A passive home is a just what it sounds like. There clearly exist a dimension of passivity that every home can be projected into. At one end, there are the traditional style homes that require lighting, plumbing, active temperature control, active humidity control, and active air ventilation. At the other end, there are homes that do not require any of these active systems and nearly no maintenance (aside for a change of filter every now and then).
I would like to extend the idea of a passive home even further, into the realm of sustainability. A sustainable home makes use of the resources in its immediate environment (photons, rain, geothermal) and minimizes waste. Ideally the sustainable home would be completely decoupled from societies infrastructure. Sustainable houses are naturally passive since passive systems are more sustainable.
So how can you create a sustainable home?
- Water catchment: store rain fall from roof in large containers for later use. In many parts of the world this can provide you with all your water needs. If the containers are stored high enough, you can use gravity to assist in transporting the water to the location of use. If you want to filter for drinking then gravity will not be enough though. For hot water heating you can create a thin chamber with a glass wall to be heating by the sun, then use gravity to feed directly from the tank for on demand hot water during the day. Otherwise an electrically powered solar hot water heater can be used.
- Passive solar: capture the suns energy in a large thermal mass with high conductivity (concrete, rock, water, soil). The basic idea is to have windows on the sun facing side of the house, with high thermal mass walls surrounded by an insulated envelope. If done right, this system evens out temperature extremes by absorbing and releasing heat to the surrounding air to reach thermodynamic equilibrium. This works excellent at night when the temperature drops. During the winter the sun is at a lower angle than in the summer, hence you can create an window overhang to keep out the sun in the summer when you don't need it. The parameters of this system (window area to house area ratio, amount of thermal mass, amount of external insulation) all depend on your climate. One idea for increasing thermal mass in an already built traditional home is to put large black barrels full of water near the windows. A word of warning: you want to avoid insulated on the inside of the thermal mass, this disrupts heat exchange. So minimize carpet area and drywall. Also make sure you insulate the thermal mass from the ground below, otherwise the earth acts as one massive heat sink!
- Solar power: solar panels for all the energy you ever need. Use a battery bank if you need to even out the extremes.
- Materials: wood, rock, concrete, soil, straw, clay, sand, used tires, cob, cordwood, earth bag, cans, used plastic or glass bottles, garbage (get creative). The idea is to use locally available resources, whatever is available! Tires can be used for structural support; just fill with soil, rock or any other material with high thermal mass. Tires can also be used for roofing, and there is probably somewhere nearby that is begging people to take them for free! Otherwise they have to transport them to a dump half way around the world. Cob is just water, clay, sand, and straw. Cob is very versatile for non-structural needs. I have seen a house that used cob and plastic bottles to make a sealed wall that let light through (free alternative to windows). Straw can be obtained for cheap or even free and makes a great insulator if kept dry.
- Passive air circulation: use natural flow of air to ventilate. There are several designs for how to do this, but the basic idea is to use air-to-air ventilation by burring several long pipes inclining up to the house. The system uses the constant temperature of the ground to moderate the temperature of incoming air. You can build a pit near the bottom outside the house to capture a cold pocket that condensates before entering the house. A filter can be placed on the house side to be easily exchanged. In general, with a sustainable house design air can be circulated more aggressive since the heat is stored in the walls and floor (thermal mass) instead of the air (like a tradition home). This is great for your health and also for decreasing stagnate, humid air. At this point I should also mention that there is not really a good way to create a passive dehumidifier that I am aware of, but this shouldn't be a problem unless you are in a tropical area.
- Geothermal: this is not about geothermal energy (though definitely make use of this if available!), but about using the ground temperature. About three feet below the earth the ground a constant 45-55 (7-13 C) degrees all year around. During extremes, this area can be utilized to moderate the temperature. This resource can be used in many ways. For instance, say you live in an area with a high variation in temperature year around and you want to build a storage shed. You could just build a hole in the ground (hobbit style) and the surrounding earth would keep the temperature steady and cool (ideal for storage purposes). The only problem with this is drainage, but if you include a drainage system then problem solved. As mentioned in the "Passive air circulation" section you can also use the ground to moderate the temperature of incoming air. Another application is to use a geothermal heat pump to help control temperatures inside the house. These systems require electricity though and add complexity to the design.
- Lighting: by limiting the length of the house in the north-south dimension you can negate the need for lighting systems. With passive solar, the sun facing side will allow natural lighting. During the night you are asleep anyway, and this system just helps to reinforce a consistent sleeping pattern. minimize internal walls and locate less used areas towards the non-sun facing side of the house
- Plumbing: separate grey and black water systems to be used in productive ways. Grey water (shower, sink), can be reused for watering your garden or for a toilet (if you do not want to use a composting toilet). Black water can be drained into a long underground pipe that can diffuse into the surrounding soils, increasing their fertility. The black water system can be completely avoided by using composting toilets. All you need is a bucket, some saw dust (or other dry, organic, absorbent material), and some green organic matter high in nitrogen. Just do your business, cover it with saw dust and the green material. If you are a guy, you can just urinate outside somewhere so that the bucket doesn't become overwhelmingly wet (this triggers the composting process and creates smell). If you do urinate in the bucket just make sure you use enough saw dust to absorb it. You can use biodegradable toilet paper if you want. No, it does not stink, trust me, I have used one. When the bucket is full just throw it into a compost pile and wait a while until you use it as soil. This is completely safe to do; as long as you are not dumping your manure directly on the actual plants that you want to eat.
The biggest downside to passive homes is public acceptance. These designs are unlikely to meet codes and regulations in many areas. This would normally be a problem for finding someone to build the house for you, but since these homes are so simple, the skill level is low enough that you and your friends could do it. Getting someone to build a house is two-thirds of the total cost on average!
So there is a brief primer to the topic. Resources are readily available by searching for key terms. Here is a list: earthship, passive home, passive solar, passive ventilation, thermal mass, straw bale construction, cob, passivhaus