For passive solar design, use a simple overhang depth rule to shade south glazing in summer while admitting winter sun.

Measure your window height and multiply by the tangent of the difference between summer sun altitude and winter sun altitude at your latitude to get a precise depth, but a practical and easy rule is to start with a depth to height ratio between one to three and one to two depending on how far you are from the equator. At higher latitudes use the deeper one to three ratio so the overhang blocks high summer sun yet still exposes lower winter sun, and at lower latitudes use the shallower one to two ratio because the sun remains higher in winter. For mixed climates or sites with variable weather we recommend adjustable shading such as exterior retractable awnings or operable louvers so you can fine tune performance seasonally. When measuring include vertical distance from the top of the glazing to the underside of the overhang and account for any soffit thickness so the effective shading edge is accurate. If you need precision calculate solar angles for the solstice and equinox days and design the overhang to block the higher angle typical of late June while admitting the lower angle of late December. Always combine overhangs with high performance glazing and insulated frames to prevent heat loss in winter while maximizing passive gain when sun exposure is achieved.

Common mistakes start with using a one size fits all depth which either over-shades winter sun or under-shades summer heat; correct this by choosing the ratio that matches your latitude or adding adjustable shading. A second issue is ignoring window head height and mounting details which shifts the shading line; measure from the actual glazing top to the overhang plane and modify the soffit if necessary. Third, people rely only on fixed horizontal overhangs in mixed climates and suffer seasonal discomfort; add operable shading or exterior shading devices that can change angle or be retracted to balance seasonal needs.

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