BERG COTTAGE, HERTFORDSHIRE

Berg Cottage is a 17 th Century timber-framed thatched detached cottage in Barkway, Hertfordshire. It is Grade II listed and has been in National Trust ownership since the 1930s. It is tenanted and not open to the public. During the 1990s the National Trust became aware that the conditions within the cottage were colder and draughtier than acceptable for a tenanted building.

Oxley Conservation were retained in 2000 to conduct a quinquennial survey of the property, and in conjunction with a buildings energy consultant, to assess the energy efficiency of the building. This assessment had two major components. Firstly fan tests were conducted on the building before and after the work. Fan tests are a direct, quantitative way of measuring the air permeability of a building, and also allow a qualitative analysis of where air is entering and leaving the building. The second component was the logging of temperature, humidity and fuel usage over a two and a quarter year period beginning one year before work commenced and ending one year after work was complete. The data collected shows how the building responds to real life conditions, and also provides a check that repair works have not inadvertently created conditions in which decay could occur.

The initial pressure tests revealed that Berg cottage leaked air at nearly four times the recommended rate for modern buildings, and almost twice an appropriate rate for historic buildings, which need more ventilation than their modern equivalents. Taping up the windows yielded little improvement: the biggest source of drafts was the walls.

The building survey had found that the front of the building had been inappropriately rendered in cement on expanded metal laths. The rear was weatherboarded, but with poor detailing. It was therefore decided to replace all the outer wall skins. Sheep's wool insulation and woodfibre boards were used in the walls to improve thermal performance; the cement render was replaced with a chalk and lime render matched to a small area of surviving historic render. The tiles in the few small areas of tiled roofing were lifted and woodfibre board and vapour permeable roofing felt were laid before the tiles were rehung. Dampers were fitted in chimney flues. Secondary glazing was installed to some of the leakier front windows. Taken together these measures reduced air leakage by a third, and heating bills by more than a quarter, while producing a warmer, less draughty home.

Other work carried out included re-thatching, repair of the timber frame, repairs to the brickwork plinth, and limewashing.

Oxley Conservation supervised all the works and designed the solutions for thermal efficiency and conservation. The principles of conservation were followed throughout, and the building illustrates how the needs of the occupants of historic buildings, the needs of the building itself, and the needs of the environment can be balanced.