Last updated: 18 May 2026
If you're specifying Smart aluminium windows or doors for a new build, extension or replacement project, the U-value is the number that decides whether the spec passes Building Regulations and how much heat your home will actually keep in. This guide explains what Smart U-values mean, the real figures across Smart Systems products, and how they map to UK compliance and energy bills.
Key takeaways
- Smart aluminium systems from Arc Glazing typically achieve U-values of 1.0 to 1.4 W/m²K, depending on frame, glass and configuration.
- A lower U-value means better insulation. Replacement windows and doors in England must hit 1.4 W/m²K or better under Part L of the Building Regulations.
- The U-value of the whole window (Uw) is what matters for compliance — not the glass-only (Ug) figure quoted by some suppliers.
- Triple glazing, warm-edge spacers and the right thermal break can push Smart systems below the 1.0 W/m²K mark for premium specs.
What is a U-value?
A U-value measures how much heat passes through one square metre of a building component for every degree of temperature difference between inside and outside. It's expressed in watts per square metre per Kelvin (W/m²K). Lower is better. A single-glazed window has a U-value around 5.0 W/m²K; a modern Smart aluminium window can come in at 1.2 W/m²K or lower.
Three U-values get used interchangeably, and that's where most confusion starts:
- Ug — the U-value of the glass only. Useful for comparing glazing units in isolation, but not what Building Control checks.
- Uf — the U-value of the frame only. Aluminium frames need a thermal break to compete with timber or uPVC here.
- Uw — the U-value of the whole window, including frame, glass and edge seals. This is the figure that matters for compliance and the one Smart publishes on its product datasheets.
If a salesperson quotes a 1.0 W/m²K U-value without specifying whether it's Ug or Uw, ask which one. The difference can be the gap between passing Building Regs and failing them.
Smart window U-values: real figures by product
Smart Systems publishes Uw figures for each window range, tested against a standard 1,230 × 1,480 mm sash. The values below reflect typical specifications Arc Glazing installs across Kent, Surrey, London and the wider South East. Actual figures vary with glass spec, gas fill, spacer type and sash size — datasheets are available on request before any quote is finalised.
| Smart window range | Typical Uw (W/m²K) | Best for |
|---|---|---|
| Alitherm Heritage | 1.4 – 1.6 | Steel-look refurbishments where Part L for replacements applies |
| Alitherm 300 | 1.4 – 1.6 | Slimline casement windows on refurbishment projects |
| Alitherm 400 | 1.2 – 1.4 | Mid-range casement and tilt-and-turn for new builds and extensions |
| Alitherm 600 | 1.0 – 1.2 | Premium thermally-broken windows for low-energy specifications |
The Alitherm 400 system was developed specifically to meet the tightened 2022 Part L limits, with thermal simulations and energy ratings published for compliance. For a new dwelling, Arc Glazing usually recommends Alitherm 400 with Ug 1.0 double glazing or Alitherm 600 with triple glazing where the SAP calculation demands a notional Uw of 1.2.
Smart door U-values: real figures by product
Door U-values follow a different rulebook from windows under Part L, because the proportion of glazing changes what counts as "good enough". A fully glazed bifold is treated like a window. A side-hinged door with a 30% glass panel sits in its own category. The Smart door ranges Arc Glazing fits cover both ends.
| Smart door range | Typical Ud (W/m²K) | Best for |
|---|---|---|
| Visofold 1000 Slim (bifold) | 1.4 – 1.6 | Indoor-outdoor extensions, garden rooms, kitchen-to-patio openings |
| Visofold 6000 (bifold) | 1.4 – 1.6 | Slimmer-sightline bifolds where light area is the priority |
| Visoglide Plus (sliding) | 1.4 – 1.6 | Larger openings with fewer interruptions; in-line sliding patio doors |
| Designer Slim (sliding) | 1.0 – 1.2 | Premium large-pane sliders for low-energy and Passivhaus-adjacent specs |
| Aluminium residential entrance doors | 1.2 – 1.4 | Front and back doors with under 30% glazed area |
For bifolds and sliders with more than 60% glazed area, the figure to check is whether the whole-door Ud comes in at or below 1.4 W/m²K for replacement in an existing dwelling, or 1.2 W/m²K for a new build under the notional specification. Most Smart bifold installs Arc Glazing completes land between 1.4 and 1.6 W/m²K depending on whether the client opts for double or triple glazing.
