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
UK Construction Week, London is here !
Debuting major international pavilions and much more.
Getting the most out of heat pumps and heating
How heat pumps work and how they work best.
Plumbing and heating for successful retrofit and renovation
Low temperature underfloor systems and heat pumps.
Cost-of-living crisis and home improvement plans
Starting on the right footing and top tips for projects.
Delays on construction projects
Types, mitigation and the acceleration of works.
From Chaucer to Fawlty Towers.
Electrotechnical excellence, now open for entries.
Net zero electricity grids BSRIA guide NZG 5/2024
Outlining the changes needed to transition to net zero.
CIOB Global Student Challenge 2024
Universitas Indonesia wins for second year running.
New project and cultural district described in detail.
The nature of EPCs, crticism and inaccuracies.
History, issues and redesign.
From waste recycling to energy performance the hierchy.
ECA 2024 Apprentice of the Year Award
Entries open for submission until May 31.
UK gov apprenticeship funding from April 2024
Brief summary the policy paper updated in March.
Comments