What is a passive house ? Principles and design

The Passive House (or Passivhaus) is a design and construction concept defined by energy efficiency, comfort and affordability. This is the strictest energy efficiency standard for buildings worldwide.

Since heating and cooling comes from passive sources such as the sun or from internal sources such as people and appliances, and not entirely by mechanical means, the energy required for active heating and cooling sources (such as as furnaces and air conditioners) can be significantly reduced. This means that carbon emissions from passive houses are close to zero.

This article explains the main characteristics of the passive house concept.

Background

The Germany-based company Passivhaus Institute (PHI) was co-founded in 1996 by Dr. Wolfgang Feist after studying super-insulated homes. 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 Passive House Institute USA (PHIUS) separated from PHI in 2012 and sets different standards for North American buildings. The PHI standard applies the same criteria all over the world; the PHIUS standard varies slightly by location, although both result in low power consumption. With growing concerns about climate change and urbanization, passive house designs can now be found on every continent, including Antarctic.

The five key principles of the passive house

Continuous insulation

Foundations, walls, floors, doors, windows and roofs are insulated with no space between them. In a passive house, the walls are thicker, often with three or more layers of material and insulation cavities between them. Cavities can be filled with up to 12 inches of dense cellulose, fiberglass or other materials. Just inside the interior wall there is often a cavity containing all the wiring and conduit. With all this insulation, Passive Houses are pleasantly quiet and comfortable.


Thick walls and foundations are essential to building a passive house.

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The windows are double or triple glazed to prevent heat escape (or, in summer, heat penetration). The space between layers of glazing is often filled with inert gases such as argon or krypton to minimize heat loss.

Window frame insulation is also important, as frames can make up to 10% of a window’s 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 during the day. On the south side, the windows will be larger to capture the heat of the sun during the cooler months. During the warmer months, adjustable shading elements on the south-facing windows prevent the house from overheating.


The south-facing windows are large, others are smaller.

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Elimination of thermal bridges

Continuous insulation eliminates “thermal bridges” with the outside world. A thermal bridge exists when heat passes through a material that has a higher thermal conductivity than the surrounding materials.

The most common thermal bridge is a void in the 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 from outside is often trapped in thermal bridges, which can lead to mold or other structural damage. A thermal bridge picks up moisture from the surrounding air, such as condensation on cold pipes or sweat on a beer glass.

Hermetic build 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 according to PHI standards . It is constructed with membranes and strips carefully installed and then tested with a blower door.

Windows and doors

Windows and doors are designed to minimize heat transfer much more effectively than normal windows, to the point that they are as warm inside as the air, eliminating winter condensation. They are airtight to maintain that layer of airtight construction, and are carefully sized and treated with coatings to admit solar gain in cool seasons without overheating in warm seasons. Neat shade designs admit the sun when you want it but control it when you don’t.

Ventilation and heat recovery

Because they are so tightly sealed, Passive House buildings need well-designed ventilation. Continuous ventilation of fresh air and exhaust of stale air from kitchens, bathrooms and basements is necessary to prevent odors, air pollutants, CO2 and humidity from building up .

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 fresh air without losing all the energy of the exhaust air.

Because so much heat is recovered from the ventilation, and so little is lost or gained through the walls, little ‘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.


Cool air can enter a Passive House even in extreme temperatures when you want your windows closed.

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Take away food

Passive house designs can resemble any conventional building, although to minimize thermal bridging they are often simpler, squarer minimalist shapes.

Passive houses are not only energy efficient and climate friendly, they are also comfortable because the interior walls are so well insulated and there are no drafts. They are also much quieter due to thicker walls and better windows.

Thanks to low energy requirements, the Passive House standard is seen as a way to cope with climate change and eliminate the need for fossil fuels, which is why the standard is adopted in building codes in Europe and some North American cities.

Passive house designs are also resilient and can act as “thermal batteries” so that if the electricity goes out, they can maintain their internal temperature for days or even weeks.

Frequently Asked Questions

  • Can existing houses be renovated to become passive houses?

    Yes. Renovations 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, adding to a home’s existing structure isn’t too much of a hassle.

  • Are passive houses suitable for hot climates?

    Yes. While the Passive House Movement has its roots in the colder climates of the Northern Hemisphere, nothing is stopping well-designed Passive Houses from succeeding. in the tropics. Proper sun protection to prevent overheating is key to adapting to warmer climates, along with protection from external moisture sources.

  • Are passive houses more expensive?

    Initial costs tend to be higher, in some cases 5-10% more than conventionally built structures. Yet the energy savings can mean that a passive house is less expensive over the lifetime of the building. If you are financing the purchase of a home, the higher monthly mortgage payment may be offset by the lower monthly energy bill. Energy prices are also subject to fluctuations and wild spikes, which makes a passive house even more profitable.

Treehugger Design editor Lloyd Alter contributed editing to this article.

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