Double Glazing

Double glazing salesmen like to pretend that the replacement windows will pay for themselves within a few years by cutting heating bills. But it could take 40 years to pay back the investment. So how does that figure for the 95 year-old man who was recently sold £8,000 worth of replacement windows? Read on ...

Q. Several years ago I had my windows replaced with double glazing. After a few years, the south-facing ones became subject to occasional, unsightly internal misting. As there was a ten-year guarantee I claimed for renewal of the windows. The insurance company claimed that the condensation was normal and therefore refused the claim.

A. One of the great unspoken truths of sealed double-glazed units (SGUs) is that eventually they will all mist up. The timescale should be twenty-ish years in a perfectly made and installed window. But in poorly made ones it can be a lot less. Five months has been reported.

So, in a way, the condensation is 'normal', in that it will happen eventually in all windows. But I would have thought you could expect that a ten-year guarantee would reasonably cover you against misting within that time. It depends on the insurers' small print, I suppose.

Q. You have advised against fitting double-glazed units in timber windows using putty or mastic, because of the problem of internal misting. Our property was built in 1987, and soon after we bought it in 1993 we had to replace 10 units which had misted up. Two different glaziers said our units could be installed only with putty, the recesses not being deep enough to accept timber glazing beads. In 1994 we installed sealed Pilkington K units in a new sun lounge, which were fitted with beads but reinforced with mastic because of the local wind-driven horizontal rain. All of these have now acquired internal misting. We would dearly love to get 20 years' use out of replacements, rather than the seven to eight years we have had so far, but how can we get a glazier to use the dry glazing method you recommend?

A. Sealed glazed units (SGUs) installed with putty or mastic can mist up within a short time because the oils in the putty or mastic dry out the edge seal, causing it to crack, and because these sealants also trap water, allowing it to penetrate the edge seal, saturate the desiccant, and cause condensation between the panes. A drained, vented dry glazing system overcomes both of these problems and should help to give SGUs a mist-free life of 20 years or more.

Any competent glazier or carpenter should be able to adapt a timber window to a dry glazing system. In the unlikely event that the existing glazing rebates are too shallow, they can easily be deepened using a router, plane or oscillating shear. The replacement SGUs should be mounted on plastic setting blocks to hold them clear of the rebate, and the bottom glazing bead should have 5 mm x 35 mm slots cut in the lower edge to allow water to drain away. This technique is described in British Standard BS6262 and in the Building Research Establishment book "Improving Durability of Sealed Units". Alternatively, a patent dry glazing system which can be used to convert existing timber windows, and which includes self-adhesive security gaskets, is supplied by Reddiseals (www.reddiseals.co.uk).

Many more readers' questions like these are answered in my book, "The Sunday Telegraph Guide to Looking After Your Property" - click on books towards the bottom of the menu for more information...

A story in the Swindon Advertiser recently described how a local man got so fed up with the persistence of double glazing salesmen that he held one of them hostage in his home until the company agreed to leave him alone. Now, if double glazing is so brilliant, why the hard sell? What, apart from the antics of the salesmen, is wrong with it?

Well, it's not the doubling up of the sheets of glass that is the problem. Twin sheets of glass, with an insulating layer of air trapped between them, are a good idea, and well worth using in new windows in new houses. In an old house, if you were to get all the existing windows re-glazed with twin sheets of glass, or sealed units, as they are known, then you should save money on your heating bill. Whether you would ever actually save enough to cover the cost of the replacement glazing would depend upon how hot you like your home, how cold it is outside, how big the windows are, and whether you live to be 104. It might just be worth it.

No, the trouble with 'double glazing' is that what is actually being marketed under this description is not just the glass but a complete replacement window system. Well, I say complete. Some cowboy operators won't even give you that. They'll simply remove the sliding sashes from your Victorian box frames and screw the replacement window frames into them. You will feel the benefits at first, largely because the new system has cut down on the draughts. But draughts equal ventilation, and if the remaining ventilation is inadequate, which it usually is, then condensation will cause wood rot in the old timber surrounds. So you will be left with new double glazed windows mounted in rotten century-old hollow box frames. Brilliant.

