Holistic Conservation…

Another listed building application has been submitted, this time for renovation and improvement works to a grade 2 listed family home in the old town of Hastings. Increasing our experience of works to listed buildings is always a welcome pleasure but this project is particularly special, for the reasons set out below.

pencil sketch by john mccart architect of the street elevation of a grade two listed townhouse on the high street in hastings old town

Something that I have always found galling is the fact that listed buildings are exempt from having to meet the increasingly stringent thermal performance requirements of the building regulations, leading to the conservation of buildings that will continue to be difficult and expensive to heat. In extreme cases, the occupants are destined to an occupancy blighted by fuel poverty, on top of the cost-laden need to maintain period fabric.

The briefing for this project included the desire to improve the thermal performance of the building as a whole and led to us researching appropriate materials for improving thermal performance and the selection of materials such as natural wool-based insulation and secondary glazing products.

The image below is a sketch detail of the existing front facade and sash window reveal with new insulation shown in yellow and a secondary glazing product installed within the existing window reveal. The broken blue and red arrows highlight the main aims; to allow moisture, in the form of water vapour, to continue to move through the fabric (blue arrow) from inside to outside; and to limit the heat-loss through the window (red arrow). Single glazed sash windows, as found on many listed properties, typically provide a U value, being the measure of how well, or how badly, a building element performs, of 4.8 W/m2K. The minimum U value for replacement windows under the current building regulations is 1.6 W/m2K. The lower the number, the better that element is said to perform in a thermal sense. The secondary glazing product that we have sourced could provide a U value of 1.5 W/m2K, beating the current building regulations requirement by a healthy margin.

sketch detail by john mccart architect showing secondary glazing combined with traditional sash windows on a grade two listed building

In a similar vein, the U values of existing walls show just how poor the building will perform thermally. The sketch below shows the corner junction of the building, with solid masonry to the left (front wall) and clay tile clad timber frame to the top (side walls). The existing U value of the front wall is around 1.3 W/m2K and the side wall is around 1.46 W/m2K. New walls need to provide a U value of 0.28 W/m2K to meet the building regulations, a long way below that of the existing walls on this building. We have specified the removal of the existing plaster and lath lining to the inside face of the front wall and the insertion of a new plaster and lath lining, this time with natural wool insulation installed between the masonry and lath, shown in yellow, to achieve a U value of 0.55 W/m2K.

The insertion of natural wool insulation between the existing timber frame to the side elevation, also shown in yellow, will provide a U value of 0.3 W/m2K. Both new U values are not quite as low as the building regulations require for new walls but are a distinct improvement on the existing values. The broken blue arrows on the sketch detail illustrate the aim to maintain the flow of water vapour through the fabric, an absolute necessity to avoid moisture build-up within the fabric.

sketch detail by john mccart architect showing the insertion of wool based insulation into the external walls of a grade two listed building

The potential to improve the thermal performance of this listed building is an exciting one and the aim of this post is to highlight the approach we have taken to this particular project without getting too bogged-down in technical detail. If you have any queries on how to improve the thermal performance of your period building, please do get in touch.

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