ACAN Scotland visits retrofit tenement project in Glasgow

Tenements are part of the architectural backdrop to Scottish towns & cities, both everyday and iconic at the same time.

Originally built to be heated by polluting open fires, tenements are facing unprecedented challenges from the energy crisis, building preservation and climate change.

Large scale strategies are required for upgrading these much-loved housing schemes to preserve them as an essential part of our cityscapes.

On Friday 11 March ACAN Scotland visited the Niddrie Road project, Glasgow, where John Gilbert Architects is upgrading a modest Glasgow tenement to EnerPHit standards providing a strategy for Housing Associations, and conservation bodies to protect both occupants and buildings.

Drew Carr, project architect, took time out of his busy day to show us around and ACAN Scotland Coordinator Alex Liddell reflects on the visit:

Initially commissioned to coincide with COP26, to demonstrate what could be achieved in upgrading a tenement, however the Covid pandemic hampered progress. Visiting the building in early March, the upper floors were largely complete, but the lower floors were still work in progress, so a good point in the building to understand the construction.

The overall size of the building has not changed. Internally the layout was adjusted with kitchens moved to form living-dining-kitchen rooms to the front, giving more space for bedrooms and bathrooms to the rear.

The EnerPHit standard places emphasis on improved thermal insulation, air tightness, and ventilation.

Front Walls

The red sandstone front elevation has been upgraded with vapour permeable wood fibre internal wall insulation with large layers of vapour permeable lime plaster between the stone and wood fibre, and a further layer as an internal finish to the insulation. Parge layers work as the air tightness layer. Hygrothermal analysis indicated that the existing joist ends that sit into the stone walls would suffer over time, so the floor structure in front of the front façade was altered. Foamglass insulation was introduced against the stone walls at floor level, with the timber floor construction bolted through to the wall behind, to insulate horizontally, and fire separate vertically.

Rear and Side Walls

200mm mineral wool external insulation with silicone render. Again, the internal plaster finishes form the air tightness layer.

Roof

490mm mineral wool insulation was introduced at ceiling level to a well ventilated attic. Ventilation is introduced at eaves by setting the new guttering off the wall and venting over the insulated wall head. Air tightness was achieved at the ceiling line, with all service penetrations air sealed.

Floor

325mm mineral wool insulation was introduced between and below the floor joists with a vapour control layer & air barrier above the insulation and breather membrane below, to reduce heat loss to the ventilated solum. A system of timber cradles was suspended below the timber floor joists to support the insulation.

Windows

Triple glazed Passivhaus windows with central transom to reflect the sash and case window pattern, resolving conflicting demands of planning and building control.

The common stair was included as part of the thermal envelope & air tightness. The concrete floor to the common stair will be replaced with an insulated solid floor construction.

Where possible chimneys were capped and the flues filled with bead insulation.

The above description is the general approach to thermal upgrade, but there were many local conditions that required a particular response. Careful detailing was required to reconcile the traditional construction with high energy performance design.

MVHR units were introduced in the ceilings above the bathrooms, which were lowered to accommodate the kit. Lowered ceilings in hallways allowed supply and extract air to the main rooms. Baths have waste water heat recovery units also.

Heating

The ground and first floor flats have air source heat pumps. The second and third floor have small gas boilers. There were a number of considerations in this mix including: ASHP are relatively expensive; there is a limit to the height that ground mounted ASHP can supply to so ASHP to second and floor flats would have required wall or roof mounted units, and there were aesthetic and noise concerns about the wall/roof mounted ASHP. Also having a mix of ASHP and gas boilers gives the research team a good testing ground for comparing old and new technologies.

Deep retrofit done properly requires careful thought and skill.

There are balances to be struck between the environmental gains and conservation losses. The external wall insulation used on the back is technically superior to internal wall insulation used on the front, but the back court elevations of tenements have heritage value as well as the front elevations. The window pattern and detail are also part of the character of a tenement but from a conservation perspective the Passivhaus windows lack the integrity of the traditional window.

Energy bills for these flats will be reduced from approx £100 per month to £10 (90% reduction) and due to the natural materials used and the MVHR these homes should have much improved internal air quality benefitting tenant health. This is key in eliminating fuel poverty and when considered against the heritage aspects, the EWI covering the stone to the rear is a small price to pay.

Inevitably some of the character of the tenement is lost in the performance gains. While such issues may seem trivial when compared to climate breakdown, the heritage value of these modest buildings is part of our cultural capital. These are decisions we need to take as a society, and Niddrie Road is a clear demonstration of what can be done if we are bold enough.

Alex Liddell, ACAN Scotland

Thanks to Drew Carr, Tom Manley and ACAN Scotland To get involved in ACAN Scotland email scotland@architectscan.org or follow them on Instagram @architectscan_scot