Draught proofing stops air leaking around a sash, while secondary glazing adds a second pane inside it. Draught proofing is cheaper and nearly invisible. Secondary glazing does more for noise and heat loss through the glass. Many London homes get the best result from doing the first and, where needed, adding the second.
What each one does
Draught proofing seals the gaps around a sash. We fit brush or compression strips into the sliding channels and across the meeting rail. The window still opens normally, but air no longer whistles through.
Secondary glazing is a separate inner window. It sits inside the original frame or reveal, so you have two layers with an air gap between them. The original sash stays untouched, which is why many people like it, although it adds a second window to clean.
These solve different problems. Draught proofing deals with air movement, whereas secondary glazing deals with the glass itself. For that reason, the right choice depends on what bothers you most.
Draughts and heat loss
Sashes are leaky because they must slide. Over the years, timber shrinks, paint builds up and gaps open. Cold air then pours in around the edges.
Draught proofing tackles that directly. It is the quickest way to make a room feel warmer, because you feel the difference at once. It also stops dust and soot blowing in from busy roads, so rooms stay cleaner.
Glass loses heat too. A single pane is a poor insulator, so a room can still feel cold near the window. In that case, secondary glazing or a sealed unit helps more.
However, neither fix works well on a window in poor condition. Loose joints and rotten timber let air through whatever you add, and a soft sill needs repair or replacement first. Our guide to the signs your sash windows need repair shows what to check first.
Noise
Traffic and aircraft noise is a real issue in many London streets. Draught proofing helps a little, because sealing gaps blocks some sound. The glass still carries most of it.
Secondary glazing does better here. The extra pane and air gap break up sound before it reaches the room. A wider gap tends to help more, and the inner window needs a good seal to work properly.
If noise is your main complaint, say so when you contact us. We can then suggest the sensible option for your windows and your street.
Condensation
Condensation forms when warm, damp air meets a cold surface. A single pane is cold, so it mists easily. However, better insulation on the glass can reduce this.
Sealing a window also changes how air moves through your home. Because less air leaks out, damp can build up unless you ventilate. As a result, you may need to open trickle vents or extractor fans when you improve a window.
We cover the causes in more depth in our post on sash window condensation. It is worth reading before you choose.
Cost
Our draught-proofing price is built into the overhaul. A full overhaul with draught proofing is £188 per window, and the draught proofing service page explains what is included. That price also covers repairing worn parts.
Secondary glazing is quoted by the window and varies widely by size and system, so we give no figure here. However, if you want sealed glass in the sash itself, we offer double glazing for existing sashes at £480 per window. A new double-glazed sash in your original frame is £680.
Both our draught-proofing system and our sealed units carry a five-year no-quibble guarantee. Our costs page lists every price in full. In addition, you can compare with our wider guide to what sash window repairs cost in London.
Looks and fit
Draught proofing is almost invisible, although you may see a thin strip at the meeting rail. From outside, the window looks the same. For a period home, that is a big plus.
Secondary glazing is visible from inside. A well-made slim frame can look neat, and some systems match the colour of your joinery. Even so, it adds a second set of frames to clean and operate.
Slimline double glazing in the sash changes the glass but keeps the look. The sash is a little heavier, so the balance needs adjusting, because the cords carry more load. Our guide to broken sash cord replacement explains why that matters.
Listed buildings and conservation areas
Rules differ for listed homes. Draught proofing is a light-touch change that keeps your original sashes. Replacing the glass or adding glazing may need consent, so you must check first.
Our article on double glazing listed sash windows goes through what to ask your council. We handle listed and conservation work case by case.
Which should you choose
Start with the condition of the window, because that decides everything else. If it is sound, draught proof it. If it is tired, repair and overhaul it first, then seal it.
Add secondary glazing or sealed units afterwards if noise or cold glass still bothers you. That order gets you the most improvement for the least money. It also keeps your original sashes in service for decades.
Looking after the window once it is sealed matters too. For example, our sash window maintenance guide lists the simple jobs. If a window rattles after the work, our post on why sash windows rattle explains the usual causes.
Child safety is another point. A sealed window is often one you open less, but upper floors still need protection. See our guide to sash window restrictors and child safety.
Local examples
The mix changes across London. Terraces on busy main roads often need noise control, so sash window repairs in Southwark often involve both fixes. Period conversions in homes across Hackney tend to start with draught proofing.
Meanwhile, period houses in Haringey include many Victorian terraces with original sashes that respond well to a full overhaul.
Next step
Send us your postcode and the number of windows through our contact page. We arrange a free site visit and a written quote. You can then see the price for each option side by side.