Window On Workmanship
31 March 2017
All over the country, windows are being installed incorrectly, causing problems for suppliers, housebuilders and homeowners alike. Chris Coxon, Head of Marketing at Eurocell explains why and offers a four-step solution for tackling this problem.
If you were to walk around twenty housing sites right now, how many of them would have windows which were installed incorrectly? Well, incredibly, perhaps all of them. A survey of 21 housebuilding projects of various sizes and run by various companies across the UK found windows installed incorrectly on all-but-one of the sites. Problems ranged from incorrect positioning of the windows in relation to the cavity to missing insulation and cavity closers.
Data the NHBC provides further evidence of poor installation. Issues with doors and windows are the most commonly reported superstructure claims, with sealing-related problems one of the most frequently occurring.
The results of these errors can be obvious, in time: damp walls where condensation forms on cold patches, mould growth and even drafts. But in some cases, householders may never know that they are paying far more to heat their home than they could be. According to the BRE, poor workmanship can lead to an increase in U-values – or heat conductivity – of up to 310%.
At the heart of the problem is a lack of understanding about how heat is lost through the fabric of a building. Those designing, installing and overseeing the installation of windows and doors need to better appreciate why the details are so important and what those details should be.
Some major housebuilders are already taking steps to tackle this problem. We know of one national builder who, having just discovered the widespread problems with window installation, is revisiting the way they specify and procure windows to achieve a more uniform and policeable approach.
Around five years ago, talk began of the ‘performance gap’ in relation to energy efficiency. Buildings that had been designed to be high on insulation and low on fuel consumption were not performing as predicted once in use.
Hence the survey referenced above, which was carried out in 2013 and published in 2014, by the now-defunct Zero Carbon Hub. The Hub was created to help all new homes to reach some definition of carbon neutrality by 2016, a mission that would later be abandoned.
The changing political landscape means that the Zero Carbon Hub is no longer in business, closed in March last year. However, the installation issue is still a current one: More Homes, Fewer Complaints, published by an all-parliamentary working group last year highlighted the problem of excessive defects in new-build homes.
Outside Scotland and south west England, where frames are mostly fitted into a check reveal, the most common error made when installing windows is to place them too far forward. The window frame should be sitting so that it partially overlaps the cavity between the skins of the wall, creating a continuous higher U-value barrier to heat loss.
In new build environments, the instinctive place to install a window seems to be so that the back of the frame is flush with the inside face of the outer brickwork. This also happens in replacement installations too. Yet this positioning can lead to the creation of the dreaded thermal bridge.
Omission of a cavity closer around the window location is another frequent cause of poorer performance. And not all cavity closers are created equal; choosing one with a PU or PIR core gives higher levels of insulation.
According to the NHBC, some installers are still failing to seal the gap between window and opening. As well as creating a pathway for the escape of heat, poor sealing can also lead to noise problems.
Good quality trims, silicants and sealants are vital but so too is attention to detail when they are being installed. Some specifications are calling for an additional EPDM fitted to the exterior of the window, lapping onto the cladding of the building; pre-treatment of any surfaces and using the right solvent is important in this situation.
Though it is not the domain of the window installer, another error is caused by the treatment of the jambs. Insulated plasterboard on the reveal rather than standard plasterboard significantly reduces heat loss.
Design on the hoof
As with jambs, problems with window installations are not solely the domain of the installer. The Zero Carbon Hub found that there was a disconnect between those carrying out the conceptual design, those carrying out the detailed or working designs and those who were installing the windows and doors.
The result of this disconnect can be details that don’t make sense resulting in RFIs (request for information) being sent to the designer, or simply creating a detail there and then on site. Time pressure is always the strongest force – waiting for an answer can be considered too costly.
The spread of BIM (building information modelling) should help address this issue: we know from the increasing number of downloads from our BIM Centre that its use is on the rise. Creating a 3D model before the physical one actually flags up interface and detail issues and may encourage earlier involvement of suppliers in the design process, avoiding those ‘by others’ notes so frequently found on architect’s drawings.
Sometimes the problem is even more basic: drawings are not being used on site at all either because they don’t exist or haven’t reached the window installer. The installers and supervisors use their past experience and instinct, yet this may lead to the thermal bridging issues already mentioned.
When there are problems with the way windows are installed it’s easy to blame poor workmanship and bemoan skills issues among the workforce. However, the reasons are more complex than that. We need to improve knowledge and competency in all the roles which impact on window selection and installation – see the four-step plan below.
Four-step solution to better window installation
Everybody involved the design and construction process should understand the importance of getting the details of window and door installation right – and the implications of getting it wrong. The NHBC produces excellent technical guidance; suppliers should also educate on issues such as energy performance, noise attenuation and product interfaces.
Workmanship, details and any potential problems such as thermal bridging should be an item on the agenda at pre-tender award meetings. The main contractor should ensure that the installer is competent and vice versa; both should check that drawings are adequate and correctly detailed.
Site managers and other supervisors need to keep a close watch on installations, especially the positioning of the frame. Larger sites could consider appointing a ‘thermal bridging champion’. Using a product such as Eurocell’s Cavalok Cavity Closer Location Sticker which marks where the frame should be positioned simplifies the inspection and assurance process.
Some housebuilders are already looking to standardise the windows and doors they use to eradicate the problems of poor detailing and confusion during installation and inspection. Wider use of BIM may encourage closer collaboration between developers and a reduced number of suppliers.