What are the best ways to reduce heat loss in original sash windows?

By Sash Windows

Posted on: 1st July 2026

Sash windows on Victorian semi detached property

Many old homes have original sash windows which, while undeniably charming, can be somewhat draughty. This can lead to considerable heat loss and make rooms feel cold and uncomfortable in the winter. At a time when energy costs are high, finding inventive ways to keep your home warm is essential.

The good news is that you don’t have to lose out on the character of your original windows. There are various cost-effective ways to improve energy efficiency and comfort. To insulate sash windows, you can use a combination of methods including applying weatherstripping and caulk to seal gaps, adding secondary glazing, or installing window insulation film or bubble wrap. Thermal curtains and cellular blinds are also effective for reducing heat loss.

According to the Energy Saving Trust, poorly insulated windows can account for up to a quarter of a home’s heat loss, and Historic England notes that draught-proofing alone can reduce heat loss through original windows by as much as 86%. The good news is that a well-maintained, draught-proofed sash window can perform close to the level of a modern replacement, without losing any of its period character.

Read on to learn the best approaches to cut the draught and keep the charm of your period property’s original sash windows.

Invest in Professional Draught-Proofing

Old sash windows can last forever if properly maintained. But without care, they get draughty, sticky and let in water that rots the wood and materials. Preventing those headaches with regular upkeep and professional draught-proofing is crucial for preserving original windows.

When your antique windows need repair, hire a professional who knows historical sash window restoration, not just a basic contractor. You want someone passionate about respecting heritage, who knows how to work with old materials the right way. Their specialised skills will protect the windows’ operation and historical look.

The right expert can replace damaged bits, refresh stuck sashes, seal draughts and make it all work like new. Prioritising quality professional care ensures your charming original windows will stay beautiful and functional for ages to come.

Add Weatherstripping

One of the easiest and most effective ways to reduce draughts in sash windows is to install weatherstripping. This adds a seal around the window sashes to close any gaps that let in cold air when the windows are shut.

There are a few options for weatherstripping original windows. Foam weatherstrip tape is inexpensive and easy to install and comes in a compressible form that inserts into the channels between sashes to block gaps and is available in various thicknesses.

Apply self-adhesive foam, felt, or vinyl strips around the edges of the sash to fill gaps, using a tool like a dinner knife to press them into place. This is especially effective for the meeting rails and the top and bottom of the window.

Alternatively, rubber or vinyl weatherstripping can be pinned or glued into place, providing a tight seal and conforming well to irregular gaps. It’s important to take accurate measurements and assess gap widths to choose the right option for each window. The channels between sashes should also be carefully cleaned and prepared before installation.

Install Secondary Glazing

Installing secondary glazing is an excellent way to improve insulation and reduce cold air around original sash windows while retaining their aesthetic charm. Secondary glazing is a second layer of glazing behind the original window to create an insulating air gap. One option is DIY window film that shrinks tightly into place with heat. However, it prevents opening the windows until it’s removed which may not be practical.

More permanent secondary glazing uses sheets of polyester, acrylic or glass mounted in tracks inside the window reveal to maintain the original appearance. If the window depth allows, secondary glazing panes can be installed between the sash boxes, which hides the panes while keeping your windows functional.

For wider gaps, secondary glazing can be installed behind curtains or blinds to optimise insulation. Combined with weatherstripping and other draught-proofing measures, it helps retain the architectural charm of sash windows while creating a warmer, more energy efficient home.

Insulate Sash Windows with Curtains and Shades

Adding insulating curtains or shades is an effective way to put an extra barrier between your windows and the cold outdoor air. Layered curtains made from a dense fabric like velvet or wool help prevent heat loss through windows. For the best insulation, hang curtains as close to the window as possible and extend them past the window edges to eliminate gaps.

Insulating cellular blinds are also excellent options for sash windows. These blinds have small air pockets built into the fabric, creating a thermal barrier, and they’re especially useful for single-pane windows since the “honeycomb” construction helps block the loss of warm indoor air.

