Material selection is one of the few decisions on a window schedule that cannot be unwound later. Change a finish and you change a maintenance cycle. Change the material and you change the sightlines, the thermal performance, the repair route and, on a heritage building, whether the scheme is consented at all.
The comparison is usually framed as timber against modern hybrid, as though hybrid were a single thing. It is not - it covers aluminium-clad timber, timber-polymer composites and a growing number of proprietary systems with little in common beyond a claim to combine the best of both. This guide sets out how each behaves, what it costs you twenty years out, and where neither is the right answer.
Understand what the terms actually cover
Specifying timber properly means selecting a species and a form of construction for the job in front of you, rather than accepting a default. In practice that covers engineered softwood, engineered hardwood, modified timbers and solid hardwoods used where a section or a historic profile demands it.
Engineered timber is the important term. It is not a lesser material than solid timber - laminating from selected sections removes the defects that cause movement, balances the grain across a section, and produces a component more dimensionally stable than the solid stock it came from. That stability is what holds a slender heritage profile true over decades.
Species selection is not purely aesthetic. Engineered softwood suits most painted finishes. Hardwood earns its cost where an exposed, stained grain is part of the design - and also where durability rather than appearance decides it, which is why we use a solid sapele section for all cills and thresholds, the components that take the most weather.
Modern hybrid covers three broad approaches. Aluminium-clad timber places a metal capping on the external face, so the cladding takes the weather and the timber does the structural and thermal work. Timber-polymer composite binds wood fibre in a polymer matrix, extruded or moulded into sections. Timber-effect systems are not hybrids in any meaningful sense, but they are frequently offered as an alternative at specification stage and worth naming for what they are.
The distinction matters because the three fail in completely different ways, and because only one of them can be repaired.
Read the product, not the material
The most common error in material comparison is treating thermal performance as a property of the material. It is a property of the product.
A window's U-value depends on frame section, glazing specification, spacer type, glazing method and the ratio of frame to glass at that particular size. Two windows in the same material, from the same maker, will not perform identically if the sizes differ.
Our whole-window U-values are published product by product rather than as a range-wide figure, and calculated to BS EN ISO 10077-1, with frame transmittance modelled numerically to Part 2 rather than taken from the tabulated default values Part 1 also allows. The defaults are generalised; the numerical model follows the actual section. When you compare a published figure from one manufacturer against another, you may be comparing two routes to the frame figure rather than two windows - so ask which sits behind any number you are asked to specify against, and at what size.
The same discipline applies across the rest of the performance set. Weather performance is measured to BS 6375, covering weathertightness, operation and strength, and it is a tested result for a specific product rather than a material characteristic.
On security, Approved Document Q applies to new dwellings only, and within them to easily accessible openings. A window counts as easily accessible if it sits within 2 m vertically of an accessible level surface such as the ground, a basement level or an access balcony, or within 2 m of a flat or sloping roof under 30° that is itself within 3.5 m of ground level. All easily accessible doorsets are in scope. Those openings must meet PAS 24. On replacement windows in existing dwellings, PAS 24 is a client, insurer or Secured by Design requirement rather than a Building Regulations one.
Our testing is predominantly to PAS 24:2016. Approved Document Q itself still references PAS 24:2012, so a later edition is being cited as meeting or exceeding the standard the regulation names - worth stating explicitly in a specification rather than leaving a reviewer to reconcile the two.
Weigh durability against repairability
The argument made for hybrid systems is maintenance. An aluminium face does not need repainting, so the external maintenance cycle extends. That is true, and it is worth something. The question is what it costs elsewhere.
Sightlines: cladding adds thickness to the external face. On a period building, or any scheme where the window detail is doing architectural work, that additional section is visible on elevation and rarely improves it.
Repairability: a timber window is a repairable assembly. Sections can be spliced, cills replaced, ironmongery renewed, finishes stripped back and rebuilt. A clad or composite system is generally a replacement item - when the cladding is damaged or the seal between cladding and frame fails, the route back is a new unit. Over a sixty-year building life this is the difference that dominates, and it rarely appears in a comparison table.
