Anyone who has ever tried to work when they are too hot or too cold will know exactly how important thermal comfort is for focus and productivity.
The problem is that the definition of thermal comfort is by its nature subjective: thermal comfort is “that condition of mind which expresses satisfaction with the thermal environment and is assessed by subjective evaluation”. Basically, it is up to each person to gauge their thermal comfort, which makes it difficult to regulate the thermal environment to satisfy everyone’s needs.
This is part of the reason why building codes for indoor environmental quality (IEQ) parameters such as indoor air quality (IAQ) aim to minimise discomfort, not eliminate it. No single set of parameters will be comfortable for everyone.
Thermal comfort can be measured as the predicted mean vote (PMV), which is a scale from – 3 (cold) to + 3 (hot) (Table 1). The value is derived from a complex equation that factors in a range of criteria, including clothing insulation, metabolic rate, air velocity, vapour pressure, and the mean radiant temperature (MRT). Generally, thermal comfort guidelines recommend that interior spaces be regulated such that the PMV is between – 0.5 and + 0.5.
Table 1: The predicted mean vote (PMV) scale for measuring thermal comfort. From ASHRAE standard 55.
The PMV can be used to calculate the predicted percentage dissatisfied (PPD). The PPD is a function of the PMV, and describes the expected percentage of people dissatisfied with their thermal environment given the PMV. As the PMV moves away from 0, the PPD increases. 100 % PPD would indicate that 100 % of people would be expected to be dissatisfied with the thermal environment. Therefore, guidelines suggest that interior spaces should aim for a PPD below 10 %.
Below are ways to ensure that through good design, construction and maintenance, you can keep PMV within a narrow range around zero, and thus minimise the PPD.
Components of thermal comfort
To begin, I will go through the six parameters of thermal comfort that have to be taken into account.
- Metabolic heat production – humans are endotherms, and produce their own heat through metabolism. Depending on activity levels and person-to-person physiological differences, metabolic heat production can vary a lot. This heat becomes an important part of how people perceive their thermal comfort.
- Clothing – the clothes that people wear in the workplace obviously play a large role in thermal comfort. Clothing affects energy transfer between the skin and the environment.
- Relative humidity – the efficiency of evaporative heat dissipation is strongly influenced by relative humidity. At high relative humidity, people will begin to struggle to lose heat via evaporative pathways. Therefore, even relatively low air temperatures will feel hot and uncomfortable if the relative humidity is high.
These first three influence people’s sensitivity to the thermal environment, and now the next three points will dictate thermal comfort.
- Convection – energy transfer via the flow of air. For people in the workplace, the convectivecomponent of thermal comfort is governed primarily by the air temperature and ventilation rate.
- Conduction – energy transfer through direct contact with surrounding surfaces. This is not generally very important in the workplace, other than in considering the material used for surfaces like chairs, which people are constantly in contact with.
- Radiation – energy transfer via radiation from surrounding surfaces. This is the largest component of thermal comfort, as humans experience the thermal environment as over 50 % the MRT.
All of these factors interact to regulate thermal comfort, which creates a lot to consider for attempting to achieve thermal comfort in the workplace. That is why I am writing this blog, to help you navigate some of these factors such that you can achieve thermal comfort for the majority of occupants.
4 ways thermal comfort can be achieved through good design, construction, and maintenance
1. Use a HVAC system that regulates MRT
This is by far the biggest way to move towards achieving thermal comfort for the vast majority of occupants.
As I explained above, the MRT is highly important to human thermal comfort. As a result, using an HVAC system that actually measures and regulates the radiant component of operative temperature goes a long way to achieving thermal comfort. The best way to achieve this is to install a radiant cooling / heating system with a means to measure and monitor the MRT. In addition to the best regulation of the thermal environment, these systems are energetically more efficient than all-air alternatives, as well as quieter and more spatially efficient.
