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    Lightweight versus heavyweight houses: talking walls with Jeremy Spencer from Positive Footprints

    Nathan Johnson

    “Mass and glass” or “light and tight” have long been the two most common strategies informing the design of low-energy buildings, and both have proven their merit in different climates.  

    The mass and glass strategy basically ensures a design with large areas of north-facing glass to collect solar energy and a heavy internal thermal mass to store it. The light and tight strategy is the opposite and implores high levels of insulation between lightweight walls, small glass areas and an air-tight construction.

    Director of Melbourne-based building design outfit Positive Footprints Jeremy Spencer says that while both strategies have proven their worth, they are by no means the only methods informing his choice of exterior wall materials – or necessarily always the best.

    “A general rule, at least in the southern half of Australia, is that mass is good as long as you have appropriate north facing windows to bring the winter sunlight in,” said Spencer.

    “However there is a balance between how much energy comes in through the windows and how much mass you have.”

    “For example a room on the southern side of the house with no light and lots of mass will not receive adequate natural light in winter and will therefore rely on lots of mechanical heating to warm that space.”

    ^Jeremy and partner Chi Lu at the 2014 Sustainability Awards.

    But what about warmer climates?

    “Once you are into a climate zone where there isn’t much range of temperature, say between 20 and 29 degrees, mass doesn’t have much benefit,” explained Spencer.

    “What you want instead is a house that cools down quickly so you’ll need a lightweight construction with lots of reflective foil to stop radiant heat and a design that you can just open up at night time and let the air pass through and get the temperature back down to 20 degrees.”

    Farnan Findlay Architects used an internal skin of brick veneer to add much needed thermal mass to this Torquay Beach House renovation project. Image: John Farnan.

    DESIGN PROCESS

    Mirvac's 'Harmony 9' is a 9-Star NatHERs rated project home that makes a feature of the reverse brick veneer system. Austral Bricks and James Hardie weatherboards and fibre cement sheeting were used. (Image: Reverse Brick Veneer - blog.)

    Firstly Spencer designs through the prism of passive solar design which considers the shading, sunlight and cross ventilation of all the rooms in the house. He then moves onto detailed assessments of each room with the aid of a software system.

    “While the design is still quite fluid –before we have done working drawings and engineering—we submit it to an energy assessment software program called AccuRate which was developed by the CSRIO and recognised by the Building Code of Australia,” he explained.

    “The software we use is really powerful in that it zones each room of the house and will give you temperature and wind predictions at different times of the year using 30 years of climatic data for area.”

    “Because we use different wall types and construction techniques outside of the norm, the AccuRate system allows you to make up your own wall segments; choose different wall thicknesses and materials etc.”

    “The great thing about doing that rating early is that you can look at two different rooms in the house and see why what’s happening in one isn’t happening in the other, and you can start to play with variables such as windows and mass or even eave size and compensating for tree shade.”

    MATERIALS:

    Spencer uses reverse brick veneer walling for a lot of his projects because of its compatibility with the Victorian climate. His system includes an exterior cladding layer, an insulation section and a recycled brick interior layer (see drawing from a Reverse Brick Veneer - blog below).

    Spencer says he uses lightweight cladding for the exterior layer because per sqm they are usually less costly than brick, block, or even lightweight Hebel and they have less embodied energy. He uses James Hardy Easylap specifically because the house can be easily configured to work around its 90mm size so there is less waste and paint adheres to them well so there is a longer span between coats.

    His insulation layer usually consists of a variety of air gaps, a layer of reflective foil and a thick layer of Fletcher Sonobatt insulation batts. Spencer notes that 20mm air gaps give a slight .5 R-Value and that the internal gap that separates the bricks and pine framing also makes it easier for the bricks to be laid.

    The internal layer consists of exposed brick or a brick with a battened out plasterboard layer. The ratio of exposed brick is guided by Spencer’s AccuRate environmental assessment and according to the specific rooms need for thermal mass.

    For a more practical overview of a Jeremy Spencer’s approach to residential wall click on the image to see a case study of Solar Sollew by Positive Footprints:

     

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