What is Passive Solar Design?
Passive solar design uses a building's windows, walls, floors and roof to collect, store and distribute the sun's heat in winter and reject it in summer, without mechanical equipment. It relies on orientation, glazing choice, shading and thermal mass working together.
Passive solar design is an architectural strategy that positions glazing, thermal mass and shading to capture free solar heat in winter and block excess solar heat in summer, reducing a building's need for mechanical heating and cooling.
- •Fixed overhang blocks direct high-angle sun
- •Low-SHGC glass reduces unwanted west/east gain
- •Cross ventilation carries excess heat away
- •Light-colored roof reflects solar radiation
- •Low winter sun passes under the overhang
- •South-facing glazing admits direct sunlight deep into the room
- •Thermal mass floor/wall absorbs and stores the heat
- •Stored heat radiates back into the room after sunset
Try it: interactive calculator
Step-by-step worked examples
A room has 8 m² of south-facing glass with SHGC = 0.6, under winter irradiance of 600 W/m². Find the instantaneous solar heat gain.
Q = A × SHGC × I Q = 8 × 0.6 × 600 Q = 2,880 W
The same 8 m² window uses low-SHGC summer glass (SHGC = 0.25) under peak summer irradiance of 900 W/m². Find the heat gain and compare to Example 1.
Q = 8 × 0.25 × 900 Q = 1,800 W Despite higher irradiance, lower SHGC glass admits less heat than the winter case — this is the point of seasonal sun control.
A designer wants to limit summer heat gain to 1,500 W through a 10 m² south window at 800 W/m² irradiance. What maximum SHGC is allowed?
Q = A × SHGC × I → SHGC = Q / (A × I) SHGC = 1,500 / (10 × 800) SHGC = 0.1875, so glass with SHGC ≤ 0.19 is needed
Flashcards
Quick quiz
Q1.Solar heat gain through a window is calculated as?
Q2.In passive solar design, glazing for winter heat gain should mainly face which direction (northern hemisphere)?
Q3.A fixed overhang sized for a site's latitude does what?
Q4.What role does thermal mass play?
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Common mistakes
Placing large glazing on the west facade for 'passive solar gain'. — Correct: West glazing causes uncontrolled low-angle afternoon heat gain in summer — south glazing with proper shading works far better.
Adding south glazing without any thermal mass. — Correct: Without mass to absorb the heat, rooms overheat during the day and lose it all by night — mass is essential.
Using the same overhang depth at every latitude. — Correct: Overhang depth must be calculated from the local summer and winter sun angles, which change with latitude.
Assuming passive solar design eliminates all heating/cooling needs. — Correct: It significantly reduces mechanical loads but rarely eliminates them entirely, especially in extreme climates.
FAQ
What is passive solar design?
It is designing a building's orientation, glazing and thermal mass to capture and store winter sun and block summer sun without mechanical equipment.
What is the passive solar design formula?
Solar heat gain is Q = A × SHGC × I: glazing area times the solar heat gain coefficient times solar irradiance.
How do you calculate passive solar heat gain?
Multiply the south-facing glazing area by its SHGC and by the incident solar irradiance in W/m².
What are examples of passive solar design elements?
South-facing windows, fixed overhangs sized for summer/winter sun angles, and thermal mass floors or walls.