Strip shape
Simple I-shaped strips are cheapest. Shaped strips with extra chambers or flags block convection and radiation inside the frame cavity, and give a lower U-value at the same depth.
Energy-efficient window systems
Thermal Break Aluminium Windows
A thermal break aluminium window has its frame split into an inner and an outer aluminium shell, joined by a low-conductivity insulating barrier, usually glass-fibre-reinforced polyamide. Because aluminium conducts heat roughly 700 times faster than polyamide, that barrier cuts heat flow through the frame several-fold and keeps the inside face warm enough to resist condensation. Metline supplies thermally broken window profiles in 6063 and 6060, with gaskets and accessories, to window fabricators for buildings worldwide.
- Barrier
- PA66 GF25 strip or polyurethane
- Strip width
- about 14 – 45 mm
- Frame depth
- about 50 – 100 mm
- Standards
- EN 14024 · EN ISO 10077 · AAMA 505
What is a thermal break aluminium window?
Aluminium is an excellent structural material for windows: stiff, light, durable and easy to extrude into precise sections. Its weakness is thermal conductivity. A 6063 extrusion conducts heat at around 200 W/m·K, so a solid aluminium frame acts as a direct bridge between a cold outside and a warm room. A thermal break interrupts that bridge.
In a thermally broken profile, the outer shell faces the weather and the inner shell faces the room. They are held together only by the insulating barrier, which is designed to carry structural loads (wind pressure on the glass, sash weight, hardware forces) while conducting very little heat. Glass-fibre-reinforced polyamide 66 conducts about 0.3 W/m·K; polyurethane barriers are lower still.
The effect is large. A non-insulated aluminium frame typically has a frame U-value (Uf) somewhere above 5 W/m²·K. A frame with a standard 20 mm polyamide break typically falls to about 2.5 – 3.0 W/m²·K, and a deep high-insulation frame with a 30 – 40 mm shaped strip and foam inserts can reach 1.0 – 1.6 W/m²·K. Combined with modern insulating glass, that puts aluminium windows within current energy limits in Europe, the UK, North America, Australia and New Zealand.
How the thermal break is built into the profile
Two manufacturing methods are used worldwide. Both produce a composite profile whose structural performance is tested before it goes into a window.
| Aspect | Polyamide strip (insert and roll) | Pour and debridge |
|---|---|---|
| Process | The two aluminium shells are extruded separately. Their receiving grooves are knurled, polyamide strips are slid in, and rollers crimp the aluminium lips tightly onto the strips | A single profile is extruded with a channel. Liquid two-part polyurethane is poured into the channel and cured, then the aluminium bridge under it is cut away |
| Barrier material | PA66 GF25 (polyamide 66 with 25 % glass fibre); some systems use PA66 with foam or low-emissivity films | High-density polyurethane |
| Barrier width | Wide range, about 14 – 45 mm and more; multiple strips possible | Typically narrower, about 6 – 10 mm per pour; dual pours used for higher performance |
| Insulation potential | High; width and shape can be increased freely | Moderate; limited by pocket width |
| Where it dominates | Europe, Asia, Middle East, Australia and most export systems | Widely used in North American storefront and commercial windows |
| Key test | Shear and transverse tensile strength per EN 14024 or AAMA TIR-A8 / AAMA 505 | Shear and dry-shrinkage testing per AAMA 505 and AAMA TIR-A8 |
Strip-type barriers also allow the inner and outer shells to be finished in different colours before assembly, for example anthracite outside and white inside. Because the strips are crimped mechanically, the quality of knurling and rolling is critical: a poorly rolled profile can slip under load or temperature cycling. That is why EN 14024 defines shear strength, transverse tensile strength and elasticity values for the composite, tested at low, room and high temperatures and after ageing.
Strip width, frame depth and typical insulation
Wider barriers increase the distance heat must travel through the insulating material and leave room for foam or extra chambers. The ranges below are typical of thermally broken casement systems on the market; each specific system has its own tested values.
| System class | Frame depth | Strip width | Typical frame Uf | Typical use |
|---|---|---|---|---|
| Entry thermal break | 50 – 55 mm | about 14 – 16 mm | about 3.0 – 3.5 W/m²·K | Mild climates, budget housing |
| Standard | 55 – 65 mm | about 18 – 28 mm | about 2.3 – 3.0 W/m²·K | Mainstream residential and light commercial |
| High insulation | 70 – 80 mm | about 30 – 40 mm, shaped | about 1.6 – 2.2 W/m²·K | Energy-efficient homes, UK, EU, Australian projects |
| Very high insulation | 76 – 90 mm | about 35 – 45 mm with foam-filled chambers | about 1.2 – 1.6 W/m²·K | Cold climates and low-energy buildings |
| Passive-house direction | 90 mm+ | wide strips plus insulating inserts | about 0.8 – 1.2 W/m²·K | Very low-energy and certified buildings |
Foam inserts
Closed-cell foam strips or co-extruded foam fill the cavity between the shells and reduce convection. They are a common route from standard to high-insulation performance.
