External Movable Solar Shading

External shading (venetian blinds, awnings) cuts solar heat gain during the cooling season by up to 80 %.

How does external solar shading work?

A large share of the solar radiation striking glazing enters the room as heat. How much is described by the g-value (total solar energy transmittance): clear triple glazing has a g-value of about 0.7, solar control glass around 0.3. Internal blinds change little, because the radiation has already passed through the glass and the heat stays in the room. External venetian blinds or awnings, by contrast, intercept the radiation before it reaches the pane. The resulting combined value g_tot for glass plus shading then drops below 0.10, in practice between 0.05 and 0.15. This reduces solar heat gains during the cooling period by up to 80 %. Ideally, the shading is automated according to global irradiance and sun position, with a wind sensor, so that the slats close under strong sunlight and reopen to admit daylight when the light is diffuse.

Which buildings benefit from external solar shading?

The measure suits all buildings with larger south-, west- or east-facing glazed areas. Typical properties are office buildings with a high glazing ratio, schools with summer overheating problems, and residential buildings. The facade must be alterable without conflicting with heritage protection; in protected zones where alterations are prohibited, the measure is ruled out. External shading is particularly effective in buildings that have so far had to manage without cooling and regularly exceed the summer comfort limits of ÖNORM B 8110-3. Summer overheating protection can often be demonstrated this way without an active chiller.

What does external solar shading actually deliver?

In cooled buildings, cooling demand falls by 20 to 40 %. The electricity saving is calculated from the sunlit window area, the solar irradiation during the cooling period (around 350 kWh/(m²·a) in Austria), the difference between the g-values with and without shading, and the SEER of the chiller. For 150 m² of sunlit window area this yields around 6,200 kWh/a of electricity and roughly 1.0 t CO2 per year. External venetian blinds including motor drive cost €180 to €380 per m² and last around 20 years. The purely energy-based payback is 10 to 20 years, although the comfort gain, the avoided air conditioning and the glare protection at screen workstations are not monetised.

Energy carrier

Strom → Strom

Savings potential

20-40 % Kühlbedarf

Worked example

An office building has 150 m² of sunlit window area on its south and west sides, so far shaded only by internal blinds. The glazing has a g-value of 0.55 and the chiller operates with a SEER of 4. Externally mounted, automatically controlled venetian blinds are retrofitted, bringing the total solar energy transmittance g_tot down to 0.08. With solar irradiation of 350 kWh/(m²·a) during the cooling period, the saving is calculated as E_saved = 150 × 350 × (0.55 − 0.08) / 4 = 150 × 350 × 0.47 / 4 = 6,169 kWh/a. Cooling electricity consumption therefore falls by around 6,200 kWh per year, equivalent to roughly 1.0 t CO2. On top of that come lower peak loads on the chiller on hot afternoons and better glare protection at the workstations.

Investment & payback

Raffstore inkl. Antrieb: 180-380 €/m² · Service life: 20 a · Payback: 10-20 (Komfortgewinn nicht monetarisiert) a

Applicability

Requirements

Exclusion criteria

Typical buildings

Frequently asked questions

Why is external solar shading more effective than internal blinds?

External solar shading blocks solar radiation before it reaches the glass; internal blinds only act after it has passed through. With internal systems the heat is already inside the room and is merely redistributed by the slats. External venetian blinds reduce the total solar energy transmittance g_tot to below 0.10 and cut solar heat gains by up to 80 %, whereas internal systems achieve only a fraction of that.

How much cooling energy does external solar shading save?

External solar shading reduces a building's cooling demand by 20 to 40 %. With 150 m² of sunlit window area and a chiller with SEER 4, that amounts to around 6,200 kWh of electricity per year. The saving depends on window orientation, the g-value of the glazing and how well the control is automated. In uncooled buildings the effect shows up as a noticeably lower room temperature.

How much do external venetian blinds cost for an office building?

External venetian blinds including motor drive cost €180 to €380 per square metre of window area, depending on slat type, guide rails and control system. With a service life of around 20 years, the purely energy-based payback is 10 to 20 years. If the shading allows an air conditioning system to be avoided or downsized, the economics improve significantly.

Do you need automatic control for solar shading?

Automatic control is recommended to achieve the full effect. Sun-controlled venetian blinds close under high irradiance even outside occupancy hours and open in diffuse light to admit more daylight. A wind sensor protects the slats from storm damage. Manually operated systems are often closed too late in practice, when the heat is already in the room.

What does ÖNORM B 8110-3 say about summer overheating protection?

ÖNORM B 8110-3 defines in Austria how summer overheating protection of buildings is to be demonstrated, for example via the maximum permissible operative room temperature without active cooling. External solar shading with a low g_tot is one of the most effective means of providing this proof. The performance characteristics of the shading are assessed according to EN 14501.

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Standards & sources

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