Old sash windows get blamed for cold rooms and high heating bills, and the usual advice is to pull them out and fit modern double glazed units. For most heritage homes that is the wrong place to start. You lose the original timber, the proportions and the character of the house, and you spend a lot of money to do it. There are cheaper ways to make an old window warmer and quieter that keep the window you already have. Here are four, from least to most expensive, with what each one actually does.
Why replacement is usually the wrong question
Swapping a sound timber sash for an aluminium or uPVC double glazed unit throws away timber that can last another hundred years if it is looked after. It usually changes the sight lines and the way the window sits in the wall, which matters on a period facade. And here is the part most people miss: most of the heat you lose through an old double hung window escapes through the gaps around the sashes, not through the glass itself. Close those gaps and you have fixed the biggest problem for a tiny fraction of the cost of replacement.
So the better question is not what do I replace this with. It is where is the heat and noise actually getting in, and what is the smallest fix for it. Work up from there and you will often stop well before you ever need new windows.
Option 1: seal the window
This is the cheapest step and usually the most effective. A draughty double hung window leaks air at four points: the bottom rail, the top rail, the meeting rails where the two sashes cross, and the sides where the sashes slide. Brush seals and compression seals close those gaps without stopping the window working.
Compression seal starts from around $6 for a roll and a V-seal is $15.99, so you can seal a window for the price of a round of coffees. If you would rather do a whole window properly in one go, the Draught Sealing Kit for a double hung window ($199.99) carries the seals, carriers and the rest of the parts to finish the job. Sealing has a bonus: it kills the rattle too, because the rattle is the same gap that lets the draught in. We have gone into the noise side of sealing here.
Option 2: secondary glazing film
A step up from sealing. Secondary glazing film is a clear film you fit over the inside of the window to trap a layer of still air in front of the glass. That trapped air is what slows heat loss, the same idea as double glazing but for a small fraction of the price. The Secondary Glazing Film is $38.36 and goes on with the window still in place, so there is no glazier and no mess.
It helps most on windows you keep shut through winter, and less on a window you open every day. We have written an honest account of what it does and where it falls short in our secondary glazing film review.
Option 3: heavier or laminated glass
If noise is the main problem rather than cold, the weight of the glass matters more than anything else. Swapping thin single glass for heavier or laminated glass cuts outside noise noticeably, because mass is what blocks sound.
The catch with a traditional double hung window is that the sashes run on cords and counterweights. Put heavier glass in and the balance goes out, so the window will not stay where you put it. The fix is simple enough: rebalance the window with heavier lead weights to match the new glass. Our sash window weight calculator works out the weight you need. Done properly, the window still runs on its cords and weights exactly as it always did, which we cover in this guide to double glazed sashes on cords and weights. This is a bigger job than sealing or film and usually one for a glazier, but it keeps the original window in the wall.
Option 4: vacuum insulated glazing
At the top end, vacuum insulated glazing is a slim double glazed unit with a vacuum between the two panes instead of a gas fill. It is thin enough to fit into some heritage sashes where a standard double glazed unit simply will not, so you get close to double glazing performance without changing the look of the window. It is the most expensive option here and, like heavier glass, it needs the window rebalanced to carry the extra weight. If you want to hold a piece before you decide, you can order vacuum glass samples.
How the four options compare
Each option does a different job, and they build on each other. In rough order of cost and effort:
- Seal the window: cheapest, a DIY afternoon, fixes draughts, rattle and a little noise. Almost everyone should start here.
- Secondary glazing film: low cost, DIY, adds warmth and takes the edge off noise, best on windows you rarely open.
- Heavier or laminated glass: mid to high cost, needs the window rebalanced, the best answer for traffic or train noise.
- Vacuum insulated glazing: highest cost, closest to double glazing, keeps the heritage look, needs the window rebalanced.
For most homes the honest advice is to start at the top of that list and only move down it if the first fix is not enough. Sealing alone solves the problem for a lot of people.
Frequently asked questions
Is sealing an old window really as good as double glazing?
Not for heat loss through the glass itself, but most of the heat and draught in an old double hung window comes through the gaps around the sashes, not the glass. Sealing those gaps fixes the biggest loss for the least money, which is why we always suggest it first.
Will heavier glass stop my sash window staying open?
It will if you do nothing else, because the sash is balanced against counterweights sized for the old glass. Rebalance the window with heavier lead weights to match the new glass and it will run on its cords and weights exactly as before. Our weight calculator works out the weight you need.
Does secondary glazing film help with noise?
A little. It traps a layer of still air that takes the edge off, but it is mainly a warmth fix. For serious noise, heavier or laminated glass does far more.
Is it worth keeping an old sash window rather than replacing it?
A sound timber sash can be sealed, reglazed and rebalanced to perform far better than it does now while keeping the original joinery, and good timber lasts for decades if it is looked after. Replacement throws that away and costs far more, so for a heritage home it is usually worth keeping the window and improving it.