Dark colours
Dark frames in sun can exceed 70 °C on the outer shell. Use strips rated for the temperature and allow for differential expansion of inner and outer shells on long frames.
Frame U-values are calculated to EN ISO 10077-2 or simulated in THERM for NFRC; always compare whole-window values (Uw) at the same reference size.
Why the thermal break matters: energy, condensation and comfort
Energy
Frames make up 15 – 30 % of a typical window area. Without a break, that area can lose more heat than the glass beside it. Energy codes now set whole-window U-value limits that a non-insulated aluminium frame cannot meet with ordinary glass.
Condensation
Water condenses on any surface colder than the room's dew point. With 20 °C and 50 % relative humidity indoors, the dew point is about 9 °C. A non-insulated frame in freezing weather can fall well below that; a broken frame stays several degrees warmer.
Comfort
Cold inner surfaces cause draughts and radiant discomfort near windows. A warmer frame and glass edge make seating next to the window comfortable in winter.
Hot climates
In the Gulf, India, Southeast Asia and northern Australia, the break limits heat flow into air-conditioned rooms and reduces cooling load, especially with solar-control Low-E glass.
Acoustics
The break itself adds little sound reduction, but thermally broken systems normally come with multiple gaskets and accept thicker acoustic glass, which together improve sound insulation.
Durability
Less condensation means less staining, mould and damage to reveals, sills and finishes around the window.
Condensation performance is often specified separately. In Europe it is expressed through the temperature factor fRsi of the inner surface; in North America through the AAMA 1503 condensation resistance factor (CRF) or the NFRC 500 condensation resistance rating. Ask for these values if the building has humid rooms, swimming pools or a cold-climate location.
Thermal break versus non-thermal break aluminium windows
| Item | Thermal break windows | Non-thermal break windows |
|---|---|---|
| Frame structure | Inner and outer shells joined by an insulating barrier | Solid aluminium from outside to inside |
| Typical frame Uf | about 1.0 – 3.0 W/m²·K depending on depth | above about 5 W/m²·K |
| Condensation risk on frame | Low to moderate with correct glass and ventilation | High when outdoor temperatures are low |
| Energy code compliance | Meets most current limits with suitable glass | Generally limited to warm climates, internal screens or exempt buildings |
| Dual colour inside/outside | Yes, with strip-type barriers | No |
| Initial cost | Higher, for profiles, strips and rolling | Lower |
| Typical use | Homes, apartments, hotels, offices, schools, hospitals | Interior partitions, shop fronts in mild climates, utility buildings |
Choose a non-insulated frame only when the climate is mild year-round, the building is unconditioned, or the window is internal. For most occupied buildings, the extra cost of a thermal break is recovered through energy savings, lower condensation damage and compliance with building regulations.
Thermal break window types
Casement
Inward or outward hinged sashes closing onto compressed gaskets. The strongest air, water and acoustic performance; available from standard to very high insulation depths.
Tilt-turn
Inward-opening sash with tilt ventilation and turn-to-clean. Common in European-style housing, schools and hotels, usually in 70 mm and deeper systems.
Sliding and lift-slide
Space-saving for balconies and wide openings. Thermally broken interlocks and lift-slide gaskets are essential for good insulation in sliders.
Fixed
Picture windows and fixed lights with the smallest frame area per square metre of glass, so the best U-value for their size.
Awning
Top-hung sashes for kitchens, bathrooms and corridors; shed rain when open.
Large custom windows
Big fixed or combined units with reinforced mullions, sized to wind load, glass weight and transport limits.
Glass options and how they combine with the frame
The glazing unit usually covers 70 – 85 % of the window, so it sets most of the thermal result. The table gives typical centre-of-glass values (Ug) for common builds; exact figures depend on coating, gas and cavity width.
| Glass build-up | Typical Ug (W/m²·K) | Where it fits |
|---|---|---|
| Single 6 mm | about 5.7 | Non-insulated or internal windows only |
| Double 4-16-4, air, uncoated | about 2.7 | Not usually enough for energy codes |
| Double Low-E, argon, 16 mm cavity | about 1.0 – 1.1 | Standard energy-efficient specification |
| Double solar-control Low-E | about 1.0 – 1.6, low g-value | Hot climates and glazed offices |
| Triple Low-E, argon (two cavities) | about 0.5 – 0.7 | Cold climates, passive-house direction |
| Laminated or acoustic laminated IGU | similar to the base IGU | Safety, security and noise control |
Two details decide whether the frame and glass work well together. First, warm-edge spacers (stainless steel or polymer instead of aluminium) reduce heat loss along the glass edge, which is where condensation usually starts. Second, the glazing rebate must accept the full unit thickness with the specified gaskets; triple glazing needs deeper systems and heavier hinges. Typical thermally broken systems accept glass from about 24 mm in standard depths to 50 mm or more in deep systems.
