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To be sustainable, a home needs to be built in such a way as to minimise its impact on the environment. This means constructing the home so that it works in harmony with its surroundings and uses up less resources, such as power and water, during its lifetime.
The added advantage of this is that the people living in the home will beneﬁt in terms of reduced energy bills, making the home sustainable from the point of view of continuing affordability - that is the cost of living in the home is able to be sustained.
One of the key ways of doing this is by incorporating passive design principles into your home.
The sun is a massive source of free energy. Use it well and it can provide natural lighting in your home, warmth to heat your home during cooler months, and energy to heat your water during the warmer seasons.
Using the sun in this way is one of the key principles of passive design and it costs little or nothing to do this. It does however require planning and thought and this applies right back to the selection of your building site. You need to consider where the sun will be at different parts of the day during each season to decide where best to position your home on your section. Then the next step is to determine the layout of the various rooms in your home.
To achieve the best orientation for passive home heating, your living, family and dining rooms should face north and have plenty of large windows. It is often a good choice to have your kitchen facing east as this will enable it to beneﬁt from early morning sun throughout the year but it will be cooler later in the day for evening meal preparation. Similarly, east-facing bedrooms will be cooler and more comfortable for sleeping, especially for children who tend to go to bed earlier.
The main additional cost that passive heating can entail is the cost of larger or additional windows. If installing skylights this can also affect the style of roof you choose which can have cost implications.
Like anything, with the sun, you can get too much of a good thing. You don’t want to save lots of energy lighting and heating your home using the sun only to have to expend it cooling your overheated home in the middle of summer. For this reason it is also important to consider shading, particularly for west-facing rooms that get low-angled sun late in the day. Shading can be in the form of trees or a covered verandah or sun-blinds. Careful placement of deciduous trees or vines can give you shade in summer to help reduce overheating, while allowing the winter sun in when you need it for heating and light.
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Once you have the sun’s warmth entering your well-designed home, the next thing to do is to store it.
The ability of a material to store heat energy is referred to as “thermal mass” and this is another important principle of passive design.
In practical terms, the idea is to expose a high-density material in your home’s interior, such as concrete, stone or yes - even solid timber - to direct sunlight. These materials all absorb heat from the surrounding air and then release the heat when the temperature in the room drops below that of the mass. When used effectively this can help maintain a more stable temperature inside your home.
A concrete slab ﬂoor is the most commonly used thermal mass, however a feature stone or concrete block wall is another option. If you are using a concrete ﬂoor as a thermal mass, you should never cover it with carpet, cork or timber as these materials have high thermal resistance. The concrete needs to be exposed (polished concrete) or covered with a high density material such as tiles, preferably dark-coloured tiles as light colours will reﬂect the heat.
It is also important to have insulation under your concrete mass so that heat moves up into your home rather than dissipating into the ground, as well as insulation on the ends of the slab to prevent heat escaping from the perimeter of the slab to the outside air.
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As we saw in the previous section, insulation works in tandem with thermal mass to maximise the beneﬁts of storing heat. Without adequate insulation, other sustainable features in a home will be rendered ineffective as heat will travel in and out of the home freely. The idea of insulation is to act as a barrier to heat, preventing heat from ﬂowing out of the home in cooler weather and reducing the amount of heat entering the home in hot weather.
Insulation is needed in the ceiling, walls and ﬂoor. In terms of product, there are a variety of different types of insulation available in New Zealand. Glasswool insulation is made from recycled glass and is the most commonly used insulation for walls and ceilings because it does not burn easily and is cost-effective. Other options include wool and polyester insulation but these all need to be separated from heat sources due to their greater combustibility.
In terms of the durability aspect of sustainability, insulation products represent good value for money as they have a lifespan of 50 years or more.
Polystyrene boards with a damp-proof membrane are generally used as insulation under concrete slabs. However research has shown that slab perimeter insulation is more essential than insulation underneath the slab as most of the heat loss from the slab occurs at the edges between the air and the ground.
According to studies carried out by BRANZ, combining under-slab insulation with insulation around the slab edge can result in improvements of thermal performance of 100% or more. Perimeter insulation is therefore really worth doing as it results in signiﬁcant gains in energy eﬃciency.
You can also improve your home’s energy eﬃciency and thus sustainability by using thicker insulation in your walls and ceiling to bring the R-value above the minimum required. In our experience, additional money spent on extra insulation is money well spent as you will get it back in reduced heating and cooling costs over the lifetime of your home.
