Quick Answer:
Timber frames insulate well and look right on period homes, but the outside needs regular repainting. Aluminium gives the slimmest frames and the least upkeep, but it only performs well with a thermal break. Alu-clad windows put timber on the inside and aluminium on the outside to get most of both. If you want low maintenance and good energy performance, alu-clad is the safest starting point for most UK self-builds and renovations. Timber is often the right call for listed buildings, and aluminium for very large, minimal glazing.
The rest of this guide explains why. It also covers the question most comparisons skip: "alu-clad" describes two quite different kinds of window.
At a glance
| Timber | Aluminium | Alu-clad timber | |
|---|---|---|---|
| What it is | A solid or engineered timber frame, painted or stained | An aluminium frame, powder coated, with a thermal break in modern windows | A timber frame inside with an aluminium skin outside; better versions add an insulating core between the two |
| Frame insulation | Good: timber is a natural insulator | Poor on its own; depends on the thermal break | Good: timber or an insulating core does the work |
| Upkeep outside | Regular repainting or re-staining | Occasional clean | Occasional clean |
| Looks | Wood inside and out | Metal inside and out; slimmest frames | Wood inside, metal outside; different colours inside and out are possible |
| Best for | Listed buildings and owners happy to maintain timber | Large glazing, minimal frames, modern designs | Self-builds, extensions and renovations wanting warmth and low maintenance |
| Watch out for | Rot at the bottom corners if maintenance slips | Cold frames and condensation without a good thermal break | How the window handles water at the bottom of the frame (see below) |
Manufacturers quote very different lifespans for each material, from 20 to 60 years or more, and much depends on build quality and upkeep. We haven't put a single number in the table because no independent source gives one. Ask any supplier what their warranty covers and for how long.
Why the frame material matters
Most of a window's heat loss goes through the glass, but the frame matters more than people expect. That's because the materials conduct heat so differently:
- Aluminium has a thermal conductivity of about 215 W/(m·K) (Engineering ToolBox).
- Generic softwood is about 0.13 W/(m·K), Norway spruce 0.11 and European oak 0.18 (British Woodworking Federation).
So aluminium conducts heat roughly 1,700 times more readily than softwood. That is why an all-aluminium frame needs a thermal break: an insulating barrier separating the outer and inner metal. Without one, the inside of the frame gets cold enough to attract condensation.
The glass matters just as much. The same Idealcombi Futura+ frame has an indicative whole-window U-value of 1.31 W/m²K with double glazing and 0.74 W/m²K with triple glazing, for a standard European test size (Futura+ specification). Always compare whole-window U-values, not frame or glass figures alone.
Timber windows
Timber is warm to the touch, can be painted any colour, and is the traditional material for most British homes. Modern engineered timber is far more stable than the single-glazed timber windows many of us grew up with.
The weakness is water. Timber shrinks and swells with the humidity, and rot in the lower corners of the frame is the most common reason timber windows need replacing (why timber windows fail). Painted timber needs repainting outside on a regular cycle to stay protected.
Choose timber if you're working on a listed building where the council expects timber, or you love the look of painted wood outside and are happy to keep up the maintenance.
Aluminium windows
Aluminium is strong, so frames can be slim and panes large. Powder coating gives a durable finish that only needs cleaning. That makes aluminium a natural fit for big contemporary windows and sliding doors.
The trade-off is heat. Even with a thermal break, an all-aluminium frame relies on that break to stop heat escaping, and the inside face is metal rather than wood.
Choose aluminium if you want the thinnest possible frames, a fully metal look inside and out, or very large glazed openings.
Alu-clad timber: not all alu-clad windows are the same
"Alu-clad" is used for two different constructions, and the difference matters for how long the window lasts.
Classic alu-clad. This is a timber window with an aluminium skin fitted to the outside. Idealcombi launched its first aluminium-clad window in 1988 (our history of the design). The aluminium protects the timber from rain and sun, so nothing needs painting outside.
