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Double Glazing, Replacement Windows -10 Frequently Asked Questions

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Special Note: - Always take specialist advice - especially with regard to planning permission and building regulations.

Q 1. Some suppliers say that internal glazing is more secure since there is no beading on the outside. However other suppliers say that external beading is more secure because the fitter will push the glass against the beading from the inside, which effectively locks the beading onto place. One supplier says that it is possible to simply "kick" out internally fitted glass panels. They also say that they are more prone to leaking since during wind loading, the flex of the glass is being pressed against beading rather than a moulded recess. I have contacted my insurance company and they have no preference. What is the truth? Which is more secure?

There are "elements" of "truth" in both "arguments".

It is difficult to argue against internal beading being the more secure of the two glazing methods and on most occasions we would favour this option.

However it is also true that some externally beaded systems (especially when combined with a double-sided security tape) are just about as secure.

It is also true that an externally beaded window is less prone to "leaks" through the beads as there is effectively a PVCu upstand on the inside of the window which any water would have to rise above before showing on inside. However leaks of this type are very rare and only happen in the most extreme of conditions (for instance if you had a coastal property or a property on the top of a hill with little or no protection.) In any case no matter what method of glazing is used your supplier should be guaranteeing against leaks. (The PVCu has drainage channels installed at time of manufacture to cope with this).

With regards to the glass panels being easy to "knock out" - its more likely the glass would break prior to being "knocked out" of the retaining beads.

We suggest make your choice based on the calibre of the company and your belief that they will be around to honor their guarantees in the future. Most companies (including the not so great ones) offer perfectly good products with optional extras such as Pilkington K Glass and better security locking. Base the majority of your "decision" on this aspect of the buying equation.

Unfortunately it's often the case that the more you investigate a product and the more opinions you get - the more confused you will be. We appreciate we may not have given you the definitive answer you have expected - but hopefully this is some help.

Q 2. I am considering having double glazing installed in an old terraced property, which currently has sash box windows. Having received three quotes, I have also received three views on whether the whole sash box should be removed or the new units simply fitted within the existing sash box, leaving the internal wood surrounding area. One company said yes must be done for a good job, a second said no need to as the wood, probably the original, is OK and a third said doesn't matter, up to you, but fitting within the sash box will be cheaper. Is there a definitive view?

We are not sure that there is a "definitive view". It is true that both methods are used and there are many that consider leaving the original "box" in place OK. Complete replacement will be a fair amount more expensive and this is one of the reasons an "in between" option is often offered. We would always prefer complete replacement - but if cost is a consideration and the existing timber is "sound" then fitting within the existing box sash should be OK.

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Q 3. What is the best option - toughened glass or laminated glass? What is the difference? I have a child - am I right in saying that toughened glass is more dangerous if smashed? Is one more secure than the other?

Both Toughened and Laminated glasses are forms of "safety" glass. People often assume that toughened glass is some form of EXTRA STRONG glass - perhaps a little like "bullet proof glass". However in our 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 shards when "normal" float glass breaks. Toughened glass is the most widely specified safety glass in conservatory construction. Laminated glass will, when hit with force "crack". However it is unlikely to smash. Being very difficult to break means that it can be dangerous in any situation where it is likely you may need to break the glass in order to escape (such as a fire). This is of course an advantage if security is a major consideration. Laminated glass is also "thicker" - usually 6.4 mm - and as such will offer better insulation. However this thicker glass is also "heavier" which will in turn mean greater "wear and tear" on opening windows / doors. It is also more expensive than toughened glass. This is probably one of the reasons it's not so widely specified in conservatory construction. (Although in our opinion it is a perfectly good alternative for conservatory glazing.)

Q 4. Is there any difference between PVCu windows fitted in a conservatory and those fitted in a house? I have had different sales people giving different responses to this question; one suggested house windows are not as strong and should not be used in a conservatory. Another said they were identical. Can you help?

The "base" PVCu extrusion used in conservatory construction is often the same as that used in house windows. That said, most conservatory companies will add to the base spec with better quality reinforcing (say fully reinforced) or thicker framing.

For instance some conservatory companies will prefer to use the outer frame section usually associated with door frames for the window framing in their conservatories. (The thickness/width of outer frame in most windows is 50 - 55 mm while the thickness of the door framing is usually 70mm) A thicker frame - fully reinforced will give better structural strength. Also some companies will use more frames to assembly a conservatory than others - often joining these frames with special structural mullions. (By this we mean that some companies will manufacture each section/frame separately while others will make one larger "window" that could be divided into say three sections) Clearly these differences have considerable effects on the prices quoted and you should expect to pay more for a conservatory made up out of individual frames.

Please note that having the conservatory made out of a greater number of frames is more important with a glass roofed conservatory than say a polycarbonate roofed conservatory. (Because of the greater weight of a glass roof) While we like using a greater number of frames to construct a conservatory you should note that using more frames will also add to the "bulkiness" of your conservatory and this may not be to everybody's taste.

When attempting to compare "like with like" in quotations you receive for your conservatory you should bear in mind the above points in particular. Just because the drawings look similar and the all companies specify PVC-U with say a polycarnonate roof does not mean that the specifications are identical. For this reason we always advise people to be especially careful if they receive one price that is considerably lower than the rest of the quotes received.

Q 5. What is the difference between u-pvc and pvc-u, one supplier claims better aging, particularly colour, for pvc-u.

