Certifier Sustainable Engineering Ltd – sustainableengineering.co.nz
Sections are hard to find in Wellington and are rarely flat and uncomplicated. The owner of this small and perfectly formed Passive House home heard about an upcoming subdivision at after-work drinks. The site is not tiny at 476m² but the T-bone shape and driveway and access left few options for siting the house; the legal build platform was a mere 187m². Proximity to the boundaries was a challenge to solve. Other difficulties came in the form of Covid-related delays and shortages, industry constraints and consenting officials continually moving regulatory goalposts. It wasn’t smooth sailing but the outcome is a lovely home-and-income set-up. It’s a welcome respite for a homeowner who was sadly well-acquainted with cold, dank Wellington rentals.
The client works in the IT sector, focused on efficiency and automation. Naturally he brought a measured and rational approach to his house build. The accountability and rigour that come with the Passive House performance standard was appealing. The benefits of prefabrication were also clear and the decision to go with SIP construction significantly shaped the project. The architect/Passive House designer was chosen on the basis on his experience in designing with SIP and his excellent technical understanding of high-performance building.
The ground floor houses a self-contained studio that is professionally managed as an Air BnB rental, plus the entrance to the main house. The next level has utilities and a bedroom, with the lounge and kitchen up another short flight of stairs. A bathroom, toilet and another bedroom are on the top floor.
The homeowner initially wanted a two-bedroom house but mortgage requirements and considerations of resale value pushed the build bigger. The floor plan has been designed with flexibility in mind; the studio unit could readily be incorporated into the main home in the future if required.
Building firms were thoroughly booked up at the time, but the client methodically interviewed five options before deciding to go with a firm that handled commercial as well as residential work. They hadn’t built with SIP or tackled a Passive House before but their enthusiastic attitude counted for a lot. So did their capacity to get a lot of hands on site for key stages such as standing the SIP panels and achieving the weathertight milestone.
The project ran into Covid-related supply issues but was fortunate to have its locally-built high-performance windows delivered shortly before the manufacturer went under. Meeting fire requirements became a painful, protracted affair that strained the budget and everyone’s patience.
The footprint is tiny and the TFA 108m², across the two units. The architect has UK experience with very small dwellings and put this to good use here. “My house is not big but it doesn’t feel small,” remarks the owner. “Every room feels like there’s enough space.”
The small and separate volumes presented challenges when it came to preventing overheating. The two apartments face north- and south-west, due to the site constraints and direction of views. A combination of sliding shutters and window boxes provide shading. Perforated aluminium sheets proved to be a fraction of the cost of timber or aluminium louvres. It suits the aesthetic of the cladding material and further has the advantage of being extremely light and readily obtainable. There were various options for opening diameter and shading percentage that could be readily selected from a sheet metal catalogue.
Small buildings have high energy intensity thanks to more people and appliances in a small area and this makes it very difficult to hit the standard certification PER target. Recognising this, there is an option to develop a project-specific requirement for the primary energy (both PER and PE), requiring more work by the Passive House designer. PHPP10 now makes this calculation easier.
The studio is heavily booked with very positive ratings and the owner says guests often comment on how nice and warm it is inside and how fresh the air is. The project won gold in its regional category in the 2023 House of the Year Awards. Seldom does a house offer so much, commented the judges, who praised its performance and aesthetics.
Passive House Metrics
- Heating Demand14.5 kWh/m2/year
- Heating Load14.7 W/m2
- Form Factor3.26
- Air leakage @ 50Pa0.5 ACH/hour
- PER demand63.6kWh/m2/year
Passive House Database - 7187
Construction Details Average Values
- U-value External Walls0.22 W/(m2K)(R4.4)
- 115mm NZ SIP panel with 45mm service cavity insulated with Knauf R1.4 batts
- U-value Floor0.18W/(m2K)(R5.3)
- Timber floor with 240mm floor joists, insulated with 22kg/m3 Jetstream blown insulation
- U-value Roof 0.15W/(m2K)(R6.3)
- 165mm NZ SIP panel with 45mm service cavity insulated with Knauf R1.4 batts
- U-value Glass0.57 W/(m2K)(R1.8)
- Low E triple glazing
- U-Windows0.77 W/(m2K)(R1.3)
- Seda Smartwin compact
- Ventilation Efficiency 80%
- Wolf - CWL-F-150 Excellent