Conservatory Glass UV

A single clear-glass pane 4mm thick has the very best light transmittance with 89% of the UV light passing through it and 85% of the solar heat falling in and transmitting through it. The problem is that heat transmittance is a two-way street and a sheet of glass like this can’t stop the heat leaving just as quickly as it arrived. Conservatory glass of a U value of 5.8 makes it one of the worst insulation products known to man.

If light transmittance through your conservatory roof is of no importance at all but you would rather have glass than a solid roof, you should go for green-tinted glass sheet. Green cuts down light more efficiently than any other shading colour and a double-glazed unit with a 6mm thick green glass outer pane will only let a mere 7 per cent of the light through. Silver reflective and bronze-shaded units, as used in designer-offices, are a very close second.

A standard double-glazed clear-glass unit of two 4mm panes does quite well with around 75 per cent of light penetrating, much better than polycarbonate sheeting; its solar heat transmission is also considerable at 72 per cent.

If you’re trying to keep out the heat of the sun, bronze-finished double-glazed units with thicker panes of 6mm or 10mm will repel all but 9 per cent of the sun’s heat back at it. The light transmission ability is about that percentage as well.

In terms of sound insulation, if you live beneath a flight path or beside a busy road, double-glazed units that employ a thick outer pane (the thicker the better) will achieve the best sound insulation – units with 12mm thick outer panes are available. Sound is always best kept out with a combination of mass and an air cavity and the heavier the product the better: a double-glazed unit with a 12mm outer pane, 12mm cavity and 6mm inner pane will weigh a staggering 45kg/sq. m, five times heavier than a single 4mm float glass pane and will require a great deal of structural support. For thermal insulation, argon filled 20mm cavities and low-e glass are about as good as you can get; and U values of down to 1.2w/sq m deg k can be achieved.

CRITICAL LOCATIONS
For any conservatory to be exempted under the Building Regulations or approved, its glass in critical locations must be safety glass, which either resists breaking on impact or breaks safely. As far as most conservatories go, critical locations are pretty much everywhere. They are zones that are at risk from accidental collision by running children, sprawling adults, stumbling pensioners and so on.

TYPES OF SAFETY GLASS

Laminated glass
Laminated glass is one of those products that involve other materials combined with glass, namely transparent plastic. It is essentially two sheets of glass bonded on either side of a clear plastic sheet which it holds together on impact. Invented in1909 by the French chemist Edouard Benedictus, it was given the product name of Triplex. Laminated glass will crack into fragments but remain in the frame, bonded together by its inner core; in this way it is harder to injure yourself by colliding with it and harder for intruders to break in by hitting it.

There are a fair number of dangerous structures caused by vehicles colliding unhappily with buildings but glass of this type hangs together very well after impact. The disadvantage of laminated glass is that it has no cavity for thermal insulation and a single sheet of glass is already around 12mm thick – great for car windscreens and shop windows but not so great for double-glazing where the insulating cavity is already pushing the pane width to the limits of acceptability.

Tempered glass

This product’s arrival was born out of research carried out for the automobile industry in 1929.The laboratories of Saint-Gobain, France were set the challenge of finding a stronger glass for car windows that would break safely. They discovered that by cooling glass rapidly from a temperature of 600 degrees C down to 300 C, the glass became stronger, but when it did break it shattered into a very large number of tiny regular-sized fragments that were harder to cut yourself on. The rapid drop in temperature had to be achieved by blast-cooling in just a few short seconds, but it worked and the product became known as Securit glass. Tempered glass today is still widely found in buildings, mostly in the panes of glazed doors.

Toughened glass

By adding chemicals and controlling the process of manufacture, glass can become toughened to resist an impact. This is the material that most, if not all, of your windows should be fitted with. In a double-glazed pane often the inner sheet is the only toughened one, but since it can achieve this status in its production without requiring greater thickness or change in appearance, it has become the normal product in critical locations for double-glazing.

This does not mean to say that you won’t have to specify it. The glazing industry tries hard to be self-regulating and mostly succeeds, but glass suppliers aren’t clairvoyant and they may not guess that your windows are below 800mm of the floor finish or beside a door if you don’t tell them. When you are using a conservatory specialist or professional glazier, the onus of responsibility can reasonably be transferred to them, but with a DIY build or a general builder the need arises to specify the glass type and to make sure that it complies before it’s installed.

Safety glass comes with a British Standard kitemark or product label etched in the corner of a pane of glass that identifies the pane’s nature. If there isn’t one, it’s not unreasonable to assume that it isn’t anything but standard float glass. Apart from holding a receipt from a glass supplier who is a member of the Glass and Glazing Federation (GGF) and has some professional integrity, the only other way to check it is safety glass is with the use of a special eye-scope tool as employed by glass suppliers, glaziers and surveyors. When held up to the surface of the glass, these little magnifying scopes can reveal what it is, and if a dispute arises over the issue this is the only way of resolving it convincingly.

Annealed glass

Annealing glass is much the same process as tempering, in that the material is hardened by a sudden drop in temperature, but in this case its strength also comes from its thickness. Given enough thickness, annealed glass can be used for shop fronts, but in our homes it is reserved usually for the smallest of panes – a Georgian-style window or door where each pane is no larger than 0.5 sq m in area and 250mm wide at the most. In these dimensions, annealed glass 6mm thick will suffice but in case you have plans to use it in larger panes minimum thicknesses will apply. Off the bottom of the scale are 4mm thick panes which can only be used in traditional leaded or copper lights. Apart from this 6mm is the bare minimum that should be used.

Glass blocks

Not only are glass blocks inherently strong and suitable for use in critical locations, some have even been tested to periods of fire resistance and could be used for boundary walls.

Although they have been enjoying a period of revival since the Millennium, glass blocks have been around for some time, even though much of their past was spent in public conveniences, which makes it all the more remarkable that they should be so popular now. They are available in colours as well as clear, and their thickness creates an obscured light that is well suited to boundaries or privacy screens.

Glass blocks have a hollow core and come in a variety of thicknesses. You should use one of at least 200mm which has a weight of almost 100 kg/sq m for an external screen that is resistant to wind loads.

Glass blocks usually look better when lit by artificial light from behind, from the outside looking in as it were. Daylight penetrates them in a considerably diffused way but this in itself could be exactly what you are looking for on one elevation – what they don’t do, of course, is offer you a view.