Why does solar shading need to be external? Won’t closing curtains do?
Shading windows and rooflights from the outside is a far more effective method of keeping out the sun’s heat than closing curtains or internal blinds.
If you are going to try and reduce the solar gain from a window or rooflight, tackle it by shading or treating the outside surface of the glazing.
CIBSE’s Guide A, Environmental Design, puts it nicely:
“External blinds are the most effective form of solar control; this is because the majority of any solar radiation absorbed by the blind will be lost to the external air. Conversely, internal blinds are the least efficient because much of the absorbed solar radiation will enter the space”..
The Building Research Establishment has measured it. The heat transmission through a glazing and blind system is often represented by a parameter called the g-value. A value of 1 means all the sun’s heat incident on the window is absorbed, and a value of 0 means that it is all reflected. Glass alone will reflect some heat and more will be reflected when one adds in a blind or glazing treatment.
| Window and Blind Combination | Effective G-Value1 |
|---|---|
| Double Glazed Window (No blind) | 0.59 |
| Double Glazed Window with light coloured internal fabric blind | 0.35 |
| Double Glazed Window with light coloured external fabric blind | 0.09 |
These figures show that the g-value for the option with the external blind is significantly lower than for the option with the internal blind; it reflects significantly more of the sun’s heat.
External Shading Measures are Particularly Effective
Therefore, whenever you can, shade the outside of the window (or apply a surface treatment to the outside of the glazing surface) to help reflect the unwan’ted heat away before it can reach the space.
Check out the solutions page for more information and practical ideas on how to do apply rapidly-deployable external shading or glazing treatments.
¹ Extract from The Building Research Establishment’s document ‘Summertime Performance of Windows with Shading Devices’, Table 1a south-facing double-glazed window.