DIYbanter - View Single Post - Putting a double glazed pane in an old frame

In article ,
(jacob) wrote:

Waste of money. If it's broken and needs replacing double glazing
still a waste of money - don't bother.

From the New Statesman 6 January 2003

It's all a lot of hot air
=========================

Resist double glazing salesmen, advises JEFF HOWELL.
They can't save us from global warming

Are you, in these midwinter bothered by double glazing
salesmen? They are annoying *******s at the best of times.
But they have become especially irritating since 1 April
2002, when they could start claiming that, as well as
fighting global warming and stopping old people dying from
hypothermia, they had the British government on their side.

April Fool's Day was an appropriate date to launch the
mishmash of convoluted reasoning that forms the new Part L1
of the Building Regulations for England and Wales, and which
decrees that henceforth all new windows shall be
double-glazed. The new rules include much genuflecting to
the Kyoto Protocol, which committed western governments
(though not the US, obviously) to reducing waste heat. Or
"carbon emissions", as we are now supposed to call it.

Other European countries have decided to crack down on
carbon emissions from power stations, factories and motor
vehicles, but the British government has decided that our
primary source of carbon emissions is poor people heating
their homes with one-bar electric fires, and that if they
would all just replace their windows with new double
glazing, then that should do the trick.

They reached this conclusion after much unselfish
committee-stage work by the British glass industry, which
helpfully suggested that a thermal insulation value of two
watts per square metre per degree centigrade would be about
right. And now that the new building regulations have been
unveiled, it must be gratifying for the glass-makers to find
that the only way to achieve this new insulation value is by
using sealed double-glazed units made with their own special
low-emissivity ("low-e") glass. And I am sure it is nothing
more than a coincidence that April saw the price of low-e
glass rise by 5 per cent, followed by a further 10 per cent
hike in September.

Low-e glass is very clever stuff. It has a metallic coating
that reflects approximately 25 per cent of escaping heat
back into the room. The downside is that the metallic
coating also reflects approximately 25 per cent of incoming
light back outside, so that the new windows appear much
darker than ordinary glass.

It is a testament to the enduring scientific ignorance of
the British public that they are consistently surprised by
this. After all, heat and light are but a few wavelengths
apart on the electromagnetic spectrum, and if you reflect
one, then you are sure as hell likely to reflect the other.
But the salesman didn't mention it, so the punters are
puzzled. It's all done with mirrors, you see.

When we say "double glazing", what we really mean is
"replacement windows". Most replacement windows are sold
over the phone by commission-paid salesmen, and are made
from PVC-U, or uPVC as it used to be called. The "U" stands
for "unplasticised", to distinguish it from plasticised PVC,
which is what raincoats are made out of. But though PVC-U is
not as bendy as raincoat material, it is still a bit bendy,
which is why the window frames are so chunky looking. They
have to be, to stop the things from flopping around when you
open them.

So the darkness inside homes fitted with replacement PVC-U
windows is also a function of at least one-quarter of the
window area being taken up with the framing material itself.
Beautiful 1930s houses with delicate timber fenestration are
having their windows replaced with PVC-U.

They look as though their front elevations have been redrawn
using a blunt pencil.

And despite their chunkiness, PVC-U window frames still have
to be reinforced internally with steel or aluminium alloy
bars. Which creates a bit of a problem when it comes to
reaching the aforementioned thermal insulation value. For,
contrary to popular perception, PVC is not a particularly
good insulator. Considerably less effective than wood, for
example. And when you poke a bit of metal down inside it, it
becomes even more likely to conduct heat out of the house;
which is why owners of new double glazing are often
surprised to find condensation forming on the plastic frames
of their new windows.

But the chief complaint of double glazed homeowners is not
about insulation, it is about internal misting. Sealed
glazed units are bound to mist up sooner or later, due to
the irrevocable forces of nature. The sealing material that
joins the two panes of glass has to be flexible, otherwise
the glass would break, and physics dictates that a flexible
seal will also be vapour-permeable. Sealed glazed units cope
with the constant seepage of moist air by incorporating a
desiccant within the perforated alloy spacer bars that run
around the perimeter. Eventually, the desiccant will become
saturated, and the double glazing will mist up. This is also
something that the salesmen never mention.

The timescale before the glazing mists up depends on the
quality of the installation. Dryglazed vented units in
German windows may last 30 years. Cheap back-street British
units stuck in with putty have been known to fail after
three months.

Double glazing won't save you any money in the long run. If
you take into account making the product, installing it,
disposing of the old windows . . . well, it's like nuclear
power; it's a net consumer of money and energy. There used
to be an advert on the telly, with dear old Ted Moult
saying: "You only fit double glazing once, so fit the best -
fit Everest." So how come Everest is now leafleting all its
customers, telling them they might wish to "upgrade" to the
new standards?

Jeff Howell is a bricklayer, and building columnist for the
Sunday Telegraph