Most homes built before 1990 have single glazing or older double glazing that no longer performs well. Modern glazing specification has improved significantly, and the difference between replacing outdated windows with good quality modern units versus mediocre modern units is significant: both in thermal performance and in comfort. This guide cuts through the specifications to tell you what actually matters.
Window Energy Ratings
Windows in the UK are rated under the British Fenestration Rating Council (BFRC) scheme, using an A++ to G scale similar to appliance energy labels. The rating combines thermal transmittance (how well the window retains heat) with solar factor (how much solar heat gain it allows through). An A-rated window is the minimum recommended for new installations under Part L of the Building Regulations.
The key specifications to look at:
U-value (Uw). The thermal transmittance of the whole window, including frame and glass. Lower is better. Single glazing: 4.5-5.0 W/m2K. Standard double glazing: 1.6-2.8 W/m2K. Good modern double glazing: 1.2-1.6 W/m2K. Good triple glazing: 0.6-0.9 W/m2K.
Centre pane U-value (Ug). The performance of the glazed unit alone, excluding frame. More useful for comparing glazed units directly. A standard 4-16-4 argon-filled double glazed unit achieves Ug of approximately 1.1 W/m2K. A triple glazed unit with warm edge spacers and krypton fill: 0.5-0.6 W/m2K.
Solar heat gain coefficient (g-value or SHGC). How much solar radiation passes through the glass as heat. Higher values allow more solar gain, which can be beneficial for south-facing windows (free heating) and detrimental for large west-facing windows (overheating). Standard low-e coatings have g-values of 0.3-0.6.
Frame Materials
uPVC. The most common choice for residential replacement windows. Durable, low maintenance, available in a wide range of styles and colours. Thermal performance of the frame depends heavily on the profile design: multi-chamber uPVC profiles with foam filling significantly outperform basic single-chamber profiles. Good uPVC frames contribute meaningfully to overall window U-value.
Aluminium. Strong, slim profiles, excellent durability. Standard aluminium frames are thermally poor (metal is a very good thermal conductor). Thermally broken aluminium profiles (where the inner and outer sections are separated by a low-conductivity thermal break) perform much better, typically achieving frame U-values comparable to good uPVC. Costs more than uPVC.
Timber. Traditional appearance, good thermal performance, and the frame is recyclable at end of life. Requires more maintenance than uPVC (repainting every 5-10 years). Engineered timber windows (laminated from multiple pieces) are more stable and less prone to warping than solid timber. Softwood is cheaper; hardwood is more durable.
Composite (timber-aluminium). Timber internally, aluminium externally. Combines the aesthetic warmth of timber inside with the weather resistance of aluminium outside. Premium product, used in high-specification projects and where appearance is a priority.
Is Triple Glazing Worth It?
Triple glazing costs 30-60% more than equivalent double glazing. For a whole-house window replacement on a standard 3-bedroom semi, that might be an additional £3,000-£8,000. Whether that premium is justified depends on:
Building fabric. Triple glazing makes most sense in a very well-insulated building where the walls and roof are already at high performance levels and windows are a relatively significant proportion of total heat loss. In a poorly insulated building, the priority is walls and roof before upgrading windows beyond A-rated double.
Climate and exposure. Scotland and northern England have significantly higher heating demand than southern England. The payback period for triple glazing in Aberdeen is meaningfully shorter than in London.
Comfort. The surface temperature of a triple-glazed window in cold weather is significantly higher than double, which reduces the radiant cold sensation near windows and allows furniture to be placed closer to them. This comfort benefit is real and not always captured in energy payback calculations.
For most properties in England with average insulation levels, high-quality A++-rated double glazing is the pragmatic choice. Triple glazing is worth considering for new builds targeting high energy performance standards, for properties being fitted with heat pumps where every thermal improvement compounds, and for cold climates.
The quality of installation matters as much as the quality of the product. A premium triple-glazed window poorly installed (gaps in the reveal, inadequate insulation around the frame, poor mastic sealing) will underperform a standard double-glazed window installed carefully. Insist on proper installation and check the reveals are fully insulated and airtight before decoration covers them.
Regulations and Certification
Replacement windows are notifiable building work under Part L and Part N of Building Regulations. Windows must meet minimum energy performance requirements (current minimum Uw 1.6 W/m2K or WER Band C) and safety glazing requirements in critical locations (low-level glazing, glazing within 300mm of door handles).
FENSA or CERTASS-registered installers self-certify that the installation complies with building regulations and issue a certificate that is registered with the local council. Keep this certificate. You'll need it when selling the property. If your installer is not FENSA or CERTASS registered, the installation must be inspected and certified by building control, which requires separate notification and fee.