Can you put double glazing in old sash windows?

Yes, old sash windows can take double glazing, either as new double-glazed sashes in the existing box or as slim units fitted into the original sashes. Listed buildings need listed building consent first, and conservation areas may have extra limits. Standard cavity units usually perform better and last longer than very narrow ones.

What are the double glazing options?

• New double-glazed timber sashes in the existing frame.
• Slim-profile units, with a cavity of 8 mm or less, fitted into original sashes.
• Secondary glazing or draught-proofing, which suits single-glazed windows you must keep.

Which option performs best?

Standard double-glazed timber sashes can reach a U-value of 1.4 W/m²K. Very narrow cavities save appearance but give up some efficiency, and large panes of 4 mm cavity can touch under load. Ask for test evidence and a guarantee.

Do you need permission?

Listed buildings always need listed building consent. Unlisted homes in an Article 4 conservation area may need planning permission. Ask your council before work starts.

What do we check first?

Jake surveys every job himself, checking sash condition, weights and the glazing bars before suggesting a route, and the work comes with a 10-year guarantee. See our sliding sash windows.

Thinking about new windows? Jake surveys every job himself, and our quotes are free, in writing and without obligation. We fit across Oxford, Kidlington, Bicester, Witney, Abingdon and the villages around. Call 01865 708464 or ask for a free quote.

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