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Window and Door Glossary

Window and Door Glossary

Buying new windows and doors can be daunting, especially when you have to negotiate you way through so much jargon. This is our guide to some of the most important references you’re going to come across and what they mean in plain English!

We’ve lumped them together by category so that related references are linked.

Energy efficient windows and doors

How does a double-glazed and triple glazed window work?

Energy efficient double-glazed windows work using two sheets of glass in the case of double and three, for triple-glazed units with a gap in between – usually about 16mm – to create an insulating barrier that keeps heat in. Performance is further enhanced using warm-edge spacers, low emissivity gases, for example, argon, xenon or krypton to fill the gap and low-iron glass. To meet building regulations, all new windows must achieve a minimum WER ‘C’ rating or U-value of 1.6 or less.

What are Window Energy Ratings (WERS)?

Operated by the British Fenestrations Ratings Council (BFRC) it provides a simple end user measure of window performance. This combines three different elements of window performance: Solar gain or how much heat the window allows into your home; and how much it lets out of it, either through draughts or thermal transfer and heat loss – the u-value. The best performing energy efficient double and triple-glazed windows will deliver WERs of up to A++.

What are Door Energy Ratings (DERS)?

Door Energy Ratings or DERs are exactly the same as Window Energy Ratings but apply to inline sliding patio doors; aluminium bi-folding doors; French Doors; and front and back doors, including low maintenance composite and PVC-U doors.

What is a U-value?

A u-value is a single but very important measure of window and door performance – namely heat loss. Heat lost through windows accounts for around 25 per cent of the total heat lost from your home. Energy efficient windows and doors can cut this heat loss by up to 75 per cent. The lower the U-value the better your new windows and doors are at keeping the heat in, with the best performing double and triple-glazed windows achieving U-values as low as 0.8W/m2K. This represents heat loss in watts (W) through an area of 1 square metre (m) for a temperature difference across the structure of 1 C degree centigrade or Kelvin (K).

What is Low emissivity (low-E) glass?

Low-emissivity (low-E) glass uses special low-E coatings which allow light and heat into your home but reflects escaping heat back inside the room. Double glazing with low-E glass cuts window heat loss by about 20% to 30%, compared to double glazing without it. Remember – you can’t see it, so check the specification of windows and doors carefully before making a purchase!

What is Argon gas (and why is it used in windows)?

Argon gas and other inert gasses including xenon and krypton are used to fill the space between glass in double or triple glazed windows to make them more energy efficient. Argon, for example is a far better insulator than air, reducing window heat loss 3% to 9% more than double glazing with air filling.

What do spacer bars do in windows?

Spacer bars are used to separate the space between the sheets of glass which make up either double or triple glazed windows. They effectively created the cavity which is then filled with an insulative inert gas to maximise energy efficiency. These are also specially designed to prevent heat transfer between panes.

Home security

What is Part Q of Building Regulations?

To a certain extent you only need to worry about Part Q if you’re building an extension or a new home as it only applies to new build properties. This essentially creates a requirement for all doors and windows fitted to new build properties to be tested to and meet the PAS24 test (see below).

The useful thing about Part Q, particularly in the absence of a regulatory requirement for replacement windows and doors, is that it gives you a benchmark. There’s also nothing to stop you from telling your supplier that you want your new windows and doors to be Part Q approved.

What is PAS24? 

PAS 24 (Publicly Available Specification) is the minimum standard set out in Building Regulations Part Q for windows and doors installed in new homes, receiving either a pass or fail – for the moment the test applies to new build only.

PAS24 tests cover everything from manipulation using tools, for example paint scrapers and screwdrivers to lever the window open or force entry by removing glazing to impact tests on doors. It also sets a requirement for the components used in the manufacture of windows and doors including strengthening of frames and in doors TS007 compliant door cylinders, which are resistant to drilling and other methods of assault.

These tests must also be independently assessed by a United Kingdom Accreditation Service (UKAS), accredited test house – in short UKAS checks the checkers to ensure competence, impartiality and performance to achieve Part Q approval.

What is Secured by Design?

Secured by Design, according to the official blurb, is ‘A police initiative to guide and encourage those engaged within the specification, design and build of new homes, and those undertaking major or minor property refurbishment, to adopt crime prevention measures’.

What it gives homeowners is an independent assurance, that their new windows and doors have been manufactured and independently tested to, pre-defined standards of performance for security. This for the most part largely mirrors updated requirements set out in building regulations for security (See PAS24/PartQ).

It does, however, place a couple of further requirements on their approvals. This includes the use of laminated glass in doors and any non-key locking window within 400mm of a door. A Secured by Design approved window or door needs to have been manufactured by an SBD license holder.

What is toughened glass?

Toughened glass has become pretty much a standard product in glass unit manufacture. Also called tempered glass, toughened glass is a safety glass made from heat treated or tempered standard or annealed glass. This heats standard glass to very high temperatures of around 650°C, so that it begins to soften before rapid cooling.

The result is toughened or tempered glass. Four or five times stronger than annealed glass, it can withstand far greater impacts.The other advantage is that if it does break it shatters into thousands of small pieces rather than forming the ‘lethal’ shards associated with standard glass. This makes it ideal for use in the manufacture of double glazed and triple glazed units.

What is laminate glass?

Security glass used in high security windows, glazed entrance doors, French doors and bi-folding doors, in addition to being toughened, features a special laminated inter-layer, which even if smashed, holds the glass together making forced entry much more difficult. Each pane is generally made up from two 3mm thick panes of glass with a polyvinyl butyral or PVB interlayer in between.

