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The new W4 range of steel window is ideal for fitting in stone mullions, since they easily replaces direct glazing.

Available from members of the Steel Window Association, the W4 has been designed as a refinement of the W30, a steel window that can meet Part L of the Building Regulations.

SWA product development group member Trevor Woskett was one of the team behind the W4. “We needed an equal leg section because in the market that we are in which quite often involves stone mullions, there aren't always rebates,” he explained.

“Sometimes where they have perhaps been direct glazed, they're just straight through. The equal leg section which we've added to the suite enables us to put the window straight in there without any infill bars behind, or anything like that.

“Quite often it's just a slot that's direct glazed into it so that we can now cut out the old glass and introduce a very slim line steel section that will hold a double glazed unit.”

W4 is an equal leg system and as a result W4 always has a 12mm gap behind it and this normally fits into a rebate in timber or in stone.

“The sight lines for this window are even smaller than the previous double glazed window. The slim sections of the steel window are one of the big positives and architects and householders love it. The bead system is different and enables us to do mitred corners on the bead as opposed to a butt joint.”

The W4 is primarily designed for the domestic replacement market but it can be used for commercial windows.

“Steel windows can get under your skin.” says Trevor Woskett. “I've been involved with them for nearly 30 years and naturally I like the product. Every time we do something that enables us to comply to the next Part L regs I think that's good news.”

SWA's W30 windows are highly energy efficient (with a remarkable 1.7 W/m²K u-value) and achieve a Grade C on the BFRC window rating scheme.

The W30 style exceeds the Building Regulation standards for domestic dwellings. The design is all about reducing the amount of material facing the elements but it also has warm edge spacers, glass that boosts solar gain as well as soft coat and low-e, with Krypton gas between the two layers.