A retrofit template for hard-to-heat tenement homes

According to Scottish Government research, tenement homes emit 5.6 tonnes of carbon a year, compared with 3.6 tonnes for those built after 1982. As well as tackling carbon emissions, the Niddrie Road project also aims to address the urgent problem of fuel poverty, which is affecting up to 30% of households living in Scottish tenement buildings. Previous tenants of the building reported high fuel bills and that the flats were hard to heat. 

 

The Niddrie Road project has received funding from the Scottish Government as part of its Climate Emergency Collaboration Challenge, in a bid led by CaCHE (UK Collaborative Centre for Housing Evidence). The four storey 8 flat building, owned by Southside Housing Association, was empty and in a poor state of repair.

PHT member John Gilbert Architects is leading the demonstration project evaluating the carbon reduction secured by a high-quality retrofit of the tenement block to the EnerPHit (Passivhaus retrofit) standard. The project aims to assess the scalability and replicability of the lessons learned for Glasgow’s wider pre-1919 tenement stock.

 

Key Stats Build start date: April 2021 No.Units: 8 flats Construction: Solid wall sandstone Construction cost per flat: £88K (of which £37K is for energy efficiency measures)

 

Glasgow is estimated to have more than 70,000 such tenement flats and Scotland as a whole, 182,000 (SHCS, 2018). These buildings have significant cultural, historical and urban value, but commonly suffer from poor thermal performance, having solid walls, suspended timber floors and high air permeability. The retrofit project has ambitions to exceed the Scottish Government’s energy efficiency standard for social housing (EESSH2).

 

Left: Existing floor plan  |  Right: Proposed floor plan

 

Retrofitting these eight Glasgow flats and the classic close stairwell is important to determining whether Scotland can reach its target for net zero climate-changing emissions, and within less than 25 years. Older homes are doing more of the damage in emissions, because they are more difficult to heat.

 Douglas Fraser BBC Business/economy editor

 

Construction

The main elements of the existing four-storey building included:

• Walls: uninsulated red and blond solid sandstone walls, lath and plastered internally

• Roof: timber framed structure, concrete tile finish

• Floor: uninsulated suspended timber floor

 

 

Hybrid EWI/ IWI strategy

An insulation and airtightness strategy was developed for the roof and the suspended timber floor. Heritage considerations meant that the project needed to adopt a hybrid strategy combining internal wall insulation (IWI) to the front and party walls and external wall insulation (EWI) strategy at the rear and gable walls. 200mm mineral wool insulation was used externally while 120mm wood fibre insulation was used internally. Lime plaster is replacing the lath and plaster internally and is acting as the airtightness barrier.

External wall insulation was brought down below floors, where it met up with the suspended timber floor insulation strategies to minimise thermal bridging at the ground floor/ wall junction

Triple glazed timber windows were chosen that were deemed acceptable for the building’s heritage requirements and have been carefully integrated into the hybrid wall airtightness and insulation strategies. Careful detailing was undertaken to ensure that the IWI and EWI strategies connected well, maintaining the continuity of insulation and airtightness.

Left: Roof strategy when connecting to external wall Insulation wall strategy  |  Right: Roof strategy when connecting to internal wall Insulation wall strategy

 

Challenges & lessons learned

Heritage and planning: Planning constraints meant that adding external wall insulation (EWI) was not a possibility on the frontage of the building. Similarly, adding solar PV to the roof was not permissible due to heritage requirements. For the most part, the window dimensions of the tenement had to be retained, even though larger window openings would have reduced thermal bridging. 

 

A balance needs to be struck between energy efficiency and the buildings' heritage. The sandstone frontage, the size and shape of windows, ceiling height and room size is barely changed, so the building still retains the "tenement feel" and manages to balance heritage requirements with the project’s stringent energy efficiency goals.

Drew Carr, John Gilberts Architects

 

Practicalities: The retrofit required gutting of internal features and re-plastering work as well as substantial changes to the floors. It was an advantage that all 8 flats were unoccupied, as works would be difficult with tenants in-situ. Where social housing associations share blocks with private owners, good communication is crucial between parties.

Financial: The project has been costed at £88K per property, of which only £37K are specific energy efficiency measures, with most of the costs going towards general building maintenance improvements. In this case, Southside Housing received financial support from a consortium of funders, including £250,000 from the Scottish government, backing from Strathclyde University, and the John Gilbert Architecture practice's research budget.

 

This is an exemplar project that Governments can draw upon to strengthen funding and ambition for urgent deep green retrofits.

Sarah Lewis, Research & Policy Director, Passivhaus Trust

 

10th October 2021

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