To answer the question very simply, there is no difference between u-pvc and pvc-u. Both terms refer to unplasticised (hard) PVC, which is used extensively in building products where rigidity is an important attribute.

Plasticised (soft) PVC on the other hand is used where flexibility is important, for example medical tubing applications, insulation sheathing on electrical wiring, etc. Pvc-u is more to do with commonality of terminology world-wide than it is to do with formulation. Any claims of better performance of pvc-u over u-pvc are therefore spurious.

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Q 6. I have a conservatory supposedly made with Pilkington K glass. I am unsure about this. How can I tell?

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 it's quite easy to use. You simply press it against the glass with the "K" installed and 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 we recommend). We 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. We 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." 

Q 7. Can you please tell what the law says regarding the replacement of windows as regards if toughened glass should be fitted. I have been told that as I am replacing windows I need not fit toughened glass even if they are of a size and area that requires toughened glass to be fitted if I was building a new structure.

You should find the information at useful:

In particular Diagram 1. The rules are quite clear on the use of safety glass - no matter if its "replacement" or "new build".

Remember toughened glass is just one form of safety glass.

Safety glass includes:

* Toughened Glass (also called tempered) categorised as Class A

* Laminated Glass available in Class A, B or C

* Wired Glass (also called Pyroshield safetyclear/textured) categorised as Class C

Certain internal and external areas are considered 'critical locations' in terms of the safety of vertical glazing, as they are at risk from accidental human impact. The critical locations defined by the standard are similar to the Approved Document N of the Building Regulations 1991.

The 'critical locations' in any internal or external domestic area are:

a) Doors - Any glazing or part of that glazing in a door, which is between the finished floor level and a height of 1500mm above the floor level, is in a 'critical location'.

b) Side Panels to Doors - Any glazing or part of that glazing, which is within 300mm of either side of a door edge and which is between the finished floor level and a height of 1500mm above the floor level, is in a 'critical location'.

c) Windows, partitions, and walls - Any glazing or part of that glazing, which is between the finished floor level and a height of 800mm above the floor level, is in a 'critical location'.

Q 8. Many firms are not very clear on what UPVC system they use but others claim to use "high quality" sections with names such as Kommerling, Rehau and WHS Halo. How do you judge the quality of a system? Which, in your opinion, are the leading contenders?

All of the companies you mention above are leading contenders - offering good quality PVCu extrusions. We suggest you examine the thickness of frame (depth from inside to outside). Generally speaking a 60-mm thickness is the lowest you should accept. Go for 65 mm and 70-mm thickness if available. Also enquire about the thickness of the PVCu walling in the extrusion. The extrusion is a multi-walled section (not solid) and wall thickness of 3 - 3.5 mm is normal.

The types of glass used will also have an effect on the overall quality and performance of the window. Pilkington K Glass (for better insulation) is worth considering.

The other issue to consider is security. There are many different types of lock etc on the market and most are not supplied by the extruder but by other hardware suppliers. Check with your supplier the security features of their window and make your final decision based on this. Remember its possible to have two suppliers supplying exactly the same extrusion - for instance the REHAU extrusion - but with entirely different locking features. To compare "like with like" is a lot more complicated than saying both suppliers use the same extrusion.

Looking for a new windows and doors? uPVC windows, aluminium windows, sash windows, patio doors, porches. All Shapes, All Sizes. To request your free brochure click here

Q 9. How do you see the advantages and disadvantages of PVCu and aluminium for replacement windows?

PVCu offers the following advantages...

* Good insulator
* Low maintenance
* Many suppliers / usually the cheapest option today
* Now available in wood grain and colored finishes.
If it has a disadvantage it maybe that it has relatively low structural
integrity. This is overcome with steel or aluminium reinforcement. Also it
can be susceptible especially in south facing situations to "expansion" in
sunlight. Again using "fully reinforced" frames can reduce this.

Aluminium offers the following advantages...

* Virtually no maintenance over its long lifetime
* Slim, strong sections that will not warp or twist
* About one third the expansion of PVC-u
Its main disadvantage is that it's a relatively poor insulator. (However
thermal break aluminium frames are available which offer better insulation)
It is also usually more expensive than PVCu

Q 10. Please tell me what the best PVC product is i.e. should it be reinforced with steel or aluminium? Any tips would be helpful.

In our opinion - there is really not much to choose between Galvanized Steel Reinforcement or Aluminium Reinforcement. Both do the job and leading companies uses both alternatives. What we do find is that "salespeople" will often make exaggerated claims one way or the other in an attempt to sell their product. Typically the arguments go as follows - Steel is stronger - but more likely to rust if the galvanization is removed and Aluminium is not quite so strong - but will not rust! (That's the simplified arguments!)

As an example of the "exaggeration" that can go on - we have know of companies to show photos of windows with rust coming from them! What is not explained is that when screws are inserted into galvanized steel is that a small amount of "swarf" is removed and with that the galvanization. It's possible for the swarf - which will always drop down to the bottom of the frame to "rust". The evidence of this quickly disappears as the swarf oxidises. This rusting is "not catching" and will not pass to the rest of the reinforcing.

Sorry if our answer is not "conclusive" - but we feel the more important consideration is the overall calibre of the company you chose and the quality/looks of the PVCu system - not whether Steel or Aluminium is used for reinforcement.

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