Sustainable design: How to achieve thermal comfort
Eight Associates’ Chris Hocknell and Yiota Paraskeva discuss how achieving thermal comfort through sustainable design involves the careful balancing of interdependent factors – and how the results, in terms of energy efficiency, productivity and wellbeing, are worth it.
BS EN ISO 7730 defines thermal comfort as ‘…that condition of mind which expresses satisfaction with the thermal environment.’ A combination of environmental and personal factors affect thermal comfort: The environmental factors include air temperature, radiant temperature, air velocity, humidity and uniformity of conditions, while the personal factors span clothing, metabolic rate, acclimatisation, state of health, expectation and access to food and drink.
Overheating is a growing problem in the UK, due to climate change, the urban heat island effect and more highly insulated, highly glazed air tight (but potentially sub-optimally ventilated) buildings. You can imagine the impact on your productivity – and employee satisfaction – of sitting in an old office in front of a sealed, south facing window, next to the printer, on a warm day. The Health and Safety Executive view that ‘reasonable comfort’ is achieved when at least 80% of its occupants are thermally comfortable, and acceptable workplace temperatures are between 13°C (56°F) and 30°C (86°F), depending on whether the work activities are active or sedentary. To put this in perspective, open plan offices – and also residential lounges – are typically between 21-23oC in winter and 22-25oC in summer. While most people feel warm at 25oC and ‘hot’ at 28oC, there is no legal maximum workplace temperature and overheating is a serious concern due to health and wellbeing implications, and also its effect on people’s mental and physical performance.
Air temperature is measured in degrees Celsius or by the dry bulb temperature (DBT). Radiant temperature (measured as mean radiant surface temperature MRT), from heat sources such as the sun, heaters or machinery and equipment, has a greater influence than air temperature on how we lose or gain heat, along with the rate of air movement or air velocity – the faster the air movement, the greater the exchange of heat between the person and air. Relative humidity is the ratio between the actual amount of water vapour in the air and the maximum amount of water vapour that the air can hold at that air temperature. Indoor environments with humidity higher than 70%, preventing the evaporation of sweat from the skin, make it very difficult to cool down.
There are many techniques for estimating likely thermal comfort: effective temperature, equivalent temperature, Wet Bulb Globe Temperature (WBGT), resultant temperature and also predicted comfort zones within ranges of conditions. Under BS EN ISO 7730 and BS EN ISO 10551 thermal comfort is expressed in terms of Predicted Mean Vote (PMV) and Percentage People Dissatisfied (PPD).
Thermal Comfort graph, showing operative temperature of 22oC, with PMV <0.5 and PPD <10%
Overheating analysis should always be conducted and methodologies include the CIBSE Guide approach to overheating which is based on not exceeding a single limiting temperature of 25oC for > 5% and 28oC for > 1% of occupied (working) hours, while CIBSE TM 52 requires temperatures above the ‘threshold comfort temperature’ for less than 3% of occupied hours, with daily weighted exceedance of less than ‘6’ in any one day and no hours at the upper limit temperature.
Other overheating standards include BB 101, SAP – Overheating Assessment, SBEM – Excess Solar Gains, PMV and PPD, ASHRAE STANDARD 55-2010, CEN STANDARD EN 15251:2007 and PHPP – Number of hours. Be aware that a ‘slight’ risk of overheating as identified by SAP analysis may actually result in a high risk when it is properly analysed, SAP is a very simplified tool that is based on average monthly (not peak) temperatures. Data suggest that the loss of productivity is between 10% and 20%, and that the CIBSE maximum is out of range 3% of the time. For a business with a £8,000,000 turnover, this equates of a loss of at least £24,000-£48,000 for a building which meets CIBSE Guide A criteria in full.
