Creating the Living House
Construction - the second factor • Infiltration or draughtiness • Mass • Structural framing • Window frames • Glazings
An energy efficient house relies heavily on the ability of the outer “skin” (floor, walls and roof ) to maintain comfortable living conditions inside the house. If the skin leaks air, conditions inside will quickly match those outside. You can either pump energy into the airconditioning and heating systems or sort the problem out at the source. Obvious weak points are around door and window frames and the doors and window sashes themselves, where electrical fittings penetrate walls and ceilings (especially incandescent downlights), ducting grilles and fireplace flues and of course suspended timber floors. All of these potential problems can be avoided by more careful fitting of wall wrap, insulation, seals, duct registers and floorcoverings. Same thing if the skin is able to conduct heat easily from one side to the other, conditions inside will quickly match those outside. The answer is to provide effective insulation. A combination of a reflective barrier (foil) and closely fitted bulk insulation (batts and blankets) to maintain a still air space is becoming common practice but care needs to be taken to install this correctly. Roofs require reflective foil (sarking) under tiles or anticon foil-faced blanket under metal roofs together with snugly fitted ceiling batts. Timber floors benefit from bulk and/or foil insulation although a bigger problem here is often air infiltration through the cracks. Concrete slabs should be insulated around the perimeter to take full advantage of earth-coupled thermal mass and to prevent excessive heat loss, especially when in slab heating is installed. We’re still carrying most of a home’s roof loads through the
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exterior walls so the structural framing members are typically part of the outer skin. They are usually either steel, which is a poor insulator or timber which is a better insulator (but nowhere near as good as the batts installed in the wall). Care needs to be taken to provide an effective “thermal break” between steel structural members and cladding materials to avoid flanking heat loss and even condensation problems. Like the building skin itself, window frames may be made of materials which are either good insulators like timber or plastics, good conductors like steel and aluminium or composites which take advantage of the best qualities of both. Glazing needs to be appropriate to the orientation, exposure and size of the window unit. There’s a huge range of options available to maximise the thermal and acoustic performance of glazed panels which includes specialised coatings, laminated films, tinting, reflective films and insulated glazing units (IGU’s). A standard double glazed IGU can more than double the insulation value of an appropriately framed window and further improvements are possible if one or more panes are thermally improved glass. Increasing the size of the air gap between the panes, or even filing it with argon gas, also substantially improves the performance of the window. The effectiveness of the combination of frames and glazings is readily comparable through the Window Energy Rating Scheme (WERS) with separate ratings for heating and cooling situations. High heating ratings are best for the Canberra region but cooling ratings can also be important for unshaded westerly windows.
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Materials - the third factor • Insulation (including framing members) • Colour • Floor coverings • Embodied energy • Toxicity and LCA with Ventilation
An effective skin with well placed windows also allows us to control the performance of the most useful tool we have for maintaining thermal comfort within a building designed for a ‘heating climate’, materials with thermal mass. They are slow to take up thermal energy and slow to give it up. This ‘reluctance’ to change temperature is known as thermal inertia or lag. We can take advantage of this characteristic with good passive solar design. Common building materials such as reinforced concrete (especially floors) and brickwork (interior walls) are placed where they can receive free solar energy during the day in winter. The stored warmth is released into the rooms through the night. In summer the same materials are protected from receiving thermal energy. They now have a cooling effect during the day after giving up any energy absorbed during the day to cooling natural cross ventilation over night.
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So we are relying on the skin of the building to act like a good esky to keep what’s inside at a constant temperature, hopefully without the need to pump energy into airconditioning and heating systems. Like an esky, the insulating materials are not particularly durable or pretty so we can choose from the entire range of exterior finishes for our building as long as the insulation is a good match. Reflective foils, cellular reflective foils, double walled reflective foils, foil faced insulating blankets, wall and ceiling batts, rigid foam panels and inserts, fibreglass, polyester, natural wool, rockwool, cellulose, EPS beads, non itch, recycled, recyclable, composite, bonded … often designated according to their heat flow resistance (called an R-value). The bigger the R-value, the better is the insulation. Insulation may even be inadvertent and detrimental. The beneficial thermal mass of an earth coupled concrete floor may be masked by choosing insulating floor coverings such as floating timber floors act winning homes 2011
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www.exemplary.com.au or carpeting; (However, if that floor is suspended and exposed to the cold air from beneath., that same insulating finish can improve the comfort of your home). If you aim to produce an energy efficient house and save a bundle on your energy bills then passive solar design and a well constructed skin protecting strategically placed thermal mass will get you there. If your aim is to build a house that is sustainable then you’ll need to consider whether the materials have come from a sustainable and renewable resource. Is it recycled or recyclable and what do I do with the waste? Is the product local and can locals install it? And how much energy is needed to produce and transport the material to site before it’s installed? Most building materials require a certain amount of energy to produce and transport to site before actually being incorporated into the building. This is called the embodied energy of the product and is measured in megajoules/kilogram (MJ/kg as in Lawson, 1996). A mud brick might be hand made on site from materials dug up on site and laid by local labour. The embodied energy in a mud brick wall that uses no cement would be almost nil, but to build a clay brick wall using cement mortar, the clay would be quarried, transported, processed, kiln fired, transported, probably up to three times before it arrives on site, moved around on site, and laid in cement mortar using sand which has no doubt travelled and cement which has been produced by a process even more energy intensive than brickmaking. However, the bricks are durable, reusable and recyclable so the life of the material relative to its embodied energy becomes a mitigating factor.