Standards and documents by market
Thermally broken windows are assessed at two levels: the composite profile (does the barrier hold the shells together?) and the complete window (how does it perform in the building?). Different markets ask for different evidence.
| Market | Window-level requirements | Profile-level and supporting standards |
|---|---|---|
| European Union | CE marking to EN 14351-1: air (EN 12207), water (EN 12208), wind (EN 12210), Uw (EN ISO 10077), acoustics (EN ISO 717-1) | EN 14024 for thermal barrier profiles; EN 755 and EN 12020 for extrusions |
| United Kingdom | UKCA to EN 14351-1; Approved Document L limiting U-values (in the region of 1.4 – 1.6 W/m²·K for dwellings); PAS 24 security; BS 6375 | EN 14024; Qualicoat / Qualanod finish approvals |
| United States and Canada | NAFS (AAMA/WDMA/CSA 101/I.S.2/A440) performance class and grade; NFRC 100/200 U-factor and SHGC labels; ENERGY STAR where claimed | AAMA 505 and AAMA TIR-A8 for thermal barriers; AAMA 1503 thermal and condensation testing |
| Australia | AS 2047 for windows; AS 1288 glass selection; NCC energy provisions and WERS ratings | AS/NZS standards for finishes and materials |
| New Zealand | NZS 4211 window performance; Building Code clause H1 energy efficiency | Equivalent profile and finish documentation |
| Gulf and Asia | Local green building codes (for example Estidama or local energy codes) often reference EN or ASTM/AAMA tests | EN or AAMA profile tests accepted on most projects |
Test reports apply to the tested size, glass and hardware. A larger sash, a different glass unit or substituted hardware may need a new test or a recognised extended-application calculation.
Finishes, colours and hardware
| Option | Choices | Advice |
|---|---|---|
| Powder coating | Polyester and super-durable powders, RAL and custom colours, matt to gloss | Qualicoat class 1 for standard exposure; class 2/3 or AAMA 2604/2605 for coastal and high-UV sites |
| PVDF coating | Two- or three-coat fluoropolymer systems | Highest colour and gloss retention; often specified on high-rise and coastal facades |
| Anodising | Clear, bronze, black and other electrolytic colours | Use 20 – 25 µm film for exterior; colour can vary slightly between batches, so order from controlled lots |
| Wood-grain | Sublimated wood pattern on powder base | Warm look for villas and hotels; check UV durability of the pattern |
| Dual colour | Different inside and outside colours | Only possible with thermally broken profiles; a strong selling point for housing |
| Hardware | Handles, hinges, multi-point locks, friction stays, rollers, restrictors, screens | Match rated load to sash weight; use stainless or coated fittings near the sea |
Ask the system supplier to confirm that the thermal barrier is approved for the coating cure cycle and the colour you choose. More on finishes in our pages on powder-coated aluminium windows and anodised aluminium extrusions.
How to choose a thermal break window for your project
- Set the climate targetIdentify the U-value and, in hot regions, the solar heat gain (g-value or SHGC) required by the local code or energy model. Cold climates push towards deep frames and triple glazing; hot climates towards solar-control Low-E glass with a standard break.
- Choose opening typesCasements and tilt-turns for the best sealing; lift-slides for wide openings; fixed lights wherever opening is not needed, because they insulate best.
- Pick the system depthMatch frame depth and strip width to the U-value target and the glass thickness. Do not buy a deeper system than the target needs; do not try to reach a passive-house target with a 55 mm frame.
- Check structureLarge windows, high-rise floors and windy or coastal sites need stiffer sections, reinforced mullions and tested wind classes.
- Detail sealing and drainageSpecify gasket lines, pressure-equalised drainage and sill details; poor drainage is a more common cause of failure than poor insulation.
- Specify finish and hardwareChoose a coating class for the exposure and hardware rated for the sash weight and environment.
- Confirm documentsList the test reports, certificates and calculations the authority or consultant will require before ordering.