Achieving thermal comfort through sustainable design
External heat gains are caused primarily by sunlight and high external temperatures, and these factors are controlled through the consideration of passive design measures (including optimising orientation and site layout, geometry, room layout and shading devices), construction measures (targeting glazing solar energy transmittance, thermal mass and air leakage) and active design measures (such as heating and cooling capacities, efficiencies and set points, ventilation rates and heat recovery efficiency).
It is worth noting that there are very different ‘internal gains’ heat profiles in hours across the day for residential and office settings. The images below show the internal gains for a north-south facing office and a west-facing dwelling during typical occupation hours. The graphs illustrate the relative importance of each gain type (lighting, equipment, occupancy factors and solar gain).
Internal gains across the day for offices and living rooms
The impact of orientation
Orientation is an important factor in providing a building with passive thermal comfort; to take advantage of solar gains to reduce heat loads or to protect against unwanted solar gains. For example, a south-west facing elevation will receive direct sunlight in the late afternoon when the ambient external temperature is at its highest. As shown on the graphs above, the west facing building (living room) is actually receiving a relatively higher amount of solar gain per m2, primarily as a result of the solar incidence. Angles of incidence closer to perpendicular to the window will result in higher transmission of the solar energy, until the angle exceeds 55°, when the transmission reduces sharply.
The images below demonstrate the sun incidence for different orientations. The solar heat gains will be higher a west-facing façade (left image) where the sunlight hits the window at an angle which is more perpendicular to the window, relative to the south-facing façade (image on right).
Solar incidence: lower angles may result in greater overheating from solar gains
Thermal mass refers to the ability of building materials to store and emit heat, so careful consideration should be given to integrating thermal mass and ventilation strategies. When the air within a space is warmed due to direct sunlight or heat gains from people and appliances, some building materials absorb heat. As the air in the space cools overnight, this heat is re-emitted into the space, thus oscillations in temperatures are attenuated somewhat, relative to a building with low mass which is more ‘reactive’ to heat input. Dense building materials which are exposed to the internal environment can absorb and release heat due to their thermal mass (heat capacity), although this is significantly reduced by for example, levels of internal insulation and service voids, which have very low thermal mass. However, it should be noted that our analysis shows that there is limited benefit between a light-weight (glass and steel) and medium-weight building (traditional cavity construction), although there is an evident benefit in medium-weight to heavy-weight (concrete structure and masonry internal elements). Moreover, achieving benefits from thermal mass requires that it is used optimally, for example the temperature attenuation from thermal mass requires that the mass be effectively ‘reset’ i.e. purged of heat, so that it has the capacity to absorb unwanted heat when most convenient. This is the theory behind night ventilation strategies, which are effective in some circumstances; increased ventilation rates are required to purge the heat from the building via the air, but consider that increased mechanical ventilation volumes significantly increases fan power consumption.
The importance of fenestration design and glazing choices
The fenestration design and glazing choices have a large impact on thermal comfort as radiation from the sun passes through glazing to heat the internal fabric of the building, accumulating inside. In a domestic setting, windows have massive implications on the heating and cooling loads of a building, as up to 40% of a home’s heating energy can be lost via windows and the required cooling capacity can be increased by up to 50%. As a rule of thumb in commercial settings, a façade with greater than 25% (glazing to floor area) should be given increased attention, particularly the glazing solar factor, which has a significant impact on overheating compliance. Also the Seasonal Energy Efficiency Ratio (SEER) of the cooling plant should be reviewed as the energy consumption for cooling will be a lot higher, which may have a large impact on carbon emissions compliance for Building Regulations Part L.
Whilst it is common practice to use the same U-value and solar heat gain coefficient (SHGC) for glazing on all elevations, this can be customised at the design stage to optimise predicted thermal comfort. The SHGC for windows measures how readily heat from direct sunlight flows through a window system, the lower a window’s SHGC (between 0 and 1), the less solar heat it transmits. Solar heat gain varies depending on location, orientation, time of day and season. Therefore the angle that solar radiation hits the glass should be considered when determining the appropriate SHGC for the façade, once the angle exceeds 35° (relative to the horizontal plane) the solar radiation entering the space falls off sharply for a vertical window, the inverse is true for a horizontal rooflight.
