Passive solar design

Passive solar design uses the house itself, meaning its orientation, layout, windows, floors and walls, to collect the low winter sun and block the high summer sun. Unlike active solar heating, it needs no pumps, fans or electric controls to move the heat. Passive solar homes range from those heated almost entirely by the sun to those where south windows cover part of the heating.

It’s cheapest to design in from the start, but existing houses can be adapted too.

The five elements

1. Aperture (windows). The main windows face within 30 degrees of true south and aren’t shaded between 9 a.m. and 3 p.m. in winter. Living areas face south and bedrooms can face north. In cold climates, keep windows on the north, east and west small. In warm climates, use north windows and well-shaded south windows.
2. Absorber. The hard, dark surface that the sun hits, such as a masonry floor or wall, turns sunlight into heat.
3. Thermal mass. The material behind or below the absorber stores that heat and releases it after dark. Concrete, stone, brick and tile are common.
4. Distribution. Heat moves from where it’s collected to the rest of the house, by natural movement of air and radiant heat, or with help from fans and ducts.
5. Control. Roof overhangs and trees shade the windows in summer. Vents, dampers, blinds, awnings and thermostats help fine-tune it.

How the heat moves

Heat flows from warmer things to cooler things. Passive design uses three ways it travels:

• Convection. Warm air rises and cool air sinks, which is why upstairs is warmer. Some designs use this loop to carry heat from a south wall into the rooms.
• Radiation. Heat radiates from warm surfaces to cooler ones. Dark surfaces absorb the sun’s heat. Light surfaces reflect it, which helps in hot climates.
• Heat capacity. Dense materials store a lot of heat and give it back slowly, evening out the temperature through the day and night.

For a Leafhouse project

Orientation is decided when you site the house, so check it on the land before anything else is fixed. Earth, cob and rammed earth walls, and slab floors, can do double duty as thermal mass. A Trombe wall is a classic passive solar system that puts the mass right behind the glass. DOE suggests working with a designer who knows passive solar for a new home or major remodel.