Solar angles and solar heat gain: azimuth, altitude, SHGC

One-line orientation

Solar geometry uses azimuth and altitude to describe the sun’s position. SHGC describes how much solar energy a window lets in. The preferred SHGC depends on whether the building should gain or reject solar heat.

Key points

• Azimuth: the sun’s horizontal compass angle. State the reference direction before calculating.
• Altitude: the vertical angle between the sun and the observer’s local horizon. Low altitude = morning/evening or winter sun (long shadows, low angle); high altitude = midday or summer sun (short shadows, near-overhead).
• Solar Heat Gain Coefficient (SHGC): a dimensionless number from 0 to 1 indicating what fraction of incident solar radiation passes through the glazing assembly (glass + frame). SHGC includes both directly transmitted radiation and the portion absorbed by the glazing and re-radiated inward.
• Hot climates: specify low SHGC glazing — less solar radiation enters, reducing cooling loads.
• Cold climates / passive solar: specify high SHGC glazing on south-facing glass — more solar radiation enters, supplementing heating.
• Azimuth and altitude together define the sun’s position vector; architects use sun-path diagrams or software to evaluate shading device effectiveness by season and hour.

Solar position needs two angles: azimuth + altitude

ANGULAR GEOMETRY

Plan measures compass direction from north; section measures height above the horizon.

View diagram Hide diagram 2 panels

2 panels side by side — swipe to compare →

Key

• Measured angle
• Sun position

SHGC governs solar radiation through glazing; U-value separately governs conductive heat flow.

Confusions / comparison

Term	Plane measured in	What it tells you	Key building use
Azimuth	Horizontal (plan)	Compass direction of the sun	Which facades get sun; orientation of shading devices
Altitude	Vertical (section)	Height of sun above horizon	Overhang depth sizing; seasonal shading analysis

SHGC target	Climate / load type	Why
Low SHGC (e.g. 0.2–0.3)	Hot climate, cooling-dominated, skin-load	Rejects solar radiation → reduces cooling load
High SHGC (e.g. 0.5–0.7+)	Cold climate, heating-dominated, passive solar south glazing	Admits solar radiation → free heating

Glazing metric	What it governs	Units	Direction of “good”
SHGC	Solar (radiation) heat gain	dimensionless (0–1)	Depends on climate
U-value	Conductive heat loss through assembly	BTU/(h·ft²·°F)	Always lower = better
VLT	Visible light transmission	dimensionless (0–1)	Higher = more daylight

Related

→ Thermal: Heat-transfer modes (SHGC vs U-value distinction) · Building loads & degree days (skin-load buildings most affected by SHGC choice) · Code: energy code (ASHRAE 90.1 prescriptive maximum SHGC by climate zone) · Construction: Enclosure (shading devices, overhangs, exterior fins).