Date posted: 29th May 2018

Different types of glass

Glass is the singularly most important element of your new windows, bi-folding door or conservatory. It’s not just that you look through it that makes it so critical. Glass is key because it is so important in defining the performance of your new windows and ultimately your home.

Your choice of double or triple-glazed unit will determine how energy efficient your new windows or ultimately your home is. The same goes for self-cleaning or solar control glass in your conservatory roof or aluminium bi-fold doors.

We may not be able to see it but the technology in glass is not only cutting-edge but the single most important factor in determining the performance of your replacement windows and doors.

How do double-glazed and triple-glazed windows work?

The starting point for most of us when we buy new windows and doors is that we want to improve the energy efficiency of our home and to reduce the cost of heating. That makes how energy efficient your double or triple glazed insulating unit is, singularly important.

Double-glazed and triple-glazed windows can cut the cost of heating your home significantly. In a detached three-bedroom home, according to the Energy Saving Trust, this can equate to an annual saving over and above single – and in many cases, older double-glazed windows - of up to £160 a year in heating bills.

Double and triple-glazed windows do this by 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.

They also use specially designed insulative warm-edge spacers, low emissivity gases, for example, argon, xenon or krypton to fill the gap in between. While you can’t see it, these gases act a little bit like wall insulation, slowing the transfer of heat from inner to outer panes of glass.

The different combinations of glass,  triple or double-glazed options, will ultimately come together to define the overall performance of the window. 

But take note. To the untrained eye, you can’t really see any of this simply by looking at it. If you’ve got a window quote that’s significantly cheaper than another go back and check the specification. The more energy efficient the window the more the cost of the glass.

Take care to make sure that you’re comparing like for like and the performance of both windows or doors is the same or at the very least, not miles apart. The best way to do this is by looking at window performance.

How is window performance measured?

There are two systems for rating the performance of windows and doors – U-values and Window Energy Ratings (WERs). U-Values assess a product on how effective it is at keeping heat in or how much heat is lost through it, as a single measure of performance.

Window Energy Ratings do the same and use the U-Value (heat loss) but also balance heat loss against solar gain (warmth which passes into the room from the sun) and also throw into the mix additional heat loss through air penetration, giving windows a ranking from ‘A/A+’ to ‘G’ using a ‘traffic-light’ type colour code.

To meet building regulations, all new windows must achieve a minimum WER ‘C’ rating or U-value of 1.6 or less. Windows fitted today - go far beyond this minimum. For example, at KJM we supply an energy saving triple-glazed window with a U-value as low as 0.62W/m².K and WER off the A++ scale.

What is toughened glass and do I need in my replacement windows?

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, so if you’re children kick a football against it, unless they have really launched it, it won’t break.

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.

While it’s now more or less used as a standard product by most window and door companies, technically it only has to be used in certain settings, namely overhead and in low-level glazing where it’s likely to come into contact with people. These are described as ‘critical locations’ in building regulations. 

Where should safety glass be used?

Safety glass should be fitted in all doors and other windows or glazed areas that are lower than 800mm from the floor level. Glass panels less than 250mm wide can be fitted with 6mm glass or laminated glass instead of toughened glass.

This includes:-

  • Glazing in doors – The regulation states that in doors any glass which is wholly or partially within 1500mm from floor level must be toughened. This means that all glass entrance doors, bi-folding doors and patio doors must use toughened glass by law.
  • Glazing next to doors – You also need to use toughened glass if you have a glass front door with a windows or side light next to it which is wholly or partially within 300mm of the edge of a door and which is also wholly or partially within 1500mm from floor level.
  • Low Level Glazing – Finally, you also need to set If you have floor to ceiling windows or your windows are set  in the wall so that they start within 800mm from floor level, regardless of how close they are to a door. This can be a requirement in older properties which often feature larger windows set lower to the floor.

This is another point to check with your supplier. Toughened glass or laminate glass goes through additional processes, so by definition, costs more. If you have one quote that comes out significantly cheaper, it’s again worth double checking that it’s based on the same glass and performance specification.

What is laminate or high security glass?

Laminate glass is another form of safety glass. 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’. That’s why it’s also used in glass balustrading. Toughened glass is a lot stronger than normal glass but at the point at which it breaks it shatters – that’s something that’s clearly no good for a product designed to stop you from falling.

This makes it a really good choice if you’re concerned about security. Recent changes in building regulations mean that in new build developments laminated glass must be specified on all windows and sidelights within 400mm of a door.

This is based on a specification developed by Secured by Design the police home security campaign. Although it’s not a requirement for replacement windows, we’d argue that it’s well worth the extra investment if you think your home might be vulnerable to burglars.

Solar Control Glass

PVB interlayers can also be used alongside solar reflective coatings to control the amount of heat your home absorbs from the sun. You remember that old conservatory that you used to swelter in in the summer and you vowed never to buy again? Well the reason why it overheated was because of solar gain and the performance of the glass.

Things, have, however, moved forward. The reason that glass can be used on a large scale in commercial building – and not least The Shard -  is that solar control technologies have moved forward massively in the last decade. These allow architects to keep enormous glass buildings cool, so keeping your conservatory at the right temperature is simple in comparison.

How does solar control glass work?

High performance solar control glass does this by controlling three factors that influence how much heat your home or conservatory absorbs.

These are:

  • Reflectance – how much radiation from the sun the surface of the glass reflects back into the atmosphere
  • Direct transmittance – or how much of the suns radiation is transmitted directly through the glass
  • Absorptance – how much heat is absorbed by the glass and indirectly radiates inside the building

All of the above contribute to the warming of the building or the ‘G’ value, cited in the Window Energy Ratings window performance measure.

A G-value of 1.0 represents full transmittance of all solar radiation while 0.0 represents a window with no solar energy transmittance. In practice though, most G-values will range between 0.2 and 0.7, with solar control glazing having a g-value as low as 0.18 – less than quarter of the G-value of standard glass.

Combined with the insulative U-values offered by energy efficient glass,  your conservatory or glazed extension becomes an infinitely more comfortable place to be, providing flexible year round living space at a significantly more affordable price point than a traditional ‘bricks and mortar’ extension. 

Self-cleaning glass

The other significant advantage that innovation in glass technology has brought to the deisng of conservatories and glazed extensions is self-cleaning glass.

While in our experience not eradicating cleaning altogether, reduces the need to do so, also keeping your view uninterrupted for longer.

How does self-cleaning glass work?

Self-cleaning glass features a special coating that reacts with UV rays in sunlight. This photocatalytic process, loosens and breaks down organic dirt particles on the surface of the glass.

The surface of the coating is also hydrophilic, which means that it behaves a little bit like soap, so that rather than attracting water to form droplets, it forms a thin layer that then sheets away quickly and dries without leaving unsightly drying spots.

Closing thoughts

This guide to glass is meant as a starting point to give you an overview of the different options available and a few pointers to think about.

What you have hopefully gathered is that glass comes in all sorts of wonderful configurations, which is what makes it such a great building material.

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