In theory, a net zero house is any house which produces as much or more power than it uses. If a household uses 1000 kWh/month (average household electrical usage in Ontario) yet also manages to produce 1000 kWh/month then it is a net zero house. In theory, you don't necessarily need a highly energy efficient house to be a net zero house. In practice, it's a different story. Unless you've got a whole whack of land to mount a large array of solar panels or put up a windmill or dam a stream, it's unlikely producing that much power is a feasible prospect. In an urban environment, it would be especially difficult given the space constraints.
The alternative then, is to decrease household energy usage. The most efficient ways to do this are to:
1. Seal up a house as air tight as possible. This doesn't mean the house gets no air penetration. It means that, ideally, the air flow is completely mechanically controlled so that you get enough air changes per hour to maintain comfort and health while not leaking too much warm air, in the winter, or cool air, in the summer, to the outside. The value passive house builders aim for is .6 air changes per hour or less. In other words, the building must not leak more air than 0.6 times the house volume per hour under a test pressure of 50 pascals when being tested by one of those blower doors.
2. Insulate the house. Taking into account the law of diminishing returns, you don't want to over insulate your house but one standard I'm hearing more about these days is the R-5:10:20:40:60 approach. This suggests that as a minimum there should be an insulation value of R-5 for windows, R-10 for the basement slab, R-20 for the basement walls, R-40 for aboveground walls and R-60 for the roof. If you don't know what all this means, it means a lot of insulation, way more than in the typical house.
3. Use passive solar gain for heating in the cold months as much as possible. This basically means you want your south facing windows to be a big as possible. In the summer, though, you'll need some kind of shading apparatus (like an awning) to keep the sun's heat from streaming in those windows. Flip side of the coin is to minimize north facing windows because they don't really get much solar heat gain in the winter (like none if you're in Toronto) and instead will just allow inside heat to escape because they are, by their nature, poor insulators.
4. Even with an air tight envelope, air (in controlled ventilation) and water will still have to cross the barrier and when this happens, a lot of heat can be carried out. This heat can be recovered using heat recovery devices. For air, a heat recovery ventilation unit, extracts the heat from outflowing air and transfers it to inflowing air. For water, a drain water heat recovery unit, extracts the heat from drain water and transfers it to the water flowing through the hot water line before it gets heated.
5. A heat pump, a device which acts as both an air cooler (air conditioner) and air heater, is an efficient way of meeting any heating demands not achieved by the other systems. Heat pumps are much more efficient at generating heat than furnaces or baseboard heaters and while their output may not be as high, in a well-insulated, air tight house, they should be sufficient to meet the demands.
6. Buy efficient appliances and lights.
There's more, but these are the main points. In designing a net zero house then, great consideration must be given to the envelope of the house, its orientation with respect to the sun, and the selection of highly efficient and complimentary mechanical systems. If money is no concern, a super energy efficient house really isn't that hard to achieve but the goal of this project is to also be cost effective and that presents challenges and some maybe not so obvious choices.
I'll detail some of those choices in future posts.