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As we have touched on already, the size and placement of windows is a critical part of passive design, particularly having larger windows in the living areas to maximise sun and light.
Windows also play the important role of enabling ventilation, which is another element of having a healthy, sustainable home. Letting fresh air into your home can help to cool your home in the summer, while opening your windows regularly in the winter helps prevent moisture from building up in your home. A dry home is easier to heat and will therefore be more energy-eﬃcient and more comfortable to live in.
With all of this in mind, it pays to not just consider sun, views and privacy when deciding where to place windows in your home, but also the direction of prevailing winds and which windows should be opening windows with stays or catches to promote easy airﬂow.
Even when windows are not open, they can account for more heat gain or loss than any other part of a well-insulated new home. As heat is transferred through both the glass and the framing we need to look at ways to minimise heat transfer through both.
Double glazed windows with aluminium frames are the standard in most new homes. While they certainly perform far better than old-fashioned single glazed windows, there are other options which can help reduce heat loss and gain signiﬁcantly compared to standard aluminium-framed double glazing.
One option is to have tinted or coated glass such as low-emissivity (low-E) glass as one pane of double glazing. Low-E glass can reduce glare, while improving thermal performance and the R-value of the window by 50%.
Another option is to choose windows with timber or uPVC frames which provide better insulation than aluminium or to select thermally broken aluminium frames. Thermally broken aluminium frames have a thermal break in the centre of the aluminium frame. BRANZ testing has shown that frames with this feature can be almost 60% more thermally eﬃcient than regular aluminium frames.
An often overlooked consideration is to complement your insulated windows with lined curtains or blinds to reduce heat loss from your windows at night.
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Appliances such as whiteware, TVs and heated towel rails can account for 29% of energy use in an average New Zealand house. It makes sense therefore to consider selecting energy-eﬃcient appliances for your home as an easy way of reducing the ongoing impact your household is having on the environment, as well as reducing your power bills.
One mistake people sometimes make is to choose ﬁttings or appliances that are too large or too small for their needs. For example, if you have a small capacity washing machine for a large family you may end up using more power and water than necessary, running more loads. Conversely if you have a heat pump with a greater output than you need, you may be using more power than necessary to heat or cool your home.
EECA estimates that by 2035, households could reduce their energy use by around 20 per cent through more eﬃcient space heating, water heating and lighting. This is deﬁnitely an area where you can make a considerable difference to your home’s environmental impact by choosing recognised brands with good energy star ratings - that way you are more likely to get the beneﬁts of durability as well as reduced energy and/or water consumption over the life of the appliance.
In terms of sustainable heating options for your home, the new ultra low emission woodburners are well worth considering. While they are more expensive than regular woodburners, unlike regular woodburners they can be installed and used even in restrictive clean air zones. Ultra low emission woodburners have two burning chambers which enables them to burn very cleanly and eﬃciently - their thermal eﬃciency rating is 65 per cent or greater. They also have a cooktop so that you can use them for boiling water or cooking and some have a wetback option. Some models even have a USB port to charge your mobile phones during a blackout!
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Solar water heating is a really worthwhile investment - it is energy-eﬃcient and has low running costs. It will add an additional $5,000 to $10,000 to your build costs, depending on the size of system that you require. However, a well-installed system will typically save you 70% on your annual water heating bill.
Because solar energy is variable, a booster system or wetback is required to provide reliable water heating, particularly in the winter.
Another very eﬃcient option that is well worth considering is a heat pump hot water system. Heat pumps are the most eﬃcient electricity-based option for heating water and are comparable to solar water heating in terms of energy eﬃciency.
As with solar water heating, it is common to include an electrical heating element to supplement the heat pump. This is because they tend to work less well in lower outdoor temperatures, such as those experienced in the coolest parts of the South Island during winter, although they are improving all the time.
If you have a building site that doesn’t get a great deal of direct sun, a heat pump may be the ideal choice for heating your water. They are also an excellent option for heating large amounts of water for an underﬂoor heating system, a spa or a swimming pool.
However, if you have a smaller household of one or two people, and don’t use a lot of water they may not be the best solution. You also need to factor in the placement of the exterior unit as they can be rather noisy.
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As we have indicated in the previous section, all appliances are not created equal in terms of water use so if you are serious about sustainability you should compare the water use of appliances such as dishwashers and washing machines carefully.
