Ask three companies to quote for the same windows and you will get three sets of paperwork full of ratings, gas fills and technical terms, very little of it explained. The prices will differ too, sometimes by thousands, and nothing on the page will tell you why.
This guide explains it the way we would explain it standing in your front room. What double glazing actually is, what makes one window better than another, what the rules ask for, and how the industry behind your windows is put together, so you know what to ask before you sign anything.
What is double glazing?
Double glazing means two panes of glass held apart by a spacer bar around the edge, with the whole perimeter sealed in the factory so nothing can get in or out. The gap between the panes is filled with dry air or a harmless inert gas.
The whole thing is called a sealed unit.
The gap is the part that does the insulating. Glass on its own is a poor insulator, so a single pane loses heat almost as fast as an open window frame. But trap a layer of still air or gas between two panes and you slow that heat loss right down.
There is also a drying agent hidden inside the spacer bar, which mops up any last trace of moisture. That is why a healthy unit stays clear inside for decades.
The sealed unit is only half the product. It has to sit in a frame, and the frame carries the weight, holds everything square, keeps the weather out and does its own share of the insulating.
Ruislip Windows fits sealed units into both uPVC and aluminium frames.
Did you know?
The part of a modern window that does the most work is one you cannot see. A very fine metallic coating sits on the glass and bounces the warmth from your radiators back into the room instead of letting it out.
What makes one double glazed window better than another
Two windows can both be described as double glazed and perform very differently. Three things account for most of the difference:
1. The gas in the gap
Most modern units are filled with argon, a harmless gas that carries heat less readily than ordinary air. It costs very little to add and it is now the standard. If a quote does not mention what is in the gap, ask.
2. The coating on the glass
This is the one that matters most, and you cannot see it at all.
Plain glass soaks up the warmth coming off your radiators and lets most of it straight back out. Modern glass carries a metallic coating far too fine to see, sitting on the inside face of the glass, which lets sunlight in and bounces your heating back into the room.
There is more than one grade of this coating, and the good ones perform a great deal better than the cheaper ones. Two quotes can both say “double glazed and argon filled” and still be a long way apart on this single point, so it is always worth asking what glass you are being offered.
3. The spacer around the edge
For years the bar that holds the two panes apart was made of aluminium, which carries heat beautifully. That gave a cold line all the way around the edge of the glass, which is why older windows mist up at the corners first on a cold morning.
A warm edge spacer does the same job in a material that does not conduct heat the way metal does.
The edge of the glass stays warmer, there is much less condensation around the perimeter, and the window performs better overall. It is a difference you can feel with your hand on the edge of a unit, which makes it much easier to understand in a showroom than on paper.
How to compare two quotes
You do not need to understand the physics to compare quotes sensibly. These questions will tell you most of what you need.
- What is the energy rating? Windows are graded on a scale running from A++ down to E, in the same way as a fridge or a washing machine. It takes account of the heat lost, the warmth gained from the sun and the draughts around the seals, all in one letter. Most of our ranges are rated A or better.
- What glass and what spacer? A company that is proud of its glass will tell you happily. A vague answer usually means the cheaper option.
- Who actually makes them, and who fits them? More on this below, because the name on the brochure is often neither.
- Who registers the work, and who do I call in five years? Both matter more than most people expect at the time of buying.
What the rules ask for
Replacing your windows is building work, and there are three things worth knowing.
There is a minimum standard for how well a replacement window has to perform and keep heat in. It does mean a company cannot legally fit you something cheap and poor.
New windows in England have needed trickle vents in the top of the frame since June 2022. These are the small vents you can slide open and shut. They are there to let damp air escape gently in the background, and they are the best defence you have against condensation and mould, so leave them open even in winter.
The work also has to be registered with your local council.
We are registered with both FENSA and CERTASS, so we certify the installation and notify the council ourselves, and your certificate arrives without you applying for anything. Keep it with your house papers, because a buyer’s solicitor will ask to see it when you move.
uPVC or aluminium?
The popular frame choices are uPVC or aluminium, both are fitted straight into the brick opening now.
The hardwood subframes used the 1980s have long gone, which is why modern windows look slimmer than the old ones they replace.
uPVC is a rigid plastic, shaped into profiles with hollow chambers running through them. Those chambers are what make it insulate so well for the money, and modern finishes give it colours and woodgrain effects that were not available a generation ago. It is the sensible choice for most homes.
