Technical Advice | Glazing Technologies
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Triple glazing reduces heat transfer caused by a temperature difference across the building envelope. In winter, this means less heat is lost from a warm interior to colder external air. During hot weather, the same insulating effect can reduce heat transfer from hotter external air into a cooler building.
Solar heat gain through the glazing is a separate mechanism. A triple-glazed unit can have a very low U-value and still admit a substantial proportion of the solar energy striking the glass. The question of triple glazing and heat transfer therefore involves two different measurements: thermal transmittance and solar gain. For large glazed elevations, both need to be considered as part of the glass specification.
An insulated glass unit restricts heat transfer through a combination of the glass panes, sealed cavities, cavity gas and low-emissivity coatings.
Moving from double to triple glazing introduces another pane and a second insulating cavity. Argon or, in some applications, krypton can be used within these cavities because these gases conduct less heat than air. Low-e coatings reduce long-wave radiative heat transfer between the glass surfaces. Cavity width also matters because the insulating performance of a gas layer depends partly on controlling conductive and convective heat transfer within the cavity.
The resulting improvement is expressed through the U-value, measured in W/m²K. A lower U-value means less heat passes through the element for a given temperature difference.
Published manufacturer data demonstrates the scale of the difference. One Guardian ClimaGuard A 1.1 double-glazed configuration has a Ug-value of 1.1 W/m²K, while a comparable triple-glazed configuration using two 16 mm argon-filled cavities achieves 0.6 W/m²K.
For an illustrative 20 m² area of centre-pane glazing with a 20°C temperature difference between inside and outside, those figures equate to approximately 440 W of heat flow through the double glazing and 240 W through the triple glazing. The calculation excludes frames, edge effects, air leakage and solar energy, but it shows clearly what the reduction in Ug represents.
Lower heat transfer can also increase the internal surface temperature of the glazing during cold conditions. This can reduce cold-radiant discomfort and downdraughts close to large glazed elevations, as well as helping to manage condensation risk around correctly designed glass edges.
The distinction between U-value and g-value is central to specifying glazing for both winter and summer conditions.
A U-value measures thermal transmittance caused by a temperature difference between the two sides of the glazing. For glass, the Ug-value describes the centre-pane performance of the insulated glass unit. A complete window or glazed system also includes the effect of the framing and the thermal bridge around the glass perimeter. These additional elements contribute to the complete Uw-value or relevant system-level thermal performance.
The g-value, sometimes referred to as the solar factor, describes the proportion of incident solar energy that ultimately enters the building through the glazing.
A g-value of 0.50 means that approximately 50 per cent of the incident solar energy is admitted under the standard calculation conditions. This includes directly transmitted solar energy and the inward component of energy absorbed by the glass.
The current ISO method for calculating glazing U-values specifically excludes solar radiation. Solar and luminous properties of glazing are dealt with separately.
A useful way to understand triple glazing and heat transfer performance is therefore to read the U-value and g-value independently. A low U-value establishes strong insulation performance. It does not establish how much solar energy enters through the glass.
Adding the third pane can affect solar transmission, but there is no universal g-value for triple glazing.
A useful controlled comparison comes from the same Guardian ClimaGuard A 1.1 glass family. Its published double-glazed configuration has a Ug-value of 1.1 W/m²K and a g-value of 0.648. The corresponding triple-glazed configuration achieves a much lower Ug-value of 0.6 W/m²K, while its g-value falls to 0.55.
The triple glazing therefore improves both figures in that particular build-up, but the change is much greater for thermal transmittance than for solar gain. A g-value of 0.55 still means a significant proportion of the incident solar energy can enter the building.
Other triple-glazed units behave differently.
A Guardian ClimaGuard A+ triple-glazed configuration also achieves Ug 0.6 W/m²K but has a published g-value of 0.611. By comparison, a Guardian SunGuard SN 70/37 double-glazed solar-control configuration can achieve approximately Ug 1.0 W/m²K with a g-value around 0.37.
The triple-glazed unit in this comparison provides substantially better insulation against temperature-driven heat transfer. The solar-control double glazing admits considerably less solar energy.
The same distinction applies during summer. Pane count contributes to the behaviour of the glass, while the complete glass make-up determines the actual solar performance.
The components responsible for a low U-value do not all have the same effect on solar gain.
Additional cavities, argon or krypton filling and low-e coatings primarily improve the resistance of the insulated glass unit to temperature-driven heat transfer. Solar-control coatings are specifically engineered to alter the transmission, reflection and absorption of incoming solar energy.
Low-e glass itself does not define a particular g-value. Manufacturer data shows triple-glazed units with similar Ug-values but substantially different solar characteristics. In published Guardian configurations, for example, triple glazing at Ug 0.6 W/m²K is available with a g-value of 0.611, while a solar-control triple configuration can reach Ug 0.5 W/m²K with a g-value around 0.304.
The solar specification also affects visible light transmission, external appearance and reflectance. Very low solar gain may be appropriate for a heavily exposed elevation, while another orientation may require a different balance between solar protection and useful winter gain.
