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Leighton Library, Dunblane

Installation of loft insulation

Overview

Due to increasing energy costs, the Trustees of Leighton Library wished to improve the energy efficiency of the B-listed building. In 2011, Historic Scotland provided grant funding to allow loft insulation to be installed. Loft insulation was determined as the most suitable way of improving the thermal performance of the building.

 

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Approach

Existing Fabric

Leighton Library is a small, domestic scale B-listed building, located in Dunblane. It was purpose built in the 17th Century to house a collection of books belonging to Archbishop Leighton, Bishop of Dunkeld.

The structure is rectangular in plan, will mass masonry walls, lime bonded and externally harled with wet dash cement, finished with modern masonry paint. While the exterior finish is not ideal as it will not allow the building to breathe appropriately, it was considered to be in good condition and having no current detrimental effect on the building.

The building has a ground floor, currently used for storage but likely to have once been the home of the librarian; the library floor and an unoccupied roof space. The walls on the library level are finished with fine joinery to house the books, as such internal wall insulation was not proposed. Hence, loft insulation was determined as the most effective way of improving the thermal performance of the library.

 

Improvement Works

To protect the books and the woodwork, all the bookcases were covered in dust sheets before work commenced, and all cutting of insulation was undertaken outside.

To ensure humidity buffering in the roof space, a hydroscopic natural material was used, namely wood fibre board. A 200mm layer of wood fibre boards was installed between the joists in the floor space of the loft. The insulation came in rigid sheets of 100mm which were cut with hand saws to ensure all corners were filled. The final level of the insulation was just above the level of the joists.

Performance

Pre and post intervention U-value testing was undertaken to quantify improvements in the thermal performance.

The thermal performance of the upper floor ceiling was measured by Edinburgh Napier University prior to work taking place, achieving a U-value of 1.3 W/m²K. This measurement was taken over a 21 day period in January 2012, when the internal average relative humidity was 69% at 12°C and external relative humidity was 95% at 2°C.

Post intervention U-value measurements showed that the addition of 200mm of wood fibre insulation boards had improved the U-value to a value of 0.2 W/m²K.

 

Moisture levels, at both the interface of the insulation and the ceiling, and within the loft space are monitored to determine if the improvement works have affected the relative humidity.

Readings taken in March 2012, following the work in the roof gave an average relative humidity of 57% at an average temperature of 15°C. Externally the average relative humidity level was 69% at an average temperature of 1°C. Additionally, the moisture level within the library itself was measured to ensure that the environment remains within tolerable levels. For more information on relative humidity and moisture levels within traditional buildings, see Technical Paper 19.

Lessons

The site trial at the Library has shown that considerable improvement can be made to the thermal performance of a traditional roof space using a natural, vapour permeable material. Whilst works to buildings with special contents need careful consideration, the principles and approach for energy efficiency upgrades remain the same.

Ventilation of the roof space was carefully considered, as a 'cold roof' was created by the installation of insulation material at the library floor ceiling joist level. To prevent the build-up of condensation, ventilation must be ensured to the roof space.

During investigations it was discovered that bituminous roofing felt was installed under the roof slates of the building. This is often found in traditional buildings that have been restored after 1945. Roofs with bituminous roofing felt do not let air and vapour transfer in the same way as traditionally constructed roofs so they require additional ventilation, often roof vents, or ridge venting.

The library had previously been ventilated using two louvered vents on the south-east gable but these had been partially closed off. To ensure suitable ventilation, these were re-opened and monitoring was undertaken to make sure that adequate ventilation was achieved.

 

Links

Historic Scotland   

Scottish Energy Centre, Edinburgh Napier University

 

Refurbishment Case Studies   

 

 

Resource Efficient Scotland supported the preparation and presentation of this case study for the Retrofit Scotland website.

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