The concealed interface: in a clad system the timber sits behind an impermeable external face. Where drainage and ventilation of that cavity are well designed, this works. Where they are not, moisture that gets in has no easy route out, and the failure is invisible until it is significant.
Finish life on timber: a factory-applied coating on a well-detailed timber window is not a three-year maintenance cycle. Our standard finish is four coats of a spray-applied, water-based microporous system, warranted for up to ten years. That period reduces on exposed sites, high elevations and coastal regions; south elevations wear faster than the rest of a building; and translucent stain finishes have shorter durability than opaque ones. Routine care is a wash down at least twice a year, more often on the coast, with recoating prompted by flaking, colour fade or raised grain rather than by a fixed date in a maintenance plan.
State the sustainability position accurately
There is a great deal of loose language in this area, so here is what can be said without overreaching.
Timber is the only one of these materials that is grown. Responsibly managed forestry replaces what it takes, and the carbon in a timber window remains stored there for as long as the window is in service. A window that is repaired rather than replaced extends that period, which is the more meaningful point - durability and repairability do more for a building's whole-life carbon than any material label. Aluminium and polymer components carry a high embodied energy at manufacture; aluminium is genuinely recyclable at end of life, while composite sections, being bound materials, are considerably harder to separate and recover.
The honest comparison is a whole-life assessment against the specific building, its exposure and its expected service life. That is a calculation, not a claim, and it belongs in your specification rather than in a brochure.
Know where timber is not the answer
Where the original windows were metal, the correct replacement is a heritage metal window. This applies to interwar and early modern buildings, to steel casements, and to any elevation where the slenderness of the original section is the defining characteristic. No timber section will reproduce it, and the local planning authority, advised by its conservation officer, will say so.
Naming that openly strengthens the rest of your case rather than weakening it.
Where Mumford & Wood sits on this
Every product we make is built in engineered timber, with solid sapele cills and thresholds. That is the material position this guide argues for, and we build to it rather than around it.
Three ranges - Conservation, Classic and Heritage - carry the same product types: box sash, spring sash and flush casement. The product type answers the elevation; the range answers what the scheme has to comply with. Conservation is the range manufactured to meet Approved Document L and PAS 24, so where either is a condition of the scheme, that is the one to specify.
When you write the specification, state the product-specific U-value with the calculation route named, weather performance to BS 6375, installation to BS 8213-4, and PAS 24 where security is a condition rather than an assumption.
Compare properly, with the figures to hand
Our Resource Centre holds product-specific performance data with the calculation route stated alongside each figure, together with CAD details, BIM objects for every Conservation range product and NBS specifications across all three ranges.
To take your team deeper on material selection, book a CPD. To test any of this against a live project, request an estimate.
Manufacturing timber windows and doors since 1954. Where a material comparison has to survive scrutiny, we would rather hand you the figure and the method behind it than the claim.
Frequently asked questions
Is engineered timber lower quality than solid timber?
No. Laminating from selected sections removes the natural defects that cause movement and warping, and balances the grain across the section. For a window or door, where dimensional stability across seasons is what keeps the unit operating properly, engineered timber outperforms solid stock of the same species.
Does aluminium cladding make a window more thermally efficient?
Not in itself. Aluminium is a conductor, so the thermal work in a clad system is still done by the timber behind it and by the glazing. Compare product-specific U-values arrived at by the same route rather than assuming a material advantage either way.
Can a composite window be repaired?
Usually not in any meaningful sense. Damage to a bound or extruded section generally means replacing the unit. This is the single largest difference in whole-life terms between a composite and a timber window, and it is worth modelling over the building's expected life rather than over a warranty period.
Which materials are acceptable in a conservation area?
That is decided by the local planning authority, advised by its conservation officer, and it turns on the building rather than on a general preference. Where the original windows were timber, a well-detailed timber replacement is normally the starting point. Where they were metal, a heritage metal replacement is the correct answer.
How do I compare U-values between manufacturers fairly?
Ask how the frame figure was arrived at, and at what size. Whole-window U-values are calculated to BS EN ISO 10077-1, but the frame transmittance within that calculation can either be modelled numerically to Part 2 or taken from the tabulated default values Part 1 permits. Two figures produced by different routes are not directly comparable, and neither is comparable across different unit sizes.
