Exterior Shading Is More Effective Than Interior Shading - Design It Into the Building, Not As an Afterthought
Solar heat gain through glazing is one of the dominant drivers of cooling loads in commercial buildings with significant window area. Interior shading devices (blinds, shades, interior roller shades) can reduce glare but do relatively little to reduce solar heat gain - the solar radiation has already entered the space and is converting to heat before the interior shading intercepts it. Exterior shading devices intercept solar radiation before it reaches the glass, dramatically reducing both glare and solar heat gain. The effectiveness of exterior shading depends on the device type, the facade orientation, and the sun angles at the project's latitude - a horizontal overhang that provides perfect summer shade on a south facade provides no shade on an east or west facade where the sun is low in the sky. Understanding which shading device works for which facade orientation and climate is a core ARE PPD sustainable design skill.
Horizontal Overhangs (South Facade)
Horizontal overhangs are the most effective shading device for south-facing facades in the Northern Hemisphere. The sun is high in the sky on the south in summer (when shading is needed) and low in the sky in winter (when solar gain is welcome for passive heating). A properly sized overhang shades the south window entirely in summer and admits winter sun entirely - achieving the optimal balance with no moving parts. Overhang sizing: for complete summer shade, the overhang depth-to-window height ratio must be calculated based on the summer solar angle at the project latitude. Overhangs are ineffective on east and west facades because the summer morning and afternoon sun is too low for a horizontal overhang to intercept.
Vertical Fins (East and West Facades)
Vertical fins are the primary shading device for east and west facades where the sun is relatively low in the sky in the morning (east) and afternoon (west) and cannot be blocked by horizontal overhangs. Vertical fins on the east facade shade the window from the south (from mid-morning forward) but cannot block the low eastern sun in the early morning hours. For complete east/west shading, a combination of vertical fins and interior shading (or movable exterior devices) is typically required. Vertical fins can also be used architecturally as a screen element, with the fin spacing tuned to the required shading performance.
Brise-Soleil and Perforated Screens
A brise-soleil is a fixed exterior shading screen - typically a horizontal or angled screen of horizontal or vertical blades - mounted in front of the glazed facade. Unlike a simple overhang or fin, a brise-soleil typically covers the entire glazed area and is designed to admit diffuse sky light while blocking direct beam radiation at specific sun angles. Perforated metal or woven metal mesh screens are a contemporary alternative - they allow views while reducing solar radiation and glare through their open ratio (the percentage of the screen area that is open). Perforated screens are specified by their open ratio (typically 20-50% open) and their solar heat gain reduction coefficient.
Automated/Movable Exterior Shading
Automated exterior roller shades, louvered screens, and Venetian-type exterior blinds can adapt to changing sun angles throughout the day and season. They can provide maximum solar control when the sun angle is direct and retract when overcast or when maximum daylight is desired. Automated exterior shading responds to light sensors, sun angle calculations, or occupant override. It is more expensive than fixed shading and requires maintenance, but it provides the best performance flexibility. Automated exterior shading is common in high-performance commercial office buildings.
Key Exam Points
- South facade: horizontal overhangs most effective; shade summer high sun; admit winter low sun.
- East/west facades: vertical fins primary device; combined with interior shading for complete coverage.
- North facade: no direct sun in Northern Hemisphere; no shading devices needed for solar control.
- Brise-soleil: fixed screen of blades; covers entire glazed area; admit diffuse light, block direct beam.
- Exterior shading: more effective than interior (intercepts radiation before entering building); design into building, not afterthought.
AREprep's PPD passive design and building envelope content covers facade shading devices - horizontal overhangs, vertical fins, brise-soleil, and automated systems - their orientation-specific effectiveness, sizing principles, and how shading device selection is documented in architectural drawings and specifications - giving ARE PPD candidates the solar shading knowledge the exam tests in building performance optimization questions where selecting the correct shading device for a described facade orientation and climate is the central design decision.
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