About ten years ago these two sash windows had double glazed units fitted after the original glass had been removed. Because the original rebates that the old glass sat in was not deep enough, the contractor machined them so that they became deeper and allowed the thicker units to be fitted. This meant that the joints were reduced and their strength compromised – these sash windows should not have had double glazed units fitted due to their construction; there is just not enough stock to be able to do this. Also the rebates were not painted prior to the glass being fitted which meant that the sealant used did not adhere to the timber. Add to this the inadequate timber beads, the end result was a weakened window at the joints and the opportunity for rot to take hold once water ingressed into the rebate, due to poor adhesion of the sealant. Also there was no throat (a groove that stops capillary action taking place) underneath the bottom rail.
Some sash windows can be successfully retro fitted with slimline double glazed units but not all; it depends very much on the thickness and width of the stiles (side members) and the top rail (meeting rail).
For those windows that are too slim or too narrow, single glazing remains the only option. After all, box sash windows manufactured in the early and mid part of the 20th century or earlier were not designed to accommodate such double glazed units, just like a car built in the 1970s was not designed to accommodate air bags or many other modern features we now take for granted.
This is where secondary glazing comes into its own, with or without brush seals on the parting and staff beads and along the meeting rail. As I have mentioned several times in previous posts, thick curtains will certainly help.
With these two sashes, I managed to replace the bottom rail and glazing bead, along with the bottom 1/3rd of the stiles. The question of weakened joints I have solved by using an adhesive as the sealant between the glass and the frame at the bottom of the rebate – not ideal because if these units need to be replaced once they blow and mist up (and all double glazed units are destined to do this, even if its years down the road) then removal may be a challenge. As you may guess, I am not a fan of double glazed units in timber windows. Normally you can strengthen joints by: 1. inserting a wooden slip, which can be seen in one of the images that accompany this post, 2. inserting a loose Tennon, or 3. inserting two wood screws that secure the stile to the rail, but on this occasion there is not enough stock to undertake any of these options.
This is something I am asked about frequently. It depends on the style of window you have and the thickness of the openers or sashes. Unless I am taking out double glazed units and replacing them, the rebates will need to be altered to accommodate the extra thicknesses of the double glazed units. The thinnest units available are 11mm, which have 3mm glass, a 5 mm spacer, and 3mm glass. These are known as a 3-5-3. The next size up, which incidentally is cheaper, is a 4-6-4: 4mm glass, a 6mm spacer, and 4mm glass, giving a total thickness of 14mm.
This extra thickness needs to be accommodated in the rebate which has been manufactured to accommodate 4mm glass or on older box sash windows 3mm glass. This alteration to the rebate can come with problems, to begin with it may weaken the actual frame that the glass sits in i.e.: the opener or sash. Timber beading as opposed to putty has to be used to keep them secure, extra width may be required to cover up the edging around the double glazed unit. This can throw your eye towards this and can take away the originality of your period windows, also if you have original glass it may well be drawn glass as opposed to float which has a regular flat appearance this again can throw your eye towards this change of glass, and to be frank it doesn’t look right. Another consideration is the extra weight particularly with box sash windows this has to be counteracted with new sash weights which can be costly particularly if the pocket that the weights go into are too small to take the extra length of a bigger steel/iron weight and a lead weight has to be used instead.
You may have gathered at this point that I am no fan of double glazed units, single pieces of glass have an approximate life span of 800 years, double glazed units 15 perhaps 20 years before they ‘blow’ and condensation then becomes a problem in the space between the glass. Replacement really does mean replacement every 15/20 yrs! Indeed with modern plastic windows the guarantee more often than not does not cover the glass units, only the plastic and window furniture.
So what’s the solution? There are ways to overcome the inherent issues that you may be experiencing with draughts and maybe sound from outside, the first one is to perhaps consider investing in some thick curtains, they really do make a difference, or perhaps the fitting of acoustic and thermal seals, rubber strips that are fitted after a suitable groove has been machined on the edges of your openers or if you have box sash windows, brush seals fitted to parting and sash beads. Another viable option is secondary glazing, either fixed to the inside of the window frame, or sliding or the manufacture and fitting of a second window within the revel that opens inwards this option is common in many parts of Europe and was also employed in this country in the late 1800s in the centre of towns and cities.