Midlothian momentum

Midlothian Council's ambitious Passivhaus social housing programme has achieved another milestone with the Passivhaus certification of 20 apartments, comprising 8 assisted living homes and 12 over 55s homes. The 1-21 St Cuthberts project in Bonnyrigg is in the vanguard of an even larger Council project in the area targeting Passivhaus which will comprise a 40-bedroom care complex, 46 extra care flats, and a day-care unit that is also now on site in Bonnyrigg.

Ground floor plan, 1-21 St Cuthbert's, Bonnyrigg. Image credit: AHR

First floor plan, 1-21 St Cuthbert's, Bonnyrigg. Image credit: AHR

The £4.4 million project was designed by PHT Patron AHR, with PHT member Pick Everard as structural engineer, and was certified by PHT member MEAD Consulting. PHT member PAUL Heat Recovery Scotland supplied the MVHR systems for the project.

Key stats

Construction: Timber frame & blockwork
Number of homes: 20 apartments
TFA: 1251 m²
Form factor: 2.3
Build start date: 2023
Completed: 2024
Certified: Passivhaus, 2024

1-21 St Cuthbert's, Bonnyrigg. Image credit: Keith Hunter Photography

Construction

The project was predominantly constructed in timber frame, with some blockwork construction. The external palette of facing brick and seamed cladding has been used to blend the building into its surroundings. The scheme is designed to avoid institutional features and to be deliberately indistinguishable from regular market housing.

U-values

Roof: 0.074 W/m²K

Timber truss with standing seam cladding. Cold roof with insulation at eaves level.

Detailing section, 1-21 St Cuthbert's, Bonnyrigg. Image credit: AHR

Wall: Various

Timber frame with brick work or cladding:

Timber-Brick: 0.102 W/(m²K)
Timber-Cladding: 0.116 W/(m²K)

Blockwork with brick work or cladding:

Block-Brick: 0.124 W/(m²K)
Block-Cladding: 0.190 W/(m²K)

Floor: 0.097 W/m²K

Concrete slab above insulation

Building performance

Designed energy performance

Airtightness n₅₀ (≤ 0.6ACH @ 50 Pa)

0.6 @ 50 Pa

Space Heating Demand (≤ 15 kWh/m².a)

15 kWh/m².a

Heating Load (≤ 10 W/m²)

9 W/m²

Primary Energy Renewable (PER) Demand (≤ 60 kWh/m².a*)

72 kWh/m².a

Primary Energy Renewable Generation

23 kWh/m².a

*+/-15 kWh/m².a allowance if offset by energy generation. See Passivhaus criteria.

Services

1-21 St Cuthbert's, Bonnyrigg. Image credit: Keith Hunter Photography

Individual Mechanical Ventilation Heat Recovery (MVHR) units were installed into each apartment, designed by PAUL Heat Recovery Scotland. The building also had a separate MVHR to feed the ground floor and first floor offices.The project has moved away from fossil fuels and is a fully electric project. Electric radiators are used for heating and hot water cylinders in each flat heat the domestic hot water. Solar photovoltaic panels on the roof provide renewable energy, with generated energy being fed to the communal corridors and stairwells. A brise soleil provides shading for the summer comfort strategy.

Challenges & lessons learned

Designing a project on a tight town centre site reduced the layout options for building orientation. This had implications for overall building performance at the design stage and needed to be carefully balanced out in the PHPP (Passivhaus Planning Package) modelling
Neighbouring buildings were built hard against the boundary lines of the project. This made the external wall detailing of the building challenging to ensure that U-values were maintained. The solution was a building with a full external leaf masonry wall very close to the neighbouring building, allowing the U-values to be consistent in the project
The project incorporates an overhang, which required steel framing for support. The steel frame introduced potential thermal bridging challenges that had to be accounted for in the PHPP. The construction of the overhang detail and associated airtightness sealing and taping brought their own challenges, but were successfully implemented by the contractor on site.

1-21 St Cuthbert's, Bonnyrigg. Image credit: Keith Hunter Photography

Architect’s view

Our design prioritises creating a comfortable and sustainable living environment that benefits both residents and the community.

By minimising its environmental impact, we ensure that these homes are not only welcoming and adaptable but also future-proofed ensuring residents live comfortably without the added stress of energy consumption. In today’s current climate, we have ensured that the homes are both affordable to live in and are environmentally responsible. To make life easier for residents, our design integrates high levels of energy efficiency to help reduce fuel costs.

From the start, our goal has always been to not only shape a space that meets the practical needs of residents but to truly enrich their lives. This project is about fostering independence, dignity and a sense of belonging for people with learning disabilities in the heart of their community.

Louise Smith, Project Architect, AHR

Stuart McKenzie, Cabinet Member for Housing, Midlothian Council:

Key team

Architect & Passivhaus Designer: AHR
Contractor: Cruden
Structural engineer: Pick Everard
M&E consultant: Hawthorne Boyle
Passivhaus Certifier: MEAD Consulting

Many congratulations to clients Midlothian Council for successful delivery of another Passivhaus social housing scheme, as part of its ambitious programme. As the Scottish Government currently consults on its Scottish Passivhaus equivalent policy it is inspiring to see Scottish local authorities driving Passivhaus forward in Scotland.