We double glaze existing sash windows by deepening the timber rebate and bedding a sealed unit into the original sash. It costs from £480 per window and keeps the original timber and period look. If a sash cannot carry the unit, a new sash in your old frame costs £680.
How we double glaze an original sash
Many London homeowners expect to replace their sashes to get double glazing. However, you can often keep them. We use the sash you already have.
First, we remove the old glass and putty. That exposes the timber rebate. Then we deepen and widen it with a purpose-made router bit, so it can take a sealed unit.
Next, we bed the unit in a quality silicone that will not break down. Finally, we fit an external timber glazing bead in place of putty. The result looks neat from the street, and the window still looks like a sash.
Which sashes can take a sealed unit
Most sashes can carry a double-glazed unit. However, the timber must be in reasonably good structural condition. A sealed unit adds weight, and a weak sash will not hold it.
For that reason we assess every window at the site visit. If the timber has rotted, we repair it first. Our sash window repairs service covers that work. Because the sashes come out, we can also rebalance the cords at the same time.
Weight matters for the counterbalance too, because a heavier sash needs the right weights, otherwise it will not stay open. We check this as part of the job.
Three routes to a double-glazed sash window
However, not every window suits the same route. We offer three, and we start with the cheapest that works.
Route one, double glaze the original sash
This costs from £480 per window. We rebate the existing sash and bed in a Pilkington low-E unit. It is usually hard to spot from the street.
Route two, a new sash in your original frame
This costs from £680 per window. We make a like-for-like sash with a double-glazed unit and fit it into your existing box frame. We choose it when the original sash cannot carry the unit.
Route three, full replacement
This costs from £1,480 per window. We supply a hard or softwood window, fully decorated, with modern security. It is a last resort because we always try to use the existing joinery first, even though the work on site is harder.
Slim units, vacuum glazing and other choices
Glass choice affects thickness, weight and cost. For example, slim units suit original sashes best because they keep the sightlines fine. Our guide to slim double glazing versus vacuum glazing compares the options in plain terms.
We fit Pilkington low-E glass in our sealed units. The coating also helps keep warmth in the room. In addition, we supply clear, laminated and satin glass for other windows. Our costs page shows the full price list.
Pair it with draught proofing
However, glass is only part of the story. A tight sealed unit sitting in a leaky sash still leaves gaps at the edges. As a result, we recommend draught proofing every double-glazed sash.
Draught proofing seals the perimeter and stops rattles. Therefore, together the two jobs give you the best result. If you are not ready for new glass, draught proofing alone is a good first step.
Listed buildings and conservation areas
Listed and conservation work needs more care, because councils decide what they allow, and the rules differ between buildings. Some homes need consent before any change to the glass.
We handle this work case by case because the rules differ. For example, read our article on double glazing listed sash windows before you apply. It explains the consent process and the alternatives. Our advice is to check with your council first, then talk to us about what is practical.
Prices at a glance
First, here are the prices for the three routes. Double glazing your original sash is £480 per window. A new double-glazed sash in the original frame is £680, and full replacement is £1,480.
Access, very large sashes and listed-building rules can change these figures. For that reason, we quote in writing after a free site visit. Your quote also includes a five-year guarantee on the sealed unit and on our labour.
Ask for a quote
Then send photos of your windows and your postcode through the contact page. We will tell you honestly which route suits you. We cover all Greater London postcodes and often work just outside the M25, Monday to Friday, 08:00 to 18:00.