In warm climates, the sun is responsible for a large share of a building's cooling load, and much of that heat enters through windows. Exterior shading is one of the most effective ways to reduce it. Perforated metal screens, including see-thru hurricane barriers, can shade glass while preserving daylight and views. This article explains how they work and how to evaluate their performance.
Why Windows Bring in So Much Heat
Sunlight passing through glass warms the floors, furniture and people it reaches, and that heat is trapped inside. On an unshaded east or west window in summer, solar heat gain can be very high for several hours a day. Exeter's sun control study, based on ASHRAE reference conditions for Miami, Florida on July 21, shows clear glass on an east facing wall admitting about 215 BTU per square foot per hour at 8 AM, and the same on a west wall in the late afternoon. Just sixty square feet of glass can admit as much as 12,000 BTU per hour, roughly the output of a one-ton air conditioner. See the full sun control study.
Exterior vs Interior Shading
Blinds, curtains and interior films help with glare, but by the time sunlight reaches them it has already passed through the glass. Much of the heat they absorb stays inside the room. Exterior shading intercepts the sun before it reaches the glass, so the heat stays outside. That is why architects favor exterior devices such as overhangs, fins, awnings and screens in hot climates. Perforated metal is a popular choice because it can block a large share of direct sun while still letting people see out and letting air move.
Measuring Shading Performance
Shading coefficient
The shading coefficient (SC) compares the solar heat gain through a glazing and shading system with that through a reference sheet of clear 1/8 inch glass, which has an SC of 1. A lower number means less heat gets in. Independent testing of STORM SHIELD barriers over clear glass found a shading coefficient of .51, meaning solar heat gain was reduced by almost 50%.
Solar heat gain coefficient
Modern energy codes and window labels use the solar heat gain coefficient (SHGC), the fraction of incident solar radiation that ends up as heat inside. SHGC and SC are related; as a rough rule, SHGC is about 0.87 times the shading coefficient. When comparing products, make sure you are comparing the same measure and the same glass.
Open area and pattern
For a perforated screen, the share of open area, the hole size and the sheet thickness all influence shading. Less open area blocks more sun but also reduces daylight and views. The InvisiPerf™ pattern used in STORM SHIELD has a 51% open area, a balance chosen to keep views clear while stopping debris. Learn more on the InvisiPerf page.
Angle of the sun
Perforated screens shade best when the sun strikes at a steep angle, because the thickness of the metal around each hole blocks more of the oblique rays. Low morning and evening sun on east and west walls still delivers large heat loads, which is where exterior screens help most.
Energy and Comfort Benefits
- Lower cooling loads on sunny elevations, which can reduce energy use and allow smaller equipment in new construction.
- Less glare on computer screens and work surfaces, without closing blinds and losing the view.
- More even temperatures near windows, improving comfort for people seated close to the glass.
- Protection for furnishings, because exterior screens reduce the sunlight that fades floors, fabrics and artwork. The original STORM SHIELD literature notes that the barriers reflect 50% of the sun's harmful ultraviolet rays.
- Daylight preserved, so occupants are not forced to rely on electric lighting.
Combining Shading With Hurricane Protection
Most hurricane shutters only provide value during a few days each year. When a shutter is also a well-designed sunscreen, it works every sunny day. Permanent see-thru barriers such as STORM SHIELD Fixed Mount, Side Hinge and Bahama Mount stay over the glass all year, providing shading, security and storm protection with one product. Bahama units can be propped open as awnings for extra shade.
The performance depends on the barrier being in place. Removable panels stored in a garage provide no shading benefit, so if energy savings matter, favor styles that stay on the building.
Where Sun Control Pays Off Most
- East and west facing windows, which receive intense low-angle sun.
- Large glass areas such as storefronts, lobbies and sliding doors.
- Buildings in hot, sunny climates with long cooling seasons, including the Gulf and Atlantic coasts.
- Rooms where glare is a problem, such as offices, classrooms and control rooms.
- South facing windows in lower latitudes during summer.
Estimating Savings
Actual energy savings depend on climate, orientation, glass type, building use and HVAC efficiency, so no single percentage applies to every building. An energy modeler or mechanical engineer can estimate savings using the shading coefficient or SHGC of the glass and screen combination. For a quick comparison, look at the hour-by-hour data in the sun control study, which shows solar heat gain through protected glass at roughly half that of unprotected glass at every hour of the day.
Maintenance and Long-Term Performance
A sunscreen only performs if it stays clean and intact. Dust, salt and pollen can build up on perforated metal and slightly reduce light transmission, so rinse screens with fresh water periodically, especially near the coast. Check finishes for chalking or damage, and make sure fasteners and hinges remain tight. Durable materials such as galvannealed or stainless steel infill, aluminum frames and marine-grade powder coat finishes help a screen keep its appearance and shading performance for many years. Our hurricane shutter maintenance guide covers routine care.
Conclusion
Exterior perforated metal screens stop much of the sun's heat before it reaches the glass, while preserving daylight and views. When that screen is also an approved hurricane barrier, a single product delivers year-round energy and comfort benefits along with storm protection and security. If you are planning hurricane protection in a hot climate, make shading performance part of the decision. Request literature to learn more about STORM SHIELD.