Effectiveness of External Venetian Blinds

Assessing the impact of window blinds on levels of heat gain through glass


Heat gains that occur within internal spaces can be classified on the basis of their origin. These may result directly from internal sources of heat or from the external environment.

A) External Blinds – Slats angled at 45 degrees

B) Double Glazing

C) Sun’s Radiation

D) Reflection

Heat sources produce thermal energy which acts to directly increase air temperature. This thermal load may be absorbed by water vapour. Internal sources of heat gain production include, but are not limited to, body heat, lighting, machinery and heat passage between rooms.

Heat gains from the external environment are caused by the passage of solar radiation through mediums and structures such as walls, ceilings, glazed window surfaces, doors and the infiltration of external air. External Heat Gains have a decisive impact on the thermal load of buildings. Window design, surface area and orientation are all factors which affect this loading dramatically.

External Versus Internal Shading Systems

Many reputable studies have been conducted into the efficiency of external and internal shading systems. Most of the research has been carried out in Europe where high energy costs make effective insulation of glass areas a must – in fact some countries have tough legislation in place against energy inefficiency. Research findings by Australian Institutions and summary figures are available through CSIRO.

The objective with any shading device is of course to keep the inside temperature in a range where our bodies feel comfortable and perform at their best. It has been well established that these temperatures are ranging between 18o and 25o Celsius. To achieve and maintain this comfort zone we have to spend valuable energy into cooling and heating our homes and workplaces.

Shading & Insulation

External shading systems that also act as barriers against cold winter nights, will save money in 2 ways:

  1. The upfront investment into a smaller cooling and heating unit.
  2. Ongoing savings in running and maintenance costs.

In case of an environmentally friendly design, cooling or heating may not even be necessary by using external shading and insulation devices (Olympic Village Design).

External Shutters Outperform Internal Shading Devices

Benchmarks set by the “German Industry Standard VDI” show external shutters to be outperforming internal venetian or vertical blind system. External covers over glass areas with a northerly aspect stop over 85% of the heat whereas the internal systems allow nearly 5 times as much energy to pass.

To achieve and maintain the same temperature with internal shading devices, you need to invest twice as much into a larger cooling system. It will cost on average 115% more to maintain and 140% more to run the larger unit.

Another important fact: In achieving these results external louvres reduce daylight only by 20%. (this figure is higher with roller shutters)

Limits of Internal Applications

To achieve similar cooling results internally the air between the glass and the internal blind needs exchanging up to 20 times per hour. In situations where the glass area is more than 65% of the total wall area satisfactory results cannot be achieved by means of internal solar control.

These results clearly favour external solar control systems. You also have the additional bonus of Security.

Passage of Heat Flow Through Windows by Convection and Solar Radiation

Heat passage by convection is determined by the window surface and the heat passage coefficient ( the higher the quality of glazing, the lower the coefficient ) and is negligible in this instance.

The greatest contributor to heat flow through windows is solar radiation. Heat passage is calculated by determining the area of the windows surface exposed to the sun and its shading coefficient ( s ). The shading coefficient expresses the percentage of radiation passing through a window in comparison to one which is single glazed. The value of this coefficient is determined by the type of glazing and shading affecting the window.

The table below demonstrates that the use of a light coloured External Venetian Blind will reduce the heat passage through glass by 85% compared to utilising no shading at all.

Please Note : The amount of decrease in the internal temperature of the space cannot be unambiguously determined by calculation. This value can only be ascertained by measuring the internal temperature before and after installation of shading equipment.

How does the S co-efficient affect a six square metre window facing the sun for four hours per day?

The dollar values provided are based on approximate costs of electrical power per kW and values will vary according to the supply type.


Table of shading coefficients for various glazing types:

The annual air conditioning energy cost saving will be in excess of $ AUD 800.00 per year and this is only for one window!

Solar Heat Gain is not appreciably reduced by using either single or double glazing if the glazing is unprotected.

Double Glazing provides the additional benefit, over single glazing, of a more stable and lower ambient temperature environment when used in conjunction with an External Venetian Blind. Light and highly reflective slat colours will act to provide maximum solar heat gain protection.

Other non-variable external shading, such as fabric blinds, window films, performance glazing or fixed shading, either block the entrance of natural light, retain and reflect heat and limit or obscure external views.

Fully Retractable External Venetian Blinds provide the perfect solution.

These Blinds are:

1. Adjustable for maximum exclusion of solar radiation relative to the suns position

2. Adjustable to reflect light internally whilst also excluding heat.

3. Adjustable to maintain a high degree of direct view.

4. Adjustable to manage natural light without glare.

5. Adjustable to maintain a high degree of privacy whilst excluding heat.

6. Retractable to allow entire views through windows.

7. Able to allow and maximise winter solar heat gain

8. Retractable for maintenance or window cleaning.

9. Self Cleaning