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The Pleasance, Edinburgh

The Pleasance, Edinburgh

Thermal insulation to an attic level flat including the typically hard-to-treat areas of coom wall and dormer window cheeks. 

Overview

This Case Study looks at the insulation of a traditional attic level flat, in a mansard slate roof in the old town of Edinburgh. The flat is a mid terrace in a late 18th Century sandstone tenement building. There are three single glazed dormer windows, the dormer cheeks are clad in lead. A light shaft provides daylight to the hall of the flat from a rooflight above.

This case study looks particularly at the insulation of the coom wall, the sloping wall within a habitable room, which is totally within the roof space, typically these are difficult to insulate without stripping out internal linings. The aim of this project was to illustrate that it is possible to effectively thermally improve coom ceilings and dormers without excessive cost or disruption to owner or tenant.  

The thermal performance of the flat was measured prior to the works being carried out and is detailed in the table below.

Building element

U-value (W/m2K)*

Notes

Coom ceiling

1.5

Slate and sarking, 120 mm void, lath and plaster lining

Dormer roof

1.5

Lead on sarking with lath and plaster lining

Dormer cheek

0.9

Lead on sarking with lath and plaster lining

*Measurement taken December 2011–the average internal temperature during testing period was 19°C and the average internal RH 58%

Approach

The work was carried out with the tenant of the flat in residence and was scheduled to last a week, this however over-ran slightly due to late delivery of materials and technical problems. The limited downtakings confirmed construction methods and informed the proposed works.

Light Well

Glass fibre insulation was removed from the light well and replaced with 180mm sheep’s wool insulation. The existing roof light was removed and replaced with a double glazed mechanical model, which allows the tenant to ventilate the flat without increasing noise pollution, as windows overlook the busy Pleasance Courtyard, a major Edinburgh Fringe Festival Venue.

Roof Space

All existing insulation to the roof space was removed, 180 mm sheep’s wool insulation was laid between the truss ties, and 100 mm insulation above, across the ties, overlapping with the bonded bead which filled the coom, which provides a completely insulated envelope.

Coom Wall

Holes were drilled through the lath and plaster in the coom wall, with care taken to address every void. Polystyrene bead Insulation was the blown in through these holes, the product was coated on entry with a PVA glue that, once in place, holds the beads together in a solid open celled matrix. This is an experimental intervention, which is being continually monitored.

   Screen Shot 2014-02-17 At 16.47.50        Pleasance blown bead

Dormers

To establish construction detailing, the linings to two of the dormers were removed, these were then fitted with wood fibre insulation and the cheeks were reinstated with plasterboard with a plaster skim. To the other two dormers the blowsn polystyrene bead insulation was fitted. This was also installed to all dormer ceilings, between the existing timbers.

                 Pleasance dormer

Performance

Following the completion of the work measurements were taken again, in the same manner as before to assess the thermal improvements, these are outlined in the table below.

Building element

Pre-intervention U-value (W/m2K)

Post-intervention U-value (W/m2K)*

Notes

Coom ceiling

1.5

0.4

Blown polystyrene bead behind lath & plaster

Dormer roof

1.5

0.5

Blown polystyrene bead

Dormer cheek

0.9

-

Not insulated

*The post-intervention U-value readings were taken in March 2012. During this period the average internal temperature was 18°C and the external temperature varied between 3°C and 10°C

The relative humidity within the unventilated cavity was measured before the works revealing the voids in the roof were dry and well ventilated. Following the installations of the insulation to the coom ceiling humidity sensors were reinstalled. Initial readings taken over a period of three months showed relative humidity at levels lower than expected.

While the immediate gains from this intervention with regards to U-Values are clear long term monitoring of the filled cavity in the coom walls will confirm whether this is the best approach. 

Lessons

Following completion of the works an incident caused water damage to the insulated areas. The volume of water was enough to penetrate through the insulation and into the internal linings of the coom and flat ceilings. In properly ventilated roof structures water should be able to dissipate freely. In this case the damage was exacerbated by the fact that the situation was not fully addressed for some time and drying of the plaster was inhibited by surface finishes that did not allow the free movement of water vapour. However, when the areas were fully uncovered, it was observed that the bonded bead remained largely unaffected although the bonding agent in the beads (a water-based glue) had partially broken down and the beads were effectively loose.

The fabric did however effectively dry out, and the presence of the bonded bead did not prevent timely drying. Humidity monitors have been installed to monitor the relative humidity at the depth of the sarking and just behind the internal lining. Results from this monitoring will show how quickly and effectively the wall is drying out to appropriate levels. 

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U-value Testing Methodology