Shading and daylight
The choice of the shading device that is used is determined by the orientation, and common ‘shading’ strategies include overhangs, external louvers, external shading and internal blinds, however, limitations may exist in the external context (for example, restrictions in a conservation area), and there are pros and cons for each shading device type such as level of occupant control, glare, and implications for views and daylight. South-facing windows need to be protected from high-level sun and this can be done by overhangs/balconies or horizontal awnings. Windows facing east or west experience the sun much lower in the sky, so the most effective shading devices such as external louvers may result in daylight implications, so careful consideration is required to ensure shading without completely blocking views.
Effective ventilation strategies
While it may be possible to limit the effects of excessive internal and external heat gains through design and construction measures, these cannot be completely eliminated. The most effective way to remove built-up heat is through an effective ventilation strategy, to replace existing warm air with fresh air from outside (when the outside air is cooler than the inside).
Research has shown that occupants of naturally ventilated office buildings are significantly more satisfied with their thermal environment than occupants in air-conditioned buildings. Natural ventilation, whilst highly desirable for a lean energy strategy, is not always an option for a variety of reasons – not least air and noise pollution in urban settings.
It is common to see residential mechanical ventilation heat recovery (MVHR) units with very low specific fan powers specified to achieve CO2 targets. However, the lower amount of ventilation provided from smaller power-consuming MVHR units might mean that the dwelling does not meet its required air change rates for air quality and overheating. For example, in a large single dwelling the volume of air required for the development’s wet rooms may mean that very low fan powers of 0.45-0.65 W/l/s are just not achievable. Also, be aware of affinity laws; if the speed of a fan is increased with 10%, the volume flow is increased by 10%, the head is increased by 21% – and the power is increased by 33%.
It is also possible that the ventilation system itself can become an unwanted source of heat, particularly if the heat exchanger is working in the wrong direction; i.e. transferring heat from the extract air stream into the supply air stream. All of these variables demonstrate the need for a ventilation strategy that can modulate according to the prevailing conditions; higher ventilation rates during periods of high solar and internal gains, with the ability to bypass the heat exchanger when it becomes detrimental, and ideally the option to switch to natural ventilation only to achieve high air change rates with no additional energy consumption.
Under the cooling hierarchy is it the intention to effectively ‘design-out’ air-conditioning through reducing all sources of heat gain, but where design measures and the use of natural and/or mechanical ventilation are not enough to guarantee the occupant’s comfort, providing the measures in the cooling hierarchy have been maximised, it may be necessary to adopt an active cooling strategy. It is now not uncommon to experience summer peak air temperatures of 28oC or more in the UK, at this point ventilation ceases to be of much benefit, and in buildings with high internal gains the thermal comfort will dramatically suffer, consequently active cooling may be inevitable in some instances. However, the approach should be to use active cooling sparingly when all other means have been exhausted, a best practice approach would be to fit sensors on opening windows which deter the use of active cooling in conjunction with high volumes of natural air changes.
A complex picture of cumulative effects
So, a complex picture of symbiotic and cumulative effects of site context, external temperature, solar gains, internal gains and building design, influence thermal comfort levels, which in turn have an impact on a range of health and wellbeing factors, such as daylight views, air quality and noise pollution. Achieving thermal comfort through sustainable design involves the careful balancing of a complex set of interdependent factors, including orientation, thermal mass, glazing, shading, ventilation. The benefits of getting it right are worth it, not just in terms of energy efficiency, but also productivity and wellbeing, which are increasingly becoming the criteria against which a building is considered whether to be of satisfactory quality.
Eight Associates offers thermal comfort overheating analysis, thermal simulation and U value calculations of fenestration and cladding systems, ventilation strategies, daylight and sunlight impact assessment – get in touch to discuss your project requirements. T 020 7043 0418