From: Mark Norcliffe, Managing Director,
Cornwall Glass Manufacturing.

The GGF issued new guidance earlier this month [July] urging members not to supply low-sightline heritage IGUs, unless they are supported by test evidence.

It follows a growing number of incidence of failure – something which has clear potential to damage the reputations of not only those IGU manufacturers and window and door companies supplying them, but also the industry at large.

We have been concerned for some time that the limits of IGU performance are being  overstretched in the pursuit of low-sightline products. This is not through any deliberate design or ill-will but simply by pushing limits of design too far, with a consequential impact on through life performance.

Cornwall Glass Manufacturing, has been supplying heritage and low sightline units since 2011. We know first-hand the pressure that is placed on manufacturers to get sightlines down to a minimum, however, there is a point where in our opinion a glazing rebate upstand, simply becomes too low and too restrictive.

As an industry, we need to be able to meet demand from the heritage sector for low sightlines but this cannot be at the expense of performance.

The sightlines on our heritage IGU are 8mm including a 4mm spacer bar and sealant, delivering U-values as low as 1.2W/m2K.

It isn’t the lowest sightline IGU on the market. It does, however, and is proven to perform.

Of the more than 42,000 heritage IGUs we have supplied, we have recorded only a handful of failures. This is because we haven’t been prepared to push sealants beyond the limit of their performance.

If you’re only using a very minimal level of sealant it takes nothing to puncture it and lead to failure. 4mm/5mm is the same as our standard unit, which allows us to underwrite the performance of the window.

With an 8mm sightline you still get a very good match to a traditional period window - but critically, you get genuinely guaranteed levels of performance.

The GGF’s new guidance is a positive contribution to the debate. While the industry remains, however, under pressure to deliver ultra-low rebate windows, there will always be those tempted to supply them at a cost to not only their customers but the fenestration industry at large.