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Passive design lies at the foundation of sustainable architecture. Using the inherent conditions of the site to a project’s advantage, the different elements of passive design work together to provide thermal comfort and achieve energy efficiency.
Passive heating and cooling is the most energy efficient way to heat and cool a home utilising the sun, shade and movement of air. Passive heating works with the sun to warm a house during winter, while passive cooling works with shading and air movement to cool a house during summer.
In winter, windows and doors are positioned to allow the lower angle of the sun to penetrate the interior, thereby heating materials that can absorb and store that heat for later release (see thermal mass below). In summer, precisely angled shading of walls and windows, through awnings for example, will block the intense sun from entering the building, allowing the interior to remain cool. Windows are also positioned for cross- and stack ventilation to encourage air movement through a house, which helps cool people and spaces and lets built-up heat escape (see ventilation below).
To make full use of the benefits of passive house design, the building’s operation should respond to temperature fluctuations: doors and windows are kept closed during warmer temperatures to keep cooler air inside, while window blinds and awnings are extended to provide shade and reduce the heat load of the sun. At night, windows are opened to encourage cross-ventilation, which purges warm air and ventilates and cools interior spaces.
Since passive design works with the local climate, the exact design approach and use of materials will depend on the location of the building. While the same overarching principles apply, a building in the temperate oceanic climate of Melbourne and a building in the humid subtropical climate of Brisbane will require different passive design responses.
How well a house retains heat in winter and stays cool in summer is described as ‘thermal performance’. Good thermal performance is achieved through the design and materials of the ‘thermal envelope’: the walls, roof, floor, glazing and insulation.
The purpose of a thermal envelope is to seal off the external environment and insulate the internal environment, minimising temperature fluctuations. Investing in a good thermal envelope will increase the building’s thermal performance, providing comfortable year-round temperatures, lower operational costs and greater energy efficiency.
Thermal mass is the ability of a material to absorb and store heat, helping stabilise temperatures and improve thermal performance. High-density materials, such as concrete, brick and rammed earth, have high thermal mass as they take a long time to heat up and cool down. Lightweight materials, such as timber, aluminium or fibre cement sheets, have low thermal mass.
In winter, elements of a house with high thermal mass, like a concrete floor or brick wall, can capture the heat from the sun penetrating through windows and doors, and hold onto that heat for many hours into the night, only slowly releasing it as temperatures cool. In summer, thermal mass that is shaded from the sun cools down over night and provides a cooling effect as air moves over it. Smart positioning and use of thermal mass can thereby act like a natural heating and cooling system, regulating temperature highs and lows to a comfortable median.
A thermal bridge is a part of a building made, for example, from metal or glass that has a higher thermal conductivity than the materials around it. It provides a pathway for heat to transfer in or out of the building envelope, thereby decreasing thermal performance and comfort, and increasing energy costs.
Thermal bridges are mitigated with thermal breaks. A thermal break is a type of insulation placed in between the building elements to prevent the transfer of heat, keeping inside temperatures cooler in summer and warmer in winter.
Glazing can be one of the main thermal bridges through which heat can exit and enter a building. Consequently, glazing has a large impact on the thermal envelope. Careful orientation and sizing of windows and using quality glazing will make a significant difference to thermal performance, comfort and energy efficiency.
Windows are desired for framing views and outdoor connections, yet large swathes of glass can result in too much heat loss or heat gain. To moderate this, the total window area should be less than 25 per cent of the total floor area of the house, and the window-to-wall ratio will differ depending on its orientation. Windows to the north will provide good solar access, while windows to the south will be a source of heat loss. Glazing on the eastern and western façades need to account for the softer morning and harsher afternoon sun. In a similar vein, skylights come with pros and cons and need to be carefully considered. They are an excellent source of natural light, but also a major source of heat gain in summer and heat loss in winter.
Installing double-glazed windows will substantially improve thermal performance. Double glazing uses two sheets of glass with an inert gas, such as argon or krypton, between them. The gas is a poor thermal conductor, slowing the passage of heat, and the entire unit is sealed in the frame for lower heat exchange rates. The window frames themselves are also thermal bridges and can lead to significant heat gain or loss — unless they are thermally broken, i.e. have an insulated barrier within the frame. Timber window frames are less conductive than aluminium frames.
According to the Australian Window Association (AWA) “up to 40 per cent of a home’s heating energy can be lost through windows and up to 87 per cent of its heat gained through them”, making double glazing an important and worthwhile investment of any sustainable home.
Insulation is the primary material used for a thermal break. Its purpose is to provide a barrier to heat loss and heat gain through the floor, walls and ceiling. Good insulation helps regulate internal temperatures and will also help with weatherproofing, sound reduction and sealing a house for air tightness.
Insulation is installed throughout the building envelope to prevent heat loss in winter and heat gain in summer: under the roofing and in the ceiling, between the framework and cavities of external walls, and on the inside or outside of solid walls. Insulation should also be installed on the underside of suspended floors, and the edge or underside of concrete slab floors, as well as in internal walls to improve thermal and acoustic compartmentalisation.
Air leakage is a year-round issue that allows hot air to escape and cool air to enter in winter, and vice versa in summer. Air will leak through any gap it can find: through doors, windows, vents and skylights, through gaps in floorboards, insulation and thermal bridging, even through power outlets and light switches. Paying close attention to properly sealing a home against air leaks improves its thermal performance and energy efficiency, and mitigates draughts and condensation.
Once sealed, the air tightness of a building can be measured by recording the number of air changes per hour between inside and out. This is a test that can be performed by a professional building envelope tester, who will also help identify where exactly leaks occur and offer advice on how to mitigate them.
Air leakage accounts for 15 to 25 per cent of heat loss in buildings in winter.
In the same way that solar energy can be used for light and heating, outside air movement as well as pressure and temperature differences can be used for ventilation and cooling. The orientation, placement and opening of windows and doors is key to encouraging and controlling natural ventilation.
Cross ventilation uses differences in air pressure, sucking air through a house towards areas of lower pressure. Larger windows or doors provide an opening for the breeze to enter a house, while smaller openings on opposite or adjacent walls suck the breeze through the house. Opening and closing various windows, fully or partially, at different times of the day/night can shift ventilation paths and control air speed.
Stack ventilation (also known as thermal chimney) uses differences in air temperature to replace warm air with cool air. Staircases, double-height ceilings, clerestory windows, roof ventilators and vented ridges draw warm air upward, while lower-level openings draw in cool air to replace the hot air as it escapes.
With different paths of natural ventilation through a house, artificial air conditioning can be avoided, and a sweep or ceiling fan, running on renewable energy, will be sufficient to cool spaces and move air on very hot days.
Glazing and outdoor spaces close to the building need to be protected against direct sun to reduce heat gain during hotter months. Fixed external devices, such as eaves, awnings, deep verandas, pergolas and brise soleil, provide shade protection against the summer sun on north- and west-facing glazing. Plantings and deciduous vines can be used to integrate greenery and provide seasonal shading. And adjustable shading, such as external blinds, on east- and west-facing openings mean glazing can be protected from the morning and afternoon summer sun as needed.
Deciduous planting offers cost effective shading with many benefits: not only does it provide a green, natural outlook that is carbon-positive with no material waste, in summer the leaves offer shade from the sun, while in winter the lack of leaves allow the sun to penetrate and warm the building.