Low sightline IGUs: Why some specifiers are on thin ice

For those responsible for the refurbishment of listed heritage buildings, a conflict often exists between the statutory commitment to restore the aesthetics and original structural qualities of a building and the desire to improve its performance while the opportunity presents itself.

This is especially true of box sash windows, often such a key element of the style and character of a building and for some time now, it has been possible to add double glazed glass units to refurbished original box sash frames, in the form of specially developed low sightline, thin insulated glass units. These overcome the apparent paradox of maintaining the aesthetics of the window and improving thermal performance.

The availability of these units is widely known; however, many of those being specified and supplied are sub-standard, non-compliant and prone to failure within a short period of time. Ultimately, their installation may actually be illegal, in contravention of the Building Regulations and prosecutable under the Construction Products Regulations (CPR).

At the root of the problem

Thin insulated glass units (IGUs) have been developed for installation in refurbished original box sash windows. Such windows feature thin glazing bars or ‘astragals’ of between 16 and 27mm that were originally designed to take a single pane of 3mm glass and which therefore, do not have sufficient cover to accept a normal compliant IGU.

The lowest compliant sightline (edge of glass to top of spacer-bar) available within an IGU is 8.5mm. However, some firms reduce the sightlines to as low as 5mm and trim the feather/glazing platform within the astragal to the thickness of a match, in attempts to ‘make fit’. These units are prone to failure, some even before they have been installed.

With regards to edge clearance (the distance between the edge of the glass unit and glazing platform) in accordance with Glass and Glazing Federation (GGF) datasheet 4.2, clause 2.4.3, whilst the recommended minimum is 5mm, this may be reduced to a minimum of 3mm for low sightline IGUs.

Given the spacer bar sightline is 8.5mm, the total tight sightline, level with the frame rebate upstand is 11.5mm. Glazing bar feather width can be reduced from a recommended 8mm to a minimum of 6mm to still enable glazing sprigs to have some purchase into the timber. If a minimum of 2mm edge cover either side is provided for to ensure the sealants remain protected, see clauses 2.4.4 and 2.4.5, the minimum astragal width that can successfully accept compliant low sightline IGUs is 33mm.


Attempting to adapt an original window to accept double glazed units can be dangerous. If a frame designed to be glazed with single 3mm glass has the load on it effectively trebled by fitting 2 x 4mm IGUs, the window will be structurally unsound and give way under minimal pressure. If someone stumbled against one of these windows a few storeys high, the consequences could be horrific. In addition, such a fragile construction is highly unlikely to offer any realistic form of fire protection. This has been demonstrated by mechanical stress testing.


Windows with such a flimsy structure have no inherent robustness or structural integrity and burglars gain entry to peoples’ homes by simply applying pressure where the astragals cross and the window collapses.

Perimeter heat loss

Regardless of the claimed U-values for individual IGUs within multipane sashes, the psi value (W/mK) of the whole window frame will be severely compromised in comparison to a single IGU per sash.

The Law

Low sightline units are covered by a harmonised European product standard EN 1279-5 and so the Construction Products Regulations (CPR) EU 305/2011 apply to their manufacture, import and distribution. These regulations set requirements for placing construction products onto the market.

Monitoring, compliance and enforcement duties fall to Trading Standards bodies in England, Scotland and Wales and District Councils in Northern Ireland. A successful prosecution for a breach of the Construction Product Regulations can result in a three-month custodial sentence and/or a fine not exceeding level 5 on the Criminal Justice Act scale.

Glass and Glazing Federation

The GGF has made it clear that all IGUs placed on the market in the UK must have achieved passes to EN 1279 parts 2 and 3 and have an EN 1279-4 component manufacturer confirmation. Any manufacturer producing these reduced sightline IGUs must meet the requirements of the CPR in Great Britain and Northern Ireland. GGF publication 70.4 (https://www.ggf.org.uk/publications/industry-guidance/insulating-glass-units-igus-conforming-construction-products-regulation-cpr/) gives unequivocal guidance on compliance and clearly states that derogations cannot be claimed under article 5. The GGF has already reported offenders to Trading Standards.

Fully compliant low sightline thin IGUs have also been developed for refurbished original windows, which will provide long term performance whilst also maintaining the aesthetics demanded by planners.

Historic England and Historic Environment Scotland

Both Historic England and Historic Environment Scotland have published guidance. Historic England’s guidance document Traditional Windows: their care, repair and upgrading, confirms the need to comply with Construction Products Regulations, on page 60. https://www.historicengland.org.uk/images-books/publications/traditional-windows-care-repair-upgrading/

Within the guidance document, Managing Change in the Historic Environment – Windows, Historic Environment Scotland makes a clear statement on page 21.  https://pub-prod-sdk.azurewebsites.net/api/file/8d219a4e-69a0-451f-9082-a87900d2ad89

Certification bodies

Censolutions, the industry specific certification body, has publicly raised concerns on numerous occasions over the illegal sale of low sightline IGUs. In its press release: Calling time on non-compliance, the organisation advises: ‘Last year a glass supplier was jailed for passing off standard laminated glass as fire rated glass, which was reported by the BBC. This same practice is occurring daily in the UK, with illegal heritage units being placed on the market by unscrupulous suppliers. These are then specified by unsuspecting or unaware specifiers, so they continue to be produced. Some of the offending companies are also guilty of misrepresentation of certification by providing test evidence for completely different products.’

Caveat emptor

Specifiers should be certain that suppliers of low sightline thin insulated glass units are offering products that are specifically compliant with EN1279 and that this applies specifically to the units being offered. These have a 4mm cavity with an 8.5mm sightline and are only available with aluminium spacer bar. No 4mm cavity IGU with a sightline lower than this, eg, 5, 6, 7 or 8mm, will have passed all the required tests and is therefore non-compliant and should neither be specified nor placed on the market.

The consequences of taking such products are premature failure at best, through to catastrophic failure of the window and even prosecution for those responsible.



Alex Gray is managing director of Saveheat Group and also serves on a number of formal glazing industry bodies and committees.