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Conservatories, Sunrooms - Your questions Answered
Low E / K Glass (1

Please note: Most of the answers we feature here are from 1999 - early 2002. We endeavour to keep all links etc up to date, however if you spot any errors please let our webmaster know at It should also be noted that some replies may change in light of changes to legislation especially with regards to Planning Permission and Building Regulations. To submit a new question or to query an existing question visit

Question submitted by Dave

I am considering a conservatory on the back of my property and have had several quotes. One company - who is a member of the Conservatory Association, has told me that there are building regulations coming into force, which would alter the way that conservatories are constructed. He mentioned "k" glass, and u values but no other company has mentioned this. Is he trying to claim exclusivity to the new regulations or is he just bamboozling me?

This question answered by the Conservatories Online editorial team - Very few conservatories are currently subject to building regulations in England and Wales. For more info on building regs have a look at

Conservatories that are subject to building regulation approval are likely to have Pilkington K Glass or low E glass specified in order to conform to the "U" values required by building regulations. We are not aware of any immediate plans to make all conservatories subject to building regulation approval - but there are often rumors.

As most conservatories are not subject to building regulations conservatory suppliers work to varying standards with regards to insulation levels/depth of foundations etc. It sounds to us that the supplier you mention is offering a higher specification and while we would consider that a good thing - only you can decide if the increased benefits of his specification are worth the extra cost that his specification almost certainly attracts.

It may be worth speaking to the other suppliers again and asking if they wished to make any particular recommendations with regards to insulation/winter use etc.

We would also say that consumer expectations vary greatly and while some people may find a conservatory with normal double glazing and say 16 mm polycarbonate in the roof acceptable in the winter months because they only intend to use during the daytime or during sunny periods - others will not find the lower specification acceptable because they intend to use the conservatory in the winter evenings also. For these people, low E glass and 25 mm polycarbonate in the roof will be important.

Question submitted by D Blake

Click here for a brochure request We are having a hard wood conservatory installed using Pilkington K Glass. In the rest of the house the double glazing has laminated glass. I think Pilkington supply K glass in a laminate form. I just wondered if this would be suitable for a conservatory

This question is answered by Brendan Bermingham  - Pilkington K glass is definitely available in laminated form and it would be more than suitable for a conservatory. I wonder why your house is glazed using laminated glass? Perhaps it was because it gives increased security over ordinary window glass or toughened safety glass, it gives superior sound insulation and it cuts down uv radiation by 99.5% which stops your furniture fading.

As your conservatory is being built now it will be to late to change the specification, but an ideal spec would be Pilks K laminated/4mm antisun. The antisun will reduce heat build up and glare.

Question submitted by Sandy

On investigating the possible use of K glass in a conservatory I have discovered that because of the enhanced insulation qualities of the double glazing, condensation may occur (on cold, damp , misty, foggy mornings) on the OUTSIDE of the glass. I have spoken with Pilkingtons and they do admit this happens. Is this a rare seasonal occurrence and would it deter you from using K glass? (Question edited)

This question is answered by the Conservatories Online editorial team - Pilkington have published some information sheets on this and the occurrence is indeed very rare. In a way it proves that the K Glass is doing its job - keeping more of the heat in your room. As K Glass can be a considerable help in reducing the risk of condensation inside we feel this is a more important advantage that the very rare possibility of condensation on the outside. It would not deter us from using K Glass.

Note we said reducing condensation above - not eliminating - no product can be guaranteed to eliminate condensation as there are many other variables such as lack of ventilation which are responsible for condensation.

Question submitted by concerned

During a recent installation, I noticed that a patterned glass section sealed unit did not have K glass fitted (although it was specified at the time of order). I understood that the K coating was normally applied to the inside face of the inside pane. During discussions with the supplier, they stated that the K coating couldn't be applied to patterned glass (i.e. the inside pane). After further discussions, their suggestion is that the K coating will be applied to the inside face of the outside pane of (flat) glass, leaving the patterned glass as the inside pane (with the pattern on the inside of the sealed unit. Is the suggested change permissible? Is the K coating directional? Many thanks in advance for any advice.

