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 reglazed 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 uPVC. The small "u" stands for "unplasticised", as opposed to ordinary PVC which is plasticised and out of which raincoats are made. But although uPVC is not as "bendy" as raincoat material, it is still a bit flexible, which is why the frames are so chunky. They have to be to stop the whole thing from flopping around when you open it. High quality uPVC 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 uPVC 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 uPVC is not strong enough to take this load and, after a short time, it 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 seriously bad news for the owners. 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't you just love it?
Another problem with replacement windows is that they are often "designed", ha ha, with scant regard for the original glazing pattern. 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 growing in the corners of rooms is usually the 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 reducing your heating bills?Reuse content