What's new in WELL v2: Thermal Comfort
What's new in WELL v2: Thermal Comfort
Tuesday, January 22, 2019
/ By: Nicholaus Johnson & Nathan Stodola
Why thermal comfort matters
Did you know that while indoor thermal comfort is one of the strongest predictors of satisfaction in the built environment, only 11% of office buildings in the U.S. achieve generally accepted parameters? Thermal comfort, overall, plays a large role in the way we experience the places where we live, learn and work and also has implications for productivity and health outcomes. One study found that increasing office temperature by only 1°C decreased productivity by 15%, while others suggest that small temperature differences (a few °C) can influence worker speed and accuracy by as much as 20%. This relationship between thermal comfort, well-being and productivity is not only limited to the workplace. Student performance, especially among children, can be significantly influenced by thermal comfort parameters such as temperature. In one study, increased temperature decreased student performance by as much as 30%. In regards to health outcomes, exposure to cold air and rapid temperature changes, for instance, are known to trigger asthma in adults. Furthermore, while cold and dry environments can facilitate the spread of viruses such as hepatitis A and influenza, overly warm and humid conditions are associated with symptoms of sick building syndrome, respiratory issues and fatigue.
Measuring thermal comfort
For mechanically ventilated spaces, thermal comfort is determined using the whole-body thermal-balance comfort (WBC) model that considers four environmental parameters – air speed, temperature, thermal radiation and humidity - in combination with two personal parameters – metabolic rate and clothing. Using these six parameters (Figure 1), the WBC model produces two indices used to assess the thermal environment of a given space.
Figure 1: Six parameters of thermal comfort.
The first of these indices is the Predicted Mean Vote (PMV), which predicts the average thermal sensation of a group of people on a seven-point scale. The second is the Predicted Percentage Dissatisfied (PPD), which predicts dissatisfaction in a given thermal environment.
Figure 2: Predicted mean vote and predicted percentage dissatisfied.
As can be seen in Figure 2, for any single thermal condition, even in an optimized thermal environment (PMV=0), 5% of occupants are expected to be dissatisfied with their thermal environment. This is due to variation in individuals’ personal temperament and preferences; thus, the only way to further reduce PPD is to provide individual control of the thermal environment.
In naturally ventilated spaces, thermal comfort is determined using the Adaptive Thermal Comfort (ATC) model that considers only indoor and outdoor temperature as its inputs.
Thermal Comfort in v2
The Thermal Comfort concept in the WELL v2™ pilot draws from features across the WELL v1 Air and Comfort concepts. It seeks to promote human productivity and ensure a maximum level of thermal comfort among all building users through improved HVAC system design and control, and by meeting individual thermal preferences. Towards this intent, features in the Thermal Comfort concept address:
Three features in the Thermal Comfort concept – including the only precondition,
Feature T01: Thermal Performance – involve on-site evaluation of conditions within the WELL project. WELL provides projects flexibility to design according to expected occupants’ activity and clothing levels. A project’s thermal performance can be evaluated using these design conditions and the environmental conditions measured on-site during Performance Verification.
As with air and water quality in their respective concepts, WELL promotes more frequent measurement of thermal conditions. Feature T01: Thermal Performance requires that all projects undergo regular assessment of their space and Feature T06: Thermal Comfort Monitoring rewards projects for conducting continuous monitoring and using the resulting data for refinement of thermal comfort parameters.
Providing individual controls and flexibility over the thermal environment is an effective strategy to promote and enhance thermal comfort. The benefit? Research has demonstrated that providing just 5°F of individual temperature control can increase productivity by up to 7%. Feature T03: Thermal Zoning complements the requirements of thermal zoning and occupant control of thermostats, through free address – the freedom of occupants to move and work in different thermal zones throughout the day.
Beyond control over the environment, another area that occupants can control is clothing. In order to facilitate this, Feature T04: Individual Thermal Control requires that projects implement a flexible dress code and provide access to blankets to further support individual thermal comfort. Furthermore, this feature also requires that projects provide personalized heating and cooling devices such as fans, heated/cooled chairs and other non-combustion sources of heating/cooling upon occupant request.
Since the goal of thermal comfort design is for the occupants of the space to find the thermal environmental conditions acceptable, the ultimate test of success is simply asking if the occupants indeed find this to be true. Thus, Feature T02 Part 2 is the only feature in WELL where results from an occupant survey determine the number of points awarded (Features C03 and C04 in Community require projects administer surveys, but the occupants’ responses do not affect compliance with the features.). Feature T02: Enhanced Thermal Performance requires that projects administer an anonymous survey (example survey is provided in appendix T1 or projects may select from a list of pre-approved surveys) to all regular building occupants. In order to ensure that the results of this survey contribute to enhanced thermal conditions, the points awarded to a project are predicated on the percentage of occupants that are satisfied with their thermal environment.
Challenges and evolution
There are several unique challenges that can inhibit the delivery of optimized thermal environments for occupants. First, thermal comfort models are only as accurate as the assumptions they are based on. For example, while many models assume that occupants will be wearing “typical business attire” (which corresponds to clothing insulation of 1.0 CLO, or 1.55 m2 °C / W), both seasonal and personal variations may weaken the accuracy of this assumption. Second, it has become increasingly difficult to provide a user-controllable thermal environment with the increased popularity of open office plans.
Fortunately, the same element that makes perfecting thermal comfort so challenging - its highly personal experience - is what has been driving advancements in the field. For instance, as thermal comfort modeling techniques become more sophisticated, there is a growing opportunity to identify changes to specific parameters that will have the most significant impact on occupant satisfaction. Furthermore, improvements to individual thermal regulating devices will continue to enhance the personalization of thermal environments with limited impacts for others.
Moving forward, the Thermal Comfort concept will continue to evolve under the guidance of the Air & Thermal Comfort Advisory. This extraordinary, global group of architects, engineers, designers and construction specialists will continue to provide invaluable guidance regarding the development and implementation of standards in the Thermal Comfort concept.