Passive homes are homes designed to meet a strict set of standards regarding energy usage, the loss of interior heated or cooled air through leaks, and a lack of sufficient insulation. These homes are referred to as passive homes because they lose so little heated or cooled air that a comfortable interior temperature can be maintained with little or no forced air, central air conditioning and heating systems that use fossil fuels. Instead, the main source of cooling and heating comes from thick insulation, energy exchange and heat transfer ventilation systems, the orientation of the building on the lot, the use of landscaping and hardscaping features to produce shade, and the use of architectural materials and features and interior furnishings to deflect, reflect, or absorb heat as well as provide shade.
Energy transfer systems also remove excess humidity from the air while most of the lighting is supplied passively through large windows of triple pane glass. Any additional cooling or heating that’s required can easily be supplied by heat pumps, ceiling fans, and space heating and cooling systems. Power for appliances, lighting, and supplemental heating and cooling comes from renewable energy sources such as solar panels or wind turbines.
Passive Solar Design Elements
Passive solar design elements include:
In order for the building to receive as much sunlight as possible, for lighting the interior and for heating during cool or cold weather, buildings are situated on the property so that they face the equator. Passive homes in the northern hemisphere are built facing south, and buildings in the southern hemisphere are built facing north.
In addition, buildings in the north may be built as close to the lot’s northern lot line as possible while buildings in the south may be built as close to the southern lot line as possible. This allows for the growth of trees and reduces the chance that a taller building will block the passive home’s access to sunlight.
Landscaping, Hardscaping, Architectural Features, and Interior Furnishings
Landscaping elements such as deciduous trees, wall or arbor climbing vines, vertical gardens, and roof gardens or plant-covered green roofs shade the windows and protect the walls and roof from absorbing so much heat that the home’s interior becomes overheated.
Hardscaping elements such as Habitat Screens, awnings, sun sails, fountains, and attached pergolas fitted with removable canvas, ripstop panels or Habitat Screens topped with plants or vines also provide shade and cooling. Potted trees and vines and fountains can be placed indoors or outdoors.
Architectural features that can reduce sunlight at midday include roof overhangs, rollshutters, standard shutters, blinds, exterior fabric screens, and insulated curtains, Brise soleil, and Trombe walls.
Brise soleil are architectural features that are designed to deflect sunlight and provide shade. In particular, they are designed to prevent large glass windows from becoming overheated. They are usually horizontal, but they can also frame a window or be built to cover an area where people gather or walk as well as the windows in the area. They can also have security shutters panel that close to provide shade during the heat of the day.
Trombe walls are used on south-facing exterior walls that are in areas of the home where a window isn’t wanted, such as in a home theater room or a bathroom. To create a Trombe wall, a single or double layer of glass is mounted so that it stands out one inch or less beyond the surface of a dark-colored masonry wall. The wall absorbs solar heat that slowly passes into the wall throughout the day until it reaches the interior of the home when the radiated heat might be needed for warmth.
Other exterior and interior features and furnishings that are used to collect or disperse heat include exterior walls and roofs and interior walls, floors, ceilings, and furnishings. Light colors reflect and disperse heat while darker colors collect and absorb it, radiating it later when temperatures cool and the heat might be needed to warm the home.
Sealing and Insulating to Eliminate Thermal Bridges
To eliminate thermal bridges (leaks that result in uncontrolled exchanges of heated or cooled interior air with air from outside the home), passive homes are sealed where pipes, wires, and vents pass from the exterior to the interior; where pipes, wires, and vents pass from heated and cooled areas of the home to areas that are not heated or cooled such as garages and attics; and where structural seams occur such as at dormers, with changes in the roofline, or around chimneys, skylights, tube lights, or windows. They also use high-performance windows with U-factors of 0.15 to 0.05 which compares to insulation R values from -6.6 to -20.
The amount of insulation used in passive homes depends on the climate in the home’s geographical location. Homes in warm or temperate climates require less insulation while those in cooler climates require more. It’s not only the walls and roof that are insulated, but also the floor. Insulating the floor eliminates a thermal bridge between the ground and the house that may drain needed heat and introduce unwanted cooling in cooler climates.
