In this post we are going to discuss how to prevent overheating in buildings through bringing in and integrating solar control methods during the architectural design process.
This is a key consideration when designing any building, but one that is often left until the end of the process or worse still left out completely.
What is overheating in buildings?
In order to design out and stop overheating in our buildings, we need to firstly understand what is meant by overheating and excess heat.
Due to varying local and regional climates, weather patterns and seasons, there are no internationally excepted figures or definitions of what exactly defines overheating. However studies have shown that temperatures over 24 degrees C can cause discomfort, especially to the more fragile members of our population.
This figure is however based on air temperature, and other factors such as humidity, air movement, direct sun light and the amount of exposure to it, also play very important factors in understanding excess heat.
Heatwaves need to also be taken into account, and although often only temporary (typically three to five days), they should play a vital role is assessing and mitigating overheating.
Impact of overheating
When overheating occurs it can have serious impacts on the buildings occupiers health and wellbeing.
High temperatures and heat increases cause us to suffer from thermal stress, such as sweating and cardiovascular strain which in extreme circumstances can lead to heat exhaustion. The elderly and very young are the most susceptible to this and can often be hospitalised.
In the worst instances, there have been reports of over 20,000 heat related deaths associated to a signal heatwave period.
It’s not all doom and gloom however, as it’s important to note that in most instances, overheating is avoidable through good and well thought out architectural design strategies.
Where do solar gains come from?
Solar gains can happen through a buildings fabric when the external surface has a higher temperature than the interior. This however rarely happens in modern buildings due to the well-insulated and air tight construction techniques used to comply with today’s modern building regulations.
However older buildings that have not been restored or are in poor condition can face severer overheating.
Negative solar gains predominately happen through South and West facing glazing during the summer months, as the warmth in absorbed through the buildings openings and released into its spaces.
Many building users will pull and close blinds and/or curtains to emit theses gains, however once these themselves have been warmed up, there is little to benefit. The most common method of cooling is to obviously open a window, but if the air temperature outside the building is hotter than the air in inside, then when windows and doors are open for ventilation, the air transition between the two spaces will increase the air temperature within.
We as humans also emit heat through the activities we do and items we use. This is dependant on the level of the activity and occupation of the building, for example someone exercising will produce more heat output than someone sitting, and an office full of computers will produce more heat than say a library.
Heat gain is also very site specific, as there can be considerable air temperature differences between rural and urban sites. Urban sites tend to run hotter due to a buildings mass absorbing the heat during the day and then releasing it during it the night. A further contribution to this is also the increased number of commercial units that may have machinery and refrigeration units running constantly.
A buildings context will also play a large role, as if the building is a noisy or high polluted area then this discourages its occupants from opening windows, is say for example they are situated next to busy main road or railway station .
Furthermore security may also be an issue, with a ground floor apartment for example.
The orientation of a building plays a large roll in heat gain, particularly on its south and west elevations.
Special consideration should be taken into account when assessing the the late afternoon heat gain, due to the suns lower orientation and the increased afternoon temperatures. This can however also be used as an advantage in the winter months when the heat gain can be used to positively warm the interior spaces.
Modern methods of construction will contribute towards overheating in the way that building are constructed almost too well, with no leaks and with such high levels of insulation, that there is simply no way for the internal heat to naturally escape with physical help.
With regards to materials, high thermal mass options such as brick and stone can collect and store the heat transferred during the day and as long as there are methods on ventilation during the night, the stored heat can be purged. Leaving the building cool for the next morning.
A light weight timber framed construction however can not use its mass to its advantage, and therefore assumes the external temperatures quicker.
Resolving the problem
When a building overheats the first thought and most common solution is to open the windows to draw in the external air and flush out the hot internal air. However if the exterior temperature is the same or similar to the interior, then this will have minimal impact, in some instances can even exaggerate the problem and increase the temperature.
With this in mind and during times of high temperatures, passive ventilation may only serve as a method of renewing the internal air and removing any pollutants and smells.
Its worth noting that the scale of this problem varies dramatically depending on geographical context, and whilst the common solution to the above it to use air conditioning, in some countries the increased running costs do not out weigh the 5 or so days a year this would be an issue in say Britain for example.
Increasing the amount of insulation in wall and roof build-up’s can slow down the heat transfer from the outside and reduce the amount of convection through to the internal spaces.
That said, given today’s strict building regulations and target energy values, most modern buildings already have a high amount of insulation through default.
The same can be applied to a buildings services, along with plenty of ventilation to maintain air flow.
Preventing solar gain through glazed openings on the south and west elevations is critical when preventing overheating, internal shutters, blinds or curtains can stop this to a degree, but as mentioned above these can quickly heat up themselves.
The best form of prevention is therefore from an external means, which is best designed into the form and massing of a building from the outset. For example by using the buildings overhangs and recesses to protect the openings at risk.
Other forms of external protection can come from a more bolted on approach, by using a bre-solay, shutter or external blind.
The choice of materials, particularly for a shutter or bre-solay approach is critical however, from an aesthetic standpoint they must tie into the building as much as possible.