Keep home temperature steady with passive solar techniques
- Sean Canning
Imagine if your home varied temperature by less than one degree Fahrenheit a day with no help from a heater or air conditioner. That is typical of a passive home design.
Passive design should not be confused with net zero design, which focuses on a zero degree plus/minus in terms of energy usage as a metric exclusively. Both would be considered extremely efficient/green well above building code standards. A passive home has three design criteria: reduce heating demand, reduce primary energy use, and a very tight envelope assembly.
A passive design requires the home to function in complete unison, from the solar orientation to the materials selected plus the way they are installed. Passive design is not an attachment or supplement to architectural design, but a design process that is integrated with architectural design. Although a combination of various sustainable techniques are needed to achieve passive standards, there are a couple consistencies that we find in these types of homes.
Passive solar is a technique that is consistent with most passive design. To maintain a consistent comfortable indoor air temperature, a thermal mass is installed. This is a component that will absorb solar radiation (heat from direct sunlight) during the day plus release that heat at night when the sun is down plus outdoor temperatures drop. The thermal mass can be as simple as a ceramic tile floor but is generally much more prominent in the design like a concrete wall up two-feet thick in some cases. The key to this component is it requires direct sunlight and the thicker/denser the mass, the more heat/energy that will be stored. In some cases the Earth itself can be used as a thermal mass.
Also, direct sunlight (which will heat up a space) should be limited in warmer seasons. In the summer months, the sun comes from a higher point in the sky. Exterior window shading devices, trellis structures and deciduous trees are common solutions. Deciduous trees will actually allow the solar heat gain in the colder parts of the year when they drop their leaves and provide shade in the warmer seasons.
The point of the solar heat gain and thermal mass combination is to regulate the indoor air temperature and reduce the need to spend energy to do this. Even so, generally a supplemental mechanical system is installed for extreme conditions and ventilation.
Ventilation can also be achieved passively through techniques utilizing stack effect (when heat rises and cold air settles) and naturally with operable windows. Ventilation should be considered from the very onset of the project.
Super insulation is the second component to passive design. Once we have the solar orientation correct and have determined which windows will allow solar heat gain, it is important to tightly seal the rest of the building envelope. This is generally achieved with a tight air barrier and an extra thick layer of thermal insulation in the exterior walls. This prevents temperature transition and air infiltration through the envelope (spray polyurethane foam insulation is ideal for this b/c it has an R-value of about 6 per 1-inch thick and doubles as a moisture barrier.) The importance and cost of super insulation will increase with harsher climates.
Once you have a comfortable indoor air quality, the super insulation will maintain that environment. It is important to note that when installing a tight building envelope the home should be checked for healthy indoor air quality. A minor mechanical vent leak from a water heater that could have went unnoticed in standard construction conditions will be exponentially dangerous in a tightly sealed home.
In order to achieve a passive home it is important to establish a qualified team of professionals starting with an architect and an energy consultant and engineer. The team (including the client) needs to establish project goals and expectations for a completely integrated design phase. Passive design is very challenging but the rewards are reduced energy dependence, reduced utility bills, and a comfortable home.
Sean is a California Registered Architect and LEED Accredited Professional. He owns 10|70architecture and specializes in process-based modern and contemporary architecture. He has a bachelor’s degree in Architecture from the New Jersey Institute of Technology and a Certificate in Project Management from San Diego State University. He has multiple certifications from the Building Performance Institute and was named the 2011 chair of the USGBC Emerging Professionals Committee.