Passive House (or Passivhaus) is a design and construction concept that defined by energy efficiency, comfort, and affordability. Worldwide, it is the strictest energy efficiency standard for buildings.
Because heating and cooling come from passive sources such as the sun or internal sources like people and appliances, and not entirely through mechanical means, the energy needed for active sources of heating and cooling (such as furnaces and air conditioners) can be cut dramatically. This means carbon emissions from Passive Houses are close to zero.
This article explains the key characteristics of the Passive House concept.
The Germany-based Passivhaus Institute (PHI) was co-founded in 1996 by Dr. Wolfgang Feist after studying super-insulated houses. It is separate and distinct from what is known as “Passive Solar” design in North America.
Today, PHI’s standards are the most widely followed internationally, while the Passive House Institute US (PHIUS) separated from PHI in 2012 and sets different standards for North American buildings. The PHI standard applies the same criteria anywhere in the world; the PHIUS standard varies slightly according to location, although they both result in low energy consumption. With increasing concerns about climate change and urbanization, Passive House designs can now be found on every continent, including Antarctica.
The Five Key Passive House Principles
Foundations, walls, floors, doors, windows, and roofs are insulated without gaps between them. In a Passive House, the walls are thicker, often with three or more layers of materials and insulation cavities between them. The cavities can be filled with as much as 12 inches of densely packed cellulose, fiberglass, or other materials. Just inside the inner wall there is often a cavity containing all the wiring and ducting. With all this insulation, Passive Houses are pleasantly quiet and comfortable.
Windows are double- or triple-glazed to prevent the escape of heat (or, in the summer, penetration of heat). The space between glazing layers is often filled with inert gases such as argon or krypton in order to minimize heat loss.
Insulation of window frames is important, too, as frames can constitute up to 10% of a window surface area and represent the greatest point of unwanted heat loss. In the Northern Hemisphere, windows on the north or west side of the house will be smaller, just enough to reduce or eliminate the need for artificial light in the daytime. On the south side, the windows will be larger to capture the sun’s heat in cooler months. In warmer months, adjustable shading elements on south-facing windows prevent the house from overheating.
Elimination of Thermal Bridges
Continuous insulation eliminates “thermal bridges” to the outside world. A thermal bridge exists where heat passes through a material with higher thermal conductivity than the surrounding materials.
The most common thermal bridge is a gap in insulation, but outlets, junction boxes, and plumbing can also act as thermal bridges. Thermal bridges are more common at corners, jogs, balconies, and discontinuities, which is why Passive House designs tend to be simpler.
Moisture coming from outdoors is often trapped in thermal bridges, which can lead to mold or other structural damage. A thermal bridge collects moisture from surrounding air, like the condensation on cold pipes or the sweat on a beer glass.
Airtight Building Layer
The airtight building layer is one of the most critical features of a passive house, as the amount of air infiltration is strictly limited to 0.6 air changes per hour under PHI standards. It is built up with membranes and tapes carefully installed and then tested with a blower door.
Windows and Doors
Windows and doors are designed to minimize heat transfer far more effectively than normal windows, to the point that they feel as warm inside as the air, eliminating condensation in winter. They are airtight to maintain that airtight building layer, and are carefully sized and treated with coatings to admit solar gain in cool seasons without overheating in warm seasons. Careful shading designs admit sun when you want it but control it when you do not.
Ventilation and Heat Recovery
Because they are so tightly sealed, Passive House buildings need well-designed ventilation. Continuous ventilation of fresh air and the evacuation of stale air from kitchens, bathrooms, and basements is necessary to prevent smells, air pollutants, CO2, and humidity from accumulating.
Ventilation can take the form of heat or energy recovery ventilators—devices that transfer heat from one medium to another—connected to fans, vents, and ducts to provide “balanced ventilation” and bring in fresh air without losing all the energy in the exhausted air.
Because so much heat is recovered from the ventilation, and so little is lost or gained through the walls, not much “active” heating or cooling was needed when the passive house concept was conceived, hence the original name Passivhaus or Passive House. However, as the concept has spread geographically, the climate has warmed, and new technologies have been developed. Today, the use of “active” air source heat pumps has become more common.
Passive House designs can look like any conventional building, although to minimize thermal bridges, they are often simpler and boxier minimalist forms.
Passive house residences are not only energy-efficient and climate-friendly, but they are also comfortable because the interior walls are so well insulated and there are no drafts. They are also much quieter because of the thicker walls and better windows.
Thanks to low energy requirements, the Passive House standard is seen as a way of dealing with climate change and eliminating the need for fossil fuels, which is why the standard is being adopted in building codes in Europe and some cities in North America.
Passive house designs are also resilient and can act as “thermal batteries,” so if the electricity goes out they can maintain their internal temperatures for days or even weeks.
Frequently Asked Questions
Can existing houses be retrofitted to become Passive Houses?
Yes. Retrofits can be complicated by the poor quality of the original building materials. However, since a key feature of a Passive House is thicker walls and windows, it’s not overly complicated to add to the existing structure of a house.
Are Passive Houses suitable for warm climates?
Yes. While the Passive House movement has its roots in colder climates in the Northern Hemisphere, there is nothing stopping well-designed Passive Houses from succeeding in the tropics. Proper solar shading to prevent overheating is the key to adapting to warmer climates, as well as protection from external moisture sources.
Are Passive Houses more expensive?
The upfront costs tend to be higher, in some cases 5% to 10% more than conventionally built structures. Yet the savings in energy usage can mean a Passive House is less expensive over the lifetime of the building. If you are financing a house purchase, the higher monthly mortgage payment can be offset by the minimal monthly energy bill. Energy prices are also subject to wild swings and spikes, making a Passive House even more cost-beneficial.
Treehugger Design Editor Lloyd Alter contributed editing to this article.