Meeting 5-Star Energy Efficiency with Timber Framing 

For new homes in Western Australia, requirements for 5-star energy efficiency ratings have been introduced through the Building Code of Australia (BCA). Energy efficient homes should be more comfortable to live in, save energy costs and reduce the production of greenhouse gases which are contributing to climate change. It is important to note that the 5 star energy efficiency regulations aim to minimize the need for heating and cooling that rely on the use of additional energy from heaters and air-conditioners. At the end of the day a household may use no additional heating or cooling however this may reflect the usage pattern of the occupiers and their tolerance to temperature extremes even if the home is very energy inefficient.

There are many ways to design successful energy efficient homes and they can be constructed from many materials. Lightweight timber construction is thermally efficient and can be designed to capture the sun in winter and prevailing breezes in summer to assist heating and cooling. Furthermore, cavity walls and platform floors of lightweight timber construction can be easily insulated.

In designing energy efficient homes a number of important considerations apply:

1. The climate zone. The BCA defines a number of climate zones used for energy efficiency requirements (see the BCA climate zone map). Thermal design will be fundamentally different in northern tropical climates than it would be in southern cooler climates for example.

2. Orientation. Most “passive solar” designs aim to orient the home to capture the sun in winter months by orienting the main windows to the North while reducing building exposure to westerly sun in summer. Note that building shading through the use of trees or awnings over windows has a significant effect on heat gain but can only be effective in conjunction with the right orientation. Orientation of the building is really only possible where the block of land makes it practicable.

3. Insulation levels. Insulation is a necessary and low cost way of improving energy efficiency of homes. Insulation stops hot air getting inside in summer and warm heated air escaping in winter. Due to framing details, a lightweight timber house is extremely easy to insulate.

4. Ventilation and sealing. In many temperate areas of Australia, for example in the Perth area, summers can be very hot pushing into the late 30°C’s and winters can be quite cold dropping to 0°C. On a hot day if the home can let the breezes through this can make the occupants comfortable. In cold weather it is an advantage to stop air leakage around doors and windows as well as chimney flues and exhaust fans. In other words, a well designed house can be opened to cooling breezes during summer and sealed from the cold during winter.

5. Material properties. The materials used in building have been extracted, processed, packaged and transported to site where they are built into the final structure. The energy used in performing these operations is known as the embodied energy and can vary considerably between materials. Timber and timber products have one of the lowest embodied energies of all materials and this energy is significant compared to the operational energy of the house. Although no account is taken of embodied energy in regulations at this time, designers and homeowners should be aware that timber based construction benefits the environment by expending least energy in production.

Building Code of Australia (BCA) Insulation Requirements

The BCA Housing Provisions enable house designers to meet the energy efficiency requirements in one of two ways:

1. The first is the use of energy rating software. The available and allowable software varies in different states, however in Western Australia AccuRate energy rating software may be used. The software works by simulating the theoretical heating and cooling energy requirements of the house for the particular climate zone and allocating a star-rating to the house based on this theoretical energy use. The use of energy rating software allows a much more flexible approach to the design of the house or building and many combinations of external fabric and construction options may be used to obtain a 5-star rating. Importantly, glazing size and performance may be inputed directly into the simulation as well as the use of suspended timber floor options, wall types, insulation and many more. Use of rating software should result in a more refined energy efficient design. The use of energy rating software and subsequent 5-star rating must be performed by accredited raters. A list of these accredited raters is available from the Association of Building Sustainablity Assessors (ABSA).
   

2. The second method is meeting the Deemed to Satisfy provisions contained within the BCA. These provisions specify the required performance of the envelope building elements for the different climate zones and provide options for satisfying the required performance. While this method gives a simple range of possible constructions that satisfy the requirements, it will not necessarily give an optimal solution as it looks at the building elements generally in isolation.

   
   

 Figure 1 – Total insulation requirements of BCA for 5-star energy efficiency in Western Australia (Deemed to Satisfy method)

 * BCA requirement is for heat flow up only in these zones

Figure 2 – Solutions for additional insulation in timber framed homes

* Reflective foil insulation can be vapour barrier with high or low emittance foil coating

** R-values for roof materials are R0.35 tiled roof and R0.37 metal roof

*** In Albany it is also worth considering reducing window sizes to supplement the performance of walls and reduce additional insulation requirement

**** Reflective foil insulation and other equivalent products with an air-gap may satisfy the BCA requirement. Consult suppliers for product performance.

Window Glazing 

Glazing is a very important element in the satisfactory thermal performance of a building. Correct orientation, size of window and type of frame and glazing must all be considered to ensure the satisfactory performance of the building. Poorly placed and sized windows can allow excessive heat buildup in summer or allow heat to escape in winter time. The deemed to satisfy provisions of the BCA indicate a range of performance requirements for various window and frame types, however, for more details and accurate performance data, the specific manufacturers data should be consulted.

Flooring 

Timber framed floor systems have several important advantages over other floor systems.

1. Less site disturbance. Timber framed floors mean that less site excavation may be required which saves on retaining costs.

2. Sloping sites. More homes are being built on marginal land and sloping sites. Suspended timber floors are a natural and much more cost effective than suspended concrete floors in these situations.
   

3. Suits timber flooring. Traditional tongue and groove timber flooring can be fixed directly to timber floor joists.

4. Dry and off the ground. Timber framed floors remain dry in any weather and allow easy termite inspection.

Further Information and Links

• Environmentally Friendly Housing using Timber
  
  This guide published by the National Timber Development Council sets out the principles for building environmentally friendly and energy efficient housing using traditional timber construction.
• Australian Greenhouse Office – Your Home Guide
  
  This guide published by the Australian Greenhouse Office has a wealth of information about environmentally sustainable homes including building lightweight timber homes. Also included are two case studies from homes built in Perth.
• Case Studies of Five Star homes in Perth - 75.a
• Case Studies of Five Star homes in Perth - 75.f