Passive House Institute

Passive House isn’t a new concept and there are many examples around the world where communities have developed traditional building styles and construction techniques that reflect local climatic conditions e.g. turf houses in Iceland and Ireland, Italian villas and residential buildings in China.  The difference with Passive House is it is a codified thermal standard that provides a much higher standard of performance and comfort than the New Zealand Building Act 2004.

The result is Woodford Grace is warm in winter and cool in summer and the mechanical ventilation and heat recovery system provides further energy efficiency and filtered air reduces allergies and reactions to airborne toxins such as particle pollution.  Woodford Grace provides a very comfortable climate that fluctuates between about 20-22 °C (68-72 °F) and a relative humidity range of 30-70%.  Living in Woodford Grace means traditional New Zealand housing issues such as drafts, mould, mildew, and high heating costs are a thing of the past.

We are very proud to be a Low Energy Building certified by PHI.  This achievement aligns with our desire to future proof the building, live in a warm and healthy home and be more sustainable and environmentally aware.  The PHI standard accurately predicts and delivers a project’s ‘as built’ performance. Its success hinges on a robust design process, employing proven methods and on¬site verification.  All certified PHI houses are included in the Passive House Database, as is Woodford Grace.

Woodford Grace is one of only 6 certified refurbishments in the world and the first constructed in the Southern Hemisphere.  It is also the only retrofit Passive Villa in the world and only the second retrofit to ever be undertaken in timber.  It is currently the largest timber certified retrofit Passive House to be constructed in the world.
A PHI certification means achieving several difficult energy performance metrics. As a PHI Low Energy Building, Woodford Grace complies with many of the features of a fully certified Passive House.  There are 7 features of Passive Houses that are described later.

Orientation and Shading

As an existing building the opportunity to design and orientate the building to optimise solar gains in winter was limited. Additionally the window sizes for the existing building were fixed due to the requirement to refurbish and respect the existing building's style and heritage.  

Fortunately, the building was (and continues to be) orientated to the north (which is best for buildings in the southern hemisphere) so enjoys a good level of thermal gain during the winter season.

Form Factor

Again, the existing building determined the shape and size of a large portion of the home.  Additionally, the new extension is a large irregular geometrical shape which doesn’t easily translate to a low form factor.  

The design of the extension had been approved prior to the inclusion of Passive House in the project and this meant it was not a design led project to achieve the criteria. That meant added complication, but also a stunningly beautiful looking building in alignment with the Architect's vision.

Insulation and Thermal Bridges

The building construction includes a ring and pile foundation, largely timber framing and steel or membrane roofing.  There are some instances where steel portals were required and thermal bridge modelling was undertaken to determine if these met heat loss and internal surface temperature requirements.  

For interest, the NZ building Act 2004 does not require a minimum insulation performance standard.  Rather the required standard is that of a minimum constructed thermal resistance of a specific R-value.  A higher R value reflects greater thermal resistance, but depending on how the construction is designed and installed, can significantly effects how the element performs.  Woodford Grace has an underfloor element of an R7.36, whereas the minimum current building standard when construction was undertaken was R1.3 (increasing to R3.0 in 2022).

Typical building element assemblies are:
• Floor – Suspended timber floor.  140mm PIR Techboard (R7.36) between joists, 17mm ply flooring. U-value – 0.246 W/(m2K).
• Walls existing house – 7mm external ply, 90mm studs no dwangs, Knauf Jetstream Max insulation R2.8, 7mm internal ply downstairs, Intello wrap upstairs, Terra Lana 45mm Masonry insulation R1.2, gib board.  – U-value – 0.437 W(m2K)
• Walls new house – 7mm external ply, 140mm studs no dwangs, Knauf Jetstream Max insulation R4.4, Intello wrap, Terra Lana 45mm Masonry insulation R1.2, gib board. – U-value – 0.204 W/(m2K)
• Roof existing house – Knauf Jetstream Max R7.0, Intello wrap, Gib board – U-value – 0.176 W(m2K)
• Roof new house - Knauf Jetstream Max R7.0, Intello wrap, Gib board – U-value – 0.144 W(m2K)

Windows and Doors

One of the most challenging compliance areas for Passive House is window performance.  

For Woodford Grace, this performance element was exacerbated by the inclusion of original leadlight windows in the home.  All of the original windows and frames were removed and replaced with Ecowood timber frames and Neutralux triple glazed windows with Low-E coating and argon.

All the windows and doors were imported via Eco Windows.  Eco Windows used to manufacture in NZ, but given the choice of window and door styles and the efficiency of buying from a mass supplier in Germany, the company now only imports and distributes.
All the windows and doors were specifically chosen because of the older square style of window and when searching for Passive certified doors and windows these appeared unique in the market.  The windows are very heavy reflecting the triple glazing and if opening, they open both vertically hung (either left or right) and tilting.  All the windows open internally.  The doors open externally or slide open at the corner slider.  Because of the weight of the glazing, it is important to give the sliders a good push.
The externally opening doors are hung from four hinges and they are effortless to open or close.  Several external doors in the house and controlled electronically.  This has the benefit that they can be locked or unlocked remotely and integrated into the security and automation systems.

The Ug value range is 0.48-0.6 W/(m2K)The leadlights were encased inside a double glazing unit with an additional glazing unit attached to the outside.  This resulted in 3 x glass panes, leadlight and argon gas.


