The ‘curve’ appeal – fitting the new Series 25 vertical sliding curved on plan unit

Selectaglaze secondary glazing systems have been designed with a flange, also known as an odd-leg, which aids in the seamless installation into reveals which have been squared off with bespoke timber grounds. This works perfectly when the window being treated is flat, but how can it be installed when a unit has been curved on plan? When you curve the aluminium extrusion with the flange, it makes the back of the window larger than the front. This makes the standard installation impossible if a timber ground is fitted into the reveal first, as it will reduce the clearance for the back of the frame to slot into place for fixing. Standard detail VS curved on plan - Series 25 In a recent treatment we were required to add secondary glazing to a Georgian curved on plan traditional sliding box sash window. Facing onto a busy main road the window not only did little by way of thermal insulation, but also let the distracting noise of passing tourists and traffic enter.  With a few ways of treating a window of this style; Selectaglaze presented the options:

• The secondary glazing could be faceted across the window, which would provide an upgrade in the thermal performance and noise insulation, but not follow the exact lines of the original window

Faceted - bay window - secondary glazing

• Use a curved on plan inward opening casement or fixed light unit, which would follow the lines of the window but if a casement; would impede on some of the space in the office as and when opened for ventilation

Curved on plan hinged casement needs room for operation

• To install the newly developed Series 25 vertical sliding curved on plan unit, which would mimic the operation and sightlines of the primary window, like for like

Curved on plan secondary glazing All options would allow full access to the primary window for maintenance, cleaning and ventilation; however due to the Listed status of the building and desire to create no distraction from this fine early example of a curved on plan opening, it was decided that the best option would be the new Series 25 system. The installation was going to present a huge challenge; given the age of the building and its movement over the years, the window reveal was no longer plumb or true, as well as the uneven flange due to the curvature of the unit. Therefore, a specialist timber ground solution would need to be developed, to ensure the tight fit of the unit to achieve the desired benefits. The radii and frame chord were worked out, laser measurements of the surrounds for exact dimensions and templates of the tapering; so that instructions for the timer shop could be produced. In a straight forward installation, timber grounds are screwed straight into the reveal and the secondary frame fixed into them. However with a curved on plan unit, this would not work. Therefore the timber was designed to be fixed in stages with some elements screwed to the frame to slot into the jigsaw puzzle. Step one: To start with, the cill needed to be built-out to accept the curved frame, due to the differential size front to back. Step 2: Some shaped timbers were produced to be screwed directly to the reveal jamb Shaped reveal jamb timber Step 3: Shaped timbers were produced which arrived fixed to the frame jambs from the factory Shaped timbers for frame jamb Step 4: Cover beads were produced for the small gap that was created when the frame was fixed into place. (step 2+ step 3) Cover beads The completed jigsaw: All elements of the frame Despite the challenges posed, the resulting insulation looks flawless. The clients are extremely happy and the sashes slide effortlessly and gracefully in the frame. Curved on plan secondary glazing installation