Caulk Window Edges

Another simple technique for optimising energy efficiency is to seal around the edges with caulk. This fills any cracks and gaps where the window frames meet the walls, sills or mouldings, helping to prevent heat loss and cold air infiltration.

When choosing caulk for windows, look for a high-quality silicone, polyurethane or latex acrylic formula that offers long-lasting flexibility to maintain the seal and tolerate temperature changes. Carefully remove any old, cracked caulk and clean the areas to ensure the new caulk adheres properly. Caulking looks neat and prevents air leaks while keeping cold air out and hot air in.

If your property is in a conservation area or listed, draught-proofing and secondary glazing are usually the safest routes to better insulation, since they don’t alter the original glass or frame and rarely require planning consent. Full replacement is a bigger step, and one we can advise on during your free survey if draught-proofing alone isn’t enough.

Old sash windows are certainly charming, but their age-old design often makes them far less energy efficient than modern counterparts. However, as we’ve shown, there are several options available to upgrade and optimise the efficiency of original sash windows. For professional advice on draught-proofing your sash windows to improve the energy efficiency of your home, get in touch to arrange a free home survey.

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Melanie Simpkin

Excellent work done by the whole team. Great customer service through the entire project. Thank you!

Megan Russell

We recently had all of our sash windows double glazed by Sash Window Experts and are very pleased with the whole process. Mark and his team were fantastic from start to finish. They were always on time, polite and incredibly considerate of being in our house. They did a fantastic job of replacing the old Victorian sashes with new double glazed units and we can't recommend them highly enough.

Geoff P-W

These guys fully deserve their 5 star status, the work that they do is exceptional. Having bought a new period property with windows that were in need of attention we contacted several companies, and chose Sash Window Experts on the basis of Mark’s original survey and quote. Mark showed his love for repair and restoration during the initial visit, and came up with options and solutions to look at windows and doors in our house. Work started on time, and was completed to a very high standard. Given the size of our project we had a variety of people on site at different times, all of whom were a pleasure to have around, and who have that same love for their craft that Mark has. A special shout out to John, who worked with us on a number of things, and always came up with a great solution. We’d highly recommend Sash Window Experts.

Robert Lawder

Mark, Lisa and their team - Steve, Matt, Ollie and Pete repaired 9 badly decayed sash windows on our Victorian house in a conservation area. The work was finished to high quality, beautifully done and beautifully painted. The team was careful and considerate throughout. They kept to time and came in slightly under the quote value, accounting for elements of the original frames that they were able to repair rather than replace. I would wholeheartedly recommend them for work in a heritage setting.

Simon Pugh

Sash Windows Experts did a fantastic job replacing our Victorian sashes. The quality of the work and the professionalism of the team was outstanding. Many thanks, Mark and the crew!

micaela hobbs

I cannot praise this company enough. Mark and the entire team, who renovated all the rotten, old and drafty sash windows in our small flat, have been amazing. Gary, Steve and Ollie were, without exception, professional, courteous, great craftsmen and clean and tidy. The quality of the windows, both from the workshop and on site renovations is excellent and the style exactly as the old windows once were. Communication was also easy. I can highly recommend this company. Thanks to all the team.

Tony Kendall

So glad we used Sash Window Experts for our job….replacing old window sashes for new double glazed ones. Gary and Steve are great craftsmen and lovely people. We would certainly use this company again, and highly recommend it to others.

Robert Hamberger

This is the second time we have used Sash Window Experts, and we went back to them for hardwood double-glazing of our second bay window because we were so pleased with how professionally they worked previously, and the excellent results. They were prompt and attentive as ever - real experts in their field. We're delighted with the standard of their work, and would highly recommend them. The windows look great. Update: We have now (May 2024) used Sash Window Experts for the third time for hardwood double-glazed sash windows for our kitchen and second bedroom, to complete window renovation in our flat. They're as professional as ever, prompt and efficient, clean up well after themselves and the work was completed to a high standard in two days, as they said. We continue to recommend sash Window Experts.

Michael Bennison

A very professional job, carried out to a high standard. No problems and courteous and pleasant craftsmen. "Did what it says on the tin" Mjb