Double Glazing & UPVC Windows FAQ

When considering the purchase of double glazing and UPVC windows, it is always useful to know something about the various options available today. Some of the most frequently asked double glazing and UPVC window questions, together with our unbiaised answers, are as follows:

Q. 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 in place. Another supplier says that it is possible to simply 'knock' out internally fitted glass panels. It is also said 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?

A. 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 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 the inside. In fact, leaks of this type are very rare and only happen in the most extreme conditions such as in a coastal property or a house 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 profile has drainage channels installed during the manufacturing process to cope with this.

With regards to the glass panels being easy to 'knock out', we feel it's more likely the glass would break before it could be forced out of the retaining beads.

We suggest you make your choice based on the calibre of the company you choose and your belief that they will be around to honour their guarantees in the future. Most companies offer perfectly good products with optional extras such as Pilkington K Glass (see our page on 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 receive, the more confused you will be. We appreciate we may not have given you the definitive answer you might have expected, but hopefully this is of some help.

Q. 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 in place. One company said yes, it must be done for a good job, a second said there was no need as the wood, probably original, is OK and a third said it doesn't matter, but fitting within the sash box will be cheaper. Is there a definitive view?

A. We're 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 perfectly acceptable. 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 an important consideration and the existing timber is 'sound' then fitting within the existing box sash should be perfectly acceptable.

Q. 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?

A. Both toughened and laminated glass are forms of 'safety' glass. People often assume that toughened glass is some form of extra strong glass, perhaps something like 'bullet proof glass'. However, in our opinion, 'shatter safely glass' would be a better description of toughened glass. It is certainly quite difficult to break, but not impossible. When it does break, it will break into very small sections. These may, if you are unlucky, give you some scratches or minor cuts, but will not pose the danger created by large glass shards formed when ordinary float glass breaks. Toughened glass is preferred for use in domestic double glazing as laminated glass, when hit with force will crack, but is unlikely to smash. This makes it dangerous to use in any situation where it is likely you may need to break the glass in order to escape in the event of a fire.

The strength of laminated glass 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 also makes it heavier with a corresponding rise in 'wear and tear' when opening and closing windows and doors. It is also more expensive than toughened glass.

Q. What is the difference between UPVC and PVC-U? One supplier claims better aging, particularly of colour with PVC-U.

A. To answer the question very simply, there is no difference between UPVC 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 such as to protect electrical wiring. It is purely a question of terminology and any claims of better performance by PVC-U is entirely spurious.

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

A. UPVC offers the following advantages:

a) Good insulator
b) Low maintenance
c) Many suppliers, competition making it the cheapest option today
d) Now available in wood grain and coloured finishes

If it has a disadvantage, it may be that it has relatively low structural integrity. This is overcome with steel or aluminium reinforcement. In addition, it can be susceptible, especially in South facing situations, to 'expansion' in sunlight. Again, using 'fully reinforced' frames can remedy this problem.

Aluminium offers the following advantages:

a) Virtually no maintenance over its long lifetime
b) Slim, strong sections that will not warp or twist
c) About one third the expansion of UPVC

Its main disadvantage is that it is a relatively poor insulator. However, thermal break alumium frames are available, which offer better insulation.

Q. Please tell me if the best UPVC product should be reinforced with steel or aluminium.

A. In our opinion, there is not much to choose between galvanised steel reinforcement or aluminium reinforcement. Both do the job and leading companies use both alternatives. Steel is stronger, but more likely to rust if the galvanisation is compromised and aluminium is not quite so strong, but will not rust.