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33 Marshall Street, Edinburgh

33 Marshall Street, Edinburgh

Wall and window upgrades

Overview

This case study is part of a new Historic Scotland publication series which presents examples of refurbishment projects of pre-1919 buildings designed to improve their energy efficiency.

33 Marshall Street is an end of terrace first floor apartment, which is accessed off a common stair, within a four-storey plus attic, mid-19th century tenement. Upgrading works were carried out to a single room (a bedroom) with two external walls and two windows.

Approach

Existing Fabric

  • Walls to part north east and part south east elevation of apartment comprising of stugged ashlar with fair-faced window surrounds. Overall masonry thicknesses approximately 750 mm and 600 mm respectively.
  • Wall linings in lime plaster on timber lath, with a plain run cornice, and ogee skirting.
  • Lime plaster ‘on the hard’ to chimney breast, in south corner of room
  • 2 no. 2 over 4 pane sash and case windows with timber tongue and groove boarding to window reveals and back, with ogee architraves, and flush timber lining to soffits. 

 

Insulation improvements

  • The open cavities around the window openings were packed with mineral wool insulation. Expanded polystyrene bead insulation was blown into cavities through the mineral wool packing to fully fill the cavity (approx. 35 - 45 mm deep).

 

Window improvements

  • Windows (2 no.) fitted with double glazed, timber, tilt and turn secondary glazed units screwed to the face of the case lining.
  • New / salvaged timber framing to the window back and reveals.
  • New timber packers were fitted to the window soffits, and as necessary, screwed to underside of timber safe lintels / battens.
  • Rigid insulation was cut and tightly fitted in between the timbers to the new framing and soffit packers, as follows: 50 mm thick to the reveals, 100 mm thick to the window backs, 35 mm thick to the soffit.
  • Window linings were reinstated and repainted.

Performance

Pre- and Post-intervention U-value testing

A solid 700 mm wall with an internal lath and plaster finish gave a pre-intervention thermal transmission value of 1.4 W/m²K. Post-intervention testing (after polystyrene bead insulation was pumped into the 45 mm slightly ventilated void behind the lath and plaster) resulted in a final 0.7 W/m²K value. This represents a 50% reduction in thermal retention.

The second wall tested was located below the window which is a thinner solid sandstone wall with a small void and a timber facing. It gave a pre-intervention U-value of 1.6 W/m²K while the addition of 100 mm of rigid Phenolic insulation behind the timber facing resulted in a drop of 1.2 to 0.4 W/m²K – a 75% reduction.

Outline costs

Secondary glazing (and associated works)

Aluminium double glazed (approx. 1.7 m high x 0.95 m wide)

£ 1,300 per window

Blown insulation

Total area: 12.20 m²

£ 40 per m²

Lessons

Post-completion feedback indicated that the outcome is seen as beneficial by the tenants, although there were a number of issues raised. As the specification was non-standard, and some elements were experimental, the level of site supervision required was beyond that of a normal window replacement contract. The duration of works was a major concern and all tenants would have preferred that the work could have been coordinated more tightly.

Due to the bespoke components used in this pilot study, and the small scale of works, the cost per property was substantial. Such an approach would not generally be affordable to social landlords unless the works attracted additional funding. However, it is believed that the tested approach is replicable, if unit costs can be substantially reduced. This could be achieved by modifying the specification of materials and by using factory-made secondary glazing units produced by volume manufacture. Cost savings might also be achieved by in-house maintenance departments being trained on how to coordinate the tested approach.

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Case Study

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