Another way to reduce your water use is to install a rainwater tank on your property. If you have access to a town water supply, the Ministry of Health recommends that you use this for drinking and food preparation. However, you may like to have a rainwater tank to provide water for your garden, washing clothes and ﬂushing the toilet. While your home may not yet be on a metered water supply, this may change in the future so having a separate rainwater supply could save you money down the track. Other bonuses are that you will have an independent supply in the case of emergencies such as earthquakes when public water supplies may not available for many days.
Greywater (such as the wastewater from showers and clothes washing) can be collected and used for garden irrigation. It is important that greywater systems are installed by a licensed plumber as they need backﬂow prevention devices to ensure that potable (drinking) water does not become contaminated.
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In order to evaluate the sustainability of building materials you need to think about how and where the products have been made. It is always better to choose locally-manufactured products wherever possible as importing materials means additional energy in the form of fuel has been used on transportation, thereby also creating more emissions.
Wood is a sustainable building material because it is a renewable resource, unlike concrete and steel. One study identiﬁed that the total energy consumption in the manufacturing of steel beams is 2-3 times higher, and the use of fossil fuels 6-12 times higher, than when manufacturing timber beams.
In New Zealand it has been estimated that a 17% increase in wood usage in the building industry would result in a 20% reduction in carbon emissions from the manufacture of all building materials. This represents 1.5% of New Zealand’s total emissions.
When choosing timber products, look for sustainably-grown timber, such as New Zealand plantation pine, which is grown on land that would otherwise be unsuitable for use.
Another obvious factor to consider is using safe and healthy materials. Research has shown that timber offers many health beneﬁts including moderating indoor humidity. Timber provokes feelings of warmth, comfort and relaxation, helping to reduce stress levels,blood pressure and heart rate.
Paints and sealing products give off volatile organic compounds (VOCs) which can contribute to asthma and other health problems so look for paints which carry the “Environmental Choice” label. Generally waterborne paint emits less fumes than oil-based paint.
Inadequate insulation and air leakage are the main causes of heat loss in homes.
One of the most recent approaches to reducing heat loss from homes is to make the home airtight so that air and moisture cannot move freely through the building structure. If air can move uncontrolled through the structure of a building then this can reduce the effectiveness of insulation and let in moisture, which can eventually cause the insulation to deteriorate.
The concept of making your home airtight is comparable to wearing a thin windproof jacket over a woollen jersey in a cold wind - an airtight membrane is wrapped around the entire building structure to protect against air movement and retain heat.
The downside of airtightness systems is unless it is paired with a mechanical ventilation system, a home will become stuffy and unhealthy as moisture and condensation can become trapped. Mechanical ventilation (or ‘heat exchange’) systems enable your home to “breathe”, but are presently quite expensive - in the vicinity of $15,000, so the actual exercise of making a home airtight can require signiﬁcant investment. Research conclusively proves that excessive moisture levels in a home will make it harder to heat and will also reduce the value of double glazing and insulation.
There have been many improvements in the sustainability of new homes being built in New Zealand due to new technologies, increased insulation requirements and more awareness of the principles of passive design. However, according to recent research published by the University of Otago, the recent trend towards building larger houses is cancelling out the gains that have been made. Figures from Statistics New Zealand show that in the year ended August 1974, the average floor area for a new home was 110 m2, whereas in the year ended August 2016, the average floor area was 182 m2. Larger houses take more energy to heat, cool and light. So if you are looking to build sustainably, it makes sense to really think about how big a house you actually need.
Of course, technology is advancing all of the time and new ways of thinking may help us to power our homes more efficiently. One exciting new product on the market is the home battery, such as Tesla’s Powerwall, which costs approximately $10,000 plus installation. This technology enables you to store power from solar or other systems giving you the potential to meet the electricity needs of your home or have back-up power in the case of major outages.
As we have seen in this research paper, a home built using sustainable principles will not only be better for the environment, it will also be healthier and more comfortable to live in than a home built as cheaply as possible with little thought to design and material use.
Passive design is an important cornerstone of sustainable building and covers many elements including: the layout of your home and orientation of your home on its building site; the size, placement and type of windows used; insulation; thermal mass; shading; and ventilation. Each of these elements works with the other elements to enable you to minimise energy use by maximising the beneﬁts of free, renewable sources of energy.
The other important considerations when building a sustainable home are choosing renewable building materials and non-toxic products; selecting energy and water-saving appliances and ﬁttings; and recycling water where possible.
While building a sustainable home may cost a little more up front, it will cost less to run and maintain over its lifetime, reducing the burden of energy bills on the homeowners at the same time as reducing the burden on the planet.