Replacement windows: checks before you sign
What to verify before signing a PVC window contract, how PVC and timber compare, and whether timber sliding sash windows suit a replacement project.
Before signing a PVC window contract, check the written specification, the U values, the ventilation provision, the guarantee terms and the installer's registration, because the surveyor's verbal promises are not the contract. The document you sign should state frame material, glazing specification, hardware, lead times, deposit protection and who is responsible for building regulations notification. If any of those are missing, ask for them in writing before the deposit is paid.
What should I check before signing a PVC window contract?
Start with the specification sheet. It should name the profile system and its chamber count, the reinforcement inside the frame, the glass make-up (for example 4-16-4 or a warm edge unit with argon), and the spacer bar type. A quote that says only "A-rated double glazing" is not a specification. Ask for the Window Energy Rating certificate and the U value of the whole window, not just the centre pane.
Then check the installation scope. Does the price include removal and disposal of the old frames, making good inside and outside, new trims, sill replacement and any lintel repairs? These are the items that turn a cheap quote into an expensive job. Confirm whether scaffolding is included and who arranges it.
Check the compliance paperwork. In England, replacement windows and doors are normally notified through a Competent Person Scheme such as FENSA, or through a building control application. The contract should say which route applies and who pays. Ask for the scheme registration number and verify it. A homeowner who cannot produce a certificate at sale may face questions from a buyer's solicitor.
Check the guarantee. Insurance-backed guarantees matter if the installer stops trading. Confirm the length, what is covered (frames, glass, hardware, labour) and whether it is transferable to a new owner. Read the cancellation rights and the deposit terms; a large upfront deposit with no protection is a warning sign. A structured guide to PVC window contract checks sets out the same sequence of documents, which is useful when comparing several quotes side by side.
Finally, check the survey. Measurements should be taken by the company, not by you, and the surveyor should note any structural issues, drainage, or restricted opening angles. Ask for the survey report in writing.
How do PVC and timber windows compare?
PVC-u is the default replacement material in England because it is cheap to make, needs almost no maintenance and comes in a wide range of styles. Modern profiles with thermal reinforcement perform well, and the material does not rot, warp or need painting. Its weaknesses are aesthetic and environmental: the surface can discolour on south-facing elevations, the profiles are bulkier than timber, and the material is difficult to repair invisibly. A scratched or cracked frame is usually replaced, not mended.
Timber windows cost more upfront, typically 30 to 60 per cent above an equivalent PVC quote, but they suit period properties and conservation areas where planning conditions may rule out PVC altogether. Softwood needs painting every few years; hardwood and engineered timber with a factory finish last longer between coats. Timber can be repaired, spliced and repainted, which matters over a 30-year horizon. Thermal performance depends on the glazing unit and the frame section, not on the material alone, so a well-made timber window can match or beat a mid-range PVC one.
Maintenance is the practical dividing line. PVC needs cleaning and occasional lubrication of hinges and locks. Timber needs inspection, filling and repainting on a cycle. For a landlord or an owner who will not maintain external joinery, PVC is the lower-risk choice. For a listed or period building, timber is often the only choice that satisfies the planner.
Are timber sliding sash windows a good replacement option?
Yes, where the building has original sash openings or sits in a conservation area, and where the budget allows. Timber sliding sash windows reproduce the proportions, sightlines and operation of the originals, including the weights and cords or the spiral balance systems used in modern versions. They are the usual route when a planning authority or a conservation officer has refused PVC on the grounds of appearance.
Modern timber sashes can be double or triple glazed with slim units, draught-proofed with brush and bubble seals, and fitted with trickle vents. The trade-offs are cost and maintenance: expect a higher price than casement PVC, longer lead times, and a repainting cycle. Ask whether the sashes are supplied pre-finished and what the finish warranty covers. Also confirm that the replacement retains the original reveal depth and that any boxed weights are retained or replaced with a compatible balance system.
Costs, energy ratings and U values
Replacement window prices in England vary widely by size, style and specification. As a rough guide, a standard PVC casement runs from a few hundred pounds per window fitted, a timber casement rather more, and a timber sliding sash significantly more again, with bay windows, dormers and access difficulties adding to the labour. Always compare like for like: glass specification, hardware, making good and certification.
U values measure heat loss through the whole window, in W/m2K. The lower the figure, the better the insulation. Building regulations in England set a limiting U value for replacement windows in existing dwellings, commonly cited as 1.6 W/m2K for the whole window, though a Window Energy Rating of band C or above is often used as an alternative compliance route. Ask the supplier for the certified whole-window figure, not the centre-pane figure, which is always better and not comparable.
Energy ratings combine U value, solar gain and air leakage into a band from A to G. A higher band does not automatically mean a better window for every orientation: on a south-facing elevation, high solar gain can cause overheating in summer, while on a north-facing wall, insulation matters more.
Ventilation and building regulations
Replacement windows must not reduce the background ventilation of a dwelling. In practice this means trickle vents in the head or frame of windows in habitable rooms, sized to the floor area and the number of occupants. If the original windows had no vents, the installer should still assess whether the property needs them, particularly where kitchens, bathrooms or bedrooms have no other opening.
Approved Document F covers ventilation and Approved Document L covers energy efficiency; both apply to replacement windows in England. In protected areas, including conservation areas, national parks and World Heritage Sites, permitted development rights for some external alterations are restricted, and a planning application or listed building consent may be needed. Check with the local planning authority before ordering, because a refused retrospective application is expensive to undo.
Keep the certificates, the FENSA or building control notification, the guarantee documents and the specification sheet together. They are the evidence a future buyer, solicitor or insurer will ask for, and they are the reason the checks before signature matter more than the sales conversation.
Window contracts sit inside a wider sequence of decisions, and the documents that govern them rarely stand alone. A schedule of works, a specification and a drawing set each carry assumptions about tolerances, fixing zones and making good that the contract must absorb. Reading those documents against the building, its light and its use is the same discipline that applies to preparing the interior works, where the brief, the missions, the order of lots and the reserves at handover are set out. The window page stops at the joinery; the interior page follows the sequence that surrounds it.
Continue with architecture and design notes and the source register.