There is a line homeowners hear over and over when they ask about double glazing their old double hung windows. It comes in a few versions. The sashes will have to be fixed shut. The cords and weights cannot handle the new glass. You will need spring balances, or new windows altogether.
If a tradesperson has told you any of that, they are not telling you the truth. The cord and weight system does not care what glass is in the sash. It cares about one thing: that the weights in the pockets match the weight of the sash. Change the glass and you change the sash weight, so you change the counterweights to suit. That is the entire job. Windows have been rebalanced this way for as long as sash windows have existed.
What actually happens when the glass gets heavier
Float glass weighs 2.5kg per square metre for every millimetre of thickness. An original 3mm pane in an 820mm by 700mm sash weighs around 3.5kg. Swap it for 6.38mm laminated and the glass alone jumps to about 7.5kg. Put in a double glazed unit and it goes higher again.
The sash that used to weigh 8.5kg now weighs 12 or 13kg. The old counterweights are still sized for 8.5kg. So the window crashes shut the moment you let go, and someone declares that sash windows and double glazing do not mix. They mix fine. The balance is wrong, and balance is fixable.
Why the fix is lead
There is one genuine constraint, and it is space. The weight pocket beside the window is only so long, and a heavier cast iron weight is a longer cast iron weight. Sometimes the longer iron will not fit.
Lead solves that. It is nearly twice as dense as cast iron, so it carries the same weight in roughly half the length. A pocket that has no room for more iron will usually take lead without complaint. It does not rust either.
That is a round extruded lead weight next to the old iron it replaces. The centre hole means it threads onto the sash cord the traditional way, and our lots stack, so any target weight can be made up in 0.25kg steps without cutting anything.
Work out your numbers with the calculator
We built a sash window weight calculator that does the arithmetic in kilograms. You measure the sashes with the window shut, pick your glazing from a list that runs from 3mm float up to 12mm and double glazed units, and it gives you a per-weight figure for each sash with the traditional balance bias built in. The top sash runs a touch heavy so it stays up. The bottom sash runs a touch light so it stays shut.
It also handles the case most calculators ignore: the weights you already have. If the old iron is still in the pockets, enter what one weighs, in kilograms or pounds, and the calculator gives you only the difference to add. Old iron weights carry a Roman numeral cast into the side, and that numeral is the nominal weight in pounds. An VIII weight was cast as 8 pounds, about 3.6kg. The calculator page has a key for the common stamps, including the odd additive forms old foundries used, IIII for 4 and VIIII for 9.
Once the figures are on screen, one button puts the whole window's lead in your cart, broken into the right lots.
What it costs
Our lead sash weights sell as stackable lots: 0.25kg at $5.95, 0.5kg at $9.95 and 1kg at $17.95. A typical reglaze to 6.38mm laminated needs 4 to 8kg of added lead across the window, so the rebalance usually lands between $80 and $150 in parts. Set that against a quote for spring balance conversion or replacement windows and the arithmetic gets easy. Lead ships by road freight only, since a window's set is heavy, and it adds a few days to transit.
The three lots, unpeened. Each has a centre hole, threads onto the cord and stacks with the others to reach any figure in 0.25kg steps.
Questions to put to anyone who says it cannot be done
If you are quoted for double glazing and the quote says the sashes will be fixed shut, or swapped to springs, ask why the counterweights cannot be increased instead. Ask what the new sash will weigh; a glazier who has weighed the sash can answer in one line. If the answer is a shrug, get another quote. Plenty of joiners and glaziers rebalance sash windows every week, and the ones who tell you it cannot be done are usually telling you it cannot be done by them.
Your windows kept their cords for a hundred years. Heavier glass is no reason for that to stop.
Common questions
Do sash windows need new weights after double glazing? Almost always, yes. The glass weight roughly doubles, and the original counterweights can no longer hold the sash. Adding lead to the existing weights is the usual fix.
Can the old cast iron weights stay? If they are sound, keep them and add lead lots on the cord above to make up the difference. The calculator works out the exact shortfall.
Will the heavier weights fit the pocket? Lead carries the same weight in about half the length of iron, and the calculator shows the stack length so you can check it against your sash height.
Does this apply to vacuum glazing? Yes. Vacuum glazing such as Glavenir 6.1mm weighs about the same as 6mm float, and it is one of the presets in the calculator.