Zero Net Energy - Building Enclosure

It is not practical to be ZNE without an efficient building enclosure.

Insulation

It is not practical to be ZNE if you don't have really good house insulation. Good insulation is needed in cold climates to keep the heat inside your house and in hot climates it is necessary to keep the heat out.

Air tight

You need to stop all the air leaks. Your building enclosure needs to be air tight. Warm air leaking out from your house is just as bad as not having proper insulation.

The obvious question is how do people breath in an air tight house. The answer is that you need a Heat Recovery Ventilator . This sucks out the bad air and uses the heat from the bad air to warm the fresh air that it blows into the house. They are remarkably efficient at doing the heat transfer so the heat lost from the building enclosure is small.

External doors

These need to be air tight and well insulated.

Windows

Windows are transparent sections of wall and walls need insulation to stop heat loss. You will not be able to get the R value of the windows as good as the R value of the walls, but you need to try to make them not too bad.

Using argon filled double glazed windows is not too expensive and is a good way to get to a reasonable R value. Triple glazed windows have a higher R value but are too expensive. Pseudo triple glazed windows that use a plastic sheet in the middle can cut down on the visibility through the window so I am not a big fan.

Make sure you fully seal all round the windows because any air leakage will have a very significant effect on your energy bill. Don't listen to anyone that says you should not fully secure and seal the bottom nail fin so that water can escape.

Thermal mass

Thermal mass is described here .

As discussed below, thermal mass works particularly well when used as part of a Passive Solar system.

The cost of electrical energy can vary depending on the time of day. It can be less expensive at night so you can use this to heat your thermal mass in the early morning and have the thermal mass release the energy gradually during the day. This is sometimes referred to as a night storage heater.

Passive solar

Shading is necessary to avoid excessive heat from the sun high in the sky in the summer (which would require energy to power an air conditioner) and yet get as much heat from the low in the sky sun in the winter.

You can fit motorized awnings to shade the sun in the summer, but it's best to design the house properly before you build it. There should be a decent sized roof overhang. Balconies above doors and windows are a great way to shade lower floors. You do have to design the balconies correctly so they are not heat fins radiating heat to the outside world.

You ideally want windows with a high solar gain. These are actually quite hard to find because most windows are designed for houses that do not have properly designed shading overhangs.

LED lighting

In winter it is not a problem having incandescent light bulbs inside your house that generate lots of heat because the heat in within the building enclosure so is reducing the amount of energy your heating system needs to use. In summer however you do not want that heat so you need LED bulbs.

Outside lights should always be LED otherwise all you are doing is paying for energy to heat the planet. Even when using LED bulbs outside it is good to put them on a motion sensor so they don't accidentally get left on for long periods of time. Here's what I do for exterior lighting .

Wall surface area

You want the biggest enclosure volume with the least amount of external wall area. Given that it is not practical to build a spherical house then a cube is the next best shape. Given that you will want a house that has a wide southerly wall (for enjoyment and passive solar) then you should stick with a simple rectangle rather than having lots of protrusions.

House size

It is true that a small house will be less expensive to heat than a large house (assuming the same amount of wall insulation). Having said that, if you do super insulate the walls then a large house can be quite efficient. A relatively small increase in external wall surface area gives a big increase in enclosed volume as long as you stick with a simple rectangle for the house shape.