Selectaglaze secondary glazing is a smart, sustainable retrofit solution that enhances thermal performance by creating an insulating air gap between the inner and outer panes of glass. This barrier significantly reduces heat loss and improves U-Values, making buildings warmer and more energy-efficient.
When fitted with low-emissivity (low-E) glass, secondary glazing reflects infrared heat back into the interior, helping to retain warmth. Thermally isolated frames further reduce heat transfer, keeping the inner glass surface warmer and minimising uncomfortable convection currents.
High-performance seals on both the outer frame and opening panels help eliminate draughts and improve indoor air quality by limiting the entry of dust and airborne pollutants.
Why it matters
With 80% of today’s buildings expected to still be in use by 2050, reducing their energy consumption is critical to achieving the UK’s Net Zero goals. Secondary glazing offers a practical, cost-effective way to improve building performance without the need for full window replacement.Thermal benefits at a glance
- Minimised draughts
- Enhanced comfort
- Improved U-Values
- Higher EPC rating
- Better BREEAM scores
- Lower energy bills
- Reduced carbon footprint
Regulations and standards
- Building Regulations Approved Document L: Parts L1B and L2B:
Approved Document L sets strict performance standards for energy efficiency in both residential and commercial properties undergoing renovation or upgrades. Controlled fittings - such as windows - must meet specific thermal performance criteria (see performance table for details). The regulations also recognise the unique challenges of working with listed buildings. Subject to Listed Building Consent, secondary glazing with low-E glass can be installed as a discreet and effective energy efficiency measure that preserves the building’s character. - BREEAM:
Recognised as the UK’s leading sustainability assessment method for buildings, BREEAM evaluates a wide range of criteria including energy efficiency, health and wellbeing. Secondary glazing can make a meaningful contribution toward achieving a higher BREEAM rating. - EPC and MEES:
An Energy Performance Certificate (EPC) rates the energy efficiency of a building on a scale from A (most efficient) to G (least efficient). EPCs are legally required when a property is built, sold or rented. Under the Minimum Energy Efficiency Standards (MEES) - in effect since 1 April 2018 - both residential and commercial properties must achieve an EPC rating of E or above before a new lease can be granted or an existing lease renewed. Retrofitting secondary glazing offers a quick and effective way to improve a building’s EPC rating, helping property owners meet regulatory requirements and making the space more attractive to prospective tenants. - Passivhaus/EnerpHit:
Passivhaus certification applies to new buildings designed to deliver exceptional occupant comfort while using minimal energy for heating and cooling. Existing buildings can also be upgraded to meet the EnerPHit standard - a retrofit equivalent to Passivhaus. While slightly less stringent, EnerPHit accounts for the practical challenges of retrofitting older structures, including heritage conservation. Achieving EnerPHit certification requires substantial insulation, high levels of airtightness and a mechanical ventilation system with heat recovery. Secondary glazing can play a key role in meeting these requirements, particularly in improving thermal performance and airtightness without compromising the building’s original character. - WELL Building Standard:
The WELL Building Standard is a performance-based framework for designing and certifying spaces that prioritise the health and well-being of occupants. It evaluates a wide range of factors, including thermal comfort and acoustic performance - both of which are essential for achieving certification. Secondary glazing can support WELL certification by enhancing thermal comfort and significantly improving acoustic insulation, contributing to a healthier and more comfortable indoor environment.
Secondary glazing U-Value performance comparisons
U-Value performance of Selectaglaze secondary glazing
The thermal performance of our product range has been assessed against both timber and metal primary windows by the Centre for Windows and Cladding Technology (CWCT). The window size is 1480m(w) x 1230mm(h) and all figures are in W/m²K.| Timber primary + secondary unit 100mm cavity | Metal primary + secondary unit 100mm cavity | |||||||||
| Product | 6mm clear float | 6mm low-E | 4-12-4 SU low - E glass | 4-12-4 SU low-E and gas | 24mm Enhanced Thermal Sealed Unit | 6mm clear float | 6mm low-E | 4-12-4 SU low - E glass | 4-12-4 SU low-E and gas | 24mm Enhanced Thermal Sealed Unit |
| S10 HS | 2.629 | 1.807 | - | - | - | 2.911 | 1.936 | - | - | - |
| S80 HS | 2.640 | 1.810 | - | - | - | 2.920 | 1.940 | - | - | - |
| S85 HS | 2.700 | 1.840 | - | - | - | 3.000 | 1.970 | - | - | - |
| S20 VS | 2.617 | 1.802 | - | - | - | 2.897 | 1.930 | - | - | - |
| S60 TVS | 2.639 | 1.812 | - | - | - | 2.924 | 1.942 | - | - | - |
| S90 VS | 2.615 | 1.801 | - | - | - | 2.894 | 1.929 | - | - | - |
| S95 VS | 2.698 | 1.840 | - | - | - | 2.996 | 1.974 | - | - | - |
| S41 SHC | 2.581 | 1.785 | 1.676 | 1.627 | 1.27 | 2.852 | 1.910 | 1.786 | 1.731 | 1.34 |
| S45 SHC | 2.576 | 1.782 | - | - | - | 2.846 | 1.908 | - | - | - |
| S50 SHC | 2.589 | 1.788 | 1.758 | 1.700 | 1.32 | 2.862 | 1.915 | 1.879 | 1.813 | 1.38 |
| S41 DSHC | 2.588 | 1.788 | 1.865 | 1.832 | - | 2.861 | 1.914 | 2.003 | 1.964 | - |
| S45 DSHC | 2.585 | 1.786 | - | - | - | 2.857 | 1.912 | - | - | - |
| S50 DSHC | 2.598 | 1.793 | 1.966 | 1.927 | - | 2.874 | 1.920 | 2.120 | 2.074 | - |
| S30 LO | 2.585 | 1.787 | - | - | - | 2.858 | 1.913 | - | - | - |
| S41 LO | 2.581 | 1.785 | 1.676 | 1.627 | 1.27 | 2.852 | 1.910 | 1.786 | 1.731 | 1.34 |
| S45 LO | 2.576 | 1.782 | - | - | - | 2.846 | 1.908 | - | - | - |
| S40 FL | 2.574 | 1.781 | 1.517 | 1.435 | - | 2.843 | 1.906 | 1.606 | 1.515 | - |
| S46 FL | 2.579 | 1.784 | - | - | - | 2.850 | 1.910 | - | - | - |
| S55 DFL | 2.612 | 1.800 | - | - | - | 2.891 | 1.927 | - | - | - |
Glossary
- U-Value:
The U-Value measures the rate of heat transfer through a building element, such as a window, wall, or roof. It is expressed in watts per square metre per degree Kelvin (W/m²K). A lower U-Value indicates better insulation and reduced heat loss, contributing to improved energy efficiency. - G-value:
The g-value represents the total solar energy transmittance of a window and is expressed as a number between 0 and 1. A g-value of 1 means all solar energy passes through (maximum solar gain), while 0 means none is transmitted. Higher g-values can be beneficial in colder months by contributing to passive solar heating, but may lead to overheating in summer. For example, standard single glazing typically has a g-value around 0.8. This can be reduced by using different glazing types or applying specialist solar control films, particularly useful on south-facing façades where solar gain is a concern.
- Low-E glass:
Low-emissivity (low-E) glass features a microscopically thin, transparent metallic coating on the surface facing the cavity. This coating reflects heat back into the interior space, significantly reducing heat loss while allowing natural light to pass through.