Passivhaus Affiliate

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. 

Niddrie Road, Image credit: John Gilbert Architects


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)

Niddrie Road rear elevation, Image credit: John Gilbert Architects


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).


Niddrie Road existing floor plan, Image credit: John Gilbert Architects   Niddrie Road proposed floorplan, Image credit: John Gilbert Architects

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



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


Existing roof, eaves insulation. Image credit: John Gilbert Architects

  Existing internal walls. Image credit: John Gilbert Architects


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.

The 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

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.

Roof strategy when connecting to external wall Insulation wall strategy   Roof strategy when connecting to internal wall Insulation wall strategy

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

Niddrie Road, Image credit: BBC


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


Managing moisture: A critical element in any retrofit project is addressing moisture risks, particularly when considering energy efficiency measures and additional insulation. PHT Patron Greengauge undertook WUFI hygrothermal modelling on the project. The different types of sandstone in the building (red sandstone at the front and blonde sandstone at the rear) had different porosity and so had to be modelled separately.

Niddrie Road, Image credit: BBC

As a result of the WUFI modelling, the design team made the decision to remove the first-floor joists from the wall to avoid potential moisture risks and decay, as well as allowing for continuous insulation and airtightness. The project has MVHR (mechanical ventilation with heat recovery) systems with summer bypass, located in the flats’ bathrooms.

Heating: Air source heat pumps could only be installed on the first two storeys so four flats will have ASHPs.  Above that, gas boilers will be installed. The research project will collect data about how they compare. Waste water heat recovery systems will be installed to capture and recycle the heat in water from baths, showers and the kitchen sink.


Niddrie Road, Image credit: John Gilbert Architects


For more information, inspiration, and EnerPHit case studies please visit our Passivhaus Retrofit Campaign page. Looking for more detail? Delve into our Passivhaus Retrofit Masterclass lecture series. The next session is planned for the 15th of December, exploring solid wall construction.

Delegates joining the November United Nations COP26 conference in Glasgow will have a chance to visit this radical retrofit currently underway at Niddrie Road. The Passivhaus Trust, in partnership with Glasgow City Council and iPHA, is organising a Passivhaus tour. Check out more Passivhaus activities in the lead-up, during and immediately following COP26 via the Passivhaus at COP26 page and get involved!


Key Team

Client: Southside Housing Association

Architect: PHT member John Gilbert Architects

Construction Costs: nbm Construction Cost Consultants

Contractor: CCG Contractors

Structural Engineers: PHT member Design Engineering Workshop

WUFI calculations: PHT Patron Greengauge

Consultant: PHT member Passivhaus Associates

Passivhaus Certifier: PHT Patron WARM

Niddrie Road

All images, unless otherwise stated, credited to ©John Gilbert Architects


Further information

Niddrie Road EnerPHit

Passivhaus Retrofit

Passivhaus Retrofit Masterclass lecture series

PHT Research Report - Passivhaus Retrofit in the UK

Passivhaus at COP26

BBC: How do we make homes fit for net zeroes? - 6 August 2021

10th October 2021

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