Hey readers! Enjoy this article by David Guion of Sustaining Our World.
“Passive house” refers to a concept of building houses––or any other buildings, actually––so that they don’t need a traditional furnace or air conditioner.
Passive building design controls the basic structure, but not the appearance. Some passive houses look bizarre. Some look like any other house in the neighborhood, no matter what style prevails there. The owner’s taste determines the look.
Passive house may seem like the opposite of an active house. An active house is not a conventional house, however, but a different green building standard.
Although passive building techniques are usually applied to new construction, it’s possible to retrofit existing houses to become passive. If you want to live in a passive house, you don’t have to build a new one.
Single-family passive house in Voralberg, Austria
A passive building is like a thermos, except, of course, much larger. They both work on the same laws of physics.
Any building requires good circulation to keep indoor air from becoming stale. A furnace or air conditioner draws outdoor air inside and heats it or cools it. A passive house has a completely different concept of ventilation. It also has much better insulation than other buildings and takes into account body heat of occupants and heat from appliances.
Passive buildings share five basic scientific building principles:
Suppose a building has a concrete floor supported by a steel beam and the beam touches the brick façade. When it’s freezing outside, the brick gets cold. So does the beam and therefore the concrete floor.
In other words, regardless of the insulation in the wall, the contact of steel and brick forms a thermal bridge along which the outside temperature travels inside. It tends to counteract the furnace and make the room colder.
Houses may not typically have concrete floors and steel beams, but thermal bridges can arise in numerous other ways.
Passive construction eliminates thermal bridges. Internal air temperature doesn’t oscillate between hot and cold. The consistent temperature keeps the building more comfortable.
Also, passive construction constantly circulates and filters the air. The ventilation system removes staleness and fumes. If the power goes out, the building stays comfortable far longer than in a conventional building.
Conventional heating and air conditioning require tremendous energy to make indoor air very much warmer or much cooler than outside air. In very cold weather, you might want indoor air 50, 60 degrees or more warmer than it is outside. That burns a lot of fuel.
A passive house uses some kind of heat recovery system. Air circulation requires fresh air to come in and exhaust air to come out. Why not heat or cool the fresh air using the exhaust air? Both air streams flow past each other in separate channels. They don’t mix, but they pass through exchange plates. Heat transfers from one to the other as appropriate. By the time the fresh air enters the house, it’s nearly the same temperature as the exhaust air.
A similar system controls humidity. Some people who live in passive houses find them so much more comfortable that they try to avoid spending much time in conventional buildings.
Image shows heat loss from the old (right) part of the building compared to newer section with improved thermal transmittance characteristics.
Because every element in a passive building has to work together to contribute to the effect, most of the cost difference between it and a conventional building comes in the design stage. Once the design work has been completed, building it uses the same materials, construction techniques, personnel, and schedule as any other building.
Passive design in building a normal-sized house adds between 5-10% to the cost. The larger the building, the less impact the design stage has on the budget. A large passive dorm at Cornell University, for example, was only 2-3% more expensive than a more conventional design would have been.
But without a traditional furnace and air conditioner, it costs so much less to operate a passive house that after a few years, it pays for itself.