Replacement windows in an older house

September 16, 2026
5 minute read
By Betsy Hines

In an older house, the frame material matters less than the opening it has to fill. Timber suits a sash opening that must match the original trim, while PVC suits a simple casement where cost and upkeep drive the choice. Before signing anything, read the contract for the frame specification, the glazing values and the ventilation detail, because those three items decide how the window performs and what you can claim later. The useful work begins by looking closely, naming the trade-offs, and keeping the next decision visible. This guide is a starting point for a conversation, not a substitute for a site visit or qualified professional advice where the work requires it.

What should I check before signing a PVC window contract?

A PVC window contract is a specification document, not a brochure. Check that it names the frame profile, the number of chambers, the reinforcement inside the frame, and the glazing unit by its U value rather than by a word such as efficient. Ask for the ventilation method in writing: trickle vents, a night latch or an opening restrictor. Confirm who measures the opening, who removes the old frame, and what happens if the new frame does not fit the reveal. Check the deposit, the delivery date and the cancellation terms, and keep a copy of the signed survey drawing. A useful checklist for these points, written for English homeowners, appears in this guide to PVC window contract checks, which sets out what to verify before the signature and what the paperwork should contain.

How do PVC and timber windows compare?

PVC and timber differ in four practical ways: movement, repair, finish and lifespan. PVC is dimensionally stable, does not need painting, and cleans with a damp cloth. It is made in fixed profiles, so it fits standard openings well and awkward ones less well. Damage is usually repaired by replacing the whole sash or frame section, because a welded corner cannot be sanded back. Colour is applied as a foil or laminate, and a dark foil on a south-facing wall runs hotter than white. Timber moves with moisture. A well-made timber frame is jointed, primed on all faces and painted after installation. It can be sanded, filled and repainted, and a single rotten sill can be cut out and replaced. It needs repainting every five to eight years depending on exposure, and it costs more up front. In an older house with deep reveals and uneven walls, timber is easier to scribe and fit because it can be planed on site. Thermal performance now overlaps. A timber frame with a multi-chamber core and a good glazing unit can match or beat a standard PVC frame. The deciding factor is often the wall, not the frame: a poorly insulated reveal loses more heat than the difference between two decent frames.

Are timber sliding sash windows a good replacement option?

Yes, when the opening is a vertical sash and the house's appearance matters. A timber sliding sash replacement keeps the proportions of the original: the meeting rail, the sash horns, the reveal depth and the shadow line at the wall. It also keeps the opening method, so the top and bottom sashes still slide and tilt as before. Three points decide whether it works. First, the box: in many older houses the sash weights and pulleys sit in a boxed frame inside the wall. A replacement can keep the box and renew the sashes, or remove the box and use a spiral balance. Keeping the box preserves the reveal depth; removing it gains a little glass width but changes the internal line. Second, the glazing. A slim double-glazed unit fits a traditional sash better than a standard unit, but it carries a lower performance figure. A single-glazed sash with a secondary pane or a well-fitted draught seal is a legitimate option in a listed or conservation setting. Third, the draughts. Most heat loss and discomfort in an old sash comes from the gaps at the meeting rail, the sides and the bottom, not from the glass. Brushes, seals and a staff bead do more for comfort than an extra millimetre of cavity.

Sash or casement: which opening suits the room?

A sash opening slides vertically and keeps the sill clear. It suits a tall, narrow opening in a wall that faces the street, and it is the usual choice where the house's front elevation is visible. A casement opening swings on a side hinge or a top hinge. It gives a wider clear opening for the same frame width, seals with a compression gasket rather than a brush, and is easier to fit with a modern multi-point lock. It suits kitchens, bathrooms and rear extensions where the frame is not part of a historic elevation. Top-hung casements are common above a kitchen worktop because they open outward and do not clash with taps. Side-hung casements are common in bedrooms because they open fully for escape. In an older house, mixing the two is normal: sashes at the front, casements at the back.

Glazing values and ventilation in plain terms

The U value measures heat loss through the whole window, frame and glass together. The lower the figure, the less heat escapes. A single-glazed pane sits around 5.0, an old double unit around 2.8, and a modern double unit with a warm edge spacer and argon fill around 1.2 to 1.4. Triple glazing reaches lower figures, but the frame has to be deep enough to hold it, and in a shallow reveal the extra depth can sit proud of the wall. The g value measures how much solar heat comes through. A high g value helps a south-facing room in winter and overheats it in summer. Solar control glass lowers the g value and is worth considering where a large window faces the afternoon sun. Ventilation is separate from both. A sealed modern window traps moisture from cooking, washing and breathing. Without a controlled air path, that moisture condenses on the coldest surface, which is usually the glass edge or a thermal bridge at the reveal. Trickle vents in the frame head, a night latch, or a background ventilator in the wall all provide that path. In a bathroom or kitchen, an extract fan is the second half of the system.

What the paperwork should contain

A window contract in England normally covers the frame specification, the glazing unit, the ventilation method, the installation date, the deposit and the payment schedule. It should also state who is responsible for building regulations notification and what happens if the work fails an inspection. Two documents matter after installation. A FENSA certificate, or a building control certificate from the local authority, shows that the work meets the applicable requirements. A guarantee names the period, what it covers and whether it is insurance-backed, which matters if the installer stops trading. In a conservation area or a listed building, planning permission or listed building consent may be needed before any window is replaced. Replacing like for like is not automatically exempt. Check with the local planning authority before ordering, because a refusal after installation is expensive to undo. Finally, keep the survey drawing, the specification sheet and the certificate together. When the house is sold, a buyer's solicitor will ask for them, and a missing certificate is a common cause of delay. The rest of the renovation follows the same discipline, and vetting the contractor and comparing quotes line by line covers what a window contract leaves out.

Close detail showing replacement window choices in context — Details are easier to judge when context and next steps stay together.

Second detail showing a practical New England home decision — Details are easier to judge when context and next steps stay together.