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Types of Glazing

When you think of glazing you might just think “single”, “double”, and “triple” – but there’s much more to window technology than the number of panes.

Here’s a rundown of the most common types of glazing you’ll find used in windows today.

What is Annealed Glass?

Annealed glass is slowly-cooled after first being formed.  It’s left to rest at a temperature where it becomes too hard to manipulate, but soft enough for any stress points to melt away.  This steady, consistent temperature at the earliest stage helps the glass resist shattering and mechanical shock during installation.  Annealed glass is a commonplace, inexpensive and tough material, but it isn’t quite as durable as tempered glass.

What is Tempered Glass?

Tempered glass undergoes a final stage of heating to make it even stronger.  While annealed glass might shatter into large, jagged fragments should it break, tempered glass will instead shatter into tiny pieces.  As such, it can be safely used in tabletops and flooring without fear of serious injury should it break.  It’s also commonly used in public facilities such as bus stops.

What is Safety Glass?

Safety glass reinforces each pane with a thin layer of tough plastic, like polyurethane.  This holds the glass aloft so that even if it does break, it won’t fall to the floor.

What is Low-E glass?

Low-E glass (or low-emissivity glass) reflects far more heat than ordinary glass.  As such, low-E glass is a sensible choice for homeowners looking to control the interior temperature of their property.  It’s coated in a microscopically-thin layer of metal that bounce the sun’s rays back the way they came.  Low-E glass comes in two types:  passive, which is built to keep heat in during winter, and solar-control, which keeps heat out during the warmer months.  Low-E double-glazed windows tend to incorporate a combination of the two.

As an added benefit, low-E glass will also prevent a majority of ultraviolet rays from entering the building, which will protect upholstery and carpets from fading.

What is Frosted Glass?

Frosted glass is covered in tiny bumps which effectively diffuse light rays.  This means specific objects can’t be seen through the glass, although light will still shine through.  This effect can either be created at factory level by blasting each panel with high-velocity grit, or in-situ, by spraying a special substance directly onto the window.

What is Self-Cleaning Glass?

The idea of glass that cleans itself might seem like something out of science fiction, but the technology has actually been around for more than a decade.  It works via a coating of titanium dioxide, which is used in a range of unusual places – from white paint and food colouring to suntan lotion.  When it’s placed on windows, it’ll respond to sunlight by transforming surrounding water molecules into hydroxl radicals, which will break down any surrounding organic matter (like the microscopic gunk from which dirt is made).  Once broken down, these tiny quantities of dirt are easily washed away by rainwater.

What is Noise-Control Glass?

One of the chief virtues of double glazing is its ability to reduce noise pollution.  The layer of vacuum (or inert gas) which sits between the two panes will reduce the amount of noise transmitted by as much as three quarters, which in most cases is more than sufficient.

That said, if you’ve recently bought a home beneath a flight path or next to a train line, you might want protection that’s a little more substantial.  This can be achieved by placing an additional layer of special acoustic-dampening plastic between the window panes.  This plastic will absorb the sound waves, thus preventing those vibrations from finding their way into your home.

What is Fire-Protection Glass?

Fire-resistant glass works by weaving a layer of special, hard resin between the panels.  It’ll vastly increase the breaking point of the glass, up to more than 800°C (ordinary glass will begin to break down at around 260°C).  This difference can buy valuable time that might make the difference between life and death. As a result the technology is particularly worthwhile in public buildings with interior windows, and in densely-packed streets where fire might otherwise spread quickly.

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What Is Low-E Glass?

Ordinarily, glass will allow solar energy to pass from one side of the window to the other.  This includes the relatively narrow spectrum of visible wavelengths, as well as those higher and lower wavelengths (known respectively as infra-red and ultra-violet light).

This is bad news!

Ultraviolet light has a habit of bleaching materials it comes into contact with.  Fabrics and wallpaper will fade much more quickly if they’re exposed to UV for a significant period of the day.

Infra-red light poses a different problem, in that it’s transmitted into the building as heat.  If you’re looking to keep cool during a blisteringly hot summer’s day, you’re going to struggle – you might even be tempted to draw the curtains in order to keep the sunlight out.

Low-E glass comes with a special coating that’s designed to act as a filter, allowing visible light through while excluding the superfluous wavelengths to either side.  This will allow your interior to enjoy the benefits of more light, without the downsides posed by ultraviolet and infra-red.

How low-E Glass Works

When heat or light strike a surface, that surface will absorb a portion of that energy and re-radiate it.  How large a portion this might be will depend on the surface in question.  If you’ve sat in a car with black leather seats on a summer’s day, you’ll know that dark materials tend to absorb and radiate more energy than reflective ones.

The amount of energy radiated by a surface is known as its emissivity. Glass, unfortunately, is naturally high in emissivity.  The more we can reduce this, the better an insulator our glass will be.

On the surface of each pane of low-E glass is a microscopically-thin coating that is designed to reflect infra-red rays.  This coating can be made from silver, or a variety of other metals, but their purpose is always the same: to deflect heat away from the glass.

Types of Low-E Glass

Low-E glass comes in two different varieties, and it’s produced using two different techniques.

A passive Low-E coating is built to contribute to the amount of heat within a home, preventing energy from leaving and thus lowering the heating bill.

A solar-control low-E coating works in the opposite way. It reduces the amount of energy entering the home, helping to keep it cool.

The right type of low-E glass for you will depend on the location of the building.

How is Low-E Glass Coated?

Each type of glass can be coated using one of two methods:

Pyrolytic

This process emerged in the 1970s.  It is applied to the glass shortly after production.  The coating fuses neatly with the hot glass before the latter has a chance to set, and stays that way for the entire lifespan of the window.  The glass is then cut to size and shipped.

Magnetron Sputter Vacuum Deposition (MSVD)

This process came a little bit later, in the 1980s.  It’s slightly different in that it’s applied to the sheets of glass after they’ve been cut to size, using a vacuum chamber and magnets to apply the coating at room temperature.

For many years, passive coatings were produced using the pyrolytic method, and solar-control coatings using the MSVD method.  More recently this line has begun to blur.

What does Low-E Glass Look Like?

So what effect, if any, does a low-emissivity coating have on the appearance of the window?  Since there are many different types of low-E coatings, and they can be applied at different thicknesses, it’s easy to be misled by reports of a set of ‘tinted’ low-E windows which block out certain colours, and distort the view of the exterior.

If, however, you want to ensure you don’t get any nasty surprises, it’s worth inspecting the sort of low-E glass you’re considering before making your purchase.

How Much UV Does Low-E Glass Block?

The thermal performance of windows is typically measured using the U-value.  This number refers to the amount of heat loss the glass permits per a given area.  According to the U-value, low-emissivity windows tend to be twice as efficient as their plain-glass counterparts – but it’s important that an impartial initiative backs up the manufacturer’s claims.

Naturally, this figure doesn’t cover the UV light that low-E glass will block.  Depending on the strength of the coating, low-E glass can prevent anywhere between 80% and 99% of ultraviolet light from entering the home (compared with around 60% by a standard window).  This extra protection is especially worthwhile if the sunlight entering a window is immediately falling on a set of curtains, a fabric-covered sofa, or a prized rug.

Many of our windows are supplied with low-E glass as standard, including our sliding sash windows and our casement windows. To view our full range, click here.

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