Cold climates
Low U-value with relatively high solar gain, so winter sun helps heat the building. Use high-transmission Low-E, argon fill and often triple glazing.
Thermal performance
Energy-Efficient Aluminium Windows
Energy-efficient aluminium windows combine thermally broken frames, Low-E insulated glass, warm-edge spacers and multi-layer gaskets so that heat stays in during winter and solar heat stays out in summer. A well-specified modern aluminium window can reach whole-window U-values of around 1.0 to 1.4 W/m²K while keeping the slim frames and strength aluminium is chosen for. Metline supplies thermal-break window profiles and accessories, selected for your climate and energy code, to window fabricators worldwide.
- Frame
- Thermal-break 6063-T5/T6
- Glass
- Low-E double or triple IGU
- Typical Uw
- About 1.0 – 1.6 W/m²K
- Ratings
- EN ISO 10077 · NFRC 100/200
How an aluminium window becomes energy-efficient
Aluminium conducts heat very well, so an ordinary aluminium frame acts as a thermal bridge between inside and outside. Energy-efficient windows solve this at every point where heat can pass: through the frame, through the glass, around the edge of the glass and through gaps in the seals.
Four elements work together. A thermal break of glass-fibre-reinforced polyamide separates the inner and outer aluminium shells. Low-E insulated glass, with an argon or krypton-filled cavity, cuts radiant and conductive heat loss through the largest area of the window. A warm-edge spacer reduces the cold strip around the glass edge where condensation often forms. And multiple EPDM gaskets, ideally with a centre gasket, stop the air leakage that can cost as much energy as conduction.
Weak links matter more than strong ones. Triple glazing in a frame without a proper thermal break, or an excellent window installed with gaps around it, will not perform as its glass rating suggests. Specifying the whole window, and its installation, is what delivers the saving.
How each component affects the U-value
The U-value measures heat flow per square metre for each degree of temperature difference; lower is better. The whole-window value Uw is calculated from the glass value Ug, the frame value Uf and the edge-of-glass linear transmittance Ψ, to EN ISO 10077 in Europe or NFRC 100 in North America (where it is called the U-factor).
| Glass build-up | Typical centre-pane Ug (W/m²K) |
|---|---|
| Single glazing, 6 mm | About 5.7 |
| Double glazing, uncoated, air-filled | About 2.7 – 2.9 |
| Double glazing, Low-E, air-filled | About 1.4 – 1.8 |
| Double glazing, Low-E, argon-filled | About 1.0 – 1.1 |
| Triple glazing, two Low-E coatings, argon | About 0.5 – 0.7 |
| Frame type | Typical frame Uf (W/m²K) |
|---|---|
| Aluminium, no thermal break | About 5.5 – 7.0 |
| Aluminium, narrow thermal break (14 – 20 mm) | About 2.8 – 3.5 |
| Aluminium, wide thermal break with foam inserts (24 – 35 mm+) | About 1.3 – 2.2 |
Indicative ranges from published glass and frame data. Actual values depend on the cavity width, coating, gas fill, profile depth and window size, and must be taken from calculations or test reports for the specific system.
Because the frame usually makes up 20 to 35 % of a window's area, the frame matters as much as the glass. A Low-E argon double-glazed unit (Ug 1.1) in a well-insulated frame typically gives a Uw of about 1.3 to 1.6; moving to triple glazing and a wide break brings Uw near 1.0. Replacing aluminium spacer bars with warm-edge spacers lowers Uw further, often by around 0.1 W/m²K on a typical window, and raises the glass-edge temperature to reduce condensation.
Choosing Low-E glass for your climate
Insulation is only half of energy performance. The solar factor, written as the g-value in Europe or SHGC in North America, measures how much solar heat passes through the glass. The right balance depends on climate.
Hot climates
Low solar gain matters most. Use solar-control Low-E (spectrally selective) glass with a low g-value or SHGC while keeping good daylight.
Mixed climates
Balanced Low-E coatings; orientation-specific glass (lower g on west and south, higher on north) can be worthwhile on large projects.
