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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Checked against published worked examples from: Michigan Technological University (MEEM 4200, after McQuiston, Parker and Spitler).

Details, formula, and sources

ScopeThe 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.

Formula or tabletan(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.

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

Free accessThe 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.

What governsEstimate. AHJ and licensed professional govern.

API field namesprojection_in, gap_in, glass_height_in, solar_altitude_deg, profile_angle_deg, shade_line_in, shaded_height_in, sunlit_fraction

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