Kilowatt House, North Road, Bathwick
Kilowatt House is a Grade II house built in 1937-1938 for the acoustic engineer Anthony Greenhill and designed by Bath architect Mollie Taylor. An unusual example of Modern Movement architecture in Bath, it is valued for its experimental use of shape and design in which its extensive use of glass is a contributing factor to its unusual façade.
In principle, BPT does not object to the sensitive and appropriate retrofit of historic windows to improve their energy performance as well as the sustainable residential function of listed buildings. Should windows be suitably proven to be of non-historic origin, we are not opposed to the replacement of window panes with slimline double glazing, so long as this option is of no detriment to existing historic fabric such as any original historic window fittings or frames.
Following consideration of this application, we appreciate the functional need to retrofit Kilowatt House’s windows in their entirety. Due to the substantial amount of single glazing, we appreciate the applicant’s report of heating difficulties and consequent moisture issues, and the impracticality of installing secondary glazing due to the size of windows along the building’s northern elevation. We note that actual Crittal-style steel windows have been rejected in favour of aluminium; we appreciate the reasons given due to thermal bridging.
However, we are unable to make a fully-informed judgement regarding the suitability of this application due to the insufficient documentation supplied. Detailed existing and proposed window sections need to be provided to assess the proposed frame and glazing bar profiles, and the thickness of the double glazing, and whether this will be compatible with the building’s existing window reveals. It is also important in confirming that there will be no change in design or appearance to ensure that the listed façade of Kilowatt House is positively retained.
Furthermore, the Trust would recommend that the applicant considers a “whole house” approach in conjunction with the potential for window replacements. We advise that factors such as current insulation, ventilation, and heating practices should be included within schemes of thermal improvement to ensure that any changes made are sustainable and without unintended consequences for the rest of the building and its historic fabric. We note there are significant humidity factors and it is important to understand how, or whether, the new windows will alter this. Air circulation plans should therefore be a consideration in this application.
Therefore, whilst we are usually supportive of sensitive retrofitting measures that do not result in a loss of historic fabric, aesthetics, or associated significance, we are unable to fully assess the suitability of this application without the provision of window sections in accordance with Policy BD1 of the Core Strategy and Placemaking Plan. We would additionally encourage the monitoring of the existing windows’ performance and ventilation before and after installation (to be established by condition), in order to improve the dataset of the impact of double glazing in listed structures, and the consideration of additional retrofitting measures within this application to formulate a more informed programme of thermal improvement that will ensure the improved health and function of the whole building.