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Housebuilders are throwing money away. If a recent survey of defects in new homes by insurance company Zurich is anything to go by, they must be parting with a fortune just to tackle problems with window defects. The survey showed that windows needed attention in a massive 43% of the cases where remedial work had to be undertaken on new homes.

This begs the question: with so many houses being built around the country, all with similar types of windows and doors, why is there such a problem getting them right? And who is to blame for this sorry state of affairs?

The truth is that an awful lot of people are to blame: designers, manufacturers, installers, even bricklayers. But it is developers themselves that are to blame for the most fundamental problem with windows – the lack of an appropriate specification. "People still fail to recognise that climate change exists," says Jay Webb, a Midlands-based specialist fenestration surveyor. According to Webb, the first step developers should take is to upgrade the exposure rating of their window specification by "one notch". He says that even in London, people fail to recognise that where a building is above five storeys high, or is located on high ground, then moderate to severe exposure ratings – as given in British Standard 6375 – can apply. "Unless the windows are specified correctly, they will suck water into a house under windy conditions," he warns.

Water is the prime culprit in most window failures. "Water is a window's biggest enemy.

It attacks the seals of double-glazed units so that their lifespan can be measured in months rather than years," says Rod Appleyard of West Yorkshire-based specialist fenestration surveyors Verification Associates. Once water has penetrated the edge seal of a double-glazed unit, it will enter the air space between the glass panes. Moisture will then form inside the unit so that misting occurs on the inner surfaces of the glass, which will give the impression, Appleyard says, that it is "permanently raining".

Almost all domestic double-glazed units are constructed in the same way. Two panes of glass are held apart by a spacer bar fitted around the perimeter of the glass. Separating the panes creates an insulating air gap between them to limit the heat loss through the glazed area. In most units, this spacer bar will also contain a desiccant to absorb any water vapour within the enclosed air space. An edge seal is then fitted to all four of the unit's sides to seal the unit, stopping further water vapour from entering (see graphic).

Preventing water contact with this edge seal is the key to avoiding a premature failure of the glazing unit. But poor window frame design and poor workmanship both on and off site means that edge seal failure is one of the most frequent defects with window installations.

Timber is still the most common material used to make window frames in the UK. According to Appleyard, it is installed in more than half of all homes. Two basic timber profiles are commonly used: a flat glazing platform (sometimes known as a fully bedded system) and a slope-drained glazing platform – diagrams X and Y highlight the difference between the two. Failures of both types of system are common, especially for homes where glazing has been fitted into the frame on site rather than in a factory.

The most common fault with timber windows is that the setting blocks, the bits of plastic that separate the double-glazed unit from the timber frame, are often omitted. "This results in the glazing unit sitting directly on the glazing platform," explains Appleyard. "If water then enters the frame, it will be in direct contact with the glazing edge seal, which could reduce the glazing unit's lifespan from 10 years to just 10 months."

Appleyard believes that the absence of this critical component could be caused by a number of factors, ranging from simple human error in forgetting to install the setting blocks in the first place, to penny-pinching by glazing installers.

The omission could also be put down to glaziers incorrectly measuring the window frame so that the glazing units are supplied too large to fit into the window aperture with the setting blocks in place.

But problems with packing pieces are not confined to site-glazed products; factory-glazed products have their problems, too. Appleyard says that, generally, factory-glazed timber windows are "wonderful animals" but can still suffer from "the human element". As with the site-glazed units, the biggest problem is the absence of setting blocks. But with factory-glazed units, the absence of the blocks is often not apparent until some months after installation. The reason for this is that with factory-glazed units, the manufacturers often use a thick, load-bearing tape instead of the setting blocks to distance the glazing from the frame and hold the glazing unit in position. After a while, in the absence of setting blocks, the tape starts to stretch under the weight of the glazed unit. This causes the unit to slip down inside the frame's glazing rebate, often damaging the weather seal and allowing water to collect in the rebate close to the edge seal.

Water is not the only enemy of the edge seal. Sunlight can also cause serious damage. A common problem with timber frames is that they are constructed from sections of timber that are too small. Webb says: "There are over 400 joiners still making timber window frames in the UK, and many of these frames are slim, timber-saving designs based on a post-Second World War specification." This can lead to a number of problems: the frames are not big enough to house today's thermally efficient glazing systems, so a smaller glazing bead has to be used, or the rebate in which the glazing unit sits is too shallow. Both of these defects will allow sunlight to penetrate the edges of the glazed unit and accelerate the failure of the edge seal.

