Window Overhang Shading (Profile Angle and Shade Line)

How much of a window a fixed overhang actually shades, as a profile angle and a shade line down the glass.

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Details, formula, and sources

The profile angle is the sun altitude projected into the vertical plane square to the wall, tan(profile) = tan(altitude)/cos(surface-solar azimuth), and the overhang throws its shade line projection x tan(profile) down the wall. This is the geometry that makes a fixed overhang work seasonally with no moving parts: a 24 in overhang set 6 in above a 48 in window FULLY shades it at a 70 degree summer sun but leaves 84% of it sunlit at a 30 degree winter sun. At normal incidence the profile angle equals the solar altitude. Reports the DIRECT-BEAM sunlit fraction - shaded glass still gets diffuse sky and ground-reflected sun, so applying this to a total gain overstates the reduction. Get the sun position from solar-times. A design aid, not a Manual J.

tan(profile) = tan(altitude) / cos(surface-solar azimuth); shade_line = projection x tan(profile); shaded = clamp(shade_line - gap, 0, glass_height); sunlit_fraction = 1 - shaded/glass_height. No direct beam when altitude <= 0 or |surface-solar azimuth| >= 90.

The ASHRAE Handbook-Fundamentals (Fenestration) and ACCA Manual J profile-angle / shade-line method for overhang shading of vertical glazing, by name.

The profile-angle relation is first-principles solar geometry; it is computed here from the entered sun position rather than read from a proprietary shade-line-factor table.

Estimate. AHJ and licensed professional govern.

Field names used by the API: projection_in, gap_in, glass_height_in, solar_altitude_deg, profile_angle_deg, shade_line_in, shaded_height_in, sunlit_fraction

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