Double glazing jargon explained

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Replacement double glazing windows are one of the biggest expenses a homeowner is likely to have, so it's important to get it right. However the subject is a bit of a minefield of confusing information, statistics and jargon – just waiting to trip up the unwary buyer. Forewarned is forearmed, so check out our jargon-busting guide...

U-Value
You'll read a lot about U-values once you start looking at brochures and marketing material – and it relates to the ability of the windows to retain heat. The lower the value the better – since the figures relate to the amount of energy able to escape through a window. It's important to distinguish between Ug and Uw figures. The former is most-commonly provided and relates to the performance of the glass unit. The latter relates to the window unit as a whole.

WERs
Basically a more user-friendly way of tackling the same subject, Window Energy Ratings run from A+ to G and are usually displayed on a rainbow chart similar to that seen showing the energy efficiency ratings of white goods. The ratings are generated by comparing standard size windows from different manufacturers and checking the U-values. They take into account both the energy gain (from the sun) and the heat loss. WERs apply to the whole unit – frame and glass – not just one element.

Profiles
This is just another word for the frames really. Double glazing suppliers source their glass from one specialist manufacturer and the frames from another – before putting them together to create your window units. Reinforced profiles, which have steel integrated into the frame, are preferably from the point of view of security and providing a strong fixing point. The width of your profile is a key element to consider – and may be influenced by the depth of your window opening and how much "making good" of the décor you wish to carry out after installation. Well-regarded profile brands include Rehau, Kommerling, Synseal and Deceuninck.

Beads
The bead is the strip that runs around the edge of the glass holding it in place in the frame.
It's usual for modern double glazing to be internally beaded, which prevents the window panes being removed from the outside by burglars.

Spacer bars
These are the bars placed between the inner and outer panes of glass to keep them the required distance apart. You should expect your units to have composite plastic "warm edged" spacer bars rather than older-style aluminium ones – in order to aid energy efficiency. This is because aluminium conducts heat out of your home better.

Self-cleaning glass
No, it doesn't have windscreen wipers attached – it refers instead to a special coating which creates a super-smooth coating on the exterior of the glass – which dirt and water struggle to stick to. They are particularly well-suited to hard-to-reach windows and to coastal properties – where salt in the air can pose a problem for windows.

Emissivity
Emissivity refers to the amount of heat which is absorbed by glass. Since it is naturally a high-emissivity material, most modern double glazing is coated with a "low-e" treatment to help retain heat in your home.

Fensa
You'll see this acronym bandied about a lot and it stands for the Fenestration Self-Assessment Scheme. Set up by the Glass and Glazing Federation, it provides customers with a way of checking that their work is being carried out by a registered tradesperson. New doors and windows must comply with local authority building regulations – and using a Fensa-registered supplier means that the registration of work being carried out will be taken care of – so no building control visit will be required.

Replacement double glazing windows are one of the biggest expenses a homeowner is likely to have, so it's important to get it right. However the subject is a bit of a minefield of confusing information, statistics and jargon – just waiting to trip up the unwary buyer. Forewarned is forearmed, so check out our jargon-busting guide...

Secured by design
A police-led initiative to certify windows, doors etc. as meeting a certain standard of security.
It's worth checking whether the locks on your windows will meet this standard.

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