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Scotstarvit Tower Cottage, Cupar

Scotstarvit Tower Cottage, Cupar

Thermal upgrades & installation of radiant heating

Overview

This case study forms part of the Historic Scotland Refurbishment Case Study series, detailing energy efficiency improvements to traditionally constructed buildings in Scotland. This property is owned by the National Trust for Scotland. The project demonstrates a range of energy efficiency upgrades that can be carried out in traditionally constructed buildings, with minimal or no loss of original fabric. The project involved the use of innovative materials, in particular loose-fill perlite wall insulation, and demonstrated the use of vapour-permeable materials and finishes.

Approach

Existing Fabric

Scotstarvit Cottage is a late 19th century detached cottage next to Scotstarvit Tower, in an elevated location above the town of Cupar, Fife. The cottage is well sheltered by a belt of trees to three sides, with a open aspect to the south. The property is built of roughly-squared sandstone rubble bonded with lime mortar, with a slate roof.

Internally, the cottage is lined with lath and lime plaster on timber battens with a timber suspended floor and single glazed timber sash and case windows. In some areas plasterboard had replaced the lath and plaster.

The property had been vacated some time earlier, and had suffered a water leak which had caused the living room ceiling to collapse. The existing heating used an oil fired boiler, with a conventional wet system. Insulation was limited to around 200 mm mineral insulation in the roof, which had begun to slump in places. The accommodation is arranged on one floor, and consists of three bedrooms, a living room, and kitchen. A later flat roofed extension (c. 1925), provides a bathroom and small front porch.

 

Improvement measures

Improvements were made to most of the building elements, namely the roof space, ceilings, rooflight/lightwell, windows, floor, external walls and the chimney. Specific measures adopted include:

  • Perlite wall insulation (a free flowing material) was used in the roof space in addition to hemp batts.
  • Hemp batts were also used to insulate below the suspended floor.
  • Perlite wall insulation was also used to insulate the external walls, filling the space between the lath and plaster lining and the external stonework.
  • Damaged ceilings were repaired using traditional haired lime plaster and clay paint.
  • Wood fibreboard was used to insulate the lightwell.
  • Polycarbonate secondary glazing was used to improve the thermal performance of the windows.

 Scotstarvit insul attic SEC_300X185

Image: Attic of Scotstarvit Tower Cottage, insulation laid between and above the roof joists

Radiant heating has been trialled in the Scotstarvit Tower Cottage, as with many other rural properties, it is off the gas grid. A low energy electric heating system was therefore required.

This is a more efficient way of heating, as the occupants are heated first, and the air temperature rises more gradually.

The radiant heating panels are slim and wall mounted andcan be supplied simply as matt white or grey panels, or as mirrors or designed as prints or pictures. For the cottage refurbishment, simple matt white panels were specified for most of the rooms with a heated mirror selected for the living room and bathroom.

scotstarvit post int with mirror rad_300X185

Image: Radiant heating panel installed in living room with a mirrored finish, seen in the right of this photograph

 

Performance

Measured thermal performance of the fabric

The thermal performance of the property was measured prior to the improvement works by Edinburgh Napier University and consisted of in situ U-value measurements of the floor, wall and the ceiling. As glazing of the type found at the cottage had been tested before, the windows were omitted from the testing process.

The pre- and post- insulation U-values

Element

Improvement type

Pre- Intervention Monitored (W/m2K)

Post- Intervention Monitored (W/m2K)

Wall surface

Loose fill insulation (perlite) poured into the cavity between the lath and plaster linings and the external stone walls.

1.6

0.5

Floor

Solum was isolated using a geo-textile breathable membrane, after which hemp batts were fitted between the floor joists, held in place by timber battens and supported by a breathable membrane wrapped around the floor joists

2.3

0.11

Ceiling

Old insulation removed  the roof was re-insulated with 270 mm
flexible hemp batts between and across joists boarded over with loft boarding

0.7

0.14

 

The relative humidity value for the void behind the existing plasterboard was also tested. These measurements allowed a baseline pre-intervention figure to be calculated, from which the effectiveness of the upgrade works could be measured.

 

Airtightness testing

The property was also tested for air leakage by Green Footsteps, using a blower door test. This returned a result of 16.9 m³/h/m² @50Pa. In comparison the Scottish Building Standards Technical Handbook (Domestic) for new dwellings which has a recommended maximum leakage rate of 10 m³/h/m² @50Pa. The air leakage testing and results are described in full in the report which can be downloaded on the right.

Following completion of the refurbishment works, the property was tested again for air leakage, which had reduced by almost 50% to a level of 10.7 m³/h/m² @50Pa, which compares favourably to the recommended maximum air leakage of 10 m³/h/m² @50Pa.

The refurbishment work was completed in June 2012, and a tenant moved in shortly afterwards. During the summer months, the heating was not used and the air temperature differential was not sufficient to undertake U-value testing using heat flux sensors. This testing will be undertaken over the winter months (2012-13) and will be reported on as the results become available.

Lessons

At the time of writing up this case study, a new tenant had only recently settled into the cottage. Whilst there were some teething problems with the radiant heating (which had been programmed incorrectly) and the secondary glazing (which on two occasions ‘blew off’ the windows), on the whole the tenant was very pleased with the internal appearance of the property.

The works to Scotstarvit Cottage have demonstrated that significant upgrade works can be carried out to traditional buildings without requiring loss of internal linings or existing features.

The success of the project will depend on the demonstration of an improvement in the thermal performance of the building, through a reduction in heating bills and improved thermal comfort for the occupant. Further testing to establish post-intervention U-value figures will also give an indication of the success of the refurbishment to Scotstarvit Tower Cottage.

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