Aluminium thermal break: how it really works and why it matters

There’s a common misconception still heard today: “aluminium doesn’t insulate”. That was true forty years ago. Pure aluminium conducts heat 1,500 times more than wood and 750 times more than PVC, so yes, a solid aluminium profile would be a thermal disaster. But since the mid-1980s, no one has been manufacturing “cold” aluminium windows for residential use. Genuine modern aluminium is thermally broken, and its Uf value can rival that of PVC in top-of-range systems. This article explains how it really works, without the marketing spin.

The problem to solve: the thermal bridge

Pure aluminium has a thermal conductivity of around 200 W/mK. PVC sits at 0.17 W/mK, hardwood at 0.15 W/mK. This means a linear metre of solid aluminium transfers heat a thousand times faster than a linear metre of wood.

In a window or door, this translates into a thermal bridge: the aluminium frame puts the cold outside air in direct contact with the warm inside air. The result: condensation on the internal frame in winter, heat loss, and possible mould on the adjoining wall. A “cold” 1970s aluminium window has a Uf of around 6-7 W/m²K — a catastrophic value.

The solution is simple in concept: physically cut the profile through its thickness with an insulating material. Hence “thermal break”.

How the polyamide strip works

A thermally broken aluminium profile is made up of:

• An external profile in aluminium (hollow section, usually 25-40 mm thick)
• An internal profile in aluminium (hollow section, similar or slightly different dimensions)
• Between the two, one or more insulating strips in glass-fibre-reinforced polyamide (PA 6.6 GF 25%, technical code)

The polyamide strips are inserted cold into dedicated grooves in the two aluminium profiles, then mechanically crimped. Not welded, not glued: the mechanical pressure of the crimping holds them permanently in place, guaranteeing structural continuity and thermal insulation at the same time.

The conductivity of glass-fibre-reinforced polyamide 6.6 is around 0.3 W/mK — 600 times less than pure aluminium. It is low enough to break the thermal bridge, yet mechanically strong enough (glass fibre makes up 25% of the weight) to bear the load of the window or door for decades.

Strip width changes everything

Thermal break is not a binary “present or absent” feature. Its effectiveness depends on the width of the polyamide strip:

• 18-24 mm strip: first generation, 1990s. Uf 3.0-3.5 W/m²K. Today used only on low-budget commercial doors or industrial technical windows.
• 24-30 mm strip: basic residential standard. Uf 2.2-2.6 W/m²K. Sufficient for mild climate zones (A-B-C).
• 30-34 mm strip: quality residential profiles. Uf 1.8-2.2 W/m²K. Standard for most modern residential projects.
• 34+ mm strip with additional insulated chambers: passive or near-passive profiles. Uf 1.4-1.8 W/m²K. For nZEB and CasaClima buildings.

We work with these systems including contract manufacturing for installers and resellers. Within our LMT Aluminium systems:

• LMT 70: 24 mm strip, 70 mm window profile, Uf ~2.4 W/m²K. Entry-level commercial.
• LMT 71: 28 mm strip, 70 mm profile, Uf ~2.2 W/m²K. Basic residential.
• LMT 91: 34 mm strip, 75 mm profile, Uf ~1.7 W/m²K. Quality residential.
• LMT 92: 34 mm strip with multiple air chambers, 80 mm profile, Uf ~1.5 W/m²K. Premium.
• LMT 93: reinforced 34 mm strip plus insulating foam in the chambers, 86 mm profile, Uf ~1.3 W/m²K. Passive.

By comparison with PVC: a BlueEvolution 72 has a Uf of 1.0, a BlueEvolution 82 has a Uf of 0.85. Top-end aluminium remains above top-end PVC, but the gap is 0.3-0.5 W/m²K — no longer the orders of magnitude seen with old cold aluminium.

What changes in practice: condensation

The most visible symptom of the difference between cold aluminium and thermal break is internal condensation. On cold aluminium, with 0°C outside and 20°C / 50% RH inside, the surface of the internal frame drops below the dew point (12°C) and condensation is unavoidable. After a few years, condensation dripping onto the sill damages the plasterwork, blackens the gasket, and can create mould on the adjoining internal reveal.

With a modern thermal break (30+ mm strip), the internal frame surface sits at around 15-17°C under the same conditions — never below the dew point in a normally occupied home. Condensation disappears.

This is something measured on site with thermal imaging. On replacements of 1980s aluminium, the customer’s first “wow moment” is precisely this: “finally, no more water on the frames in winter.”

