FAQ's
What are the requirements for shading a conservatory roof or sloping glazing ?
The saying goes that people that live in glass houses should not throw stones – but they should also not be surprised if they get hot!
The science is this - The suns rays are not hot. In the spectrum most that prenetrate the atmosphere are short wave and are cold until they come into contact with an object. That is when they convert into heat and change wavelength and are reflected as long wave radiation.
On the beach that is you. Some of the energy is reflected and some is absorbed by your body.
The same happens in a window. Some heat is reflected by the glass, some is absorbed and some is transmitted through. Of the absorbed energy some is reflected but most is transmitted into the room.
When the transmitted energy reaches an object in the room, say the wall, some is absorbed heating the wall and some is reflected back to the glass. Because it has changed to long wave radiation most of it cannot pass back through the glass and is retained in the room as heat. This is known as the greenhouse effect.
An internal blind will have the same effect. Some heat will be transmitted through it, some will be absorbed and some reflected back to the glass. Generally the lighter the colour of the fabric the more it will reflect back. Except that, because it has changed wavelength, only a certain amount will pass back through the glass. The remainder will be trapped as hot air between the blind and the glass.
If it is not vented it will build up and as the blind heats it will be transmitted into the room.
For the most effective reduction in solar gain external shading should be selected. Most of the heat that is transmitted through or absorbed by the external blind changes wavelength and then will not pass through the glass. It becomes trapped between the blind and glass and - natural science again - as hot air rises convection draws cooler air in at the bottom and draws the heat upwards between the blind and the glass and vents it out at the top.
For internal blinds to work effectively where external blinds are not practical an extractor or opening light at the top will be needed to remove the heat build up between blind and glass. Effectively venting the hot air will prevent the blind from becoming a radiator and whilst it will still not be as effective as an external system will significantly improve on the amount of heat rejected.
As a generalisation a material that rejects 40-45% of solar gain internally would reject 80-85% fitted externally.
External blinds are the most effective – where internal blinds are the only practical option extractors or vents will improve their performance
Linked Information:
Sloping glazing and horizontal rooflights what are the options?
Awkward shapes?
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