Soft-coat Low-E
Sputtered coatings with the best performance; always placed inside the sealed cavity.
Hard-coat Low-E
Pyrolytic coatings, more durable and can face the air, but with higher emissivity and lower performance.
Acoustic and safety
Laminated glass can be combined with Low-E for noise reduction, security and safety without losing thermal performance.
Which window types perform best?
| Window type | Airtightness | Thermal potential | Notes |
|---|---|---|---|
| Casement | Excellent | Excellent | Compression gaskets seal as the sash closes |
| Tilt-and-turn | Excellent | Excellent | Multi-point locking pulls the sash tight all round |
| Fixed | Best | Best | No moving parts; lowest frame-to-glass ratio |
| Lift-and-slide | Very good | Very good | Sash drops onto gaskets when closed; suits large openings |
| Sliding | Fair to good | Good | Brush seals leak more air; choose systems with gasket seals |
For the most energy-efficient building, use casement, tilt-and-turn or fixed windows where possible, and lift-and-slide rather than standard sliding systems for large openings. Where standard sliders are chosen for cost or space, specify a system with good tested air-permeability results (EN 12207 class 3 or 4, or a low ASTM E283 leakage rate).
Energy codes and window ratings
- Europe: Uw declared under EN 14351-1 and calculated to EN ISO 10077-1 and -2; national rules set maximum values for new and replacement windows
- United States and Canada: NFRC-rated U-factor and SHGC, used to comply with the IECC, ASHRAE 90.1 or ENERGY STAR criteria for the climate zone
- Australia: whole-window U-value and SHGC rated under the Window Energy Rating Scheme (WERS) for NCC compliance
- Middle East and Asia: green building schemes and local codes often set glass U-value and SHGC limits, with solar control a priority
Ask for the U-value of the complete window at the standard reference size, not just the glass. Glass-only figures look much better than the real window and are a common source of disappointment on site.
Installation and maintenance tips
- Insulate the gap between frame and wall, and seal it airtight inside and weather-tight outside with tapes or membranes
- Align the thermal break of the window with the insulation layer of the wall to avoid a thermal bridge
- Check that sashes close evenly on all gaskets; adjust hinges and keeps after installation
- Keep drainage slots clear and clean gaskets annually; replace damaged gaskets promptly
- Lubricate hardware periodically so multi-point locks keep pulling sashes tight
References and standards
- EN ISO 10077-1 and -2 – Thermal performance of windows, doors and shutters
- EN 14351-1 – Windows and doors: product standard, performance characteristics
- NFRC 100 and NFRC 200 – Fenestration U-factor and solar heat gain coefficient
- ENERGY STAR – Residential windows, doors and skylights criteria (US EPA)
- EN 673 – Glass in building: determination of thermal transmittance (U value)
Energy-efficient aluminium (aluminum) windows: FAQ
Are aluminium windows energy-efficient?
Modern thermally broken aluminium windows with Low-E insulated glass are. They can reach whole-window U-values of about 1.0 to 1.6 W/m²K. Old aluminium windows without thermal breaks are very poor, at around 5 W/m²K or worse.
How can I make aluminium windows more energy-efficient?
Choose a wide thermal break, Low-E argon-filled double or triple glazing, warm-edge spacers and a casement or tilt-and-turn opening type with good gaskets, and make sure the installation is insulated and airtight.
Are aluminium or uPVC windows more energy-efficient?
At the same glass specification, uPVC frames usually have a slightly lower frame U-value. High-performance thermally broken aluminium closes most of the gap and allows slimmer frames and larger glass, which can offset the difference.
Which Low-E glass should I choose?
In cold climates, choose a high-solar-gain Low-E with argon to keep heat in and admit winter sun. In hot climates, choose a solar-control Low-E with a low g-value or SHGC to keep solar heat out.
Is triple glazing worth it?
In cold climates and on low-energy buildings, usually yes. In warm climates the extra cost and weight rarely pay back; a good Low-E double-glazed unit with solar control is normally the better choice.
Need energy-efficient aluminium windows?
Send your window schedule, location and target U-value. We will propose thermal-break profiles to meet it and quote delivered supply.
