The term passive house is used increasingly these days as builders, developers, and homeowners all begin to take note of how construction impacts the environment.
In short, a passive house is a house that features sustainable design elements and ultra-efficient systems, which equates to a home that uses minimal energy and offers huge monetary savings in reduced utility bills.
Definition of a Passive House
There are two definitions of a passive house. The first is a house that implements certain foundational philosophies about home energy efficiency.
This term is used commonly when referring to construction techniques such as orienting a home towards the sun to gather maximum passive solar energy, or positioning landscaping in a way it naturally shades the home.
These passive techniques are used in conjunction with active systems within the home to minimize energy use.
The second definition of a passive house means it meets passive house standards by one of the organizations that evaluate the home and offer passive house certification.
For our discussion, we’ll use the first definition, however, with all the elements in place, a passive house may qualify for the second distinction.
What Are the Benefits of a Passive House?
Why does it matter? Is it worth the extra effort? A passive house is a house that uses minimal energy. Although some of the systems cost more up front, an efficiently built and equipped house will save you tons of money over the life of the home.
While everyone else is talking about the high energy costs, you’ll barely notice a bump.
Passive house design is also great for the environment. The construction industry is responsible for 40% of the world’s carbon emissions.
This is a combination of the materials used in construction, commonly referred to as embodied carbon, and the carbon emissions released during the life of the structure, known as operational carbon.
Passive homes address the issue by nearly eliminating operational carbon. In fact, studies show passive houses are up to 90% more efficient than a standard home, using just 10% of the energy most homes require.
That’s a massive, and measurable, savings for your pocketbook and the planet.
The truth is, homeowners who are willing to put in the time and energy to prioritize a passive design for their home, are more often than not also concerned about other aspects of environmental protections.
By selecting natural materials, installing wind, hydro, geothermal, or solar power, and harvesting rainwater, the footprint of the home is further reduced.
Although these systems aren’t strictly part of the passive home criteria, they further lower the carbon footprint of the home by addressing embodied carbon through sustainable material selection.
Not only do passive homes save you money and produce fewer carbon emissions, but they also require less from the planet in the form of petroleum based fuels.
In other words, lower heating and cooling requirements mean less reliance on fossil fuels. That’s good for the planet all the way down the pipeline.
What Are the Disadvantages of Passive Houses?
In truth, not much. They clearly lower energy bills and save on natural resources. About the only disadvantage one might find is an increased initial cost. Although, this investment will pay off over time.
What Criteria Defines a Passive House?
Technically, any natural element that saves energy in the home contributes to a passive house. A passive house isn’t a brand or a title as much as it’s a building philosophy. It means a house is uber-efficient, comfortable, and good for the environment. Specifically, there are five primary criteria that designate a passive house.
- No thermal bridging
- Superior windows
- Mechanical ventilation with heat recovery
- Quality insulation
- Airtight construction
Let’s discuss each of these ideas further.
What Is Thermal Bridging and How Do I Avoid It?
A thermal bridge is an area in the construction of the home that causes heat loss. You might be picturing a wide gap in the wall, and that would qualify, but traditional building styles and materials are also to blame.
Think of a typical wall framing with 2x4s spaced sixteen inches on the center. In between those 2x4s, pink insulation is rolled into place.
While the insulation is great, the 2x4s allow air passage. If 25% of your wall is made up of wood framing, that means one-quarter of your wall material is uninsulated.
This problem is solved through effective insulation inside or outside the wall, but in order to achieve passive house status, it should be worked into the architectural design. As a retrofit, a homeowner can break thermal bridges by installing insulation beneath exterior siding.
It can also be achieved by properly insulating behind drywall, but that is an expensive and laborious endeavor in an existing home.
Windows are not all created equal. Drop by a window showroom and you’ll get a quick taste of this fact. Windows are directly responsible for 25-30% of the average home’s energy costs.
Improving the efficiency of those windows takes that number down to nearly nothing, which is why it is a foundational component of passive design.
Removing all windows from the home is one option for reducing heat gain and loss, but it’s not exactly part of a biophilic design.
As humans, even our time indoors is connected to the natural world through plants, open windows for fresh air, and views of the surrounding landscape. So the windows stay.
Actually, windows can significantly reduce energy consumption requirements, particularly when they are placed facing south (in the northern hemisphere). Southward facing windows collect and harness heat from the sun. Other systems within the home then support the retention and distribution of that heat.
Although there isn’t an official “passive house window,” certain specifications indicate a high level of efficiency, which is what we’re talking about here.
The most efficient windows are typically triple glazed, with layers of argon or krypton gas in the gaps. These windows are incredibly airtight and efficient, not allowing the loss of air through or around the frame of the window.
Low-E glass is another feature to be on the lookout for. This technology coats the inside of the glass with a thin, and unnoticeable, layer of metal oxide that maximizes heat consumption without blocking natural light.
