There are several factors you should consider in deciding whether or not to replace single panes of glass with sealed double glazing units. Your existing window frames must be in excellent condition as there is little point in fitting sealed units into frames which may themselves have to be replaced within a few years.
If the frames are more than 60 years old they are not likely to be of a standard size and so they would need specially-made sealed units. These would not be reusable in a new standard replacement frame and so this would have to be made specially, too. Standard size sealed units are, in effect, mass-produced and so are cheaper than specially made ones. They are obtainable virtually ‘off the shelf from many suppliers, particularly for use in wooden-frame windows.
The rebate in the existing frames must be deep enough to accommodate the thickness of the sealed units and still allow them to be puttied in place or fixed with a glazing bead. You are likely to be changing from a single glazing thickness of 3 or 4mm to at least 12mm, rising to 18mm if you want units with a 12mm gap. For the latter, therefore, you would need a rebate measuring some 30mm from front to back, and not all old frames have this.
It is possible, however, to overcome the problem of too narrow a glazing rebate by using what are known as ‘stepped’ sealed units. These have one sheet of glass smaller than the other, the larger pane being fitted exactly in the same position as the original single pane with the smaller one on the inside, overlapping the back of the glazing rebate. Such stepped units are readily available to fit standard modern window sizes, or they can be made specially. They can be used in wooden frames but not in steel, which are generally unsuitable for sealed unit double glazing. The same can be said of any windows incorporating a large number of small panes, such as Georgian styles.
The cost of replacing all the individual panes with sealed units would be extremely high, even if the glazing bars were of a suitable size. However, if you wish to keep this appearance, complete sealed units are available that reproduce the Georgian or leaded-light look quite effectively.
If you are quite satisfied that sealed units can be fitted to your existing frames, the first step is to measure the rebate so you can order the correct size. Take great care to get the correct dimensions because, once made, the size of the sealed unit cannot be altered. With standard sizes this is not so much of a problem, but if you are having the units specially made it could prove to be an expensive mistake if you get it wrong. The height and width of the rebate should each be measured in at least two places. If there is a difference between any of the measurements, work with the smaller size.
Deduct a further 3mm from both the selected height and width to allow for clearance around the unit, and this will be the size you should order. Once you have the new sealed units, remove the putty from the window frame using an old chisel or similar tool and taking care not to damage the wood. Pull out the glazing sprigs with a pair of pincers and carefully lever the glass from the frame. Wear thick gloves or wrap a towel round the edge of the pane to prevent cuts as you lift it clear. If the glass is stuck fast to the old bedding putty, you may find that it can be tapped out from inside by a helper.
Only gentle taps should be used to avoid breaking the glass accidentally. If all else fails, break the glass from the inside with a hammer (making sure there is no-one outside who might be injured by the flying fragments) and pull out any remaining glass with a gloved hand or pair of pliers. Clean out the remains of the putty and brush any dust or dirt from the rebate. Reprime any areas of exposed wood and allow the primer to dry before fitting the new unit. Line the rebate with a bedding layer of fresh putty, inserting rubber spacing blocks at intervals along the bottom and at each side.
These should be cut to a thickness that will centralise and square the double glazing unit in the frame. Offer up the new unit bottom edge first and gently press it into place with the palms of your hands so that the bedding putty oozes out round the inside edges of the sealed unit. Apply pressure only to the edges of the unit to prevent the glass breaking where it is unsupported in the middle. Check inside that there is about 3mm of putty between the inner face of the glazing unit and the rebate. Next, very carefully tap in the glazing sprigs, using a cross-pein hammer. Use at least two sprigs per side and slide the head of the hammer across the glass to avoid breaking it.
Drive each sprig in squarely so that it does not pinch the glass until only 6mm (1/4in) remains visible. If you can’t obtain proper glazing sprigs, you can use 19mm panel pins with their heads nipped off
Apply a finishing fillet of putty all round the rebate, pressing it into place with your thumb so that it covers all the edges of the glass. Smooth this off to an angle with a putty knife, making sure it does not project above the level of the rebate otherwise it will be visible from inside the room. Mitre the corners carefully and clean off any excess putty from both inner and outer panes of the unit.
Leave the putty to harden for two weeks before applying a coat of primer and finally a finishing coat of paint. The latter should overlap onto the glass by 3mm to ensure a watertight seal. If stepped double glazing units are to be fitted, a rebate for the stepped portion of the unit can be made by pinning lengths of beading around the inside of the window frame. Extra putty will be needed around this stepped rebate to provide a bed and surround for the inner pane of glass.