But even before timber frames have been installed, poor storage on site can create the potential problems. Often, timber frames are delivered to site weeks or even months before they are due to be installed. "This is because transport costs are reduced by taking the frames in one large delivery, rather than as the frames are required," explains Appleyard. Once the frames are delivered, it is essential to protect them from the weather, but according to Appleyard, this is rarely done. "If left out in the rain, the wooden frames will become wet and swollen, which could lead to the frames warping," he says.

The problem of site storage is often exacerbated by poorly prepared timber. "Frames delivered to site often only have a splashing of primer applied," says Appleyard, "because some firms maximise profits at the expense of quality." Once these frames are installed, their high moisture content means that the wood is often saturated and cannot be decorated properly to seal and protect the timber, leading to early deterioration. And for all timber frames supplied to site unfinished, the quality of the site-applied paint finish will be down to quality control on each site.

Poor quality glue used to assemble the frame's timber components is another problem. Paul Wornell, head of the technical audit unit at Housing Association Property Mutual, says: "Ten years ago, a lack of timber preservation was a problem, but now many units are failing because their joints are opening."

But it is not all bad news: Webb says that good quality, timber-framed windows are available under the British Woodworking Federation Timber Window Accreditation Scheme. Windows produced under this scheme can be supplied fully factory-finished, and include a 30-year guarantee against rot.

In any case, it is not just timber windows that are creating problems for housebuilders; PVCu has its problems too. Unlike most timber windows, PVCu frames are supplied to site with the glazing unit installed. Webb says there are three grades of plastic used in the manufacture of windows, produced by 67 different plastic extrusion companies. He classifies them as: totally inadequate, mediocre and OK-to-excellent. "The largest number of windows fall into the mediocre category, the next largest group is the totally inadequate and by far the smallest group falls within the OK-to-excellent category," he says.

The problem with using a "totally inadequate" thin grade plastic section is that the window frame will flex more in high winds. "The more the frame flexes, the more it will allow water on to the glazing platform," Webb explains. To shed the water, PVCu manufacturers cut drainage slots into the frame. But both Webb and Appleyard agree that these slots are often not big enough, or in some cases are blocked by poorly located setting blocks so water collects in the frame, damaging the glazing unit's edge seal.

To add rigidity and provide a fixing point for door hardware to PVCu sections, the British Plastics Federation issues recommendations calling for aluminium reinforcing sections to be inserted inside some extrusions, depending on their application. But, as Appleyard says, the window manufacturer often "might not install the reinforcing". His solution for developers is to stipulate the use of reinforcing and then test for its presence by drilling into the frame.

Many of the problems with plastic windows are unique to the UK. "In the UK, we've adapted plastic windows to the British style of thin window frames," says Webb. He claims this makes the frame sections "difficult to reinforce" and so they will flex more in high winds. Webb contrasts frames used in the UK with those installed in Germany: "In Germany, high-performance plastic windows have been in use for the past 40 years without any problems because they are made from thick plastic frame sections," he explains.

The installation of reinforcement in PVCu is particularly important where plastic frames are used to construct a projecting bay window with a roofed area above. "PVCu windows are not structural," says Appleyard. He explains that it is essential that the weight of the roof must be transferred to the masonry below the window using galvanised steel or aluminium "bay poles". These bay poles must be attached to the structure at the top and bottom of a window. Often, he says, these poles simply rest on the PVCu window sill, which, over time, will compress and cause the roof above to sag.

A recent development in the PVCu market is the introduction of plastic profiles to form a window opening in a masonry wall, rather than construct a timber former – but this introduces another set of problems. The bricklayer uses a profile to construct a correctly-sized opening in a wall into which a window can be fitted. Once the profile is installed, the PVCu window is simply clipped into this ready-made opening. But, Appleyard says, bricklayers often distort the opening formed by the profile so that the window installers have to knock the frame into place using a mallet. "This compresses the frame and stresses the window unit, and could result in the frame cracking under forces created by thermal expansion".

Despite all these problems, Webb is optimistic that future surveys by Zurich will show a drop in complaints about windows. "Some contractors have realised that it is actually more cost-effective to spend the money up front on a quality window unit, than pay for a series of return visits to rectify problems created by using poor quality windows," he insists.

So how can developers ensure that their windows are the correct specification? Unsurprisingly perhaps, given their occupation as fenestration specialists, both Appleyard and Webb agree the best way to avoid future problems with any window installation is to employ a specialist to draw up an appropriate specification in the first place. Once work starts on site, they say, it is essential that the specialist is employed to inspect the works, and keep them trouble-free.