The sash sticks every July. In January there is a thin film of condensation on the inside of the glass. A painter mentions, almost in passing, that the sill feels soft under the paint. And somewhere in the back of your mind a number is already forming — the number a window company will quote for eleven new units — which makes the whole question feel much larger than it needs to be. In most older homes the honest answer is neither "replace everything" nor "live with it." It sits in the middle, and it gets decided one window at a time, usually by the frame.

Start With the Frame, Not the Glass

Glass is the part people notice, but the frame is what decides the argument. Before you think about thermal performance or appearance, spend twenty minutes with an awl, a small knife, and a torch.

Probe the sill first, especially where water sits: the outer edge, the corners, the joints under the bottom rail. Then check the bottom rail itself, the lower jamb ends, and the head. Paint failure is cosmetic. Soft, crumbly wood that the awl sinks into is not.

What you find will usually fall into one of three categories:

• Sound wood with tired paint. Scrape, prime, fill, repaint. Nothing else is required, and any replacement argument is really an appearance argument.
• Localised rot. A short run along a sill or a corner of the bottom rail can often be cut back to solid timber and rebuilt with a structural epoxy, or spliced with a matching piece of wood. This is skilled work, but it is ordinary work for a joinery carpenter.
• Rot through the joints. When the sill has gone at the corners, or the tenons have opened and water is getting into the wall, you are weighing a rebuild against a new unit. Get a joiner's opinion before a salesperson's.

Aluminium frames from the middle of the last century are usually still structurally sound; their weakness is thermal bridging and corrosion at the sill line. Old uPVC is the least repairable of the common types — faded, brittle profiles with sealed units that are no longer made to fit are rarely worth saving.

Reading the Energy Loss Honestly

Heat leaves an old window by three routes: air leaking around the sashes, conduction through single glass, and conduction through the frame and its connection to the wall. Most homeowners assume the glass is the whole story. It usually isn't.

On a cold, windy day, hold the back of your hand near the meeting rail where the two sashes overlap, then around the perimeter of the frame where it meets the wall. A steady cool draft there is infiltration, and it is the cheapest problem in the building to fix. New weatherstripping, a properly aligned sash lock that pulls the meeting rails tight, and a bead of sealant at the interior trim can transform how a room feels for a fraction of what replacement costs.

The glass itself is the harder part. A single pane has a poor insulating value by nature, and there is no repair that changes that — but a well-fitted storm window or an interior secondary glazing panel adds an air space and cuts both conduction and drafts considerably. On many older homes this is the single most cost-effective improvement available, and it keeps the original sashes and the character of the facade intact.

Condensation is worth reading carefully. Water running down the glass on a cold morning tells you the surface is below the dew point, which is a combination of cold glass, high indoor humidity, and poor air movement. It is a comfort and durability problem, but it is not automatically a reason to replace a window.

Operation: What Sticks, What Sags, What Won't Budge

Stiffness is the complaint that pushes most people toward replacement, and it is also the one most often solved with a screwdriver and an afternoon.

A sash painted shut needs the paint line cut along the stops and the parting bead before anything else is attempted. Pounding on it will crack glass and split rails. Once it moves, a candle of paraffin or a dry silicone lubricant on the runners makes an enormous difference.

Double-hung windows with weights and cords are remarkably serviceable. Broken sash cords can be replaced, pulleys can be freed or swapped, and a sash that will not stay up is usually a weight problem rather than a window problem. Later spiral balances and tilt-pack hardware are replaceable parts with standard sizes — take the old one to a window hardware supplier and match it.

Where a sash has dropped and no longer meets the frame squarely, the cause may be a failed hinge on a casement, a settled frame, or a swollen sash in a humid climate. A carpenter can plane and re-fit a swollen sash; a settling wall is a different conversation, and one worth having before you spend money on new units that will also go out of square.

What Repair Genuinely Cannot Fix

Some problems do not respond to skill or patience, and it is worth knowing which ones before you commit to a repair programme.

• Failed sealed units. The cloudy, milky look between two panes of a double-glazed unit means the seal has gone. This is a glazing replacement, not a window replacement — a glazier can usually supply a new unit to the same dimensions.
• Structural rot through the joinery. If the frame no longer holds its shape, no amount of epoxy will restore the geometry.
• Bad installation, not bad windows. Units fitted without proper flashing, sitting in a wall that leaks, or sized too small for the opening will keep causing trouble whatever you do to the sash.
• Comfort targets that single glazing cannot meet. If the goal is a quiet room on a busy road or a warm room in an extreme climate, the physics eventually wins.

One caution while you are working: in homes built before the 1980s, assume old paint may contain lead until it has been tested. Do not sand or heat-strip it without proper advice and protection, and check the local rules on disposal.

The Long View on Cost

Replacement windows are sold partly on energy savings, and it helps to be sceptical about that pitch. For a typical older house, the payback from new glazing alone can run well beyond the life of the units, particularly once you account for the fact that a large share of heat loss is going through the loft, the walls and uncontrolled infiltration elsewhere. New windows make rooms quieter and nicer to use. Claiming that they will pay for themselves is a different and much weaker claim.

Repair, by contrast, is usually a fraction of the cost and can be staged. Spend this year's budget on the three windows that actually cause trouble — the north-facing bedroom, the one over the front door, the one with the rotted sill — and the rest can wait. Many owners find that a proper overhaul and draught-proofing of a good wooden window delivers most of the comfort of a new one at a modest price, while keeping the proportions and profile that make an older house worth living in.

Do check whether your property sits in a conservation area or has any listing or local design designation before ordering replacement units. Authorities can be strict about frame material, glazing bars and sightlines, and a refusal after the order is placed is an expensive lesson. Ask your local planning office, and get written quotes that separate frame repair from glazing work.

A Simple Way to Decide

Work through the windows one at a time, in this order:

1. Probe the frame. If it is structurally sound, repair stays on the table.
2. Fix the cheap things first: weatherstripping, sash locks, sealant, a freed pulley, storm windows. Live with the result for one heating season.
3. Re-glaze failed sealed units rather than replacing whole windows.
4. Rebuild or splice local rot with a joiner before considering replacement.
5. Replace only where the frame has lost its integrity, the installation is wrong, or comfort demands cannot be met another way — and replace those windows, not all of them.

Older windows were made to be taken apart, adjusted and put back together. That is an advantage, not a defect. Use it before you spend the money.

Photo: TheOtherKev / Pixabay