G-Value and G-Tot allow us to understand the total solar energy transmission through the transparent parts of a building - the windows, both with and without shading. This therefore demonstrates the benefits of shading in limiting solar gain through the glazed areas with many positive knock on effects.
G-Value is simply a measure of the total solar energy transmission through just the glazing, whereas,
G-Tot is a measure of the total solar energy transmission through both the glazing and shading device.
Both G-Value and G-Tot figures are expressed as a fraction. So 0.87, the g-value of single clear float glazing, simply means that 87% of the total solar energy is being transmitted through this glazing type.
The lower these figures, the more of the solar energy that is being blocked and subsequently the lower the heat gain.
Both G-Value and G-Tot figures will vary dependent on the glazing type and for G-Tot the shading system too (internal or external and type of fabric).
Below are some indicative figures for three common glazing types, with 1. no shading, 2. internal blinds and 3. external blinds.
Single Clear Glazing
Double Low-E Clear Glazing
Triple Low-E Clear Glazing
These key energy performance figures show that exterior blinds far outperform internal blinds in stopping solar gain and subsequently reducing internal temperatures. Double low-e clear glazing in combination with exterior blinds sees a staggering cut in solar energy transmission of 90%. Additional resultant impacts of installing exterior blinds include:
Glazing with exterior blind (g-tot)
replied on 01|10|11
There are many factors to consider when selecting any form of shading device for a building: the building type, location, orientation, ratio of glazing to solid structure, the use of the space, number of people occupying the space etc.
As well as the solar gain (g-value / g-tot) figures, the heat loss 'u-value' figures and also light transmission 'tv' figures must be considered. Any shading device, internal or external will cut down light levels and it is important these are cut down to an acceptable level and not too low as this would increase the use of artificial lighting.
Solar control glazing, which has a g-value figure of around 0.32, meaning 32% of the heat element in the suns rays transmits into the building, has a negative impact on a building in the winter months as it stops valuable free solar gain getting into a building and therefore increases the load on the installed heating unit. On top of this light level figures of this type of glazing are low (in comparison to let's say double low-e glazing), having a darkening effect on the room, increasing the use of artificial lighting and creating a vision barrier from inside to out. Plus if we take into account the embodied carbon to produce this type of glass (and also the cost of this type of glazing) it is just not practical.
An all glass building such as a conservatory is a more delicate situation, solar gain, heat loss and light transmission figures are all naturally greater and therefore a holistic approach should be taken and usually this would mean a combination of external and internal blinds to provide both passive cooling and passive heating methods.
A dynamic system is a must, as the ambient conditions change the blind system must be able to react automatically to these changes keeping the internal thermal and visual comfort conditions at the right levels and the energy balance of the building correct.
All in all this requires a specialist in the field of shading to correctly discuss and ascertain the correct shading system or indeed systems, for that particular application and requirements.
replied on 01|10|11
I also think that the obvious conclusion regarding the effectiveness of exterior blinds mainly applies to commercial spaces, or buildings with predominantly a solid structure, especially where there is a high reliance on mechanical ventilation, as opposed to natural ventilation from opening windows.
In addition, to get a complete picture, I believe that you have to consider the issue of heat loss.
The extent of heat loss is directly related to its exposed area: so the cost of compensating for heat loss in a conservatory (where the structure is almost entirely glass) especially here in the UK, is considerable and so makes a case for internal blinds as well.
Not that I disagree with your comments, and I applaud any dissemination of information regarding heat loss terminology for specifiers and the buying public, I’m just saying that, as always, there are many things to consider and there is no single answer.