Species and the way it is cut
Most failures we are called to look at are moisture failures at the bottom rail. Slow-grown softwood with tight growth rings, properly seasoned, outperforms fast-grown timber by a wide margin, and engineered laminated sections are more stable again because opposing grain resists movement. Accoya and other modified timbers offer very long guarantees at a price. Whatever the species, end grain sealing and a drained, ventilated bottom rail detail matter more than the specification sheet.
Glazing: single, double or triple
In a conservation area or on a listed building the glazing is often prescribed. Slim-profile double glazing with a cavity of 8–12mm can usually be accommodated in a traditional sash section and gets close to a heritage sight line; standard double glazing rarely can without thickening the members visibly. Triple glazing is heavy, and the weight has to be carried by the box, the cords and the weights — which is why triple-glazed sashes are a design decision, not an upgrade. Where consent prevents any change, secondary glazing usually outperforms the alternatives for both heat and noise.
Draught-proofing does most of the work
In an uncomfortable period room, the air moving through the window typically costs more than the glass. Brush and compression seals in the meeting rail, staff bead and parting bead transform comfort and are frequently allowed where new glazing is not. On a refurbishment where the existing sashes are sound, overhauling and draught-proofing them is often the better value proposition than replacement, and the better conservation answer too.
Ironmongery, cords and balances
Traditional weight and cord boxes can be maintained indefinitely; spiral balances cannot always be. On a house that will be kept for a generation we generally specify weights and cords, with waxed cotton or braided cord and solid bronze or brass fittings. Fitch fasteners, restrictors, and where required window locks to satisfy insurance, are chosen at the same time as the profile rather than added later.
Finish and the first five years
A factory-applied microporous finish under controlled conditions outlasts a site-applied one, and the maintenance interval is realistic — five to eight years for a wash and recoat on an exposed elevation, longer on a sheltered one. Windows fail at the joints and at the bottom rail, so those are the areas to inspect. It is worth agreeing, at handover, who will be doing that inspection.