This question answered by Tina Dunlop - The suggested change is permissible and is the best alternative as Pilkington K glass cannot be applied to textured (patterned glass).

It is correct that K Glass coating is normally applied to the inside pane of glass. However as its also normal for the pattern glass part of a sealed unit to be on the inside it will not be possible to have K glass fitted to the inside when patterned glass is specified.

We quote below from the Pilkington glass manual........

"In certain instances where a degree of privacy is required, Pilkington K glass may be incorporated as the outer pane of a Pilkington Insulating Unit, whilst incorporating Pilkington Texture Glass as the inner pane. The thermal insulation is the same regardless of whether Pilkington K Glass is positioned as the inner or outer pane. Any discernible difference in external appearance which may otherwise occur as a result of incorporating Pilkington K Glass as the outer pane will be nullified as a consequence of the Pilkington Texture Glass inner pane."

Question submitted by AOL user

I am considering purchasing a Four Seasons sunroom. You have stated that you recommend "Pilkington "K" glass for best results. Four Seasons uses MC wonderglass. In your opinion, do you think this type of glass is the quality of Pilkington "K" glass? I am planning to use this room year round. Can you also rate Four Seasons for their quality and workmanship?

This question is answered by the Conservatories Online editorial team - MC Wonderglass is a product unique to Four Seasons. In our opinion its an excellent type of glazing to use in your conservatory and we are confident you could use the conservatory all year round with this glazing installed.

Four Seasons products are of an excellent quality and standard. Be aware however that they retail their products via a local Franchise dealer. Ultimately your degree of satisfaction with the product will be dependent on the quality of installation offered by this dealer - so research the dealer and their business thoroughly before purchase.

Question submitted by Peter

Have you ever heard of a problem with Pilkington K glass units breaking when being installed in a conservatory roof or side frames? Our conservatory - 15 x 15 is nearing completion and four units have broken as being installed. The person responsible for the project tells me that the manufacturers of the glass units - Darby West of Bristol - have said that they have had a problem with the glue that they have been using but they denied this when I contacted them? They have said that maybe the units were slightly oversize so that are now making the replacements one slightly smaller all round? (This question edited).

This question is answered by the Conservatories Online editorial team - I'm familiar with Darby Glass - they are one of the UK's largest producers of toughened glass and a company of excellent reputation.

The problem you are experiencing is not unique to Pilkington K Glass units but can apply to all sealed units. One of the reasons that a lot of conservatory suppliers do not in particular favour glass roofs is that it can be more "problematical" to install. Although the glass is toughened it is in fact quite easy to break during installation just by putting too much pressure especially on the edges. Quite often I find that people assume toughened glass is some form of EXTRA STRONG glass - perhaps a little like "bullet proof glass". However in my opinion "break safely glass" would be a better description of toughened glass. Sure it is quite difficult to break - but not impossible. When it does break it will break into very small sections. These small sections may if you are unlucky give you some scratches or minor cuts but will not pose the danger created by large glass shreds when "normal" float glass breaks. Toughened glass is the most widely specified safety glass in conservatory construction.

Despite the potential installation problems and the fact that installers need to be even more "diligent" in their installation efforts I still favour glass roofs. Their looks/appearance makes up for the possible "problems" on installation especially as your supplier should take responsibility for breakage's etc.

I have known installers to go for many installations without any unit breakage's - then their "luck" runs out and they experience breakage's several times in a row. There is often no logical explanation. For what its worth I think its more likely that if there is a fault it lies somewhere on the installation side - it could be that the units where slightly oversize or perhaps hitting some obstruction on the rafters. I would not be too hard on the installers themselves - this may not be a common problem - but it does happen. I think you (and the installers) have just been unlucky. What matters are that your supplier remakes the units at no additional cost to you. I don't really think you can expect more than that.

Question submitted by Stephen

I have a conservatory supposedly made with Pilkington K glass. I am unsure about this. How can I tell?

