Martins Farmhouse Whanganui
Architecture & Passive House Design Energy Architecture NZ Ltd - energyarchitecture.nz
Main Contractor : David Brown Building Ltd - davidbrownbuilding.co.nz
Builder (natural materials): Straw Homes Hawke's Bay - strawhome.co.nz
Certifier Sustainable Engineering Ltd – sustainableengineering.co.nz
This rural home outside of Whanganui is a complex blend of traditional building and hi-tech performance, proof of the adage that Passive House is agnostic with regard to materials. It was inspired by and builds on the success of Hiberna, the first strawbale Passive House building in New Zealand and seemingly the first in the world to rely on natural plaster over straw as an air control layer.
The project was complex for many reasons, starting with being in a regional centre. The architect worked remotely after a single site visit. Covid drew out the design process. Two builders were needed. The main contractor was local and the site foreman had Passive House experience. They periodically handed over the site to an extremely experienced strawbale builder, who travelled from the Hawke’s Bay with two apprentices. The combination of Passive House detailing and traditional building materials was challenging for sub-trades unused to either.
Internal walls were built from LEM (Light Earth Material), in this case leftover straw mixed with a clay slurry, plastered once dry. The earth plaster used internally (sourced from local clay and not requiring cement or lime) is an effective buffer of interior humidity. The MVHR is running at a lower setting than originally planned as the RH and CO2 levels show this is adequate. The owners track key indoor air quality data via a portable monitor.
The final airtightness result delighted everyone involved in the project. It was truly a team effort, requiring the two building teams to closely collaborate and carefully think through sequencing. At junctions between internal and external walls, a first coat of plaster was applied to the strawbale wall before the last internal stud was fixed, in order to ensure a continuous airtight layer. Experiments during both blower door tests showed the need for two coats of plaster. A happy surprise was that a transparent casein wash, a home-made proto-paint applied to harden the plaster surface, also assisted with airtightness.
The degree of owner involvement in construction is unusual for a Passive House project but very typical for an earth building. Volunteers (including all the professional staff from the architect’s firm) came from around the lower North Island for the customary ‘bale raising’, when the strawbales were fitted into the hand-built timber frames in a single day. The owners and a few friends did almost all the LEM building and applied finishings to the appearance timber used extensively inside and out. The owners are involved with a specialist timber merchant, MacBlack Timber, and were determined to use local, untreated timber everywhere possible.
Passive House Metrics
- Heating Demand14.7 kWh/m2/year
- Heating Load9.7 W/m2
- Form Factor4.0
- Air leakage @ 50Pa0.3 ACH/hour
- PER demand49.8kWh/m2/year
Passive House Database - 7246
Construction Details Average Values
- U-value External Walls 0.16 W/(m2K)(R6.23)
- 60mm Earth Exterior Plaster / 350mm Strawbale / Interior Earth Plaster
- U-value Floor0.33W/(m2K)(R3.06)
- Concrete slab / 90mm XPS internal insulation / Plywood / Flooring
- U-value Roof 0.12W/(m2K)(R8.63)
- Truss Roof with 610mm Terralana insulation
- U-value Glass0.53 W/(m2K)(R1.9)
- Triple glazed low e argon
- U-Windows1 W/(m2K)(R1)
- EcoWindows IV78 (4XN -18 ar - 4 - 18 ar - XN 4)
- Ventilation Efficiency 80%
- LWZ 180 (Balance)