Listed Victorian Townhouse Retrofit in Clapham
In 2012, Arboreal Architecture was engaged to carry out the deep retrofit of a 170-year-old Grade II listed Victorian townhouse in a conservation area in Clapham.
The new owners had worked with Arboreal to find a property that would be their last home, and it accordingly needed to be made fit for comfortable, low-energy living. The house was of solid brick construction with single-glazed sash windows and lots of internal decorative details. Its protected status would limit the changes that could be made to these historic features.
Challenges: maintenance and materials
When the project began, the house was in a poor condition: floorboards abutting the side wall were rotten and some of the walls were soaked. Several causes for this state of disrepair were found: lapses in maintenance had allowed timbers in the modern fibre-cement roof to rot, and against the back wall a rainwater downpipe had been allowed to leak. Equally, moisture had been trapped in walls by poorly-thought-through work on the house using modern materials: modern waterproof plaster applied to the semi-basement walls, and a hard cement render used on the external brickwork plinth.
Historic England has published guidance highlighting the risks of combining some modern materials in traditional buildings. Traditional building materials such as brick are vapour permeable; they continuously take up and release moisture in response to humidity changes in the surrounding air. Damp can develop when the airtightness of a building is increased without providing enough ventilation, or when new materials used reduce the capacity of the host fabric to release moisture freely.
In this case, the existing moisture issues were resolved by carrying out overdue maintenance, removing inappropriate plaster and render in favour of original materials (lime mortar and render), and the relaying of the roof in natural slates.
Solutions: airtightness, insulation and conservation
To achieve a comfortable and appropriate level of thermal performance the house had to be properly understood in its present condition. Specialist equipment was used to test the thermal conductivity of walls and to pressure test the building’s airtightness. Thermographic surveys identified draughts and cold spots.
Amongst a range of measures, the building’s original single glazed sash windows needed particular attention if airtightness and heat loss were to be improved. Although they couldn't provide a good enough thermal performance, they had to be kept as historically important features. The solution found was to install tilt-and-turn secondary double-glazing (with thermally broken frames) that precisely matched the sightlines of the original windows.
Sufficiently insulating the house without risking future damp and damage issues was a major challenge to the project. Using the results of the thermal conductivity testing, 12 types of insulation were identified for use, including wood fibre, aerogel and cellulose, each applied to the part of the house it was most suitable for.
Harry Paticas (Arboreal Architecture) explained that in the loft of the house, for example, blown cellulose was used:
The cellulose is hygroscopic, so absorbs moisture (water vapour) in winter and dries out (desorps) in summer
The house’s solid brick facades were of varying thicknesses, and aerogel boards of appropriate thicknesses were applied to ensure ceiling cornices were not compromised by insulation thickness.
Where no original features existed, wood fibre was used, as Paticas explained:
We wanted vapour-permeable construction to allow the building to work as it always has – which is to dry both inwards and outwards.
Results: award winning energy performance
As a result of the retrofit (described in more detail in CIBSE Journal), the estimated Space Heat Demand of the 170 square metres building has been reduced by over 75% from 180 kilowatts per square metre per year (5,631 kilograms of carbon dioxide equivalent) to 40 kilowatts per square metre per year (1,251 kilograms of carbon dioxide equivalent) and the air leakage reduced from 9.6 air changes per hour to 1.8 air changes per hour.
In 2016, the project was awarded the CIBSE Building Performance Award – Residential Building of the Year. The award recognises the marked improvement in measured energy performance achieved without compromising the building’s historic character.
The future: monitoring and research
To monitor the longer term performance of the house a network of 22 wireless sensors were embedded in the walls of the house. With a five- to 10-year battery life, these sensors will generate valuable data about the viability of the methods used at this site. Indeed, data from these sensors has already been used to identify and correct an issue. It brought to light higher-than-predicted relative humidity in the lower ground floor which was then corrected by slightly increasing the ventilation rate from the mechanical extract ventilation. A research project is being planned that will use data from these sensors to examine the hygrothermal performance of the house over the last six years.
Adding to the array of sensors, Historic England has installed a weather station on the house’s roof to monitor external conditions. This forms part of a project to monitor the behaviour of heat, moisture and air in roof spaces under changing conditions to improve our understanding of condensation risk and moisture accumulation in pitched roofs.