The Best in Passive Solar Materials

Of the five elements of passive solar design, two deal primarily with materials. These are the absorber and thermal mass and are often two characteristics of the same material. The absorber represents the surface of the material (e.g., masonry wall, water wall, floor, etc.). The thermal mass retains or stores the heat from sunlight. It is the interior, the substance behind that outer, absorbent surface.

Glass, of course, cannot be ignored as a passive solar material. But the effectiveness of a window in passive solar heating has as much to do with aperture – or placement relative to the sun – as it does with material. All windows, however, are not created equal. Windows that are double- or triple-glazed with low-e coatings and efficient frames are best for passive solar design.

There are three essential materials behind passive solar design: masonry material, glass, and water. Furthermore, there are also three ways to harness solar energy passively: direct gain, indirect gain, and isolated gain. Which materials you use, and how you use them, will depend largely on which approach you take.

Direct gain is the simplest approach. It is a matter of allowing heat in through the windows. A dark colored masonry floor will absorb that heat and slowly release it throughout the night. While this technique may not eliminate heating bills, it makes for an excellent complement.

Indirect gain often involves at least two of the above materials. Take a Trombe wall, for example. It is typically a thick concrete wall (up to 16″) with a layer of glass on the exterior, south-facing side. This glass absorbs solar heat (while simultaneously preventing heat from escaping), which then passes through an airspace and into the concrete wall via convection. The masonry wall then stores the heat, slowly emitting it into the home through its other side. Concrete is advantageous for passive solar heating because it is an excellent thermal mass and releases heat slowly and consistently, well into the night.

Passive solar walls may also be constructed using water tanks. Water itself is an excellent heat transfer medium. The design of a water wall is similar to a typical Trombe wall except it is the water that stores and transfers the heat. In moderate climates, water walls have advantage over masonry walls because water offers faster heat exchange. A water wall also presents some unique aesthetic possibilities.

An isolated approach simply uses the materials already described above in a space (sunroom, greenhouse, etc) that is isolated from the main house. Any heat collected is transferred via vents or air ducts.

It is difficult to set a hierarchy on passive solar materials. Much depends on each home’s unique situation. In an arid climate you’d want a masonry wall because it delivers slow and steady, and you don’t want bursts of heat during the day. The key to using proper solar building materials is plenty of research and subsequent knowledge about the details regarding your home’s location. When building a new home or upgrading an existing one, iron out the details with your architect and building contractor to make sure the home is drawn up and built in the best way with the best materials.

Posted on December 30th in Passive Solar by Dan.