Passive design strategies use building materials and climate-specific features to keep a space comfortable without the need for energy. They involve strategies like orienting buildings to the sun, managing solar gain and utilizing balanced ventilation.
Thermal mass—like masonry walls, concrete floors or water containers—have a high capacity to absorb and store excess heat, helping to reduce indoor temperature fluctuations.
Insulation
In passive design, the key to achieving near-zero energy consumption is an airtight and highly insulated building envelope. A high level of insulation reduces heat loss during the winter and prevents unwanted summer heat gain, thereby saving on heating and cooling costs. This is achieved by installing very efficient windows, ensuring the ventilation system has a high degree of heat recovery and by eliminating thermal bridges (i.e. areas where a different material bypasses the insulation, such as aluminium frames and uninsulated concrete slab edges).
Highly insulated construction also minimises temperature fluctuations between internal and external surfaces of walls and ceilings – hence reducing the demand for mechanical heating systems and avoiding damage to interior finishes by condensation. The result is a comfortable indoor environment throughout the year with internal surface temperatures that vary little from indoor air temperature and requiring no heating or cooling.
A range of different construction materials is suitable for passive houses, including masonry constructions with cavity walls (as well as external thermal insulation compound systems), prefabricated elements from porous concrete, timber frame construction with classical studs or using light weight trusses, lost form from rigid insulation filled with concrete on site, and metal frame construction. Attention must be paid to the detailed design of the slab / wall joint, window and eave details to avoid thermal bridges.
Ventilation
For passive buildings, airflow is a big issue. In order to keep indoor temperatures stable, it’s necessary to minimize air leakage, which accounts for 15-25% of winter heating loss and significant loss of cooling. Passive homes need to be very tightly sealed and insulated.
Low-conductivity insulation materials, such as cellulose and fiber glass, are used in the walls and roofs of passive houses to stop heat from moving through the envelope. These insulation materials also help to reduce energy costs.
The LS1 House, designed by Hive Architects in Sarasota, FL, incorporates an innovative breezeway with wooden shutters, allowing low winter sun to penetrate the home while blocking intense summer sunlight. Other strategies include trellises, large overhangs, and louvres that can help to store peak conditions while venting them at night. Planting deciduous trees that shade buildings from summer sun and allow winter warming through their bare branches is another solution.
To ensure the best possible performance of a passive building, architects often combine a variety of design strategies. This can lead to complex mechanical systems and can add to the learning curve for contractors unfamiliar with passive design. However, the process can expand skill sets, and it offers a great payoff in terms of energy efficiency. By reducing heating and cooling requirements, passive buildings can significantly reduce carbon footprints.
Shading
Good shading of the building fabric can help prevent overheating in summer. This can be accomplished through careful design of the building layout, including positioning of rooms and openings. It can also be achieved through the strategic planting of trees on site, with particular consideration to the type of trees and their growth pattern.
It’s important to incorporate passive design principles in the planning stage of a new building, as it is difficult and costly to retrofit these features into completed buildings. However, with some creative thinking and the use of simulation tools it can be possible to achieve the benefits of passive design in existing buildings – for example, by adding better insulation, improving window performance, or changing room layouts to better align with solar access.
A key aspect of passive design is ensuring that living areas and bedrooms are located on the cooler, shaded side of the building. This helps maintain comfortable temperatures while reducing the need for mechanical cooling systems. It is also essential to take advantage of nighttime cooling techniques, which involves opening windows and vents during the night to displace warm air that has accumulated throughout the day. This is a relatively inexpensive option that can be combined with other passive design strategies to reduce the need for mechanical cooling. These student materials complement our Renewable Energy and Environmental Sustainability Instructor Materials.
Windows
A high-quality window helps a passive house achieve its goal of zero net energy consumption by reducing the need for artificial lighting during the day and introducing natural daylight, creating a healthy environment. It also allows for air ventilation, making it easy to control the interior temperature and air quality.
A low U-value for a window (or more generally, the whole window system) shows that the materials and design of the windows and frames help to keep heat inside in winter and out in summer. This is achieved by a combination of insulation and thermal breaks such as draught sealing.
Ideally, a window should have a ratio of glazed area to the building’s heated floor area of 7-8%. However, the optimum ratio depends on your individual needs and the climate where you live.
The window-to-wall interfaces need to be carefully designed to minimize the transfer of heat. This involves minimizing the size of gaps and seams, overinsulating around the frame, and ensuring that closure flashings are not too close to the rough opening. It is also important to avoid the formation of thermal bridges where the frame meets the wall, which can reduce a window’s energy efficiency. Using timber frames can be beneficial, as they are a natural insulator. However, it is important to ensure that the timber is from a species with high durability and that it has been treated to protect against weathering.