Aluminium is much stronger, so the frame can be far thinner for the same size of opening. More glass and less frame is why it suits big sliding doors, bifolds and picture windows. Aluminium does carry heat, so a good aluminium window has an insulating barrier built into the middle of the frame, known as a thermal break, which stops warmth travelling straight through the metal.
We do not install timber. For a period frontage, The Residence Collection R7 and R9 give you the deep sightlines and detailing of timber joinery, and Rehau Rio Flush and Heritage Flush casements sit flush in the frame the way the originals did, with nothing to rub down and repaint, ever.
Is triple glazing worth it?
Triple glazing gives you three panes of glass and two gaps, and it does keep more heat in than double glazing. In a very cold or very exposed home it can be the right choice.
It is worth knowing what it does not do. Triple glazing is a good deal heavier, so the frames and hinges have to be built for it, and the improvement over quality double glazing is smaller than most people assume.
It also does very little to reduce noise intrusion.
If noise is your real problem, the answer is not a third pane. It is a double glazed unit with two different thicknesses of glass, one of them laminated, which breaks up the sound far more effectively than a third sheet of the same thickness ever will. Tell us what you can hear and from where, and we will come up with a solution.
How the double glazing industry actually works
The name on a brochure is often not the company that made your windows, and almost never the company that fits them. There are three tiers, and knowing them gives you a very useful question to ask.
Systems companies design and make the lengths of profile: the frame, the opening sash, the beads that hold the glass in. They engineer how it all locks together and test the finished design. Rehau, Deceuninck, Kömmerling, Aluk and Smarts are systems companies.
Fabricators buy those lengths and turn them into actual windows, cutting the sections, joining the corners and fitting the hardware, seals and glass.
Installers survey the opening, take the old windows out, fit the new ones, make good and register the job. Until the early 2000s plenty of small firms did their own fabricating too. Self-certification arrived with FENSA in April 2002, and most installers have concentrated on the survey, the fit and the aftercare since.
So when a salesperson leans on a well known name, ask whether that is the system, the fabricator or the installer, and who is actually turning up on the day. Then ask who you ring in five years if a handle sticks. We employ our own service engineer, so that side of it is not subcontracted.
A short history
Double glazing is older than most people think.
An American engineer patented the sealed unit in 1934 and it went on sale in 1938, only to be withdrawn a year later because the seals kept failing. It came back in 1946, cheaper glass made it affordable through the 1950s and 60s, and British homes took it up in numbers after the 1973 oil crisis.
Frames went their own way. Steel and joinery gave way to aluminium in hardwood subframes through the mid 1980s, and uPVC took over from the late 1980s.
Today 89% of homes in England have full double glazing, up from 81% ten years ago.
Conservatories
The walls
The sides of a conservatory are window and door frames holding exactly the same sealed units as the windows in the rest of your house. Everything above applies to them: the gas in the gap, the coating on the glass, the warm edge spacer.
Below them there is often a dwarf wall of brickwork.
The roof
A glass conservatory roof uses sealed units like the walls, but specified for the job. The panes are toughened or laminated, because this is glass above your head. Most are made with a solar control coating, which lets the daylight through while turning back a good share of the sun’s heat, and many also have a self-cleaning coating on the outer face that uses daylight and rain to shift the worst of the dirt.
Blinds
One more option worth knowing about for roof glazing and big glazed doors: blinds sealed inside the glass unit itself, worked by a slider or a motor. There is nothing to dust and nothing hanging in the way of a bifold.
Come and see it for yourself
All of this is far easier to understand with the product in front of you.
Our showroom at Unit 34 Orbital 25, Dwight Road, Watford has the ranges set up so you can compare uPVC and aluminium in the same room and ask about anything above.
Ruislip Windows has been family run since 1986, covering Middlesex, Hertfordshire, Buckinghamshire and the surrounding areas. We are FENSA and CERTASS registered, our work is covered by insurance-backed warranties through QANW, and our lead time from order to installation is normally three to four weeks.
There is no hard sell, and never has been.
Call us on 020 8868 1133 for a free, no-obligation quotation, or come and see us in Watford.