Comparing the LMT 70/71/91/92/93 systems

The five systems we manufacture cover different use cases:

LMT 70 (basic residential, commercial)

• Profile thickness: 70 mm
• Polyamide strip: 24 mm
• Uf: ~2.4 W/m²K
• Uw with double glazing 4-16Ar-4 BE: 1.5-1.8
• Applications: commercial entrance doors, entry-level residential in zone C-D, secondary openings

LMT 71 (standard residential)

• Thickness: 70 mm
• Strip: 28 mm with central chamber
• Uf: ~2.2 W/m²K
• Uw with double low-emissivity glazing: 1.3-1.6
• Applications: standard residential replacements in zone D, mid-budget detached houses

LMT 91 (quality residential)

• Thickness: 75 mm
• Strip: 34 mm
• Uf: ~1.7 W/m²K
• Uw with triple glazing 4-12-4-12-4: 1.0-1.3
• Applications: quality residential, energy class A, major refurbishments in zone D-E

LMT 92 (premium)

• Thickness: 80 mm
• 34 mm strip plus insulated chambers
• Uf: ~1.5 W/m²K
• Uw with triple glazing: 0.95-1.15
• Applications: energy class A+, quality new builds, prestigious villas

LMT 93 (passive)

• Thickness: 86 mm
• Reinforced 34 mm strip plus injected polyurethane foam in the chambers
• Uf: ~1.3 W/m²K
• Uw with triple Ar/Kr glazing: 0.85-1.00
• Applications: passive house aluminium, nZEB, CasaClima Gold

All systems are compliant with EN 14351-1 CE marking, air permeability class 4, water tightness class E1200, wind load resistance class C5.

Real-world examples: the condensation that disappeared

Case 1 — 1980s villa in Bergamo, zone E

The customer had original cold aluminium, with heavy condensation all winter long. 18 windows plus 4 French doors. Replaced with LMT 91 plus triple glazing.

• New median Uw: 1.1 W/m²K (versus an estimated original Uw of 5-6)
• Annual gas savings: approximately €1,800
• Internal condensation on the frames: gone from the first winter
• Sensation of “draughts” at the window: gone (the passive acoustic performance of the triple glazing also contributes)

Case 2 — City-centre shop in Rome

Commercial shopfront plus entrance door. 1990s aluminium with minimal thermal break (18 mm strip). Replaced with LMT 70.

• Uf from ~3.2 to 2.4
• Marginal thermal improvement from the replacement
• Main benefit: a more modern appearance, integrated anti-break-in hardware

Case 3 — Energy class A new build in Trento, zone F

200 m² CasaClima Silver villa. 22 windows and doors in total. LMT 93 with Krypton triple glazing.

• Average Uw: 0.90 W/m²K
• Uf of 1.3 sufficient for energy class A certification
• Cost of the premium aluminium material: 35-40% more than the equivalent PVC, but an aesthetic/architectural choice made by the client

When to choose aluminium and when PVC

Given that top-of-range PVC beats top-of-range aluminium on pure thermal performance (Uf 0.85 versus 1.3) — as covered in detail in our point-by-point PVC versus aluminium comparison — there are good reasons to choose aluminium:

1. Large dimensions: aluminium copes better with large windows, lift-and-slide doors 4-6 metres wide, and panoramic glazing. Above certain sizes, PVC needs too much reinforcement.
2. Extreme exposure: coastal salt-air zones, high mountains. Aluminium doesn’t degrade under UV the way dark-coloured PVC can.
3. Contemporary architectural aesthetics: ultra-slim frames (60-70 mm of visible sash versus 120-130 mm for PVC). An “industrial” or modern look.
4. Security: aluminium is structurally more rigid and copes better with professional anti-break-in classes RC4-RC6.
5. Full recyclability: aluminium can be recycled 100% indefinitely without loss of quality.

The trade-offs are the price (30-50% more than the equivalent PVC) and the residual thermal gap of 0.3-0.5 W/m²K.

Reference standards

Residential thermally broken aluminium windows and doors must comply with:

• EN 14351-1: CE marking for external windows and doors
• UNI EN 12412-2: Uf measurement in a climatic chamber
• UNI EN 10077-1/2: thermal transmittance calculation using standardised methods
• Italian Ministerial Decree 26/06/2015 “Minimum Requirements”: maximum Uw values by climate zone

For up-to-date conditions on accessing tax incentives — including the maximum permitted transmittance by zone — read our 2026 window and door tax deduction guide and always refer to ENEA and the Italian Revenue Agency.

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Request a quote to assess whether your project is better suited to PVC or LMT aluminium. For large formats and panoramic glazing, aluminium is often the more sensible choice; for standard residential windows, PVC usually is.