Secondary glazing in an exceptional school for exceptional children

Education Tue, May 4, 2021 11:30 AM

Set in the grounds of the imposing 600-year-old Chester Cathedral, are two Georgian buildings which have been carefully and sympathetically renovated to become ‘Abbey School for Exceptional Children’. Grade II* Listed and within the surrounding conservation area, Cheshire West and Chester Planning Authority, Chester Cathedral's Fabric Design Committee and Historic England had to be consulted and approve any changes that were to be made to the buildings.

Some of the most striking external features are the windows; fine examples including 12-pane Georgian box sashes, as well as a couple of tripartite 4-12-4 pane sashes. Most of the windows were original and single glazed, which provided little thermal insulation for the students who would be occupying the spaces and allowed external noise to enter the rooms.

Cardiff-based Architectural Consultants Barron Design were brought in to create the plan for the conversion due to their pedigree in devising exemplar buildings for people with challenging behaviours. Because of the Listing, many period features had to be retained and restored.

Airflow was one of the fundamental parts of the design, so a sustainable multiroom ventilation and heat recovery system (MVHR) along with an air source heat pump was installed to establish a comfortable environment. However, without making the original glazing more efficient, some of the thermal benefits of these new systems would be ineffectual. Up to 60% of internal heat can be lost through ill-fitting single glazed primary windows and will do little to prevent draughts. As secondary glazing is a fully independent, and tightly sealed internal window system, it will minimise draughts and stop heat loss, creating a more even internal temperature. Secondary glazing is generally accepted by heritage agencies for use in heritage buildings as it is a reversible adaptation; so, Barron Design contacted Fenestral, trusted partner of Selectaglaze, to discuss options, suitable treatments and install the units thereafter.

The specification of the secondary glazing included 6.8mm laminate K glass. The choice of glass thickness and type, and installation of the units with a cavity of well over 200mm from the primary glazing will result in a noise reduction of around 45dB. Lower U-values were achieved due to the low-E coating and the laminate interlayer provided protection against impact and met safety concerns for the pupils.

Altogether, 70 units were installed over three floors in the two buildings, in classrooms, sensory rooms, dining areas and accommodation. The arrangement of the secondary glazing units to meet the original sightlines of the primary glazing, saw seven different systems used, some of which had to be coupled together.

The operation of the historic shutters had to be retained, so this required the secondary glazing to be fitted to the room side of them with the best possible cavity. To facilitate this, a special rebated timber ground was used. At this reveal position there was no window board, so rather than enlarging the existing board it was decided to extend the line of the secondary glazing to the floor. This had the added advantage of encapsulating the below window wall area, thought to be an additional thermal weakness. Series 46 fixed lights were used below sill level transom coupled to either Series 60, where there are off set sashes, or Series 20 where the sashes are of equal size. For the very large windows, over 3m, Series 90’s were specified, with their larger balance capacity these heavy-duty sliders can easily handle the additional glass weight.

All units were fitted with locks to stop unauthorised operation by pupils; these included fitch catches for the Series 20 and 60, flush locks for the Series 45 and Series 90. U-channels were also incorporated in all the vertical sliding units to not only keep running tracks clean and clear but also to include another element of safety.

“We needed a special secondary glazing system that not only protected the existing historic windows but also improved acoustics and the thermal efficiency of the school, without compromising on their functionality and design. Fenestral worked very closely with the design team to ensure the selected products only served to enhance the build. We have been delighted with the service and quality of product provided but more importantly, so are the end-users.” Comments Barron Design

The Abbey School is of exceptional design, catering for exceptional children whose needs are complex and challenging. The sympathetic and creative conversion of these Georgian townhouses has created comfortable and resourceful areas for the staff and pupils alike. It is now a more sustainable environment, creating a sense of well-being and pride for all; set to deliver state of the art education for up to 75 children per year aged between 4 and 19.

Selectaglaze is a specialist designer, manufacturer, and installer of bespoke secondary glazing systems across the UK. Selectaglaze seeks to provide their customers with the best in class product and service to meet all challenges, which is achieved by a process of continual improvements. Selectaglaze has the widest range of secondary glazing units providing a vast range of solutions for projects.

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