Passive Solar


Passive Solar

Passive Solar Design

Passive solar energy means that mechanical means are not employed to utilize solar energy.

Passive solar systems rules of thumb:

  • The building should be constructed on an east-west axis.
  • The building's south face should receive sunlight between the hours of 9:00 A.M. and 3:00 P.M. during the heating season.
  • Interior spaces requiring the most light and heat should be along the south face of the building. Less used spaces are located on the north.
  • An open floor plan optimizes passive system operation.
  • Use shading to prevent summer sun entering the interior

Passive Solar Heating

Two primary elements of passive solar heating are required:
  • South facing glass
  • Thermal mass to absorb, store, and distribute heat

There are three approaches to passive systems - direct gain, indirect gain, and isolated gain. The goal of all passive solar heating systems is to capture the sun's heat within the building's material and release that heat during periods when the sun is not shining. At the same time that the building's material is absorbing heat for later use, solar heat is available for keeping the space comfortable.

Direct Gain

With direct gain , the living space is a solar collector, heat absorber and distribution system. South facing glass lets solar energy into the house where it strikes directly and indirectly thermal mass materials in the house such as masonry floors and walls. The direct gain system will utilize 60 - 75% of the sun's energy striking the windows.

Passive Solar 1aPassive Solar 1b

Thermal mass in the interior absorbs the sunlight and releases the heat at night

In a direct gain system, the thermal mass floors and walls are functional parts of the house. The thermal mass will regulate the intensity of the heat during the day by absorbing the heat. At night, the thermal mass releases heat into the living space.

Direct gain system rules of thumb:

    • A heat load analysis of the house should be conducted.
    • Do not exceed 6 inches of thickness in thermal mass materials.
    • Do not cover thermal mass floors with wall to wall carpeting; keep as bare as functionally and aesthetically possible.
    • Use a medium dark color for masonry floors; use light colors for other lightweight walls; thermal mass walls can be any color.
    • For every square foot of south glass, use 150 pounds of masonry.
    • Fill the cavities of any concrete block used as thermal storage with concrete.
    • Use thermal mass at less thickness throughout the living space rather than a concentrated area of thicker mass.
    • The surface area of mass exposed to direct sunlight should be 9 times the area of the glazing.


Indirect Gain

In an indirect gain system, thermal mass is located between the sun and the living space. The thermal mass absorbs the sunlight that strikes it and transfers it to the living space by conduction. The indirect gain system will utilize 30 - 45% of the sun's energy striking the glass adjoining the thermal mass.

Thermal storage wall systems:

The thermal mass is located immediately behind south facing glass in this system.

Passive Solar 2

Thermal Mass Wall

Operable vents at the top and bottom of a thermal storage wall permit heat to convect from between the wall and the glass into the living space. When the vents are closed at night radiant heat from the wall heats the living space.

Indirect gain system rules of thumb for thermal storage walls

    • The exterior of the mass wall (toward the sun) should be a dark color.
    • Use a minimum space of 4 inches between the thermal mass wall and the glass.
    • Vents used in a thermal mass wall must be closed at night.
    • A well insulated home will require approximately 0.20 square feet of thermal mass wall per square foot of floor area.
    • If movable night insulation will be used in the thermal wall system, reduce the thermal mass wall area by 15%.
    • Thermal wall thickness should be approximately 10-14 inches for brick, 12-18 inches for concrete, 8-12 inches for adobe or other earth material.


Isolated Gain

An isolated gain system has its integral parts separate from the main living area of a house. Examples are a sunroom and a convective loop through an air collector to a storage system in the house. The ability to isolate the system from the primary living areas is the point of distinction for this type of system.

The isolated gain system will utilize 15 - 30% of the sunlight striking the glazing toward heating the adjoining living areas. Solar energy is also retained in the sunroom itself.

Sunrooms employ a combination of direct gain and indirect gain system features. Sunlight entering the sunroom is retained in the thermal mass and air of the room. Sunlight is brought into the house by via conduction through a shared mass wall in the rear of the sunroom, or by vents that permit the air between the sunroom and living space to be exchanged by convection.

The use of a south facing air collector to naturally convect air into a storage area is a variation on the active solar system air collector. These are passive collectors. Convective air collectors are located lower than the storage area so that the heated air generated in the collector naturally rises into the storage area and is replaced by return air from the lower cooler section of the storage area. Heat can be released from the storage area either by opening vents that access the storage by fans, or by conduction if the storage is built into the house.

Passive Solar 3

Operation of a Sunroom Isolated Gain System

Isolated Gain rules of thumb for sunrooms:

    • Use a dark color for the thermal wall in a sunspace.
    • The thickness of the thermal wall should be 8-12 inches for adobe or earth materials, 10-14 inches for brick, 12-18 inches for (dense) concrete.
    • Withdraw excess heat in the sunroom (if not used for warm weather plants) until the room reaches 45 degrees F. and put the excess heat into thermal mass materials in other parts of the house.
    • For a sunroom with a masonry thermal wall, use 0.30 square feet of south glazing for each square foot of living space floor area. If a water wall is used between the sunroom and living space instead of masonry, use 0.20 square feet of south facing glass for each square foot of living area.
    • Have a ventilation system for summer months.
    • If overhead glass is used in a sunroom, use heat reflecting glass and or shading systems in the overhead areas.


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