Details

The major obstacle to specifying a drained and ventilated timber double glazing system - cost - has just been removed. Open Rugby Joinery’s new catalogue and one highlight is a new range of timber window called Stormsure, an updated version of B&P’s High Performance Window, aka Sovereign 22. The biggest difference between the new range and previous versions is not trumpeted in the features bullets at all, but is buried in the pricing information - the premium which had been more than 90% has been reduced to between 15% and 20%.

Translated into whole house costs, this should reduce the cost penalty for specifying a high performance timber window on a detached 12 window house from about £600 to no more than £120.

The driver behind this change is competition from PVC windows. Whereas the likes of Boulton & Paul were in the past intent to charge premium prices for what was regarded as a premium product, the performance specification of the old standard windows has been left way behind by today’s typical PVC product, much of which has 22 mm thick glazing units, shoot bolts and BS7950 security built-in as standard.

The most obvious difference between Rugby’s standard timber windows and the Stormsure is found in the glazing detailing. The old standard allowed for a maximum unit depth of just 14 mm (in a 4-6-4 configuration); it is now accepted that 20 mm or more is a much better depth - the 12 mm air gap is more energy efficient, absorbs sound better and allows the cavity to be filled with argon.

What has also come to light over the past few years is that the size of rebate on the standard timber window has been barely adequate to take double glazed sealed units of any description and that much of the recent spate of sealed unit failures have occurred in this type of window. The 18 mm upstand is insufficient to take enough packers around the glazed units and the external beading is often forced too tight against the glass in order not to project over the front of the bottom rail.

The Stormsure range addresses these issues by providing a significantly larger rail with an 18 mm upstand and a fully drained and vented bottom rail. Also for the first time, Rugby are supplying glazing kits with spacers, dri-glaze tape and pins so that housebuilders choosing to fit and glaze timber windows on site should be able, at last, to specify cheaply and simply without fear of defect.

Both Zurich and the NHBC recently upgraded their standards for on-site glazing in order to offer guarantees against double glazing failure. The NHBC’s new guidelines, set to become a standard in January 2000, notes that “in external situations the bottom bead should project slightly over the rebate edge.”

With a drained and vented design, it is impossible not to do this. Paul Davies, Zurich’s surveying services manager says it all: “The drained and ventilated glazing system is the ideal because you are not dependent on standards of workmanship or on the use of the correct materials.”