Overheating - a growing concern that must not be ignored
The world of the internet is littered with countless articles, blogs and studies on the subject of overheating in buildings. Overheating issues have been the subject of discussion amongst those that have had to live with it for years. But the carrot was never going to beat the stick in getting those who had the power to do something about it to actually do anything other than pay lip service to it and do the bare minimum. If worst came to the worst, you could always use the blunt instrument of air conditioning to solve all of your overheating “problems”.
In a warming climate already, overheating buildings will be limited in solutions for the problems they face. It is estimated that there are 1 billion single air conditioning devices in use on the planet today, a figure that is projected to rise to 4.5 billion by 2050 and consume 13% of all electricity supply. The world warms so our individual response to this warming is to use devices that increase the warming and hence evermore binding us to the original problem.
The time has come for us to break our dependence on fossil fuel and the use of technology to “correct” our fundamental design problems and begin to design buildings around the user and the environment in which they live. The human is inherently adaptable and comfort is no more than a state of mind.
Overheating in modern homes
There are two main factors which cause overheating in modern buildings. The first is global warming. This is rendering more extreme weather events, including heat waves, more common. The second factor is the increase in the amount and efficiency of thermal insulation in modern buildings. With airtight construction and windows designed to trap the sun's heat, buildings can be very susceptible to overheating.
Overheating in buildings is caused by both external and internal gains. The conditions that cause external gains are sunlight and high external temperatures. Sunlight passing into the house through windows is absorbed by the building and this heat is trapped. High external temperatures exacerbate this by reducing heat losses through the walls and opening a window may not give relief. This problem is typically worse in urban areas where surrounding buildings reflect the heat during the day and release stored heat at night. Air conditioning units produce heat as a by-product further contribute to this problem.
Internal gains are caused by a combination of building services, lighting, appliances and occupants. To reduce this, lighting and appliances should be as low energy as possible. An average person at rest gives off 100W of heat energy. As rooms in low energy houses typically only need a 300-400W of heating in mid winter, it is easy to see how a room can quickly become too warm. In addition, building services such as hot water, mechanical ventilation systems and poorly installed heating systems can contribute to overheating.
So what can we do to limit the effect of overheating? The site context, orientation, design and thermal mass of the building are all very important, but in this article I will concentrate on the part that building services play in minimising overheating. Passive building service design is focused on 3 areas: minimising the internal gains from services, control of shading to prevent sunlight entering the building and purge ventilation. Good control of services can minimise internal gains. The importance of your heating system. Choosing the correct heating system is also key. If a heating system is too slow to respond even in mid winter overheating can become an issue.
For example: it is winter and a room is heated with the under floor heating to 21°C. The floor is at a slightly higher temperature so that it supplies heat to the room to compensate for the losses through the walls and ventilation. The sun comes out and 3 people enter the room adding extra heat sources to the room. That room will now be 21°C and becoming warmer. The under floor heating will turn off but the floor will continue supplying heat for hours until it cools down. Windows will need to be opened to cool the room down.
Had a more responsive heating system been installed this problem would not have arisen. This would increase occupants' comfort and reduce running costs. It is also likely that alternative heating systems will be cheaper to install.
Automatic shading of the building
Shading will reduce the amount of sunlight that enters a building. Shading is most effective when put on the outside of a building, with awnings. This stops the light and hence the energy entering the building. Using blinds and curtains is also an option, but will only reflect a small portion of the light energy entering the building. Curtains and blinds do prevent the heat reaching the centre of the room, allowing them to remain at a more comfortable temperature while also reducing bleaching or furniture and floor coverings.
Purge ventilation reduces overheating by moving high volumes of air through a building. The two purge ventilation strategies that can be employed; passive stack and cross ventilation. Passive stack ventilation is the opening of ground floor vents, and a high level window or roof light when the outside temperature is lower than the inside temperature. The warmer internal air will rise up and out of the roof light or high up vent, while the cooler internal air will enter the low level vents.
When vents are opened on either side of the building providing a through draft, this is referred to as Cross ventilation.. With Cross ventilation, security can be a challenge as windows need to be left open, and consequently we often use oversized DCV (demand control ventilation) vents instead. These allow reasonable air volume flows without compromising security. Incorporating window motors is on selected windows is also effective, and not too expensive. When linked with other vents in a room, automated windows can be effective in flats that have only a single aspect as this can limit cross ventilation.
Managing overheating risk in buildings is a growing challenge. Intelligent smart control with correctly specified mechanical systems like ventilation, windows, shading, heating and hot water can solve the problem. However, it is important that the design team collaborate as early as possible to ensure the best results. Smart control should simplify a building, not complicate it, and it should run in the background to make the building more comfortable, secure and energy efficient.