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Passive cooling is a design approach that uses natural methods to regulate temperatures in buildings without the use of mechanical systems such as air conditioning.
To achieve thermal comfort in cooling applications, building envelopes are designed to minimise daytime heat gain, maximise night-time heat loss, and encourage cool breeze access when available.
Considerations include:
- Designing the floor plan and building form to respond to local climate and site.
- Using and positioning thermal mass carefully to store coolness, not unwanted heat.
- Choosing climate appropriate windows and glazing.
- Positioning windows and openings to enhance air movement and cross ventilation.
- shading windows, solar exposed walls and roofs where possible
- Installing and correctly positioning appropriate combinations of both reflective and bulk insulation
- Using roof spaces and outdoor living areas as buffer zones to limit heat gain.
1) Building envelope design
Floor plan and building form
- Envelope design is the integrated design of building form and materials as a total system to achieve optimum comfort and energy savings.
- Heat enters and leaves a home through the roof, walls, windows and floor, collectively referred to as the building envelope.
Internal layout
- Walls, doors and room arrangements — also affects heat distribution within a home.
- Good design of the envelope and internal layout responds to climate and site conditions to optimize the thermal performance. It can lower operating costs, improve comfort and lifestyle and minimize environmental impact.
Maximize the indoor
- Outdoor relationship and provide outdoor living spaces that are screened, shaded and rain protected.
- Maximize convective ventilation with high level windows and ceiling or roof space vents.
- Zone living and sleeping areas appropriately for climate – vertically and horizontally.
- Locate bedrooms for sleeping comfort.
- Design ceilings and position furniture for optimum efficiency of fans, cool breezes and convective ventilation.
- Locate mechanically cooled rooms in thermally protected areas (i.e., highly insulated, shaded and well-sealed).
2) Thermal mass
Thermal mass is the storage system for warmth and ‘coolth’ (the absence of warmth) in passive design.
- Climate responsive design means positioning thermal mass where it is exposed to appropriate levels of passive summer cooling (and solar heating in winter).
- Badly positioned mass heats up and radiates heat well into the night when external temperatures have dropped.
- As a rule of thumb, avoid or limit thermal mass in upstairs sleeping areas. In climates with little or no heating requirement, low mass is generally the preferred option.
- Earth-coupled concrete slabs-on-ground provide a heat sink where deep earth temperatures (at 3m depth or more) are favourable, but should be avoided in climates where deep earth temperatures contribute to heat gain. In these regions, use open vented floors with high levels of insulation to avoid heat gain.
- In regions where deep earth temperatures are lower, consider enclosing subfloor areas to allow earth coupling to reduce temperatures and therefore heat gains.
3) Windows and shading
- Windows and shading are the most critical elements in passive cooling. They are the main source of heat gain, via direct radiation and conduction, and of cooling, via cross, stack and fan-drawn ventilation, cool breeze access and night purging.
- Low sun angles through east and west-facing windows increase heat gain, while north-facing windows (south in tropics) transmit less heat in summer because the higher angles of incidence reflect more radiation.
4) Air movement and ventilation
- Design to maximise beneficial cooling breezes by providing multiple flow paths and minimising potential barriers; single depth rooms are ideal in warmer climates.
- Because breezes come from many directions and can be deflected or diverted, orientation to breeze direction is less important than the actual design of windows and openings to collect and direct breezes within and through the home.
- Use casement windows to catch and deflect breezes from varying angles.
- For breeze collection, window design is more important than orientation.
- Wind doesn’t blow through a building — it is sucked towards areas of lower air pressure. To draw the breeze through, use larger openings on the leeward (low pressure or downwind) side of the house and smaller openings on the breeze or windward (high pressure or upwind) side. Openings near the centre of the high-pressure zone are more effective because pressure is highest near the centre of the windward wall and diminishes toward the edges as the wind finds other ways to move around the building.
- Airflow pattern and speed for different opening areas.
- In climates requiring winter heating the need for passive solar north sun influences these considerations; designers should strive for a balanced approach.
- The design of openings to direct airflow inside the home is a critical but much overlooked design component of passive cooling. Size, type, external shading and horizontal/vertical position of any openings (doors and windows) is critical — as shown in the diagrams below.
- Airflow pattern for windows of different opening height. Louvre windows help to vary ventilation paths and control air speed.
- Consider installing a louvre window above doors to let breezes pass through the building while maintaining privacy and security. In climates requiring cooling only, consider placing similar panels above head height in internal walls to allow cross-ventilation to move the hottest air.
- Position windows (vertically and horizontally) to direct airflow to the area where occupants spend most time (e.g. dining table, lounge or bed).
- In rooms where it is not possible to place windows in opposite or adjacent walls for cross-ventilation, place projecting fins on the windward side to create positive and negative pressure to draw breezes through the room, as shown in the diagram below.
- Design and locate planting, fences and outbuildings to funnel breezes into and through the building, filter stronger winds and exclude adverse hot or cold winds.
5) Insulation
- Insulation is critical to passive cooling — particularly to the roof and floor. Windows are often left open to take advantage of natural cooling and walls are easily shaded; roofs, however, are difficult to shade, and floors are a source of constant heat gain through conduction and convection, with only limited cooling contribution to offset it.
- Insulation levels and installation details for each climate zone are provided in Insulation and Insulation installation. Pay careful attention to up and down insulation values and choose appropriately for purpose and location.
- In climates that require only cooling or those with limited cooling needs, use multiple layers of reflective foil insulation in the roof instead of bulk insulation to reduce radiant daytime heat gains while maximising night-time heat loss through conduction and convection. This is known as the one-way insulation valve.
- Reflective foil insulation is less affected by condensation and is highly suited to cooling climate applications as it reflects unwanted heat out while not re-radiating it in.
6) Roof space
- Well-ventilated roof spaces (and other non-habitable spaces) play a critical role in passive cooling by providing a buffer zone between internal and external spaces in the most difficult area to shade, the roof.
- Well-ventilated roof spaces form a buffer between internal and external areas.
- Ventilators can reduce the temperature differential (see Passive heating) across ceiling insulation, increasing its effectiveness by as much as 100%. The use of foil insulation and light coloured roofing limits radiant heat flow into the roof space.
- Use careful detailing to prevent condensation from saturating the ceiling and insulation. Dew-points form where humid air comes into contact with a cooler surface, e.g. the underside of roof sarking or reflective foil insulation cooled by radiation to a clear night sky (see Sealing your home).
By incorporating these principles into the design of a building, it is possible to create a comfortable living or working environment without the need for expensive and energy-intensive mechanical cooling systems.