Early versions had a known weak spot: the bottom rebate, where water collects. Water drains through channels and holes in the frame. If dirt or dead insects block them, water can find its way to the timber and cause problems (how composite windows evolved).
Composite windows with an insulating core. Here a separate insulating material sits between the aluminium and the timber. It acts as a thermal break and keeps water away from the wood. In Idealcombi's Futura+, Frame IC and Nation IC windows, this core is Idealcore™. It is a moulded polyurethane (PUR) profile that replaces the timber in the bottom rebate. It has no drainage holes or air chambers for dirt to collect in, and no timber is exposed where water sits.
We measure Idealcore's thermal conductivity at 0.074 W/(m·K), against 0.13 for pine (Idealcore). The frames carry a 25-year warranty.
[ADD FIRST-HAND: photo of a cut cross-section of a Futura+ frame beside a traditional alu-clad frame, labelled to show the Idealcore rebate and where water drains in each]
Five questions to ask any alu-clad supplier
- Is there a thermal break between the aluminium and the timber, and what is it made of?
- How does water drain from the bottom of the frame, and is any timber exposed there?
- What is the whole-window U-value, and for what window size? Small test sizes and large windows give different figures.
- Where does the timber come from? Ask about FSC® certification and whether the outside-facing parts are heartwood. Ours is FSC-certified pine, with at least 90% heartwood in all external surfaces and rebates (materials and maintenance).
- Can the inside and outside be different colours? Most alu-clad ranges allow it. Idealcombi offers 15 standard RAL colours and different colours on each face.
What the Building Regulations ask for (England)
All three materials can meet the regulations. What matters is the whole-window U-value. Under Approved Document L, volume 1 (2021 edition incorporating 2023 amendments), as summarised by the British Woodworking Federation:
| Project | Requirement for windows |
|---|---|
| Replacement windows in an existing home | No worse than the window being replaced, and no worse than 1.4 W/m²K (or Window Energy Rating band B) |
| New-build home: limiting value | No worse than 1.6 W/m²K |
| New-build home: notional dwelling (the design target used in the energy calculation) | 1.2 W/m²K |
Two points are often misunderstood:
- The 1.2 figure for new builds is the target used in the dwelling's energy model, not a legal minimum for every window. Most new homes aim for it or better, because worse windows have to be made up for elsewhere.
- If replacement windows in a historic building can't meet these values without harming its character, the guidance allows glazing with a centre-pane U-value of 1.2 W/m²K instead.
These figures are for England. Scotland and Wales set their own standards, so check with your building control body.
Which should you choose?
| Your situation | Our suggestion | Why |
|---|---|---|
| Listed building | Ask your conservation officer first; they may require timber | Timber-look alu-clad windows suit many period homes but won't suit every listed building |
| House in a conservation area | Alu-clad flush casement, if the council agrees | Frame IC and Nation IC copy the flat face of traditional timber casements, with indicative U-values of 1.32 (double) and 0.77 W/m²K (triple). Check for an Article 4 direction before ordering |
| Self-build or extension aiming for low energy or Passivhaus | Composite alu-clad with triple glazing | Insulating core plus triple glazing; Futura+ reaches an indicative 0.74 W/m²K |
| Contemporary design with large panes and slim frames | Aluminium, or a slim composite | Futura+ has a 53 mm external aluminium profile with timber inside |
| You want inward-opening tilt and turn windows with metal on both faces | Aluminium with an insulating core | Futura+i is aluminium, Idealcore and aluminium, with the same 53 mm sightlines as Futura+ |
| You want wood inside but never want to paint outside | Alu-clad timber | The aluminium takes the weather; the timber stays inside |
Next step
Every window we make is built to measure, so the best way to compare options for your project is to send us your plans. We'll recommend a frame and glazing for each opening and explain the U-values you can expect. Send your drawings for a free quote, or read more about aluminium-clad timber windows.