This question answered by Tina Dunlop - The best way to tell is to use a coating detector. This is a unit which manufacturers and installers should have available. A detector costs about 60 and its quite easy to use. You simply press it against the glass with the "K" installed and from memory it either turns a red or green light on to confirm installation.

A detector can usually be bought from specialist glass merchants or glass wholesalers. Frankly it is rare to use a detector as the glass will usually arrive on site with Pilkington K Glass "stickers" on the sealed units. These stickers" also tell the installer which side of the glass unit should face inwards. Some installers will give you the "stickers" as proof. (This I recommend) I suggest they are kept safe and given to any new owners of your property as proof also.

Sometimes you can visually detect the K coating but this is more difficult. I quote below what Pilkington themselves have to say....

" Pilkington K Glass has high light transmission and appears virtually the same as clear float glass. However, in rare instances of strong oblique lighting, the coating may be seen as a transparent film. This is simply a transient visual effect which can be considered positive evidence of the coated surface being present.

Further evidence of the coating's presence is through the very minor effect it has on white light transmission. This effect is so small as to be generally unnoticeable However, when a light coloured object or material is in close proximity to the glazing, dependent on local circumstances and conditions, a slight darkening can be noted."


Question submitted by David

I have recently ordered an 8mx3m conservatory on a south-east facing site. However I am unsure whether I should be opting for a traditional tiled roof with velux windows or a 25mm polycarbonate conservatory roof. The pros for a tiled roof are that it would retain a higher percentage of it's initial cost if I was to sell my house in the future, removes glare and retains heat better than a 25mm polycarbonate roof. The cons are that there would not be the same amount of light as a traditional conservatory. The Conservatory Company is providing 2 No 3 kW heaters. My dilemma is that I would like to use the conservatory all year round could you tell me what would be the best option for all year use?

Also I have asked the company about Pilkington K glass but was told that the finish on the glass is sometimes indifferent i.e. cracks and ripples and it also costs about 25-30 per m2
This question answered by Tina Dunlop - I describe "conservatories" with solid roofs as sunlounges. They are not conservatories in the normal sense. They do however have the additional advantages you mention.

I think you can best decide what is good for you by first deciding what type of appearance you prefer. Do you like the idea of "living under glass" or is what you really want - an extension. Both alternatives can with careful planning be made usable all year round. As you might expect - I'm a "conservatory person". I would not normally make any comparison between a traditional brick built extension and a predominantly glass conservatory. The two types of buildings act / perform and feel quite different.

If "living under glass" and adding light to your home appeals then a "normal" conservatory is the only real alternative. Conservatories are in my opinion for people who like the feeling of the outdoors - indoors. If this is not really that important for you a "normal" extension will be more appropriate.

For a conservatory I would in the roof recommend the 25 mm polycarbonate alternative as a minimum. (The other alternative being a double glazed glass roof with Pilkington K Glass)

You mention "cracks and ripples" in Pilkington K Glass. This is not something I've ever come across. It is true however that Pilkington K Glass is very slightly "tinted". Pilkington mention this in their literature and claim (quite rightly in my opinion) that the effect is negligible.

With reference to the cost I think the point you may be making is - whether or not "K" glass is worth the extra cost? From a purely economic point of view it will be some time before the additional cost of Pilkington K Glass is "repaid" by savings in your heating bills. For me the economic savings are not the reason to invest in Pilkington K - rather it is the fact that you will be able to use your conservatory as a four seasons room - remaining comfortable without heating for longer.

I sometimes find companies/salespeople try to persuade clients not to invest in K Glass. Why they should do that - especially when all the evidence in favour of K Glass and its extra performance are so great - I do not know. Most top professional companies certainly offer this product and a few now do it as standard. I would be "suspicious" of any company who tried to persuade you against it. Perhaps they are worried that the additional cost of it may lose them a sale?


Question submitted by Mark

I am looking to use Pilkington K glass. I have however been warned that it will reduce the clarity of the glass and hence give a cloudy view out when the sun is at certain angles. Please can you give me some more detail on this. The Conservatory will face west.

