What you need to know about conservatory glass

The most important two factors to consider when designing your glazed extension, whether it be a glass roof conservatory, glazed lantern roof orangery or tiled roof garden room, is the glass specification and ventilation. If you get this wrong you will have an expensive space that won't work for you and will probably be too hot in the summer and too cold in the winter.

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Too Hot in the Summer or Too Cold in the Winter?

It's still a commonly held belief that a glazed extension, especially if it has a glass roof will be too hot in Summer and too cold in Winter. This is reinforced by marketing during the past few years where the conservatory industry, desperate to sell more product has been encouraged to replace their glazed roofs with solid roofs, most frequently with less desirable imitation plastic or metal tiles.

It is absolutely true that if the incorrect or a low specification glass is provided it WILL be too hot in summer or too cold in winter. Unfortunately, there are very few regulations about most glazed extensions. Unless it is a predominantly solid roof, over 30 sq.m. floor size or open-plan to the house, it will be exempt from building regulations and the only regulation which will apply is that it must have safety glass. Even for building regulations projects, there is no regulation covering the use of solar control glass.

Hallmark Glazed Extensions - Conservatory Glass

These beliefs are based on the fact that prior to 2002 solar control glass was not readily available for the domestic market. So, the choice was limited to multi-chambered polycarbonate, clear glass or inefficient bronze or grey anti-sun glass. At that time, polycarbonate provided the best option. However, since then, the best insulating glass sealed units have become twice as efficient at keeping the heat in on a cold day and combined with the best solar control glass can reflect up to 86% of the sun's heat on a hot day.

However, it is important to appreciate that many companies and their salespeople rely on the fact that despite the information being available on the internet, a typical customer doesn't know a lot about glass and offer low specification, lower-cost glass which is 'not fit for purpose' and, as a result' their glazed extension will be unfit for use for much of the year.

Since 2002, all glass used for a habitable dwelling has to have a minimum efficiency to meet building regs requirements and has to have argon gas filling and a low-emissivity (metallic) coating on the inside face of one of the panes of glass, which on a cold day, reflects convected heat back into the interior of the house. This is commonly called Low-E glass.

How Glass efficiency is Measured

Let's take a quick guide on how double glazing efficiency is measured. The efficiency of a sealed unit is measured in u-values, which for comparative purposes measures the amount of heat in Kilowatts that will be lost through a 1 square metre of glass during an hour. It can be compared with a value of approx. 6.0 for a single glazed pane down to 1.0, which is the best currently achievable for a double glazed unit. Most glass sold in windows and roofing has a u-value of between 1.2 and 1.4. The 0.2 difference, when compared to a u-value of 1.0 may not sound very different, but a 1.2 u-value is 17% less efficient than 1.0. At Hallmark, we provide 1.0 u-value glazing as our standard option and a minimum option of 1.2 u-value where 1.0 u-value is impractical. Unless it is a building regulations project, there is no regulation requiring a minimum u-value and much of the glass units provided in conservatories have u-values as high as 2.7.

Is it worth considering Triple Glazing?

We have carried out research into triple glazing taking into consideration the types of glass available, the gap between the glass, the gas within the unit and the type of spacer between the glass. We found that unless very expensive Krypton or Xenon heavy gases are used, making the glass units up to 5 times more expensive, using Argon gas required a minimum of 16mm gaps and an overall unit thickness of 44mm. This could achieve a further improvement in efficiency of approximately 20%. However, at what cost? A typical triple glazed unit costs 75% more than double glazing. Why? It has 50% more glass, 100% more gas and spacer bar and takes longer to make. It is 50% heavier to handle and as a result, only 2/3rd the number of units can be carried on a lorry. If fitted in a roof, the glazing bars have to be of a heavier duty and extra manpower and lifting equipment is required. Triple glazing is used in Scandinavian & Central European countries where the temperature can be significantly lower and for longer during the winter, justifying the cost. However, because of the Gulf Stream, in the UK our weather is much more temperate and begs the question of whether the additional costs associated with triple glazing can be justified?

Hallmark Glazed Extensions - Conservatories

Solar Control Glass

Unless your project faces North or East and sheltered from the sun for most of the day, it is essential to use a good specification Solar Control glass. For maximum heat reflection on your roof, we recommend blue tint Ambi-Sunshade Blue, which has an 82% heat reflection. Whilst, there is a glass available with a 91% heat reflection it is a very dark blue with only 8% light transmission. It also has a u-value of 1.0 to keep the heat in on a cold day. For the frames, we recommend Ambi-Clear solar control glass, which not only keeps out over 50% of the sun's heat on a hot day but, even on a north-facing property is 20% more efficient at keeping the heat in than the best non-solar control glass currently available.

Security & Sound Reduction

It is not true that triple glazing will significantly reduce sound from a busy road or flight path. What is required is a variation in the thicknesses of glass used. So, we can offer alternative glass which has a 6.8mm laminated glass pane and which increases security, being virtually unbreakable, plus it significantly reduces sound and keeps out up to 98% of u-v light, reducing damage/fading of furnishings.

Blinds Between the Glass

Where required sealed units can be provided with integral blinds to provide privacy where overlooked and shading on sunny days. These units avoid the usual problems of insect and dust collection with typical surface mounted blinds. Manual, cordless and motorised options are available with a wide range of coloured slats. 


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