G-value in buildings
Understanding the solar transmittance through translucent and transparent materials such as glass is important for determining the solar heat gain into the space they enclose during sunny conditions. Solar heat gain can be beneficial in the winter, as it reduces the need for heating, but in the summer can cause overheating.
The total solar heat transmittance through transparent and translucent materials is equal to the solar heat that is transmitted through the material directly, plus the solar heat that is absorbed by the material and then re-emitted into the enclosed space.
Traditionally this was expressed in terms of a shading coefficient which described the amount of solar heat transmitted through a material compared to the amount of solar heat transmitted through a standard sheet of clear float glass 3 mm thick.
However, manufacturers are now moving away from shading coefficients. In the USA, they are moving towards the use of solar heat gain coefficients (SHGC) and in Europe, g-values (window solar factors, solar factors or total energy transmittance (TET)). In essence, these both represent the fraction of incident solar radiation transmitted by a window, expressed as a number between 1 and 0, where 1 indicates the maximum possible solar heat gain, and zero, no solar heat gain.
g-value = total solar heat gain / incident solar radiation
The difference between g-values and solar heat gain coefficients is that they use a different value for air mass.
g-values can refer to the centre-of-glass g-value, or can relate to the entire window, including frame (gglass or gwindow). Generally, a higher g-value will be beneficial in cooler climates and a lower g-value in warmer climates. Typically g-values will range between 0.2 and 0.7, with solar control glazing having a g-value of less than 0.5.
[edit] Related articles on Designing Buildings
- BREEAM.
- Code for Sustainable Homes.
- Computational fluid dynamics (CFD).
- Emission rates.
- Emissivity.
- Energy certificates.
- Environmental legislation.
- Green deal.
- Leadership in Energy and Environmental Design.
- Low-e glass.
- Shading coefficient.
- Solar heat gain coefficient.
- Solar transmittance (gtot).
- Sustainability.
- Thermal bridge.
- Thermal resistance.
- U value.
- Zero carbon homes.
- Zero carbon non-domestic buildings.
Featured articles and news
ECA Industry Awards 2024 shortlist revealed
22 leading businesses from across the electrotechnical and engineering services sector.
Government unveils Skills England strategy
Skills England to transform opportunities and drive growth.
New Government Hub for York Given Planning Green Light
For up to 2,600 civil servants, due for completion by 2028.
Construction Skills Certification Scheme cards
July update on Professionally Qualified and Academically Qualified Person Cards.
BSRIA Briefing 2024, November 22
Sustainable Futures: Redefining Retrofit for Net Zero Living.
The CLC on driving competency in the retrofit sector
Previously published roadmap on skills for net zero.
The first labour government King's speech in fifteen years
Construction industry reactions, support and some concern.
CIOB Retrofit of Buildings Technical Information Sheet
What retrofit is, the approach to be taken and processes to be followed.
Adapting Historic Buildings for Energy and Carbon Efficiency
Historic England advice note 18, free download published.
10 retrofit projects revisited 10 years after completion.
Information orders, building liability orders and SPVs
Key BSA terms and how they impact special purpose vehicles.
Listed despite problems with its design.
Zen and the art of cycling exploration.
Design Council Homes Taskforce launched
To support government 1.5 million homes target within UK climate commitments.
Comments