Passive Solar Heating Is Free - If the Building Is Designed Right to Collect, Store, and Distribute the Sun's Energy
Passive solar heating uses the building itself - its orientation, glazing, thermal mass, and shading - to collect solar heat in winter and distribute it through the building without mechanical systems. When well-designed, passive solar can provide 25-75% of a building's space heating needs at no operating cost, reducing mechanical system size and energy bills for the life of the building. When poorly designed, passive solar creates disabling summer overheating that requires mechanical cooling to remedy, eliminating the energy savings. The difference between successful and unsuccessful passive solar design is in the details: proper south orientation, correct glazing-to-floor-area ratios, adequate thermal mass, and carefully sized summer shading. The ARE PPD tests passive solar design as part of the sustainable design and building performance content area, testing both the design principles and the tradeoffs between system types.
Direct Gain
Direct gain is the most common passive solar strategy: south-facing windows (in the Northern Hemisphere) admit winter sunlight directly into the occupied space, where it strikes thermal mass surfaces (concrete slab floors, masonry walls, water walls) that absorb and store the heat, slowly releasing it to the space through the evening and night. Design rules of thumb: south glazing area should be approximately 7-12% of the heated floor area (more in colder climates); thermal mass should be 3-6x the south glazing area; slab thermal mass should be dark-colored (to maximize solar absorptance); summer overhangs must be designed to shade south windows in summer but admit sun in winter (overhang sizing: overhang projection = window head height × tan(latitude + 23.5°) for equinox-based design). Advantages: simple, no separate system; highly efficient when well-designed. Disadvantages: can create glare and uncomfortable temperature swings without adequate thermal mass.
Trombe Wall (Indirect Gain)
A Trombe wall is a masonry or concrete wall located directly inside south-facing glazing, separated from the glass by a 2-4 inch air space. Solar radiation heats the wall surface; the wall then slowly conducts heat through its mass and releases it to the space on the interior face. The 8-18 hour thermal lag between solar gain and heat delivery means the Trombe wall releases heat in the evening, long after the sun has set - better matching the heating demand profile than direct gain systems that may overheat the space when occupants are present and then lose heat when the space is unoccupied. Vents at the top and bottom of the wall can allow the heated air in the air space to convect into the room more quickly when immediate heating is needed. Trombe walls eliminate the glare problem of direct gain but reduce natural daylighting.
Sunspace (Attached Greenhouse)
A sunspace is an attached, heavily glazed, solar-collection space (greenhouse, conservatory, or solar room) that is separated from the main building by a wall of thermal mass or a conventional wall with operable windows. The sunspace collects solar heat during the day and distributes it to the main building through the wall openings. Advantages: can be used as living space when temperatures are comfortable; buffers adjacent interior spaces from cold. Disadvantages: requires careful thermal design (sunspaces can overheat in summer and freeze in winter without shading and ventilation); integration with the main building requires careful detailing.
Key Exam Points
- Direct gain: south windows + thermal mass; 7-12% south glazing to floor area; thermal mass 3-6x glazing area.
- Trombe wall: masonry behind south glazing; 8-18 hour thermal lag; evening heat delivery; no glare.
- Sunspace: attached glazed buffer space; heat distributes to main building; summer shading essential.
- All passive solar: northern hemisphere = south-facing glazing; summer overhang sizing critical.
- Summer overheating: the failure mode of undersized overhangs or oversized glazing; avoid with proper shading design.
AREprep's PPD passive design content covers all three passive solar heating system types with design rules, climate applicability, and the calculations the ARE PPD exam tests - giving ARE PPD candidates the passive solar design knowledge they need to answer building performance questions that ask about passive solar system selection, sizing, and performance optimization for described climate conditions and building programs.
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