Window frame materials compared: wood, steel, uPVC and aluminium

Heat doesn’t pass through every material the same way

When people talk about how well a window insulates, the number that matters is thermal transmittance U: the flow of heat that passes, per square meter of surface, through a structure separating two environments at different temperatures. It’s measured in W/m²K, and the lower the number, the better the structure insulates. For a whole window this overall transmittance is called Uw, and it’s a weighted average between the transmittance of the frame (Uf) and that of the glazing (Ug), plus a small contribution from the interaction between frame, glazing and spacer along the edge of the glass pane.

The reason wood, uPVC, iron and aluminium behave so differently in this calculation is basic materials physics: every substance has a different thermal conductivity (λ), meaning it transports heat at a different speed whenever there’s a temperature difference between its two faces. It’s the same principle that governs how thermal energy enters and leaves any physical system, from a window frame to the planet’s atmosphere described in the article on the greenhouse effect: the materials and the scales change, but heat always moves according to the same laws.

Wood: it insulates well because it conducts little heat

Wood has an intrinsically low thermal conductivity: around 0.126 W/m·K for spruce, 0.18 for oak, down to 0.079 for chipboard. Translated into frame transmittance, a softwood profile (pine, spruce, larch) has an Uf that starts at 2.0 W/m²K for a 50 mm thickness and drops to 1.8 W/m²K for 70 mm profiles; hardwood (oak, mahogany) starts higher, from 2.4 W/m²K at 50 mm down to 2.1 W/m²K at 70 mm. Other technical estimates place wood in a wider range, between 1.5 and 2.6 W/m²K depending on the profile’s thickness.

uPVC: internal air chambers and a recycling supply chain few expect

uPVC has an equally low thermal conductivity, between 0.12 and 0.17 W/m·K. Modern profiles aren’t a solid block of material but a structure with two or three internal chambers, separated by webs that trap still air: this lowers the frame’s transmittance further, ranging from about 2.8 W/m²K for a single-chamber profile down to 2.0-2.2 for two- or three-chamber profiles, and as low as 1.2-1.7 W/m²K in thicker profiles (58-80 mm).

uPVC is often seen as a plastic destined for the landfill, but its recycling supply chain is far more structured than it looks. In 2016 alone, under VinylPlus, 568,696 tonnes of PVC were recycled in Europe; of these, 256,607 tonnes — over 45% of the total — came from window profiles and related products, thanks to programs such as Recovinyl and EPPA. A dismantled frame doesn’t become waste; it becomes raw material again: once cleaned and ground down into a fine powder, it can be reworked with virgin resin into a completely new profile. A dedicated initiative, the Hybrid Project, also tracks the recycling of composite PVC-aluminium and PVC-wood profiles.

Iron and steel: high conductivity, and corrosion that can be prevented

Iron and steel conduct heat far more than the materials above — between 50 and 73 W/m·K, against 0.1-0.2 for wood and uPVC — and a whole metal frame, with no interruption at all, has a transmittance (Uf) of around 7.0 W/m²K: the highest value among the materials considered here.

Iron’s best-known problem, though, isn’t insulation but corrosion. The oxidation of iron on contact with air and moisture — a phenomenon that depends on the presence of water in the environment, the same element that circulates constantly between the atmosphere, the surface and the ground in the water cycle — is countered with hot-dip galvanizing, governed by the EN ISO 1461 standard. The mechanism is a “sacrificial” protection: the zinc oxidizes in place of the underlying iron, because it oxidizes much more slowly than bare iron. The EN ISO 14713-1 standard assigns each environmental corrosivity class a thickness loss in micrometers per year; for a steel item thicker than 6 mm, in a moderately corrosive environment (class C3, typical of urban or non-aggressive industrial settings), a zinc thickness of 40-121 µm and an average annual loss of 1.4 µm give a calculated durability of around 60 years. Real-world durability, however, varies a lot with the environment: a coastal, saline or aggressive industrial setting consumes the zinc much faster than the example above.

Aluminium: the metal that conducts the most, but can be interrupted

Aluminium is the material with the highest thermal conductivity among those considered here, 209 W/m·K — almost three times that of iron. An unprotected aluminium frame therefore has a very high transmittance, between 5.2 and 7.0 W/m²K depending on the technical source.

