How a Passive Solar House Works

A house intended to take advantage of the solar heat streaming through a window is actively saving energy. The passive solar house can be designed to work hard in every season to maintain a comfortable temperature. Passive solar houses typically exhibit large openings to the south and small openings to the north. A passive solar house does not require special mechanical equipment to provide energy to the house.

A passive solar house collects heat through the south-facing windows while the sun shines throughout the day. The heat collected is kept in materials that store heat, known as thermal mass. The portion of the home’s heating that the passive solar design can fulfill is called the passive solar fraction and depends on the area of glazing and the amount of thermal mass. Different climates require different ratios of thermal mass to glazing.

Key features that make a passive solar house work:

Aperture

Properly arranged windows: Windows or other devices that gather the sun’s energy should face within thirty degrees of true south and should not be shaded between 9 a.m. and 3 p.m. during the heating season. Conversely, during cooling season, spring, and fall, windows need shading to prevent overheating.

Distribution mechanisms

Solar heat is transported from the collection site and different areas of the house by conduction, convection, and radiation. In some homes, small fans and blowers help distribute heat. Additionally, darker colors take in more heat than lighter colors, so darker thermal mass is a better option for passive solar.

Thermal mass

Concrete, brick, stone, and tile typically serve as thermal mass in a passive solar house. These materials absorb heat from sunlight during the heating season and absorbs heat from warm air in the house during the cooling season. Water and phase change products can also serve as thermal mass and are more efficient at storing heat. However, masonry has the advantage of also serving as a structural or finish material. Well-insulated homes in temperate climates may not need additional thermal storage materials aside from furnishings and drywall.

Some ways of adding thermal mass:

A 4-inch-thick concrete slab foundation is an economical way to add thermal mass. Some homes will have 4 inches of brick or stone veneer on interior walls, or 8-inch-thick free-standing concrete block walls that have both sides exposed and finished with thin stone or tile.

Absorber

Sunlight hits this surface and is absorbed as heat.

Control strategies

  • Roof overhangs to provide shade to vertical south windows during summer months.
  • Electronic sensing devices
  • Awnings
  • Vents and dampers that can be closed and/or redirected

Misconceptions about passive solar houses

  1. More expensive – this is not true, since the design might involve shifting some elements, but not adding any mechanical or electrical equipment.
  2. Overheating in summer – while this is possible, the opportunity to have overheating in winter is a positive aspect of the passive solar house.

Steps to retrofitting your house to become passive solar

Because of the way passive solar houses work, many houses can be retrofitted for passive solar. Houses need to already have room for windows on the south-facing wall. There should be no trees or any other shading on the south side of the house. Some homeowners choose to add solar rooms or simply add windows to the south wall of the house and increase thermal mass to absorb the heat from the sun.

 

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