A common and popular metric is the air turnover rate.  All houses to a greater or lesser extent leak air through closed windows, closed doors, walls and vents. That is because although the surfaces are solid, there are gaps between surfaces that allow air ingress and egress.  Unsurprisingly most New Zealanders have a story about growing up in a cold and often damp house.

The air turnover rate means ‘how many times per hour the volume of air inside the building is completely replaced’.  A low rate is preferred because it reduces the need to heat replacement cold air and the inherent loss of energy to the atmosphere. Most developed countries specify a minimum air turnover rate within their building standards.  

The NZ building standard does not specify a minimum standard. Unsurprisingly then it is thought most new builds in NZ have a turnover rate of about 4 and given the age of NZ housing, some of which is Victorian (built between 1837 and 1901) the turnover rate can be as high as 40.  

The criteria for PHI Low Energy Building is 1.0 air changes per hour (n50=1.0/h).  The blower door test for Woodford Grace resulted in n50=0.93/h.  During the refurbishment all of the cladding, linings, roof, windows and floors were removed and the underlying structure was exposed.  Achieving airtightness was still challenging as it is very difficult to get a continuous layer around existing structure.  Intello wrap, ply, glue and many rolls of Tescon Vana tape were used in an attempt to fill any gap in the airtightness layer.  

Numerous blower door tests, smoke detection and thermal imaging cameras were also used in the detection of air leaks. The areas that were most problematic were:
- the existing staircase wood panelling as this was left in place during the build and sealing the area could only be completed from the outside.
- The lift and slide doors that form the corner of the kitchen.  These do not seal as well as standard doors.
- The midfloor of the existing house was particularly difficult due to the structure and penetrations through where the layer should be continuous.


Given the house is designed as a Passive system, the house includes a MHRV system that mechanically sucks external air in and pushes that air around the house via a series of ducts.  It was challenging to find space in the existing house to run the large ducts required for MVHR.  Some solutions included a thickened wall downstairs for the ducting to get upstairs and at one point having to run the ducting under the floor in a fully insulated, airtight box.

Sometimes a HRV system can be confused with a MHRV system.  A MHRV is a balanced pressure system meaning the incoming and outgoing air are balanced and typically includes a heat transfer system.  A HRV system is a positive system meaning air, and heat, escapes through windows, doors and the building envelope.

The ventilation system is a Zehnder ComfoAir Q600 MVHR unit.  Air is filtered before being circulated and the heat in the outgoing air transfers to the incoming air via a heat transfer system.  For additional heating a 3kW post heater has been installed to the MVHR and although possible, it is not used.   

The MVHR has been coupled with a ground source heat exchange unit, a Zehnder Comfo Fond with 200m of pipe buried beneath the lawn.  The glycol filled ground loop functions either as a heating or cooling loop dependent on the ambient air temperature.  The ComfoFond pre-heats the air in winter and pre-cools the air in summer,  helping to maintain a comfortable indoor temperature year round.   Again, these small additions create for a more energy efficient and consequently environmentally sustainable home.

Hot Water and Heating

A 16kW air to water heatpump is linked to a 300L hot water cylinder and central heating system.  The heatpump provides three times power efficiency to heat the water to 50C. The hotwater cylinder electric coils top up the water to the required temperature.

The central heating radiator system produces 5.52kW of heat energy.  The system was originally specified to provide 9kW of heating but this has been scaled back due to the efficiencies of the house. Typically the radiators are used during the 3 winter months – June, July and August for a few hours per day.

NZ Green Building Council - Homestar

Woodford Grace has been certified as a 'built' Homestar 8 (assessed as 85.8 points).

Homestar is an independent rating tool developed by the the non-profit organisation, NZ Green Building Council and is used to assess the health, efficiency, and sustainability of homes across New Zealand.  The rating tool is from 6 to 10 Homestar with 8 Homestar being considered ‘best practice’. Key features of an 8 Homestar building include:

- Improved thermal comfort with a maximum heating and cooling demand of 61kWh/m2 for colder climates.  Woodford Grace heating and cooling demand is28kWh/m2
- Sufficient heating to meet the demands of the building.
- Sufficient ventilation to achieve the standard –MVHR exceed the base requirement.
- Water efficient taps, shower heads and toilets.
- Sustainable materials and low VOC paints
- Solar PV system and electric car charging points
- Dedicated cycle parking
- Recycling facilities
- Access to amenities and public transport

In addition to meeting the above features, Woodford Grace has applied for a number of innovations:
- Water efficient appliances – washing machine Water Efficiency Labelling and Standards (WELS) 4.5 star and dishwasher WELS 5 star to reduce onsite water consumption
- Energy Efficient appliances – Fridge 4 Energy Star, Freezer 3 Energy Star and Dishwasher 4.5 Energy Star to reduce on site energy consumption
- First PHI Low Energy Building – recognition of Woodford Grace achieving a refurbishment to world leading standards.
- Automation – Fully automated lighting system that provides energy efficiency through programming lighting circuits for auto cut off.  Integration with alarm system to enhance security settings. Integration with other products including blinds, pool cover, garage doors and other features to improve ease of use within the home.
- Ground loop – use of ground source heat exchange to improve efficiency of MVHR system by pre-heating and pre-cooling air.  This also improves overall comfort within the home.