley’s CBE has further made a web interface where we can check likelihood of thermal comfort with givens for the six variables listed above. The results were surprising to me at first, just how sensitive are we humans, how narrow is the path to comfort. As a helpful reminder of shelters’ task, another tool lets you visualize the range of outdoor conditions throughout the year at any location. From there you can overlay ASHRAE 55 comfort conditions. Simply stated, when outdoor conditions are within the green band, we can be comfortable without buildings; the rest of the time we need help.
This is a psychrometric chart showing Philadelphia’s range of outdoor conditions in blue squares, with darker blue showing more frequent occurrence. The ASHRAE 55 comfort range is shown in green. The crosshairs shows one temperature and humidity condition. It barely falls within the comfort range. The range changes to include that point with increased air velocity or reduced metabolic rate or reduced clothing level, or reduced mean radiant temperature, or a combination.
Complex as it is, this is just one point in time. A generous assumption is that mechanical equipment keeps interior conditions constant throughout a space. Exterior conditions change as a rule, however. Solar gain drastically increases mean radiant temperature on one occupant but leaves another untouched. Conversely, cold nights make glass surfaces cold and steal heat from nearby occupants’ skin. In the past, engineers developed clever ways of evaluating these human needs and meeting them without hourly evaluation. As people notice and even occupy the boundary conditions, a more-robust method is needed to predict how a building provides comfort.
Recently, dedicated practitioners combined increased computing power and improved software to allow thermal comfort to be predicted in space and time. The image below was made using Ladybug and Climate Studio. A simpler web interface is available from Berkeley CBE. You can add windows, change their properties, and move the sun around to see how comfortable different parts of the space are.
The colored dots show how thermal comfort varies in the space depending on time of day. Five of the six thermal comfort elements are held fixed, with radiant temperature allowed to fluctuate according to solar gain through windows. At 9 AM, for example, areas near the east-facing windows are showing overheating (red dots) when receiving low angle sun. Indeed, the real people who sit in that location experience real overheating, not helped by the mechanical system’s inability to provide targeted cooling at that location. Providing shading to disrupt the beam of direct sun is usually enough to deal with this condition. At 3PM however, the lack of west-facing rooms, smaller window sizes at south, and building orientation keep the south-facing areas comfortable even though daylight is still coming through those windows. No red overheating dots in the 3pm view, thankfully.
Got a challenging condition? Let us know how we can help you!