Orientation therefore matters. South-facing glass experiences a different solar profile from east and west elevations, where lower morning and afternoon sun angles can produce significant gains. Roof glazing has another exposure condition again.
The glass schedule for a highly glazed building may consequently use different solar-control specifications on different elevations.
The Ug-value describes the central glass area rather than the entire glazed construction.
Heat can also travel through the spacer around the perimeter of the insulated glass unit and through the surrounding aluminium or other framing. The junction between the glass, spacer and frame creates a linear thermal bridge, commonly represented by a psi value.
Thermally improved spacers can reduce this edge loss considerably. Manufacturer calculations for one reference triple-glazed construction, for example, show a psi value of approximately 0.120 W/mK with an aluminium spacer and 0.031 W/mK with a thermally improved spacer. These figures are specific to the manufacturer's reference construction, but they demonstrate why centre-pane Ug cannot describe the whole façade condition.
The same principle applies to framing. Aluminium is highly conductive, so thermally broken profiles are used to interrupt the conductive route between inside and outside.
This becomes increasingly important when specifying very low-U-value glass. Improving the centre-pane performance while leaving substantial conductive paths around its perimeter reduces the benefit achieved at system level.
IQ Glass developed Invisio+ as a thermally broken structural glazing fixing system capable of accommodating deeper triple-glazed units.
In testing reported by IQ Glass, Invisio+ was used with triple glazing having a Ug-value of 0.7 W/m²K. The 3 m by 6 m test assembly achieved a verified system value reported as Uw 0.5895 W/m²K. The system is also designed to support structural glass elevations up to 6 m high without internal mullions.
The thermal break within the structural fixing addresses conduction through the aluminium profile, while the triple-glazed unit controls heat transfer across the transparent area.
The published Invisio+ information does not currently provide a g-value for the tested glass specification. It would therefore be incorrect to assign a generic triple-glazing g-value to that assembly. Solar performance would depend on the actual glass make-up and coating selected for the project.
This provides a useful example of whole-system triple glazing and heat transfer design. The glass and structural fixing can be developed for very low temperature-driven heat transfer, while the required solar performance is established through the separate glass specification.
Overheating depends on glazed area, orientation, g-value, shading, ventilation, thermal mass, internal heat gains, room geometry and external climate, alongside the insulation level of the envelope.
A recent London modelling study examined a two-bedroom flat using triple low-e glazing with a U-value of 1.058 W/m²K and a solar heat gain coefficient of 0.579. The flat passed the least severe current Design Summer Year considered by the researchers but failed the two more severe current weather files, with further overheating risk appearing under future climate scenarios. The study was examining weather data rather than comparing double and triple glazing directly, but it demonstrates that a well-insulated triple-glazed envelope can still experience overheating.
UK overheating guidance follows the same broader approach. Approved Document O considers glazing area, orientation, g-value and shading alongside methods for removing excess heat. CIBSE TM59 similarly identifies high proportions of glazing, excessive solar gain and inadequate ventilation among the factors that can contribute to overheating in homes.
Summer performance is normally resolved through a combination of the glass specification and the wider façade strategy.
Solar-control coatings can reduce the proportion of incident solar energy entering through the glass while retaining useful levels of visible light. External shading can intercept direct sunlight before it reaches the glazing. Overhangs can be particularly effective where their geometry responds to seasonal solar angles, while east and west elevations may require different approaches because of lower sun angles.
Ventilation deals with heat that has already entered or been generated inside the building. Its effectiveness depends on opening area, cross-ventilation, external temperature, security, noise and whether openings can realistically remain in use.
Marlborough Avenue provides a useful built example of this combined approach. IQ Glass specified solar-control glazing throughout the package, while an automated opening rooflight provides natural ventilation and the architectural roof overhang adds external shading.
More specialised projects can also use ceramic fritting or dynamic electrochromic glazing. Electrochromic glass provides a particularly clear example of U-value and g-value operating as separate design variables because the solar transmission of the glazing can change while the insulated glass construction continues to provide thermal resistance.
Approved Document O treats limiting solar gains and removing excess heat as complementary parts of overheating design. It also recognises the seasonal value of solar gain during colder periods. The appropriate g-value therefore depends on the elevation, glazing area, building use and wider environmental strategy rather than a universal lowest-value target.
A triple-glazing specification for a highly glazed building should establish the thermal and solar requirements separately:
Edinburgh Georgian Villa shows how these requirements can coexist within one glazing package. The extension uses thermally broken triple-glazed sliding doors alongside structural roof glazing and solar-control coatings specified to manage warmer-period solar gain.
This approach reflects the separate functions of U-value and g-value within the specification.
Triple glazing can help retain heat during winter and reduce temperature-driven heat entering a building during hot external conditions. Its ability to control direct solar gain depends on the glass make-up selected for the project.
A robust triple glazing and heat transfer specification therefore combines thermal transmittance, solar performance and whole-system detailing, assessed against the orientation, glazed area and overheating strategy of the building.
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