This uses the same technology that car manufacturers will use in your windscreen. The PVB interlay creates an ultra-strong bond between the individual sheets of glass so even if the glass were to break, the window itself would remain ‘intact’.

What is a composite door?

Composite front doors offer more flexible aesthetics than PVC doors and seek to emulate the appearance of a natural painted timber woodgrain in a low maintenance and energy efficient and low maintenance door.

There are two types, solid timber core doors and slightly more energy efficient foam core doors but what both share-in-common, is a robust glass reinforced plastic (GRP) composite ‘skin’.

As highlighted, this skin looks like a traditional painted timber door but that’s where the similarities end. Water tight, robust and more or less, maintenance-free, GRP composite doors deliver a wood-effect but without the associated maintenance or maintenance related problems of wooden doors.

Design and aesthetics

What is a casement window?

Odds are, you probably have them! This is the most common window type of window supplied in the UK, it’s just you may not have come across the terminology. Casement windows are distinct from sliding sash windows (these do what they say on the tin, sliding up or down to open), in that traditionally, they’re side-hung i.e. hinged on the side, opening out or in. meeting the mullion or the up-right between them (or each other in the case of French windows) when closed.

What is a window or door sash?

A window sash is the opening part of the window or a door as opposed to the frame, which is fixed to the wall or substrate.

What is a flush sash casement window?

The opening sash of a PVC-U window almost always over-laps the frame to form a weather seal. A new generation of flush sash casement windows give homeowners the option of specifying a casement window where the opening sash sits flush with the casement when it’s closed, removing the over-lap. This more closely replicates a traditional 19th Century timber window, allowing owners of period properties to choose a low maintenance and energy efficient window but still choose a design that is sympathetic to their property.

What are PVC-U foils?

PVC-U foils are applied to standard PVC-U profile to deliver a specific finish. This is done using a lamination process. Finish and colour options are diverse including woodgrains, which are designed to replicate timber windows to a wide choice of flat-finish colours including greys, which replicate aluminium.

What is a Graf seamless weld?

The Graf weld delivers an almost invisible corner joint in PVC-U windows. PVC-U windows are almost always welded together. This gives them exceptional strength and performance. This is done by heating and then pushing four lengths of window profile together.

This creates a ‘splodge’ of molten plastic, which then has to be cleaned leaving a line which indicates where the join has been made.  

Window machinery leader Graf uses a process called ‘zip-welding’ which pushes the molten plastic back inside the window which it means more of the internal surfaces are joined, creating an exceptionally strong weld.

The other, advantage is that Graf-welded windows delivers a surgical and almost invisible joint. This is a key advantage if you’re choosing a foiled window as it delivers a better finish.  

Also useful to know if you’re buying new windows

What are trickle vents? 

There are three types of trickle vent – through frame, over frame and glazed-in. Glazed-in ventilation sits at the very top of the double or triple-glazed unit, while over-frame routes ventilation over the top of the frame into the property.

But by far and away the most common form of window ventilation is through-frame. This routes fresh-air through the top of the window frame and can be adjusted through a simple open and closing mechanism to provide as much ventilation as required.

The other advantage of through frame trickle vents is that they can be left fully open without impacting on home security, maintaining air flow and reducing the build-up of condensation, regardless of whether you’re at home or not. 

They’re important because not ventilating your home properly can lead to build up of moisture, condensation and potentially, damp and mould problems. 

What is secondary glazing?

Secondary glazing systems allow you to add a second slim-line window to the inside of your existing window and are manufactured either as timber secondary glazing or aluminium secondary glazing.

Most secondary glazing systems are built around a sliding horizontal or vertical design but hinged and ‘lift out’ units are also available.

We supply, aluminium secondary glazing because it features exceptionally slim sightlines and delivers high levels of security.

This can be fixed either directly to your existing windows or manufactured with a MDF or hardwood sub-frame, which means that they can be reveal or face-fixed.

What is acoustic glass and how does it work?

Acoustic glass uses multiple layers and different thicknesses to disrupt sound waves as they travel from one side of the glass and through the glazing unit to the other. Put very simply, acoustic glass creates more hurdles for the sound wave to pass through, reducing the amount that ultimately gets through.

It does this by splitting the glass into multiple layers using specially developed laminate interlayers. These ultra-thin sheets sit invisibly between the glass blocking sound. 

The critical thing is that the glass which makes up double or triple glazed units should not only be as thick as possible but also be of differing thicknesses. The additional sheets of glass used in triple-glazed units for example, won’t make things quieter because the line of travel of sound won’t necessarily be disrupted.

What are Dummy Casements

Dummy casements (also know as dummy openers) are used to create an even sightline where non-opening sections are balanced with opening sections. The dummy casement will not normally have any window furniture

What are Mullions and Transoms?

A mullion is a vertical devision in the window, a transom is a horizontal division

What does internally glazed mean?

The glass is inserted in the frame and the glazing beads (which hold the glass in position) are on the inside of the frame. Aluminium systems are often externally glazed.

What is face-drainage?

If you are fitting a window directly onto a stone or tile cill, drainage slots are cut into the front face of the bottom section and covered with caps. This can sometimes be avoided by using a small stub-cill. If the window is fitted with a purpose-designed cill, this allows for concealed drainage as the drainage flows out from the very bottom of the frame.


If you’re looking for help in planning your next home improvement, or simply want to talk about any of the points raised in this discussion, please don’t hesitate to contact me or a member of the team for more by calling 01264 359355 or emailing email sales@kjmgroup.co.uk

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