Energy efficiency benefits: what a lower U-value actually does
U-values get treated as an abstract number, but the practical effects are concrete. A whole-house glazing upgrade from a 1980s aluminium spec (Uw around 4.0 W/m²K with no thermal break) to a modern Smart spec at 1.2 W/m²K cuts heat loss through the glazing by roughly 70%. According to the Energy Saving Trust, upgrading single-glazed windows to A-rated double glazing saves around £140 a year on energy bills across Great Britain. The savings on a like-for-like Smart aluminium upgrade are smaller, but still meaningful — and the comfort and condensation benefits show up immediately.
The less-obvious benefits matter more day-to-day than the bill figure:
- Lower cold-spot effect. Sitting next to a high-U-value window feels cold even when the room thermostat reads 21°C, because radiant heat moves from your body to the glass. Drop the Uw to 1.2 W/m²K and that effect more or less disappears.
- Less condensation. Cold internal glass surfaces drop below the dew point and run with water. Better-insulated frames and warm-edge spacers keep the inner pane warmer and condensation off the sills.
- Better acoustic performance. The same thermally broken frames and laminated/asymmetric glazing units that drop the U-value also drop the dB rating — useful in urban locations or near busy roads.
- Higher EPC band. Replacement glazing is one of the cheaper ways to move a property up an EPC band, which matters for rentals and resale.
Applications and UK compliance
Building Regulations Part L was last revised on 15 June 2022, and the U-value limits Arc Glazing works to depend on what you're doing:
| Project type | Element | Limit (W/m²K) |
|---|---|---|
| Replacement in existing dwelling | Windows | 1.4 (or WER Band B) |
| Replacement in existing dwelling | Doors (over 60% glazed) | 1.4 (or DSER Band C) |
| Replacement in existing dwelling | Doors (under 60% glazed) | 1.4 (or DSER Band B) |
| New dwelling (notional spec) | Windows and doors over 60% glazed | 1.2 (limiting value 1.6) |
| New dwelling (notional spec) | Doors under 60% glazed | 1.0 (limiting value 1.6) |
| New dwelling (notional spec) | External fire doors | 1.8 |
Two compliance notes worth knowing. First, in a new build the whole dwelling is assessed via SAP — individual windows can exceed the notional U-value if the rest of the fabric compensates, up to a hard limiting value of 1.6 W/m²K. Second, replacement windows can never be worse than what they replaced, even if they meet the 1.4 W/m²K headline figure.
Arc Glazing is CERTASS-registered, which means our installations are independently assessed to meet the latest Part L, Part F (ventilation, including trickle vents) and Part Q (security) standards. Every window or door we fit is self-certified and registered for the homeowner — no separate Building Control application is needed for the glazing element, and a compliance certificate is issued automatically after install.
Looking ahead, the Future Homes and Buildings Standards come into force on 24 March 2027 (with a 12-month transition period) and are expected to push notional U-values further still as part of the route to net-zero by 2050. Specifying a Smart system at 1.2 W/m²K today rather than the 1.4 W/m²K replacement minimum means the glazing won't look dated against the next regulatory cycle.
Check your spec before you sign anything
The most common mistake we see on incoming quotes from other installers is a U-value quoted as glass-only (Ug) without a whole-window figure (Uw) — and on close inspection the Uw fails Part L. The second most common is a U-value that's compliant in isolation but undermined by a poor install: cold-bridging at the reveal, missing perimeter insulation, or trickle vents specified incorrectly for Approved Document F.
If you've got a quote on the table and want a second pair of eyes on the spec, or you're at the design stage and want help matching the right Smart product to your project, request a free no-obligation quote and we'll talk you through the U-values that actually matter for your build.
Related reading
- Smart Aluminium Windows & Doors: Systems, Prices & Installation — the hub page covering the full Smart product range, pricing logic and installation process.
- Aluminium Doors in Kent — full breakdown of Smart Designer, Visofold, Visoglide Plus and Cortizo Slimline-20 ranges.
- Aluminium Bifold Door Installation — installation guide and project examples for Smart Visofold bifolds.
- Aluminium Sliding Door Installation — Smart Cortizo Slimline-20 and Visoglide Plus sliding systems with U-value detail.
Check your Smart spec. Send us your drawings or a current quote and Arc Glazing will return a side-by-side comparison with the right Smart products, real Uw figures, and a price. Request a free quote.