Dunedin City Council Community Housing Dunedin
Certifier Sustainable Engineering Ltd – sustainableengineering.co.nz
Here’s another significant Passive House milestone met: New Zealand’s first certified community housing project. Dunedin City Council (DCC) shares the glory with the experienced Passive House design team at Architype.
This project owes a lot to the Toiora High Street co-housing development for which Architype provided architectural services. Firstly, DCC was deeply involved during that project’s long gestation period and it raised the profile of Passive House among council staff and improved their understanding of the benefits.
DCC subsequently included “high-performance” in its tender specifications for two five-unit social housing blocks on the corners of School and Eton Streets. Architype responded with a proposal for units that met DCC’s tight budget and aimed for full Passive House certification.
The second influence of Toiora was felt during the extremely short time period Architype had to produce detailed drawings, a mere four months. On other projects, a design might be worked through, priced, then reiterated and priced again. Not here. There was no time to mess around, says Tim Ross, Archetype’s director. “We had to reach into our bag of tricks and very rapidly put together solutions that we knew from the outset would work and would fit the budget.”
The layout and form of the two identical buildings is exceptionally straightforward: two rectangles, four corners each. The long forms are softened by entry canopies, pergolas and fencing but all of these design elements serve multiple functions, providing vitally important shading, weather protection and occupant comfort.
Due to earning Passive House certification status, the project was exempt from the Healthy Homes regulatory requirement for heat pumps. Each unit is warmed as needed by a single panel heater on a thermostat. Electricity costs are bundled with rent, so the council makes immediate savings on operating costs.
The wall system differed from that used at Toiora but the same strategy was used to eliminate thermal bridges. Hot water is centralised, as was successfully done also in the Leith Street apartments, which the design team was working on at the same time. Ventilation systems were designed to fit in a kitchen cabinet, giving the shortest possible duct runs. This placement also saved floorspace, an important consideration given each unit is merely 41m2.
A TFA this small is always going to make air tightness targets harder to reach simply due to building physics—the ratio of exterior surface to interior volume is high. The further issue with units is that no party wall system currently available is designed to create an airtight barrier; re-engineering is required.
In a multi-unit building, each unit is individually blower door tested and the average result is used for certification purposes. The target was met and the team were pleased with the airtightness of the party walls. Adjoining units could have been pressurised also during each test, which would have improved the result of each individual unit, but this was not necessary.
Budget constraints dictated the choice of some construction details (eg a truss roof), insulation products and finishings. Council property managers also had very specific requirements about detailing, in order to produce durable, easily maintained finishes. The highest priority tenants are those aged over 55 and/or with disabilities and accordingly the units conform to Lifemark standards and feature fully accessible bathrooms.
The units face north-west and needed to be carefully designed to prevent overheating. DCC is looking to replicate this design but performance of future units may vary if different site considerations are not modelled.
Passive House Metrics
- Heating Demand19.5 kWh/m2/year
- Heating Load10.0 W/m2
- TFA358m2 Total
- Form Factor3.62
- Air leakage @ 50Pa0.6 ACH/hour
- PER demand69.9kWh/m2/year
Construction Details Average Values
- U-value External Walls 0.19 W/(m2K)(R5.2)
- 165mm NZ SIP wall / Plasterboard (some of the walls have an additional 50mm service cavity)
- U-value Floor0.22W/(m2K)(R4.6)
- Concrete Slab with 120mm XPS internal insulation
- U-value Roof 0.98W/(m2K)(R10.1)
- Truss Roof with 360mm insulation
- U-value Glass0.55 W/(m2K)(R1.8)
- Metro Low E(Xcel +Xcel) Triple Glazing
- U-Windows0.91 W/(m2K)(R1.1)
- Smartwin Compact
- Ventilation Efficiency 81%
- Zehnder ComfoAir 180