Radiant cooling / heating systems do not directly affect air temperature, and do not control ventilation or indoor air quality (IAQ). Therefore, they have to be used in conjunction with a system that fulfils these purposes, such as a dedicated outdoor air system (DOAS).
2. Minimise leakage
Depending on the outdoor conditions, your HVAC system may be heating up and humidifying cold, dry air, or it could be cooling down and dehumidifying hot, humid air. Either way, the air needs to pass through the HVAC equipment for this to happen efficiently and effectively. If there is leakage in the building envelope and air is transferring in and out of the building other than through the HVAC system, IEQ will be lowered.
Essentially, air could be coming in that is below or above the desired temperature and relative humidity. This will substantially lower thermal comfort. Moreover, at the site of a leak, the pressure or temperature differential between indoor and outdoor conditions can create drafts, which can further lower thermal comfort.
As a side note, leakage causes other reasons for concern. Perhaps most importantly, leakage significantly lowers the energetic efficiency of an HVAC system. This is because air transfer in and out of the building happens without passing through the air handling unit (AHU) and the energy recovery wheel (if one is installed). This will cause the HVAC system to have to work harder to regulate indoor conditions.
3. Design and build for some occupant control
Often, people will be the most comfortable when they have control over some aspect of their IEQ. Therefore, allowing access to the thermostat, or operable windows and blinds, might boost perceived thermal comfort. Part of this is designing the building to maximise the potential use of natural ventilation and radiation from the sun. These will not only lower the energy load of the HVAC system, but also allow occupants to more precisely control their environment as they desire.
The challenge here is that in spaces occupied by multiple people, allowing individual control can actually lower IEQ for the majority of people and cause conflicts. For example, studies have shown that males and females have slightly different requirements of their thermal environment, and feel comfortable under slightly different conditions. Not to mention the effects of individual health and metabolism differences on perceived thermal comfort. Moreover, as stated above, minimising air flow that is not specifically regulated by the HVAC (leakage) can be beneficial in terms of regulation IEQ and energy consumption.
Therefore, while designing for the potential incorporation of user control and use of natural radiation and ventilation might be an option for some construction schemes, it is definitely not advisable for all of them. What is often better for thermal comfort is simply allowing people to wear clothes that are thermally comfortable, and regulate their comfort by either removing an outer coat, or putting on something warmer. This may prove a better means to provide fine adjustments to the personal thermal environment, rather than altering the overall heating and ventilation of the space.
4. Maintain the thermal environment, and make changes as necessary
Good maintenance is key to properly functioning HVAC equipment. In this regard, installing a radiant cooling / heating system is once again useful, as maintenance costs and effort are much lower than all-air systems.
Maintenance may also require being aware of, and reacting to, seasonal changes. For regions that experience hot summers and cold winters (as is typical of temperate zones), seasonally adjusting temperature control of the HVAC system is vital to maintaining thermal comfort. In fact, differences in the regulation of relative humidity and air temperature between the seasons appear in most international building codes.
For example, ASHRAE standard 55 suggests slightly different indoor air temperatures between summer and winter (Table 2). This change will not only help maintain thermal comfort, it will also amount in significant energy savings; the HVAC system will not have to do as much work each season to maintain a desired temperature, if the difference between the outdoor and indoor temperature is smaller.
Table 2: The suggested indoor air temperatures for summer and winter depending on relative humidity. Adapted from ASHRAE standard 55.
Thermal comfort is paramount to good IEQ, and one of the two major factors that are mostly responsible for the overall perceived comfort of occupants in the workplace (the other being IAQ). That makes designing for, regulating, and maintaining good thermal comfort essential for keeping workers generally comfortable.
As I have covered previously, good comfort leads to optimal performance and profitability, so it’s definitely worth your time.
How to achieve thermal comfort is a complex issue. However, through good design, construction, and maintenance, it is possible to maximise the number of occupants that are comfortable. Always remember that it has been repeatedly shown through scientific study that making everyone comfortable is virtually impossible, but you can take measures to make sure that the vast majority are satisfied.