Most replacement windows are made from PVC-U. The 'U' stands for 'unplasticised', as opposed to ordinary PVC which is plasticised, which is what raincoats are made out of. But although PVC-U is not as bendy as raincoat material, it is still a bit bendy, which is why the frames are so chunky looking. They have to be to stop the whole thing from flopping around when you open it. High quality PVC-U frames are stiffened internally with steel strips. Cheap ones may just have steel reinforcement up the sides for fixing to the brickwork, or none at all. The salesman didn't mention that? Well, there's a thing.

Probably the worst scenario with replacement PVC-U windows is when the original timber or steel windows had, by accident or design, been supporting some of the weight of the brickwork above. The PVC-U is not strong enough to take this load and, after a short time, will start to bend at the top. A characteristic cracking pattern will develop, with a triangle of brickwork detached from the rest of the wall and resting on top of the sagging window frame. This phenomenon can be observed in homes all over the UK, and it is serious bad news for the owners concerned. They have paid thousands of pounds to some chancer who has inflicted serious structural damage upon their homes, and then probably done a runner. Free enterprise; don'tcha just love it?

Another problem with replacement windows is that they are often 'designed' - ha ha - with scant regard for the original glazing pattern, or fenestration. This can knock thousands off the price of a decent Victorian or Edwardian terraced house - the price, in fact, of removing the double glazing and replacing it with the original draughty sliding sash windows.

But the most widespread problem is condensation. Replacement windows tend to make a house airtight, and when that happens condensation follows. Black mould growth in the corners of rooms is the usual first sign, which is an indication of damp walls. Damp walls are poorer insulators than dry walls, so they allow more heat to escape through them. So what was that about double glazing making your heating bills lower?

Most people who have double-glazed replacement windows fitted to their homes do so under the impression that they will last till the end of their days. Unfortunately this is often not the case. Sealed double-glazed units have a limited life-span, and the seal will eventually fail, resulting in misting-up between the two panes of glass. In the highest-quality installations this may not happen for 30 years. Some experts say 20 years is a reasonable life expectancy. But poorly- installed windows can fail much sooner - sometimes within a year - which can come as quite a shock to people who swallowed the sales pitch that their new windows would be maintenance- free forever.

The sales techniques used to flog double glazing are much the same as those for any other building gimmick, except that double glazing is now apparently approved by the World's leaders. The Kyoto accord on reducing carbon emissions committed nations to cutting down fuel consumption, and the British government is amongst those concentrating on reducing home heating energy in order to reach their Kyoto targets.

The only problem is, double glazing requires energy to manufacture and install, and the pay-back period, in terms of both energy and money, is very long. Taking the average British home, and replacing its existing windows with new double-glazed units, it would probably take around one hundred years of reduced heating bills to cover the cost of the installation. So the salesman who recently persuaded a 95- year-old man that new windows would save him money in the long run surely deserves some kind of award.

In any case, there is no way that new sealed double-glazed units are going to last 100 years. They are doomed to eventually fail because of the way they are made. The two panes of glass are joined at the edge with a polymer compound, which by its very nature is slightly vapour-permeable - ie it will always allow a small amount of water vapour to enter from outside. So to keep the glass from misting up, each double- glazed unit also incorporates a desiccant - a drying agent - housed within a perforated alloy strip running around the edge. This desiccant absorbs the invading moisture and prevents misting. But eventually there will come a day when it can absorb no more. It will be saturated, and then there will be free water between the panes, which will form as mist on the glass. How long this takes depends upon the quality of the materials and workmanship.

In ideal circumstances the sealed glass unit will be mounted in drained and ventilated recesses in the window frames, positioned on special setting blocks, and with spacers at intervals around the edges to support it when the window is opened or subjected to wind loads. But when this is not done, the resulting stresses on the glass can break the seal. Units fitted into timber windows using ordinary putty or oily mastics can also fail quickly, as these dry out the sealant and cause it to crack.

When the seal is broken, any water collecting at the bottom edge of the glass will find its way through and saturate the desiccant. And once these double glazed units mist up inside there is nothing - but nothing - that can be done to remedy it.. Thousands of windows in hundreds of homes already have this problem, and the numbers are set to grow.

Do the boffins at the DETR know about this - the ones who are madly promoting the use of replacement double glazing in existing buildings? I very much doubt it.