This Question answered by Tina Dunlop - It is true that Pilkington K Glass is very slightly "tinted" Pilkington mention this in their literature and claim (quite rightly in my opinion) that the effect is negligible. In fact I would say very few people would notice it and those who do might be grateful to have such an effect in a conservatory.

It sounds to me as though someone is trying very hard to persuade you not to have K Glass. Personally I would way up the many benefits of Pilkington K Glass (extra insulation / more likely to be able to use room all year round) versus what I can only perceive of as a very minor possible disadvantage. I have K Glass installed in my own conservatory at home and have never experienced any cloudy effect.

Question submitted by Paul

I have a south-facing conservatory full of plants and it gets too hot. (50 c) I want a glass roof, and presume I will need blinds. Will anti sun/Pilkington K preclude the need for blinds, while NOT darkening it on dark days?

This Question answered by Tina Dunlop - Anti-sun and Pilkington K Glass are great products - but will not in my opinion give the "shading" effect you require for plants. You could consider putting Anti-sun glass in the side frames - but I would consider it best to have Blinds or some other method of shading in the roof - especially on the hottest days of summer. (You can have both Anti-sun and blinds together)

I would suggest you obtain a sample of Anti-sun glass from your supplier before you buy - at least then you can decide if its effect would be to "darken" too much on cloudy days. Speaking personally I would not say that Anti-sun darkens very much at all - basically its best for reducing glare.

Question submitted by John

Pilkington K Glass: Some suppliers say using "K" is in itself sufficient at a 6mm gap. Others say we should use 16mm. The former says the difference is negligible. Pilkington's technical web site shows the "U" value of 6mm gap "K" at c. 2.6 falling to c.1.7 for 16mm. Is this difference really worth it? Our conservatory will face NW.

This Question answered by Tina Dunlop - A 16 mm air gap is generally speaking the "optimum" air gap to use with a double glazed unit and "K" glass. The lower the "U" value - the better the insulation and a "U" value of 1.7 is considerably better than 2.6. I think the difference is worth it.

Question submitted by J Chard

My conservatory supplier says that if I have 25mm polycarbonate roofing instead of 16mm it will be noisier when it rains as being thicker it creates a drum like effect. do you agree? They also say I will not need Pilkington K glass. I see that a lot of suppliers say it is not needed, as it will be too hot in the summer. How do I know Pilkington are telling the truth when they say it won't be too hot?

This Question answered by Tina Dunlop - I have never heard of 25 mm polycarbonate being noisier than 16 mm polycarbonate. Polycarbonate is generally noisier than glass roofs - but if any thing I would have thought 25 mm would have been less noisy than 16 mm due to the extra sound insulation afforded by the thicker material. I really don't think there can be that much in it.

I do think you need to ask yourself why your supplier is so determined to sell you "solutions" which clearly are less efficient and of less practical benefit in the long term. I have answered questions on Pilkington K glass previously and all I will say here is that if you want an all year round conservatory then low E or K glass is virtually a must. If you compare the experiences of owners with normal double glazing and 16 mm polycarbonate with those who have K glass and either glass roofs or 25 mm polycarbonate installed then generally you will find that those with the higher specs., report the most satisfactory results. (Less condensation or damp with better heat retention in the winter months / evenings.) Owners / Users of conservatories are the real test. There are still a few companies who for whatever reason are reluctant to offer these higher options. Their motivation is not clear to me. I believe in consumer choice - you should at least be offered the extra options - if you don't take them up because of reason of cost or what-ever, then so be it.

One final thought - If it was true (which it is not) that K glass makes your conservatory excessively hot in the summer months - which would you prefer? A conservatory streaming in condensation during the winter, requiring constant heating which "disappears" out the window? Or perhaps an overly warm conservatory during the summer when you at least have the option of opening the doors or windows?

