Design With the Sun: Passive Solar Building

Green Architect

While there's been a lot of excitement recently about passive homes and Passive House Certification, one of our favorite go-to building techniques is passive solar design. It is actually different from Passive House and the technique has been around for ages. It is a very simple design concept wherein the building relies on its local climate and the sun for heat, instead of mechanical or electrical systems.

The basic principle of passive solar building is to design the house along the east-west axis, so that the long end of the house is facing south. This provides the most surface area for south facing windows, which allows solar heat to enter the house. This sunlight is then absorbed into the interior materials and heats the space. There are a few key design elements to achieve a successful passive solar design:

Building Orientation: If possible, orient the building along an east-west axis, so that its long side faces south. This is not always possible for certain properties, but is desirable for the best solar gain.

Windows: Locate the majority of windows on the south facade to maximize winter solar gain. The windows should be designed and collected based on the local region and climate. For example, according to the US Department of Energy, for climates that require heating most of the year, "south-facing windows usually must have a solar heat gain coefficient (SHGC) of greater than 0.6 to maximize solar heat gain during the winter, a U-factor of 0.35 or less to reduce conductive heat transfer, and a high visible transmittance (VT) for good visible light transfer." For even further refinement, the windows on the other facades could also be designed with the proper SHGC values, otherwise they could be the same as the south facing windows. Low-E coatings are also recommended for control of solar heat gain and loss.

Solar Gain Control: Passive solar design typically benefits homes in cooler climates that are highly dependent on heating in the winter. Keeping clear and clean south facing windows allow for solar heat gain from low winter sun, however it is possible that during very sunny days, rooms can overheat. Therefore it's best to utilize window shades or blinds to control the amount of heat entering the house. Additionally, it is important to block solar gain in the summer to reduce cooling needs. This is easily accomplished by permanent overhangs that are correctly sized and located to block the high summer sun.

Solar Storage: While it is not necessary, a well-designed passive solar home will use various materials and objects to absorb and store solar heat. The best materials to use are dark and opaque as these are the best at absorbing solar radiation. Good objects and materials to use for thermal mass are stone tiles such as slate, concrete, brick or other masonry, plaster walls and even water tubes. These items are good at absorbing solar heat, and then slowly release it throughout the day and night when temperatures are lower. This reduces dependency on mechanical heating and cooling systems, and also moderates indoor temperatures, which results in a more comfortable environment.

Insulation & Air Sealing: Just like any other eco-friendly home, a passive solar house should have the best insulation and air sealing as possible. It is important to reduce opportunities for heat loss and unwanted air and moisture infiltration.

While passive solar design is easiest to integrate into new construction, there are some strategies that can be used for existing homes and renovations:

  • Install more south facing windows
  • Use window shades and/or overhangs to block unwanted sun and heat gain
  • Install high-mass and dark colored materials to absorb and slowly release heat.
  • Improve existing insulation values

Find out more: US Department of Energy

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(Images: Nathan Kipnis Architects)

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