Why would I need a cill extension for secondary glazing?

Cill extensions for secondary glazing installations


Secondary Glazing in Practice


fixing methods

Attention to design detail and gaining the maximum benefits from secondary glazing is paramount when determining the best window treatment. After establishing the client’s requirements and desired outcomes, configurations are designed, and the most efficient method of installation decided. This ensures the units blend in seamlessly, as if they had always been there and not an afterthought, and of course, guarantees the client’s needs are met.

What is a cill extension?

It is exactly what it says on the tin! Selectaglaze manufactures a timber window cill to either replace or add to the existing cill to enable the fixing of the secondary glazing. Very narrow cills, sometimes referred to as ‘nib cills’ can be as short as 25mm, leaving little to no space for the secondary glazing unit to be installed. It arrives fully fabricated to site, with mouldings such as a square edge or bullnose finish to match the detailing of the window surrounds and painted in the same colour as the secondary or bespoke at the request of the client. 

Why would I need a cill extension? 

In general, secondary glazing is fitted within the reveal of an existing window, which has no effect on the window cill. Secondary glazing installed in deep window reveals Secondary glazing fixed within a deep reveal, bearing no impact on the cill Depending on the performance requirement and design configuration, the optimum desired cavity is often 100mm and can be as much as 200mm. On top of this, the size of the secondary glazing profile needs to be taken into consideration. However, in some instances, the depth of the window cill or window board is less than the reveals, so would need to be extended to ensure performance is not compromised. Some window openings have no reveal and in this instance, a face fix sub frame needs to be introduced to accent the secondary glazing unit, but that does not mean a feature window cill cannot be achieved. Original windows with no cill and shallow reveals which need secondary glazing Images showing windows with narrow cills which are to have Selectaglaze secondary glazing

Our secondary glazing approach 

Where an extended cill is required, the survey size of the unit will be adjusted to accommodate it. The cill size will be set accordingly projecting forward of the secondary glazing to look just right.  The timber shop produces these ready finished for installation on site, to fix with the secondary units. Sometimes clients carry out refurbishment works and decorating to the original windows, having the existing cill removed completely prior to the installation. If they are in good condition and left in place, then Selectaglaze will adapt and work with what is already there. Installed secondary glazing with new cill extensions Completed installations of secondary glazing with a cill extension As seen in the images, you would never have known that the cill had been adapted or even replaced, due to the sensitive finishing of the timber and careful installation. The aim is to satisfy clients and make our secondary glazing as unobtrusive as possible yet providing maximum benefit – all of which were met in the projects above.