August 4, 2015

What is a Net Zero Home?

net zero house

There’s been a lot of talk lately about net-zero energy (NZE) homes but what exactly are they? The idea is they generate enough energy to offset the amount they use. But most homes are unable to produce that much, so most of the equation is based on energy conservation. This means the house must be insulated to the max and anything that uses power must be as energy-efficient as possible.

When it comes to producing energy the key is harnessing the sun. It can be used to generate electricity, heat water and warm the air inside. When the sun is too hot, ways to cool the house without using power are required. Other renewable energy sources include wind power, geothermal and even water turbines in the mountains.

When designing an NZE home, orienting the building to optimize exposure to the sun is key. In colder climates, the emphasis is to preserve heat so that the long axis of the building faces the sun. In hot climates – where cooling is more important – it’s the opposite way round.

The roof itself becomes important for energy generation. A large area of southern exposure is needed to accommodate photovoltaic solar panels to generate electricity, along with solar thermal panels to heat up collected rainwater. The roof pitch would also affect how efficient these panels would be.

Other design elements that would be affected are the number and size of windows and amount of roof-overhang. Overhangs should protect windows from the heat in the summer but not block too much sun in the winter, which is helped by the sun being lower in the sky.

In cooler climates, about half the window area should be on the south side of the building.  This maximizes the sun’s warmth entering during winter without losing too much heat from the other sides of the house.

Insulation is another key factor. Large quantities will reduce heat loss in the winter and heat gain in the summer and needs to be added to the top and bottom of the house as well as the sides. A properly vented attic will keep the house cooler in summer and prevent moisture problems. This requires soffit vents or grilles and roof vents or ridge vents.

Double exterior walls with offset studs can help reduce thermal bridging. This happens when there is direct contact between the interior of a building and the outside; for example wall studs and window and door frames. The double walls also produce a large interior cavity to fill with insulation, perhaps up to a foot thick.

Lots of insulation will not make a lot of difference if there are air leaks through the exterior of the house so having an airtight building envelope is crucial. There needs to be a continuous air barrier from the basement slab to the attic ceiling. This includes caulking top and bottom plates and sealing the perimeter of the attic and along the foundation.

What about existing homes? Although many considerations are not possible in an existing build, rigid foam insulation added to the exterior is one retrofit option that reduces thermal bridging and increases the R value of exterior walls. This can be incorporated into a rainscreen but wall caps that can accommodate the additional space will be necessary. More insulation can also easily be added to the attic and crawl spaces.

Finally, all appliances should be high efficiency Energy Star-rated and replacing incandescent lighting with LED will also reduce energy consumption by a large amount.

Using a heat recovery ventilation system will also conserve energy from outgoing exhaust air and a drain water heat recovery system will do the same with hot water.

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