When you buy new windows, you will see two key numbers on the energy label: the U-value and the g-value. Most homeowners understand that a low U-value is good because it stops heat escaping. The g-value can be more confusing because a higher number is not always better.
The target g-value for UK windows is ≤ 0.4 under Building Regulations Part L. This number measures how much solar heat passes through the glass, balancing winter warmth with summer overheating. Check your window's energy label for the exact g-value.
- G-value measures solar heat passing through glass, from 0 to 1.
- UK Building Regulations Part L targets a g-value ≤ 0.4.
- Higher g-values (0.5-0.6) reduce winter heating demand.
- Lower g-values (0.2-0.3) prevent summer overheating.
- SAP 10.2 uses g-value to calculate your EPC rating.
- The g-value measures how much solar heat passes through your window glass
- Why g-value matters for UK homes — heating vs. overheating
- The g-value range you should target for double and triple glazing
- Quick numbers — typical g-values and their effects
- How to read a window’s g-value on its energy label
- How to choose the right g-value for your home and orientation
- How to verify your installer and window certification (MCS, FENSA, Part L)
- The direct answer to “what g-value should I choose for windows in the UK?”
The g-value measures how much of the sun’s heat passes through your window glass into your home. For most replacement windows in the UK, the target g-value is ≤ 0.4, as set by Building Regulations Part L (2021) (GOV.UK, 2026).
The g-value measures how much solar heat passes through your window glass
The g-value, also called the solar heat gain coefficient (SHGC), is a number between 0 and 1. It shows the fraction of solar radiation that passes through the window as heat. A g-value of 0.6 means 60% of the sun’s heat passes through the glass. The rest is reflected or absorbed.
This is distinct from the U-value, which measures heat loss through the window. A low U-value is always better. The best g-value depends on your home and climate. In the UK, Building Regulations Approved Document L (2021 edition, updated 2022) sets target g-values for new windows. The typical target is ≤ 0.4 for most new-build and replacement glazing (GOV.UK, 2026).
Why g-value matters for UK homes — heating vs. overheating
A higher g-value, for example 0.5 to 0.6, allows more solar heat into your home. This reduces heating demand in winter. It is useful for north-facing rooms or homes with high heat loss. A lower g-value, for example 0.2 to 0.3, blocks more solar heat. This helps prevent overheating in summer. It is critical for south-facing or large glazed areas, especially as the UK climate warms.
The UK’s energy modelling software, SAP 10.2, uses the g-value to calculate solar gains for your home’s Energy Performance Certificate (EPC) rating (BRE, 2026). Overheating risk is highest in homes with large south-facing windows, poor ventilation, or high insulation levels. The g-value trade-off is between winter heat savings and summer discomfort.
The g-value range you should target for double and triple glazing
For double glazing, typical g-values range from 0.5 to 0.7 for standard low-emissivity (low-E) coating. They drop to 0.3 to 0.4 for solar-control coating. For triple glazing, g-values are generally lower, at 0.4 to 0.6, because extra panes and coatings reduce solar transmission.
A common UK specification for new double-glazed windows is a g-value ≤ 0.4 combined with a U-value ≤ 1.2 W/m²K, as required by Building Regulations Part L (2021) (GOV.UK, 2026). BRE’s SAP 10.2 (2022) Table 5a shows default g-values for common glazing types (BRE, 2026).
Quick numbers — typical g-values and their effects
| Glazing type | Typical g-value range | Solar heat transmitted | Best use case |
|---|---|---|---|
| Single glazing | 0.8–0.85 | 80–85% | Not recommended (poor U-value) |
| Double glazing, standard low-E | 0.5–0.7 | 50–70% | Mixed climates, average homes |
| Double glazing, solar-control | 0.3–0.4 | 30–40% | South-facing, overheating risk |
| Triple glazing, standard | 0.4–0.6 | 40–60% | High insulation, cold climates |
| Triple glazing, solar-control | 0.2–0.35 | 20–35% | Overheating-prone homes |
These ranges are based on Glass and Glazing Federation (GGF) technical data sheets (2023) and manufacturer specifications from Pilkington and Saint-Gobain (GGF, 2026).
How to read a window’s g-value on its energy label
UK window energy labels use the BFRC rating system. The label shows the g-value as part of the “solar factor”. Look for “g” or “SHGC” in the technical data section. The BFRC rating runs from A++ to G. It includes the g-value, U-value, and air leakage. A higher BFRC band does not guarantee a specific g-value. You must check the numbers themselves.
For replacement windows, the installer must provide a certificate with the g-value and U-value. This is a Part L compliance requirement (GOV.UK, 2026). The British Fenestration Rating Council (BFRC) labelling scheme explains how to read these labels (BFRC, 2026).
How to choose the right g-value for your home and orientation
For north-facing windows, aim for a higher g-value of 0.5 or above. This captures limited winter sun for heat gain. For south-facing windows, consider a lower g-value of 0.4 or below. This prevents summer overheating, especially with large glazed areas. For east and west-facing windows, a moderate g-value of 0.4 to 0.5 balances morning and afternoon sun exposure.
Use the SAP 10.2 methodology from BRE to model your home’s specific solar gains. A higher g-value can improve your EPC band in winter but may trigger overheating risk in summer (BRE, 2026, Section 7.2).
how to choose the right windows for your home
How to verify your installer and window certification (MCS, FENSA, Part L)
For replacement windows in England and Wales, the installer must be FENSA-registered or use a Local Authority Building Control (LABC) scheme. This self-certifies compliance with Part L. FENSA-registered installers provide a certificate showing the window’s g-value and U-value. It confirms the window meets the target of ≤ 0.4 for most new double glazing.
For new-build or major renovation, the building control officer checks g-value compliance through the SAP calculation for the whole dwelling. This is set out in Approved Document L (2021 edition, with 2022 amendments), Section 4.7 on glazing (GOV.UK, 2026).
FENSA and building regulations for window replacement
The direct answer to “what g-value should I choose for windows in the UK?”
For most UK replacement windows, the target g-value is ≤ 0.4, as specified in Building Regulations Part L (2021) for new double glazing. Choose a higher g-value of 0.5 to 0.6 if your window is north-facing or your home has high heating demand and low overheating risk.
Choose a lower g-value of 0.3 to 0.4 if your window is south-facing, large, or your home is well-insulated and prone to overheating. The trade-off is between winter heat savings from a higher g-value and summer comfort from a lower g-value. Use the SAP 10.2 model or consult a certified energy assessor for your specific property (BRE, 2026).
Frequently Asked Questions
A good g-value for UK windows is ≤ 0.4, as set by Building Regulations Part L (GOV.UK, 2026). This balances winter heat gain with summer overheating risk.
The U-value measures heat loss through the window, while the g-value measures solar heat gain. A low U-value is always better, but the best g-value depends on your home's orientation and climate.
For south-facing windows, a lower g-value of 0.2 to 0.3 is recommended to prevent overheating, especially as UK summers warm (Energy Saving Trust, 2026).
A higher g-value (0.5-0.6) can reduce winter heating costs by allowing more solar heat in, but it may increase summer cooling needs. The optimal g-value depends on your home's insulation and orientation (Ofgem, 2026).
The g-value is used in SAP 10.2 software to calculate solar gains for your Energy Performance Certificate (EPC) rating (BRE, 2026). A higher g-value can improve your EPC score in winter but may reduce it in summer.