Energy Exchange and Heat Exchange Ventilation Systems
Energy exchange and heat exchange ventilation systems are necessary to introduce fresh air into tightly sealed passive homes and to maintain the air’s quality. Both systems filter the air through HEPA filters. However, these systems also play a role in heating and cooling the home, and they operate at an efficiency of 75 to 95 percent warming cold winter air to temperatures of 11°C to 18°C (52°F to 65°F). Energy exchange ventilation systems also act as dehumidifiers in humid climates.
On average, the temperature inside a passive home remains around 22°C (72°F). That temperature will be cooler than the outside temperature in the summer and warmer than the outside temperature in the winter. In a heat exchange system, the duct that draws fresh air into the house from the outside sends that air into a heat exchange system that uses the temperature of the stale air from inside the house to warm or cool the outside air. Solarscreen provide superior solar protection, mitigate UV damage, optimize comfort, and provide daytime privacy.
An energy exchange system adds an extra humidifying or dehumidifying step to the process. Some of the stale air from the interior house is drawn into a system which takes the air from outside of the home through a preliminary heating or cooling. In the summer, any excess moisture in the outside air condenses, and the heat from the condensed water is then drawn off and used, for example, to help heat the water in the hot water tank. In the winter, the dry, cool air from outside the home causes the moisture in the air from inside the home to condense, and that moisture is the used to humidify the outside air.
The Goals These Passive House Systems Are Designed to Meet
Passive house systems are intended to reduce energy consumption, reducing fossil fuel use while providing a comfortable and healthy living environment.
To be certified as a passive house, the home should not require more than 15 kilowatt-hours (kWh) per square meter per year (1.4 kWh per square foot per year) to heat the home in the winter, and it should require no more than that same amount of energy to cool the home in the summer.
The total amount of energy required to heat and cool the home should not exceed 11.1 kWh per square foot annually (119.4 kWh per square meter), and the total amount of energy required for heating, cooling, heating hot water, lighting, and other uses of electricity should not exceed 60 kWh per square meter per year (5.58 kWh per square foot).
The peak load required at times of the highest usage for heating or cooling should not exceed 10 watts per meter (3.05 watts per foot).
In addition, passive homes should not allow any more than 0.6 cubic meters of air changes per hour (ACH) or the air inside the home to be exchanged with the air outside of the home each hour through uncontrolled leaks. If you have a decorative outdoor pond, you can compare this to the filtration rate for your pond’s water. Your filtration system should be powerful enough to recirculate all the water in your pond once each hour. The features which insulate and seal a passive home should be efficient enough to allow only 0.6 of the air in your home to escape through leaks per hour.
What It Means for You and the Environment
Taxes on energy use in Europe have been encouraging Europeans to build passive homes for about 20 years. Thanks to this experience, Europeans have found ways to reduce construction costs so that construction costs for private passive homes are only about 5 percent higher than the cost of a standard home.
There has been less incentive to build passive homes in countries that don’t have high taxes for energy consumption. Consequently, lack of building experience and volume for the industry means costs of building a passive home can be 10 to 15 percent above the cost of building a standard home, mainly due to the cost of the windows and increased insulation. It can take 10 years to recoup this initial investment in construction costs through energy savings.
Another issue is that Europeans have not had to deal with the combination of tropical warmth and humidity.
However, home builders who have built one successful, certified passive home say that they believe that they learned enough to be able to reduce the cost from 10 percent above the cost of a standard home to 5 to 6 percent above the cost of a standard home. Home builders who have failed are working with either the European and US passive home associations or with both to understand how their homes failed.
Further, the US association is working to create CAD programs similar to the ones used in Europe that will help passive home designers and builders balance the various elements of a passive home to create passive homes that succeed in climates unfamiliar to Europeans.
Given the statements of successful passive home builders, it doesn’t seem that it should take long to reduce the cost of constructing passive homes.
Passive homes are about more than recouping construction costs through energy savings, though. They are about reducing and attempting to reverse the harmful impacts of human activity on the environment that currently sustains us. It seems that a little extra investment in construction costs in exchange for sustaining that environment ought to represent a good return on investment for anyone interested in the future of the planet, as well as anyone whose main focus is on their own personal future, the future of their children, and the future of their children’s children.