Glazing not amazing

Double glazing can cause more harm than good

Why not just look after your existing windows?

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 …

Double glazing – misting up

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 time scale 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.

Dry glazing system

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 (