Double Glazing
made clear



The double glazing industry is a minefield for the average homeowner. Don't let a salesman pull the wool over your eyes. First establish the facts. Find them here from someone with more than thirty years in the business after an apprenticeship in industrial research.



simple double glazing

The most important part of any double-glazing is the second pane of glass. No surprise there. However, glass is not cheap, so making sure that the glazing will last, and last, and last, is no small matter.  Never mind the cost of replacing windows, glass takes a lot of expensive energy to make by melting sand, so while reducing the cost of central heating, making glass in the first place is far from being 'green'.

The second most important factor is the gap between the two panes of glass. It's the trapped air between the sheets of glass that do the insulating. If the gap is too small, the air doesn't do enough insulating: If the gap is too large, the air tends to start transferring MORE heat by convection inside the sealed unit, making things worse again.
It's not an exact science, but the best air-gap is between 12 and 15mm.


low emissivity glass
hard-coat
soft-coat

These are types of thin coatings of silver that reflect heat back from whence it came, in either hot or cold climates. Covered in more detail in the appendix, they both add substantially to the thermal efficiency of the glazing, with varying success.


Other small improvements can be made by using a different gas inside, rather than dry air. The most economic of these is argon, a plentiful inert gas at a modest cost. However, it does leak out over time, and it's a moot point as to whether it's worth the added cost for such small gain.  It may get a window past the Building Inspector in the short term, so maybe that's what counts.


slim double glazing

Let's be clear:
This, in general terms, is an abomination.
All of the issues around double-glazing were resolved over some thirty years from the early 1960s. Slimline double glazing appeared from nowhere in the 1990s invented by someone ignorant of all the technical issues and promoted heavily by a television presenter also ignorant of the underlying technical issues.

It appeared to solve some tricky problems with old rising sash windows, but simply added a few more  without addressing the fundamentals. They got away with it for years, aided and abetted by the Glass and Glazing Federation that held its nose while its members broke the law, and worse, by The British Standards Institution that allowed its Kitemark to be traduced.

Recently exposed in the Glazing press, both organisations were forced to eat crow pie, but whether we will see an early end to this pointless irrelevance is still unclear. Bad ideas can become entrenched and difficult to root out.

Any Conservation Officer needs to understand the facts, addressing both sides of the argument.  While superficially attractive, the totally inadequate edge-cover meets no recognised industry standard, possibly ensuring early failure.

So what, exactly, is the problem with Slimline?

There are two main issues...

1 Meeting the Building Regulations
2 The life expectancy of the glazing

The fact is that historic sash windows are not covered by the Building Regulations. Any work on the sashes is 'repair' or 'refurbishment'. It may be that the Conservation Officer might be interested, but the pressure from owners to do something - anything - about the winter comfort of their otherwise delightful homes has placed a terrible burden on authority trying to preserve our beautiful ancient towns and cities from the march of upvc.

The plain fact is that solid-bedding a sealed unit with inadequate edge-cover will probably ensure its early failure. With badly-made sealed units made by one company and fitted by another, possibly by an untrained worker, the likelihood of getting a great result is not good.

The fact is that the sealed unit should be surrounded by nothing but fresh air. This is not possible in a solid-bedded window.

There are better answers for glazing the rising sash, so just look harder. The internet makes research easy, and it will pay off in spades if you get it right.  Do your research.


triple glazing
glass/thin-film triple
downsides and upsides
(re: weight and benefits)

Make no mistake: Glass is heavy stuff. It also absorbs light to some degree, and triple glazing absorbs more of it. It's also expensive. Any window using triple-glazing is likely to need heavy-duty hardware. The worst of British winters only lasts a few months. Even double glazing - of any sort - is of no value in the warmer months, apart from keeping out noise. (This may be wot swings it for you.)

You gather that the writer is not a fan.

However...
Many years ago thin-film triple glazing appeared. It has come and gone from time-to-time, but it's still around. It's probably not cheap and there are tricky issues for the fabricator. But there are a number of benefits if it works...
The thin film - the third layer suspended inside the sealed unit - adds little additional weight and can be very useful in other ways. It can, for example, be designed to absorb UV, thereby protecting carpets, furniture, and paintings from the damaging component of sunlight.
So do your research...


safety glass
laminated glass
toughened glass
safety film

It sounds straightforward, but there are two issues here: Safety and Security.

Laminated glass and safety film are pretty much the same thing. The only difference is that laminated glass is made with the tough plastic film laminated between two thin sheets of glass, while safety film is applied retrospectively to one of the surfaces.  Each has advantages and disadvantages... In particular, laminated has two thin sheets that are easily cracked with expensive consequences.

On the other hand, toughened glass - while not a cheap option - once installed will withstand a great deal of rough handling. This writer never suffered a failure in more than thirty years of using it.


inert-gas-filling
argon   krypton   xenon

After many years in the business, having served an apprenticeship in industrial research as a young man, this writer has become sceptical about the claimed advantages of gas-filling. You will have to make up your own mind.


value for money

In the end, this is what really counts. Is double-glazing worth the cost?  My view is that the incomparable improvement in comfort, security, and noise reduction says "Yes". Are some of the optional extras worth the money? Well, maybe or maybe not. Do your research and make up your own mind.

Good Luck


Keith Nurcombe
after more than 35 years in the glazing business


Read more....

  Technical stuff

  Pros and cons

  lessons from the past

  more lessons from the past

  yet MORE lessons from the past