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Wee Causeway, Culross

Wee Causeway, Culross

Insulation to walls and roof

Overview

This report forms part of a series of Refurbishment Case Studies and describes work undertaken to improve the thermal performance of the walls and loft space of a detached cottage in Culross, Fife.

One of the main objectives was to investigate various fabric interventions which could be used to improve the thermal performance of traditionally constructed buildings. A specific aim was to bring about the greatest possible improvement in thermal performance using vapour permeable materials.

Approach

Existing Fabric

The cottage, Wee Causeway, is formed of sandstone rubble masonry bound with lime, although it has been re-pointed with cement. It has a pitched pantile roof of a type common in many parts of the east coast of Scotland. The windows are single glazed sash and case and are likely to date to an earlier refurbishment in the 1960’s. The walls were lined with lath and plaster in one ground floor room, all others are plastered ‘on the hard’ (i.e. directly onto the masonry). All rooms had a standard cornice detail in place.

During the course of the improvement works, the tenant vacated the property. Situated in a small village on the Firth of Forth, the property is exposed to only modest wind driven rain.

Improvement Measure: Aerogel Blanket

Aerogel blanket insulation (10mm) was used in all external walls in the first floor rooms (excluding the bathroom). Previous Historic Scotland tests had shown aerogel to have a reasonable level of vapour permeability and it was therefore felt appropriate to apply this directly to traditionally constructed mass masonry.

The blanket insulation was secured to the wall using thermally-decoupled expansion fasteners, then a metal mesh was added to allow a plaster finish to be applied.

Improvement Measure: Calcium Silicate Board

Calcium silicate board insulation (15mm thick) was applied directly to the internal face of the masonry in one ground floor room. This insulation board is manufactured from sand and lime and treated with heated steam to produce an open-pored structure which is permeable to water vapour. The board therefore has a high capillary action and is vapour open, which helps regulate internal humidity.

To apply the board, the existing wallpaper and paint was removed from the wall to give a good base for an adhesive mortar to fixe the board to the masonry. As each panel was placed on the wall, mortar was applied to the edges of adjacent panels and the new panel slid to sit tightly against it. The walls were finished with two coats of plaster - a rough base coat and a smooth top coat.

Improvement Measure: Blown Polystyrene Bead

In the other main room on the ground floor, a bonded polystyrene bead insulation was used. This was blown into the void behind the existing plasterboard wall lining.

First, skirting boards were removed and all voids at floor level were filled to ensure the insulation material did not disperse to other areas of the building. At this point, electrical work was also undertaken. Holes (26 mm in diameter) were cored approximately every 200 mm from the ceiling and centred between existing timber straps. The bead was then blown into the cavity between the plasterboard and the masonry. As the bead passed through the nozzle it was coated in a water-based adhesive to ensure the beads formed a cogent mass. The holes were then filled and a skim coat of plaster applied.

Improvement Meaure: Roof Insulation

Work was also undertaken to improve the roof thermal performance of the roof space. Hemp wool insulation (250mm thick) was laid in the loft. It was selected because of its ability to absorb and release moisture and its ability to be installed relatively simply and quickly. Previous trials by Historic Scotland had used sheep’s wool insulation, thus it was felt that a different material should be tested.

*UPDATE* 2014 Improvement Measures

In 2014, thermally insulated shutters, designed by National Trust for Scotland were installed at Wee Causeway, and low-e film was also applied to the rear windows. These measures were modelled by Scottish Energy Centre, and the results are to be published in due course by Historic Scotland.

the addition of thermally insulated internal timber shutters, designed by The National Trust for Scotland applied to the front windows, and low-e window film applied to each glazed pane on the back of the building. - See more at: http://www.retrofitscotland.org/news-events/2014/05/thermal-enhancement-of-pre-1919-traditional-dwelling/#sthash.LS5NZ9u2.dpuf
the addition of thermally insulated internal timber shutters, designed by The National Trust for Scotland applied to the front windows, and low-e window film applied to each glazed pane on the back of the building. - See more at: http://www.retrofitscotland.org/news-events/2014/05/thermal-enhancement-of-pre-1919-traditional-dwelling/#sthash.LS5NZ9u2.dpuf

Performance

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

The thermal performance of the external walls in both the ground and upper floor rooms and upper floor ceiling was measured prior to work commencing and after the improvement measures were made. The work was undertaken by Edinburgh Napier University and the results of this are shown in the table below.

Culross wall logger SEC_300X185

Image: Heat flux sensors installed on ground floor walls after retrofit measures had been completed

The improvement made by the polystryene bead insulation was considered to be good, particularly as this intervention allowed the retention of existing fabric. A lesser improvement was made by the introduction of the aerogel blanket, but this is related to the thinness of the material, a benefit of which is that the cornice could remain in place. The thermal improvement, as a result of the hemp loft insulation was considerable.

 

The pre- and post- insulation U-values

 Building element

Pre- intervention U-value (W/m²K)

Post- intervention U-value (W/m²K)

Measure undertaken

Ground floor wall, west

1.5

0.5

Polystyrene bead behind plasterboard wall lining

Ground floor wall, south

 

1.3

0.5

Calcium silicate board insulation applied directly to masonry

First floor wall, north

1.6

0.9

10 mm aerogel blanket applied directly to masonry

First floor ceiling

1.5

0.2

275 mm hemp wool insulation

Lessons

This case study has shown several methods of insulating traditional mass masonry walls. Two options, demonstrated at Wee Causeway, show materials can be applied directly to masonry, whilst retaining the vapour permeability inherent within traditionally constructed mass masonry.

Modest insulation work within the loft space gave a significant thermal improvement, indicating that where this can be easily achieved, it is a good place to start when looking to improve the thermal performance of traditionally constructed buildings.

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