Here comes the most counterintuitive figure of all: despite aluminium conducting heat almost three times more than iron, a whole iron frame and an aluminium frame without a thermal break end up at practically the same Uf, around 7.0 W/m²K. What really decides a metal frame’s heat loss, then, isn’t so much the metal’s “base” conductivity as whether it has a thermal break: a plastic interruption that breaks the continuity of the metal profile and drastically reduces the thermal bridge. With a thermal break, aluminium’s transmittance drops into a range between 2.2 and 3.9 W/m²K (technical sources don’t fully agree on the exact limits: 2.2 to 3.8 according to ENEA, 2.4 to 3.9 according to ACCA) — a level comparable to wood and uPVC, achieved not by changing the metal but by interrupting it.

Aluminium also has another strength, this time tied to its end of life: it can be melted down and reworked without losing its mechanical properties, so in principle it can be recycled indefinitely. According to European Aluminium data cited by the Italian consortium CIAL, producing one tonne of primary aluminium takes around 157,000 MJ of energy, against the 8,540 MJ needed for one tonne of recycled aluminium: an energy saving of about 95%. In 2020, recycling 47,400 tonnes of aluminium packaging, with an 87% melting efficiency, avoided the emission of 355,000 tonnes of CO₂ equivalent. These figures are specific to packaging, not window profiles, but the underlying chemical principle — aluminium can be remelted without degrading — holds for any object made of this metal.

Practical example: ENEA estimates that, for the same single-pane glazing, an existing window with a whole metal frame has an overall transmittance (Uw) of around 6.0 W/m²K, while one with a wood frame comes in at around 5.0 W/m²K: the difference between the two comes entirely from the frame’s contribution, since the glazing is identical in both cases.

The comparison in a table

Material	Material’s thermal conductivity (λ, W/m·K)	Typical frame transmittance (Uf, W/m²K)
Wood	0.08 – 0.18	1.5 – 2.6
uPVC	0.12 – 0.17	1.2 – 2.8
Aluminium with thermal break	209 (of the pure metal)	2.2 – 3.9
Aluminium without thermal break	209	5.2 – 7.0
Iron/steel without thermal protection	50 – 73	around 7.0

Values vary from source to source because they depend on profile thickness, wood species and the number of uPVC chambers: they should be read as indicative orders of magnitude, not as single absolute figures.

The frame material doesn’t decide everything

The same technical sources that report these values flag an important limit: the frame material alone doesn’t determine how well a finished window insulates. The thermal-break technology (how wide it is, what plastic it’s made of) and the type of glazing — single pane, double glazing, with or without inert gases such as argon or krypton in the cavity — matter a great deal too. A 4-12-4 double-glazed unit, for example, has a Ug of around 2.8 W/m²K against 5.8 W/m²K for single-pane glass: a difference that can matter as much as, or more than, the chosen frame material.

“Composite” or “mixed” profiles also exist, combining two materials in the same frame — typically aluminium and wood, to bring together the metal’s resistance to the elements with the wood’s insulating capacity — with a frame-only transmittance (Uf) of around 1.7 W/m²K; a complete window with this kind of frame and double glazing reaches an overall transmittance (Uw) of around 2.5 W/m²K instead.

Regulation: when transmittance becomes a legal requirement

In Italy, CE marking of windows has been mandatory since 1 February 2009 for all manufacturers, regardless of company size, under the product standard EN 14351-1. Legislative Decree 192/2005 (Annex C), which transposed European directive 2002/91/EC, also set maximum transmittance (Uw) limits for transparent closures including window frames, in force from 1 January 2009 and differentiated by climate zone: from the 5.0 W/m²K allowed in zone A (the mildest) to the 2.2 W/m²K required in zone F (the coldest). These values are calculated following the EN ISO 10077-1 standard (simplified method) and EN ISO 10077-2 (finite-element calculation, for more complex cases).

Looking at these conductivity and transmittance numbers also helps illustrate a broader principle of materials science: every substance responds to heat differently because its internal structure — how its atoms are arranged, how freely its particles can move — decides how quickly thermal energy can pass through it. It’s the same principle, applied at a completely different scale, that explains why chocolate melts at one precise temperature and not another: a material’s chemical composition, not just its shape, decides how it reacts to heat.