Question submitted by Lindsay

A conservatory company has given me a quotation for a conservatory. I was very pleased with everything about the company, except for one thing - they are insisting that I do not need Pilkington K glass plus Argon gas. This has always been my top requirement, from all the homework I have done on conservatories. My garden is north-facing, and only gets sun for six months of the year. Therefore I think Pilkington K is even more essential. The company insists that I will need to keep my conservatory cooler rather than hotter. Please advise.

This Question answered by Tina Dunlop - You are right to be concerned. Everybody knows conservatories can be very hot - but it is equally important to realise they can be cold and damp - especially if unheated or without good quality insulating glass such as "K" glass. I have known supplier's salespeople to suggest that fitting "K" glass will make the conservatory too hot in summer - this is not true as this glass works best when there is a substantial difference in temperatures between outside and inside - as would be the case in winter. Also ask yourself - who is more likely to be telling the truth Pilkington - with their vast resources or your conservatory supplier. Also why do so many "premium" conservatory companies promote "K" glass if it is no good?

Pilkington "K" Glass is a really worthwhile option, especially if you hope to use the conservatory all year round. This glass varies from normal Pilkington clear glass in that one side of the glass has a special metal coating, technically known as a low emissitivity, or Low E. coating. This microscopic metal coating (which is almost indistinguishable from normal clear glass) allows through shortwave radiation, like sunlight, but reflects longwave radiation, such as heat from fires and radiators, back into your conservatory making it easier to keep warm. As the Low E. coating on the glass actually makes the surface warmer to the touch, condensation is also reduced. I especially recommend this option if you decide to have a double-glazed roof. Argon filled units are nice to have also (even better insulation) but not as important as having "K" glass.

I would be very concerned about buying a conservatory from a supplier who does not feel "K" glass is worthwhile. Sure - it is more expensive - but to suggest its not worthwhile is an error in my opinion. Ask them to prove "technically" that they are correct and to guarantee that in the winter you will be able to use the conservatory any time you want - without fear of being cold. Ask for that in writing and see what happens. Also are you having a glass roof? If that is what you hope to have - make sure your potential supplier has experience with glass roofs - a lot only use polycarbonate in the roof and fitting glass in the roof is technically more difficult.

Click here for a brochure request Question submitted by Paul

What is the differences between North, South, East and West facing conservatories? What are the usual traits of conservatories with these aspects and what are the recommendations?

This Question answered by Tina Dunlop - You could write a whole book on this - Here however are my short answers.

North Facing - Coldest - least sun. Less likely to need blinds in roof - especially if you use tinted opal polycarbonate. Strongly recommend use of Pilkington K Glass in frames for extra insulation. Will need heating - especially in the Winter and evenings. Please note that with bronze tinted polycarbonate - the tint reduces the amount of light that can enter the room your conservatory is attached to. This results in a darker room - often requiring lights on during the day!

South Facing - Hot - Very hot - so essential to have adequate ventilation. Go for plenty of opening windows and maybe even roof vents. Will almost certainly require blinds in the roof. You may also consider anti-sun glass (tinted) in the roof. Other options include air conditioning.

East Facing - Similar to north facing. You have the morning sun and conservatories in this location make great Breakfast rooms.

West Facing - Similar to South Facing. However you do have the benefit of evening sun. Wonderful to relax in at the end of the day.

As a generalisation - I do recommend Pilkington "K" glass in all conservatories - no matter the location. It's just more important in North Facing Locations. Additionally if you use Polycarbonate - go with 25mm.

Question submitted by Eric

The representative from xxxx conservatory company tried to put me off Pilkington K glass - e.g. it will cost 500 more and it will make the room to hot! What do you say?

This Question answered by Tina Dunlop - Why do they do this? What is their agenda? - yes it will cost more - but quality nearly always does. If you want to use your conservatory all year round then "K" is virtually essential in my opinion. If you don't expect to use conservatory all year round then you don't need "K. If you get the PILKINGTON brochure on "K" - available from most glazing companies - you will see that Pilkington specifically mention the issue of being too hot in summer - basically they claim it will not do that.



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