Design principles



Insulation reduces the rate of heat flow between inside and outside. Heat flows when the outside temperature is hotter than the inside, like on a hot summer day. Heat flows outward when the inside temperature is hotter than the outside, like in winter or at night.

There are two main types of insulation:

  • Bulk insulation acts primarily by reducing heat transfer by conduction and convection. Use bulk insulation at ceiling-level, in walls and floors.
  • Reflective insulation acts primarily by reducing heat transfer by radiation. Use reflective insulation at roof-level and in walls, especially in hot climates.

To lower summer heat gains through the roof, also consider:

R‑Value: thermal resistance

The effectiveness of insulation is related to its thermal resistance, called R‑Value, and is measured in m².K/W.

The Total R‑Value of a composite building element is the sum of:

  • the System R‑Value of construction, which includes:
    • the Inherent R‑Value of the construction materials; and
    • the resistances of any airspaces (considering reflective surfaces); and
  • the surface resistances, which includes:
    • indoor and outdoor air films; and
    • for floors, the ground surface resistance; and
  • any Additional R‑Value provided by bulk insulation materials.

Note that the performance of reflective insulation products is typically stated as a Total R‑Value or System R‑Value because these products require the adjacent airspace(s) to perform as tested.

Direction of heat flow

This is the required direction of heat flow in which the R‑value must be achieved. The direction is dependent on the climate zone.

In summer heat flows down through the roof and up through suspended floors whereas in winter it flows up through the roof and down through suspended floors.

Foil insulation products installed in floors, ceilings and roofs have significantly higher R‑values for heat flowing down than up. BASIX provides the direction of heat flow so you can select the best insulation for your climate. For walls the R‑value is the same in summer and winter because the heat is flowing horizontally.

Bulk insulation products can also be used because they will meet the required R‑value in both directions.

Check the manufacturer’s product information for the direction of heat flow. Some manufacturers might show summer and winter R‑values on their product literature. They will be able to clarify the direction of heat flow for these values for roofs and floors.

Condensation in buildings

Condensation, in relation to buildings, is the process of water vapour in moist air changing to liquid form when it comes into contact with cold surfaces (surface condensation) or is cooled while permeating the building fabric (interstitial condensation). Surface condensation forms visible droplets of condensate. Interstitial condensation releases the moisture in permeable materials such as bulk insulation, timber and masonry.

The ABCB has produced the "Condensation in Buildings" information handbook to explain the design strategies available to minimise the risk of condensation. The handbook is available on the Australian Building Codes Board (ABCB) website.

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The design should manage breezes and allow for controlled ventilation while reducing uncontrolled air leakage. Ventilation is important for thermal comfort, air quality and moisture control.

Managing breezes involves:

  • using operable windows;
  • having windows on both sides of the dwelling for cross-ventilation;
  • landscaping and window layout to capture cooling breezes in summer;
  • landscaping and window layout to deflect cold breezes in winter; and
  • having high-level windows to allow hot air to escape and draw in cooler air from low-level windows.

Controlling ventilation involves:

  • using operable windows that seal well;
  • locating windows so they can be open and secure regardless of the weather;
  • installing weatherstripping and draught-sealing doors;
  • sealing the dwelling to minimise uncontrolled air leakage.

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The glazing design needs to balance heat, light, ventilation and access to views.

The heating load can be reduced by:

  • choosing a glass and frame type with a lower U-value, as this will reduce heat loss;
  • choosing a glass and frame type with a higher Solar Heat Gain Coefficient (SHGC), as this will increase the solar gains (but will increase cooling loads);
  • using adjustable shading instead of fixed shading to allow winter solar access;
  • raising the height of shading or lowering the sill of windows to avoid permanent shading at the top of the window;
  • reducing shading overhangs (but be aware that this will increase cooling loads);
  • not placing windows where they are overshadowed;
  • moving windows to north, northeast or northwest for more solar gains; and
  • reducing the area of windows facing south, southeast and southwest to minimise heat loss.

The cooling load can be reduced by:

  • choosing a glass and frame with a lower U-value to reduce conducted heat gain;
  • increasing shading overhangs or using external louvres or awnings (but be aware that this will increase heating loads);
  • choosing a glass and frame type with a lower Solar Heat Gain Coefficient (SHGC) to reduce heat gain from sun striking the glass in summer (but be aware that this will increase heating loads);
  • reducing the area of windows facing west, east, northwest.

Other things to consider include:

  • choosing a glass and frame type with an acceptable Visible Transmittance for natural lighting;
  • choosing shading, rather than tinting, can reduce cooling loads with less of an increase in heating loads;
  • choosing operable windows for ventilation and cooling potential; and
  • overshadowing by adjacent buildings and existing vegetation;
  • locating thermal mass appropriately relative to windows to capture useful heat gains but avoid overheating.

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Orientation and siting

Use orientation and siting to balance solar access for winter warming (if required) while avoiding overheating in summer.

The heating load can be reduced by:

  • locating private open space to the north in order to allow sufficient space from adjacent buildings for winter solar access;
  • locating living areas to the north (opening on to the private open space), and provide them with thermal mass and north-facing windows for passive heating; and
  • locating windows on the north, northeast and northwest orientations.

The cooling load can be reduced by:

  • locating laundries, bathrooms, garages and sheds on the south, east and west to shelter from excessive sun and wind; and
  • reducing windows on the east, west and northwest orientations.

Also consider balancing the optimal orientation for solar access with optimal orientation for ventilation.

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