Imagine it is winter. You wake up in the morning, put on your favourite hand-knitted wool socks and walk to the kitchen to have breakfast. But something is different today, your left toes are cold, you start to shiver and feel uncomfortable. What happened? The fabric on the toes has worn-out, there is even a little gap. The socks that used to keep your feet warm and cosy have a leakage and they are not able to keep you warm any more.
The same principle applies to a house. The building envelope’s task is to protect its occupants from the environment and to keep them warm. The building envelope needs to be a continuous shell, each little breach will negatively influence the overall performance and reduce the insulation’s potential benefits.
The following will explain where air-leakages in a building usually occur and how to prevent them.
Air-Leakage And Thermal Bridges
Thermal bridges and air leakages will increase the need of supplementary mechanical cooling and heating, but they will also increase the Relative Air Velocity and the Mean Radiant Temperature which will negatively influence the well-being and the comfort of the residents. By applying the right design features, natural ventilation and cross ventilation can be used to control indoor temperature and therefore reduce energy bills significantly. For these reasons, controlling the air movement is essential.
Draughts + air-leakage
Older style buildings commonly have draughts and air-leakages due to unsealed windows and doors, and unsealed vents and exhausted fans, therefore, heat and air can escape. It is difficult to control the air movement. Other sources for draughts are gaps within or around insulation, vented skylights, gaps between floorboards, open fire places, around air conditioners and heaters, gaps around other wall penetrations, such as down lights, pipes, cables etc. .
A thermal bridge is an element or part of a building, which allows heat to travel through it more quickly than through other parts and is therefore responsible for unwanted heat loss or gain. A thermal bridge arises for instance when poor insulative materials touch each other, when gaps occur between insulative materials and structural surfaces, and when materials with different R-values/U-values come in contact with each other. These thermal bridges allow heat transfer from a warmer to a cooler material. The main thermal bridges in a building are located at the junctions of floor to the wall, wall to the roof, balconies and window and door frames.
Avoiding gaps and thermal bridges is essential to minimise unwanted heat gain and loss and to utilise the full potential of the insulation. Retrofitting draught-proofing measures is essential to increase the performance, particularly for older homes, such as sealing doors and windows, closing of wall and ceiling vents and installing self-closing exhaust fans.
Most new buildings waive wall and ceiling vents due to different construction methods and use self-closing exhaust fans as a matter of fact. If these measurements are applied correctly the amount of air leakage is reduced enormously.
How to locate draughts?
– Are there any visible gaps? For example is light coming through gaps around windows and doors?
– Are blinds or curtains moving when the windows are closed?
A lit candle can be used to check air movement, such as around windows and doors, vents, floorboards, junctions of floor to wall and wall to roof connections.
Ventilation is the active process of “changing” or replacing air to regulate temperature and moisture. It should always occur under controlled conditions, by opening windows or with ceiling or exhaust fans, NOT through gaps and air-leakage. Ventilation is important to support the ability of thermal mass to absorb and release heat in order to regulate the indoor temperature. It is necessary to ascertain where natural breezes are and to locate the windows accordingly. Landscaping and other buildings can influence and obstruct air flow; therefore it is necessary to visit and check the site before locating windows. Cool summer breezes in and around Melbourne usually come from south; detailed information can be found from the Bureau of Meteorology (BOM) web site.
The less gaps and air-leakage occur the air-tighter a building gets. Therefore regular ventilation is mandatory to renew oxygen and discharge odours, water vapour, carbon dioxide and other contaminations. For instance, in Germany it’s recommended to cross-ventilate every day for a few minutes, even in winter. If the occupants forget to ventilate regularly, water vapour will be trapped inside and will lead to mildew and mould on the walls and the ceilings.
Openable windows and external doors should be located on different sides of the home, with less than 8 metres distance between them to allow for adequate and effective air flow. Cooler air enters the building where the breeze is loctated, passes through the building and exits on the other side. The warm air inside gets replaced by fresh and cooler air.
The term “stack-effect” goes back to the chimney. The heat source – in this scenario, the fire – heats up the air. Hot air rises and is discharged through the chimney, as it has a lower density than cold air. This effect can be used to replace air inside a house. For instance, when it’s colder outside the windows can be opened to let in cooler air. Warmer air inside the room will rise towards the ceiling, exiting via high openable windows and skylights. Warm air inside is replaced by fresh and cooler outdoor air.
Exhaust fans should always be self-closing, so that the replacement of air is controlled and not accidental. They are an effective way to replace air, especially in rooms where no natural ventilation is available, or where natural ventilation might not be sufficient, such as kitchens or bathrooms.
Ceiling fans are an easy and cost effective way to improve the indoor air quality in summer and also to gain points towards the desired energy rating stars. Ceiling fans provide additional air movement/wind, increasing the Relative Air Velocity (‘wind chill factor’) resulting in the apparent temperature felt on exposed skin to be 3 °C colder than the actual air temperature, thereby reducing the need for additional cooling.
Thermal Bridges And Ventilation Overseas
In Europe, strict regulations are in place to control thermal bridges and air-leakages to minimise the energy needed for heating and cooling. Furthermore, due to the colder climate, a lot of structural damage can occur if heat and vapour is able to ‘travel’ through building materials.
Unlike in Europe, the significance of avoiding gaps and thermal bridges is commonly unknown and not a regulatory requirement in Australia. Common practice often shows that there is barely attention paid to minimising gaps and thermal bridges, leading to unwanted thermal bridges and air-leakages and therefore increases the need for cooling and heating.
Requirements to minimise thermal bridges
Maximum values of heat transfer through thermal bridges are specified and need to get incorporated into the energy ratings. Windows and doors as well as junctions of different building parts and materials require much detailing during the working drawing stage, as well as on the building site. It’s the architect’s/designer’s responsibility to find and draw solutions to overcome thermal bridges, and the builder’s to build accordingly.
Infrared thermal image
In Europe, infrared cameras are often used to locate the misapplication of materials and resulting thermal bridges. The lighter the colour the warmer the materials, the darker the colour the colder the materials. Great differentiation between colours means great temperature difference.
The first picture below shows a typical German home. Although double glazing and thermally improved window frames are used, the windows have a higher U-value than the walls, as the required U-value for the external walls is 0.24 W/(m²K) and the U-value for the windows 1.30 W/(m²K). Expectedly, the windows present in a lighter colour as they let more heat escape through them than the walls. Determining if a thermal bridge is within the allowed limits requires meticulous measurements and comparison of internal and external material and air temperatures, humidity levels and following calculations of heat transfer. In this particular case, the thermal bridges occurring due to different U-values are within the allowed limits.
The third picture shows an obvious thermal bridge where the wall meets the ceiling. The dark colour indicates that there are gaps in the insulation as that corner is significantly darker and therefore colder than the rest of the room. It will require further investigation, if the architect/designer or the builder is liable for this structural damage.
Although air-leakages and thermal bridges are not accounted for in energy ratings, they can majorly limit the ability and the potential benefits of insulation and other passive solar design solution. Consequently even a house with a 6, 7 or 8 star-energy rating could be draughty in winter. Avoiding air-leakages and thermal bridges means minimising unwanted heat gain or loss and therefore reduces the energy needed to cool or heat a building.