Specifying timber windows well protects three things at once: your design intent, your programme, and your standing with the client who signs it off.
This guide sets out how to specify timber windows with confidence - from frame and material through to performance, statutory detail and installation - so the product that reaches the site is the one you drew.
Start with the frame and material
Timber window frames carry everything else - the sightlines, the mouldings, the performance - so the material behind them is your first decision. Engineered timber, laminated from selected sections, offers superior stability over solid wood windows, resisting the movement that undermines slender heritage profiles over time.
On the question of the best timber for windows, engineered softwood suits most painted finishes, while hardwood earns its place where a stained, exposed grain is part of the design. Ask your manufacturer to match profiles and mouldings to your drawings, not to a standard catalogue section. The detail is where design intent survives or is lost.
Specify for performance, not just appearance
Energy efficient timber windows have to answer Approved Document L without compromising the elevation. In England, Approved Document L Volume 1 (2021 edition, incorporating the 2023 amendments) sets replacement windows in existing dwellings at 1.4 W/(m²·K). For new dwellings the limiting standard is 1.6 W/(m²·K), with a notional specification of 1.2 W/(m²·K), demonstrated by whole-dwelling calculation rather than window by window. Approved Document L applies in England only. Where you are working on a historic or traditional building, paragraphs 0.8 to 0.13 allow the standards to be relaxed to preserve character and appearance - worth citing explicitly in your specification rather than leaving to interpretation on site.
Specify the glazing to the U-value the project demands, and note that slim double-glazed units preserve the narrow sightlines the local planning authority, advised by its conservation officer, will expect. Our U-values are published product by product rather than as a range-wide figure, and calculated using the detailed method in BS EN ISO 10077-2 rather than the simplified method in Part 1 that much of the market relies on. The two aren't directly comparable, so check which method sits behind any figure you're asked to specify against.
Beyond thermal performance, set out your requirements for weather performance to BS 6375, for acoustic reduction, and for security. Approved Document Q applies to new dwellings only, where windows must meet PAS 24:2022+A1:2024. On replacement windows in existing dwellings, PAS 24 is a client, insurer or Secured by Design requirement rather than a Building Regulations one - so if the scheme calls for it, it has to be in your specification, because nothing else will put it there. Our Conservation range is manufactured to meet both Approved Document L and PAS 24. Classic and Heritage are not, so where compliance is a condition of the scheme, specify Conservation.
A clear, unambiguous performance specification is the document your contractor prices against and your client relies on, so leave nothing to interpretation.
Carry the statutory detail into the specification
Two requirements catch replacement window schemes more often than any others, and both are yours to resolve.
Background ventilation: Approved Document F Volume 1, paragraphs 3.14 and 3.15, requires replacement windows to carry background ventilators with an equivalent area of at least 8,000 mm² in habitable rooms and kitchens and 4,000 mm² in bathrooms, and in no case smaller than the ventilator in the window being replaced. Settle the ventilator type and position at design stage - retro-fitting a trickle vent into a finished heritage sash is not a conversation you want to have with the client.
Means of escape: Approved Document B Volume 1, paragraph 2.10, sets escape windows at a minimum 0.33 m² unobstructed openable area, with no dimension less than 450 mm and a cill no more than 1,100 mm above floor level; paragraphs 2.18 to 2.20 deal with replacements. Identify which openings serve escape before you fix the sightlines, because the opening light has to satisfy both.
Both are far easier to resolve on the drawing than on site, and both are yours to state. A manufacturer builds what the specification asks for.
Match the range to the context
Custom timber windows should answer the building, not the other way round.
Mumford & Wood manufactures three ranges - Conservation, Classic and Heritage - and each carries the product types below with the detailing already resolved.
For period and conservation work, box sash windows reproduce traditional weight-and-cord operation with modern performance. Where a concealed spring balance suits the detail or the budget, spring sash holds the same lines with simpler operation. For contemporary and cottage-style schemes, a flush casement profile sits cleanly within the reveal.
Range and product type are separate decisions. The product type answers the elevation; the range answers what the scheme has to comply with. Where there's a Part L or PAS 24 obligation, that points to Conservation.
Detail the installation early
Timber window installation belongs in the specification, not on the snagging list. Specify installation to BS 8213-4, confirm cavity closers, sequencing and weathering details at design stage, and set out delivery and storage requirements so factory-finished units reach site undamaged.
This matters more than it looks. A window that performs in the test house and is installed badly performs like a bad window. Agreeing the programme with your manufacturer early protects both your finish and your contractor's certainty on site.
Specify with Mumford & Wood
Our Resource Centre carries CAD details and performance data across the ranges, BIM objects for every product in the Conservation range, and NBS specifications for every product across all three - built to drop straight into your model and your specification.
To take your team deeper on timber specification, book a CPD; or to price a live project, request an estimate.
Manufacturing timber windows and doors since 1954, we work alongside architects to honour the intent precisely - and to give you the confident, quotable detail you can carry to the client who holds the decision.
Frequently asked questions
What information do you need to specify a timber window?
Specify the range and profile, the moulding detail, the glazing and target U-value, the sightlines, the ironmongery, the paint or stain finish, and the performance ratings for thermal, acoustic and security. Add the background ventilator equivalent area and identify which openings serve means of escape, as both are set by the Approved Documents rather than by preference.
What is the best timber for windows?
Engineered timber offers the best stability for painted finishes and slender profiles; hardwood suits exposed, stained grain. Both outperform solid wood windows worked from a single section.
Do Mumford & Wood windows meet Approved Document L?
Our Conservation range does. Approved Document L Volume 1 (2021 edition, incorporating the 2023 amendments) sets 1.4 W/(m²·K) for replacement windows in existing dwellings in England, and Conservation is manufactured to meet it while preserving heritage sightlines. Classic and Heritage are not certified to Approved Document L. Historic and traditional buildings may qualify for the relaxation at paragraphs 0.8 to 0.13.
Are timber windows suitable for conservation areas?
Yes, where timber is the appropriate material. Traditional profiles and slim glazing suit period and listed work, and the local planning authority, advised by its conservation officer, will look for a like-for-like match to what was there. Where the original windows were metal, a heritage metal replacement is the correct answer.
