The truth about low sightline IGUs

The use of slim insulated glass units in refurbished box sash windows has boomed in recent years. However, buyers and specifiers should tread carefully when choosing non-compliant products, explains Fraser Nicol, managing director of The Original Glass Company, a division of Ely-based Holloseal Group.

The controversy surrounding the manufacture and sale of low sightline, slim insulated glass units for the heritage market is way out of proportion with the number of units sold. But, arguably, so is the potential damage to the IGU sector overall with non-complaint products being chosen for, installed, and failing in a number of important heritage buildings by unsuspecting specifiers and owners.

In recent years technological advances have enabled the production of high performance slim sealed insulated glass units for installation in the renovated windows of historic buildings. These enable owners of such properties to retain the appearance of their windows, while greatly reducing heat loss.

Indeed, most owners, advisors and contractors undertaking the renovation of their windows will be aware of the availability of such units. What they are unlikely to know, however, is that while these glass units are quite widely available, an estimated 98% of slim insulated glass units produced for this purpose do not comply with the required standard EN1279 Parts 1-6.

But what are the consequences of installing products that do not have this certification?

The key significance of using non-compliant slim IGUs is failure to comply with clearly defined laws that govern the quality of building products and their application. Slim or low sightline insulated glass units are covered by the harmonised European product standard EN 1279-5. In turn, the Construction Products Regulations (CPR) EU 305/2011 applies to the manufacture, import and distribution of these products, and a successful prosecution of CPR can result in serious fines and even custodial sentences for offenders.

Crucially of course, such units are prone to premature failure, manifesting itself not only in reduced thermal performance but also in the appearance of condensation within the unit which, given the usual complexity and sensitivity of such renovation projects, will be unacceptable.

In certain circumstances, safety and security may also be compromised by the use of non-compliant IGUs as the strength of the astragal bars – the sub-frame into which the units are glazed, – may be seriously compromised. Without appropriate installation the bars are severely weakened so that anyone stumbling against a non-complaint window could easily fall through, while intruders have been known to gain entry to a building simply by pushing against the astragal bars.

The reason why so many non-compliant units are flooding the market is that they are difficult to produce and few manufacturers have the capability of doing so, despite what they might claim. It is, however, a worthwhile and potentially profitable niche market, and the producers of what are effectively illegal units are prepared to run the risk of detection and conviction under Construction Products Regulations (CPR) in pursuit of profit.

Some companies that display a EN1279 compliance certificate, are using certificates relating to other products in their range. If the certificate does not show the cavity width specifically as 4mm, it is probably only for general IGUs, not slim units, which require special skills to produce. Such certificates do not comply and those suppliers are breaching the law by placing their product on the market.

Buyers should understand the key issues that are creating what has become contentious within the glass and glazing industry itself. A good place to begin is the Glass and Glazing Federation, which has issued a statement that may be found at

The key problem is that the depth of sightline, or sealed edge, of a slim insulated glass unit is critical to both its longevity and performance. This element means that, due to the limitations on the amount of desiccant, or drying agent, that may be used in the sealed edge, the inert gas pumped in to the unit during manufacture is liable to dissipate quickly when it is installed, and moisture will permeate into the unit. This in turn leads to a loss of performance and the unit will fog up, which is unacceptable.

The only EN1279 – and consequently CPR-compliant – low sightline IGU with a 4mm cavity available in the UK has an 8.5mm sightline and is only available with aluminium spacer bar. No 4mm cavity IGU with a sightline lower than this – eg, 5mm, 6mm, 7mm or 8mm – has passed the required tests, and is therefore non-compliant, and should neither be specified nor placed on the market.

Only units produced by manufacturers with the skills and experience to manufacture these units have been awarded the all-too-important EN1279 Parts 1-6. And fewer than a dozen of these companies exist in the UK.

What can be done? It is quite simple, in principle at least. Buyers and specifiers should not just accept either the word of the sales person or a cursory glance at their EN1279 certificate. If the certificate does not refer to a cavity width of 4mm then it is not relevant to the product you wish to buy. They should be shown the door.

Although there are just three producers of compliant units at the time of writing, more may become approved in due course. It is essential to choose certified, guaranteed products that will perform perfectly for years, thus preserving the crucial aesthetics of the heritage property in which they are installed. And, of course, the reputation of the bona fide IGU industry.