Passive Design – Building Thermal Rating Assessors

The ideal percentage of glazing to wall—often referred to as the window-to-wall ratio (WWR)—depends on several factors, including the building’s orientation, climate, and intended use. Here’s a general guideline for different orientations:

North-Facing Walls:

Ideal WWR: 20-40%
Reason: North-facing windows (in the Southern Hemisphere) receive the most consistent and beneficial solar gain throughout the year. A higher glazing percentage can optimize passive solar heating and daylighting, which can enhance energy efficiency and comfort.

South-Facing Walls:

Ideal WWR: 10-20%
Reason: South-facing windows receive less direct solar gain, especially in winter. Limiting glazing helps to reduce heat loss and prevent excessive cooling loads. However, strategic placement of high-performance windows can still contribute to daylighting without compromising energy efficiency.

East-Facing Walls:

Ideal WWR: 10-25%
Reason: East-facing windows benefit from morning sun, which can be useful for passive solar heating in the morning but can also cause overheating in the summer. Balancing glazing with shading devices is important to manage heat gain and glare.

West-Facing Walls:

Ideal WWR: 10-25%
Reason: West-facing windows receive strong afternoon sunlight, which can lead to overheating and glare issues. It’s crucial to use shading devices or limit glazing to manage these effects effectively.

General Considerations:

Climate: In colder climates, a higher WWR can improve passive solar heating. In hotter climates, a lower WWR helps reduce cooling loads.
Shading and Glazing Performance: The use of high-performance glazing and shading devices can allow for a higher WWR while managing solar gains and heat loss more effectively.
Building Use: The intended use of the building (e.g., residential, commercial, or office) can influence the optimal WWR based on factors like lighting needs and privacy.

These percentages are starting points and should be adapted based on specific design requirements, local climate conditions, and performance goals. Energy modeling and consulting with building professionals can provide more tailored recommendations for achieving the best balance of daylighting, thermal performance, and energy efficiency.

As part of our preparation for the proposed change to a minimum 7-star NatHERS rating, we revisit the benefit of passive solar design. Not only is its application fundamental to achieving a 7-star energy rating, but the benefit is clear: passive solar design adapts a building to its environment creating a comfortable home throughout the year; minimising the need for mechanical heating and cooling; resulting in lower household energy use, lower energy costs, and smaller carbon footprint.

At the concept stage of a building project, an understanding of climate location is fundamental to creating a design that adapts to environmental factors. For example, a building designed for a cool climate harnesses and stores heat from the sun during winter and provides shading and ventilation during summer.

Here are some of the key principles of passive solar design to keep in mind for building projects in the southern half of Australia:

Orientation and Siting – a rectangular building where the longest walls face north/south, concentrating the habitable room area, utilising the suns heat during winter whilst minimising during summer through shade and ventilation. Take into account shading from neighbours, vegetation, landform, setbacks and easements.
Layout – locate living areas to the north to benefit from daytime sun, bedrooms rooms to the south and unconditioned utility rooms to the west.
Thermal mass – use and expose the building slab to winter sun, so that it stores heat throughout the day and releases that heat throughout the night. In summer the slab provides a cooling benefit by drawing heat from the rooms above during the day. Natural ventilation can then transfer the releasing heat during the night to the outside. Slab insulation is becoming more common to optimise the benefit of thermal mass.
Building materials and insulation – choose building materials for walls and ceiling combined with insulation to retain heat within the building during winter and provide protection from daytime heat during summer.
Windows – as a rule of thumb 50% of the northern wall area should be glazed, whilst 15% to the east and west and 10% to the south. Openable windows provide natural ventilation during summer. Select windows that provide a balance of properties that insulate the home (low U-value) and allow the sun’s rays to provide warmth during winter (solar heat gain). Double glazing has become more common, and some building designs also require high-performance low-e glass. Timber and uPVC have better insulation properties than aluminium frames. Also bear in mind options for tinting and shading.

Including these fundamentals into your next building project will provide a comfortable home environment to the occupants, whilst minimising their energy costs and environmental impact. This is also necessary to meet a 7-star rating, so feel free to contact us if you’d like assistance.

Tag: Passive Design

Window-to-Wall Ratio