In short, passive house windows maximize solar gain and minimize heat loss. In more detail, that means they have a low U-value, which measures how conductive the windows are, and a high R-value, which measures how well the window retains heat.
The same characteristics that ensure the best heat absorption also work to keep warm air from escaping the home, unlike traditional windows.
During the summer, efficient windows keep cooled air inside too, further reducing cooling and heating costs.
In addition to the type of window, placement is essential. We already mentioned windows on the south side of the home are most effective in collecting passive solar heat. A sunroom in that location would multiply the effects of solar gain.
Well-planned window placement also promotes natural ventilation.
Mechanical Ventilation with Heat Recovery
Mechanical Ventilation with Heat Recovery (MVHR) is a necessary component of the passive house system. This is the process that ensures fresh, filtered, healthy air throughout the home.
It’s a modern system that works in conjunction with the other components of passive house design such as eliminating thermal bridges, installing highly efficient windows, and maximizing the benefits of insulation.
MVHR works by pulling previously heated or cooled air from areas of the home, filtering that air, and returning it to other areas of the home. Its efficiency comes from eliminating the cooling and heating as it filters.
It’s quiet and doesn’t create drafts in the home.
MVHR is a whole-house system, best supported during the design stages of the home and most efficient when the home is tightly sealed.
Insulation is a key component in the passive house design. Investment in the best insulation for every space in the home results in what’s known as a tight envelope--a structure that allows minimal air leaks into or out of the building.
Insulation throughout the building reduces the amount of cold and hot air transferred through windows, walls, vents, gaps around doorways, etc.
That helps keep the area cool when it’s hot outside and warm when there’s a chill in the air. Effective insulation results in low heating and cooling costs, especially in conjunction with energy efficient windows and an MVHR system.
Insulation can be placed in the roof/attic, flooring, and walls. Additional insulation around doorways, windows, and even outlets is a form of passive solar heat retention.
Every small space within a home can be put to work in an effective passive house design. Airtight construction, along with the other elements, means further guaranteeing temperature control in the home.
Outside of the five primary characteristics of a passive house, there are specific targets to meet for comfort and if applying for passive house certification.
For example, the temperature in the home needs to be naturally moderated by these passive design features in every season of the year. The target is to fall within the comfort zone 90% of the time, year round.
The phrase comfort may seem a bit subjective. After all, what’s comfortable for one person may be too warm or cold for others.
As a target for Passive House Certification, however, comfort is defined as maintaining a cozy 20°C/68°F in winter and a comfortably cool 25°C/77°F in summer months.
In addition to the design of the home and selection of efficient HVAC systems, material selection throughout the home is a major factor in passive house design.
For example, heat retention is essential in conjunction with those windows drawing solar heat.
Inasmuch, energy storing materials such as stone, concrete, brick, and tile are used for surfaces in areas where the heat enters the home.
Those materials hold onto the heat until the sun goes down and then slowly release the heat into the cooler air during the night to keep temperatures moderated without adjusting the thermostat.
Home Orientation and Shading for Passive Design
We mentioned this briefly earlier. The way the home is oriented on the lot is crucial to the success of the passive systems in the home.
Taking advantage of heat from the sun is at the heart of passive solar heating. When the sun is high in the sky, between the hours of 9 am and 3 pm, a properly-oriented house will capture large quantities of sunlight directly through the windows during the winter.
In contrast, when the seasons change and the summer heat rolls in, the goal is to shade the house from the intense rays that cause excessive heating.
A well-designed passive house responds to the angle of the sun during the summer, with overhangs that create natural shading and deflect the sun from entering the home, while still allowing natural light in (which further reduces the costs of powering artificial lights).
Architectural Elements for Passive Design
Many homeowners employ architectural details to further amplify the passive design benefits. Double facades, such as slatted boards that offer shading, are one example.
Homes can be equipped with other forms of exterior shades to protect the home during hot weather too.
A good architectural design will also include buffers around the home for airflow and efficient space planning indoors too.
Landscaping for Passive Design
Even factors outside the home can be part of the overall passive house design. Landscaping is commonly used for light and temperature control.
For example, large trees may be strategically placed for home shading during the summer months. In contrast, tall landscaping is avoided in areas where sunlight entrance is the goal.
Green roofs may also be used in passive design, with rooftop gardens helping to lower the temperature of the building below.
Airflow in Passive Houses
With the MVHR in place and a high focus on restricting unwanted airflow between the outdoor and the indoor environment, many people wonder if it’s okay to open the windows in a passive house.
The answer is a resounding yes! Of course, it’s okay and encouraged, to open the windows wide. In fact, most passive homes are planned so that cross-breezes between open windows and doors can naturally cool the space without relying on air conditioning.
Find out more by answering the question What is Passive Solar Heating? and consider The Efficiency of Ancient Passive Heating and Cooling Techniques.