2. Ways To Achieve Thermal
Comfort Through Good
Design, Construction and
Maintenance
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 minimize 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,
vapor 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 minimize the PPD.
3. 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 convective component 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. 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).
5. 2. Minimize 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.
6. 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 maximize 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, minimizing 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.
7. 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.
8. Programs can be used in simulation:
• 3D CAD Design
• MaCAD
• Master in advanced
• Computation for architecture and design
