Why Period Windows Lose Heat

If you live in a Victorian terrace, a Georgian townhouse or a 1930s semi, you already know the charm of original sash windows or leaded casements. You probably also know the draught that whistles past the sash cord on a windy night. A single-glazed timber window typically has a U-value of around 5.0 W/m²K, compared with roughly 1.4 for a modern double-glazed unit. In plain terms, that's a lot of heat leaving the room and a lot of your heating budget going with it.

But here's the important part: the glass is only one culprit. In older windows, most heat loss and discomfort comes from gaps — around the frame, between the sashes, past the meeting rail, and through the box frame itself. Fixing those gaps is often cheaper, less disruptive and far kinder to a period property than ripping everything out.

Start With Draught-Proofing

Before you spend thousands, spend a weekend. Draught-proofing is the cheapest energy upgrade available to most older homes, and it's reversible, which matters if you're in a conservation area or simply want to keep your options open.

• Sash windows: fit brush or wiper seals into the staff beads and meeting rails, and consider a discreet draught strip where the sashes meet.
• Casement windows: self-adhesive or screw-fixed rubber seals along the rebate work well, provided the hinges aren't binding.
• Box frames: seal internal gaps with a flexible filler or narrow foam strip, but never block the box completely — sashes need to move.
• Around the frame: fill the gap between frame and plaster with a suitable sealant or lime-friendly filler rather than squirts of expanding foam.
• Beyond the glass: a heavy lined curtain, a well-fitted blind or internal shutters can cut perceived draughts and radiant heat loss noticeably.

A competent DIYer can do most of this for £100–£300, or a joiner might charge £150–£400 per window. Expect to notice the difference immediately, even if the energy figures look modest.

Secondary Glazing: The Quiet Achiever

Secondary glazing means adding a second pane on the inside of the existing window, usually as a hinged or sliding panel in its own slim frame. It can bring a single-glazed window down to roughly the performance of a basic double-glazed unit, and it is the go-to solution for listed buildings, conservation areas and anywhere the original joinery is worth keeping.

It suits period homes particularly well because it doesn't alter the external appearance at all. You keep your sash windows, your glazing bars and your street view. It also cuts noise remarkably well — a real bonus on a main road. Costs typically run from about £250 to £600 per window, depending on size, frame finish and whether you choose magnetic, hinged or sliding panels. Do leave a small ventilation gap or ensure the window can still be opened, otherwise you risk condensation and stale air.

When Replacement Is the Right Call

Sometimes the original windows are beyond saving, or they've already been replaced with something unsuitable. Replacement makes sense when frames are rotten through, when sashes no longer operate safely, or when you're renovating a room and the budget allows. It isn't a magic fix for every draught, and it rarely pays for itself quickly, but it does bring comfort, easier maintenance and better security.

Be realistic about payback. A whole-house replacement can cost £8,000–£20,000 or more, while the annual saving might be a few hundred pounds. Think of it as a comfort, appearance and maintenance decision that also happens to reduce your bills.

Getting Replacement Right in an Older Home

Where you replace, do it thoughtfully. In England and Wales, replacement windows must meet Part L of the Building Regulations, which for existing dwellings generally means a U-value of 1.4 W/m²K or better, or a comparable window energy rating. Part F also matters — you'll usually need background ventilation such as trickle vents, even though they look out of place on a period facade.

• Listed buildings: you'll need listed building consent, and like-for-like timber is normally expected.
• Conservation areas and Article 4 directions: check with your local planning authority before ordering anything.
• Materials: slimline double-glazed units (12–14mm) can sometimes be fitted into existing sashes, preserving the original timber.
• Specification: low-emissivity glass, argon fill and warm-edge spacers give the best performance for the thickness available.
• Installation: use an approved installer and keep the paperwork — it matters at sale time.

A Sensible Order of Work

My usual advice to anyone in a period property is this: draught-proof first, add secondary glazing to the coldest or noisiest rooms next, and only replace original joinery when it's genuinely at the end of its life. Reinstating a sash window that a previous owner ripped out is far more expensive than repairing one you still have. Take your time, work room by room, and you'll keep the character that made you fall in love with the house — while making it noticeably warmer and cheaper to run.