Specification items to confirm before quotation
| Item | What to confirm |
|---|---|
| Window type | Casement, tilt-turn, sliding, lift-slide, fixed, awning or combinations |
| System depth and barrier | Frame and sash depth, strip width and shape, foam inserts |
| Profile alloy and walls | 6063-T5 / T6 or 6060-T66; minimum wall thickness for frames and mullions |
| Glass | Unit build-up, coatings, gas fill, spacer type, safety and acoustic layers |
| Performance | Uw, g-value or SHGC, air, water and wind classes, sound reduction, condensation resistance |
| Finish and colour | Coating type and class, colour inside and outside, gloss |
| Hardware | Brand or equivalent, locking points, restrictors, screens |
| Documents | Section drawings, test reports, thermal calculations, finish certificates, mill test certificates for profiles |
| Supply scope | Profile lengths, or profiles cut and machined to drawing, with accessories |
| Packing and delivery | Protective film, labels by schedule reference, export crates, destination and Incoterms |
Quality control from extrusion to finished profile
- Profile dimensions: sections checked against drawings and the tolerance standard (EN 12020-2 for precision architectural profiles, EN 755-9 otherwise).
- Composite strength: rolled profiles sampled for shear strength to the system's EN 14024 or AAMA requirements, so strips cannot slip in service.
- Finish: film thickness, adhesion, gloss and colour checked against the approved sample and coating standard.
- Processing: where profiles are cut and machined to order, cutting length, mitre accuracy and hole positions are checked against the drawing.
- Accessories: gaskets, cleats and hardware checked for fit against the profile sections before packing, ready for your fabricator.
- Documents and packing: mill test certificates for the alloy, finish records, labels matching the window schedule, and edge-protected export packing.
Metline is ISO 9001 certified, and every profile shipment carries a mill test certificate. Third-party inspection before shipment can be arranged when the project requires it.
Installation details that protect thermal performance
A thermally broken frame only performs as tested if the joint between frame and wall is treated with the same care. Heat and moisture both look for the weakest path, and in many buildings that path is the installation gap rather than the window itself.
- Position in the wall: align the thermal break with the insulation layer of the wall so the insulating line is continuous; a window set fully in the outer masonry leaf creates a cold reveal.
- Seal inside and outside: use a vapour-tight membrane or sealant on the warm side and a vapour-open, weather-tight membrane on the cold side, with insulating foam or mineral wool between them.
- Fixings: use thermally isolated brackets or packers where possible; steel fixings bridging the break reduce its benefit.
- Sills and drainage: provide a sloped external sill and keep drainage slots clear so water never reaches the interior seal.
References and standards
- EN 14024 – Metal profiles with thermal barrier: mechanical performance, requirements, proof and tests
- EN ISO 10077-2 – Thermal performance of windows, doors and shutters: numerical method for frames
- EN 14351-1 – Windows and doors: product standard, performance characteristics
- FGIA – Thermal barrier technology (AAMA 505 and TIR-A8)
- NFRC 100 / 200 – Procedures for determining fenestration U-factors and SHGC
- AAMA/WDMA/CSA 101/I.S.2/A440 – North American Fenestration Standard
Thermal break aluminium (aluminum) windows: frequently asked questions
How much does a thermal break improve an aluminium window?
Typically it reduces frame heat loss by half to three-quarters. A non-insulated frame is usually above 5 W/m²·K; a standard 20 mm polyamide break brings it to about 2.5 – 3.0, and a deep high-insulation frame to about 1.0 – 1.6 W/m²·K.
Do thermal break windows stop condensation?
They greatly reduce condensation on the frame because the inner shell stays warmer, but no window stops it in every condition. High indoor humidity, poor ventilation and basic aluminium spacers in the glass can still cause condensation.
Polyamide strip or pour and debridge: which is better?
Polyamide strips allow wider barriers, better insulation and dual colours, and dominate in Europe, Asia and Australia. Pour-and-debridge is a proven, economical method widely used in North American commercial windows. Both are reliable when tested to EN 14024 or AAMA 505.
Are thermal break windows worth it in hot climates?
Yes. In air-conditioned buildings they reduce heat flow inwards and lower cooling demand, especially when combined with solar-control Low-E glass. They also stop condensation on the outside face of cold, air-conditioned frames in humid climates.
Can the inside and outside be different colours?
Yes. Because the two aluminium shells are separate until the strips are rolled in, each can be powder coated, anodised or wood-grain finished in a different colour.
What do you need to quote thermal break windows?
Project location, building type, window schedule or drawings with sizes and quantities, opening types, target U-value and other performance requirements, glass build-up, finish and colour, hardware preferences, required test documents and the delivery destination.
Can you supply thermally broken profiles only?
Yes, profiles are what we supply. Metline provides thermally broken profiles in mill finish, anodised or powder coated, as lengths or cut to size for your own fabrication, with mill test certificates and export packing; we do not supply complete windows.
Need thermal break windows or profiles?
Send the window schedule, U-value target, glass, finish and destination. Our team will recommend a system depth and quote profiles and accessories.
