Achieving thermal comfort inside buildings starts with good architectural design and leads on to include well-considered passive elements integrated with mechanical controls that consider the building in its entirety.
When designing sustainable buildings, achieving thermal comfort is one of the most important goals, not only so that people would enjoy using the building, but also because it will reduce the energy demand for heating or cooling.
Daniel Rimbault, a sustainability consultant at Ecocentric, a green building consultancy, states that the Leadership in Energy and Environmental Design (LEED) certification programme, for example, prescribes design according to ASHRAE Standard 55, an American standard that provides minimum requirements for thermal indoor environments. This standard considers six factors that influence people’s perception of thermal comfort, namely:
– Air temperature.
– Air speed.
– Radiant temperature.
– Clothing of the occupants.
– Metabolic rate of the occupants.
“By considering these six factors, one can come up with a range in which humans should be thermally comfortable,” Rimbault states.
While building regulations such as SANS 10400 XA, which refers to SANS 204, also have prescriptive minimums for the building envelope performance, Rimbault advises that the best approach is to conduct energy modelling of the overall building, which results in a better design both in terms of energy efficiency and thermal comfort. “Especially when facing constraints on projects, focusing on the overall building is better rather than following prescriptive values in isolation and hitting targets that might not have as much of an impact,” he notes.
For example, just as too many windows or incorrect orientation can lead to a hot interior, Rimbault points out that a building that has very few windows but is well insulated can become a system that keeps all the cold in because there isn’t enough solar energy to keep the space at a comfortable temperature. “This is why an integrated approach is so important – to consider everything about the building rather than just individual elements,” he states.
Thermal comfort by design
Ecocentric principal, Jutta Berns-Mumbi, adds that first of all, a building needs to be designed in such a way that it reduces the heating or cooling demand. “If the building isn’t designed efficiently, HVAC people have to take care of those excessive demands to meet the comfort of the occupants,” she explains.
According to Francois Joubert, managing director of a passive and low energy building modelling and consultation firm, Greenplan, one factor that hugely impacts thermal comfort is the sunlight penetrating a building through windows. “Although a building can benefit considerably from sun penetration in the winter, it is crucial to have some form of control to prevent excessive sunlight on glazed surfaces during the hotter months of the year,” he states.
To accomplish this, Joubert recommends movable external shading devices or low-emissivity glazing to reduce solar ingress through east, west and northern windows, but he stresses the importance of ensuring that the selected glass type allows adequate visual light transmittance to avoid a dark and gloomy interior environment.
Another important aspect is air-conditioning, whether it is mechanical or natural ventilation based.
“Here, both the air temperature and velocity play important roles,” states Joubert. “Relatively recent studies regarding thermal comfort have led to a concept called adaptive thermal comfort, which indicates that the temperature band can be adjusted throughout the year. This immediately creates an opportunity to make more use of natural ventilation systems when appropriate and to save energy when using mechanical systems,” he says.
Naturally ventilated buildings
Since natural ventilation systems have a limited cooling capacity, Joubert recommends that such systems should only be considered in cases where heat loads in the building have been reduced to low levels, typically 40 to 50W/m², by means of the inherent design of the building.
“This implies very careful attention to solar loads in terms of window sizes, insulation and shading,” he says.
When designing ventilation openings such as windows, vents or louvres to augment the movement of air through a building, Joubert also stresses the necessity of controllable openings to limit ventilation rates and completely stop the system when required due to wind or extreme external conditions.
Rimbault further warns that while green rating systems encourage naturally ventilated spaces, one of the biggest challenges in designing these is that most commercial clients are unforgiving of being outside of a comfortable condition, which is a high probability in naturally ventilated buildings.
“As for actual airflow, we find that buildings over the last 20 years have become more and more airtight with a much bigger emphasis on mechanical ventilation and conditioning,” he states.
Mechanical ventilation and conditioning
The design of a heating, ventilation and air-conditioning (HVAC) systems should be an integrated approach, according to Berns-Mumbi. Often HVAC or mechanical engineers work independently without giving attention to the specified insulation and other specifics of the building, either trusting another consultant’s work or using conservative rules of thumb. The result is an oversized system that is more powerful than it needs to be, which leads to inefficiencies in a building, and costs the client more to install.
“This is why green rating systems encourage energy modelling of the building that considers everything from the architectural design to the occupancy, electrical and mechanical design, and you get a much more accurate estimate of what is required from your HVAC system,” she states
Radiant heat is one factor that is sometimes overlooked, but a crucial consideration especially in perimeter zones, highlights Rimbault. “Even if the correct air temperature is maintained by a well-designed HVAC system that is operating as planned, but surfaces such as windows are radiating heat, building occupants in that vicinity will feel uncomfortable,” he explains.
This again refers to the importance of appropriate building orientation and efficient design.
The role of the roof
Radiation also plays a role in the thermal performance of roofs, together with heat transfer through conduction and convection. Joubert points out that on tall buildings, the thermal performance of the roof primarily has an effect on the top floor, but on single-storey buildings the roof represents a significant percentage of the building surface area, and hence sub-standard thermal performance will have a pronounced influence on annual energy use and thermal comfort.
“Because of radiative effects, roof colour can play an ‘insulating’ role,” says Joubert. “As a general rule-of-thumb, lighter colours, especially white, absorb less heat during the day and radiate less heat at night, while dark-coloured roofs usually represent ideal solar absorbers and easily reach temperatures of up to 70°C. It is obvious that starting with such extreme surface temperatures places additional and unnecessary demands on the insulation system under the roof surface,” he explains.
“Many different material types and installation options exist for insulating roofs, and both soft, bulk-type and rigid insulation types are commonly employed in South Africa,” Joubert continues.
Correct installation, however, is crucial. Joubert advises that flexible and loose fill insulation that is installed directly on ceilings, need to cover the entire ceiling area and shouldn’t be compressed over time as this will lead to deterioration in effectiveness. Also when installing a high-performance bulk insulation material over rafters or purlins, local compression between the roof sheet and the purlin severely compromises the overall performance if suitable spacers are not used.
Rimbault adds that the location of the insulation is just as important. “Ideally one should place the thermal envelope outside of as much mass as possible. For example, if the building has a concrete roof, the insulation should be placed above the slab. The effect of this is that the insulated mass can hold the temperature longer and reduce temperature swings within the building.”
Climate is another factor that should be considered when selecting insulation since humidity can negatively impact the performance of some insulation materials.
However, Rimbault stresses that insulation is not the only factor that influences thermal comfort in a building. “Once a certain amount of insulation has been installed, adding more is not going to drastically improve conditions,” he says. “It is even more important to consider the solar gains through the windows. And in commercial buildings that are almost completely airtight, it is incredibly important to get the HVAC system correct.”
All things considered, achieving thermal comfort inside buildings starts with good architectural design and leads on to include well-considered passive elements integrated with mechanical controls that consider the building in its entirety.
Full thanks and acknowledgement are given to Ecocentric and Greenplan for the information given to write this article.
Factors to consider when designing for thermal comfort:
– Solar gain.
– Ventilation, natural or mechanical.
– Radiant heat.
– Thermal performance of the roof.