Norris Strawbale Passive House Christchurch

A one of a kind, two-storey strawbale passive house on a tricky sloping site in the Port hills of Christchurch.
​The house achieved Homestar 10 Certification as well as Passive House Certification and Life mark. 
Featuring earthen floors and playfully decorative earthen and lime plasters, the house is the forever home of Elizabeth and Everett who have wanted to build and live in a strawbale house since they first stepped into one over a decade ago.

The building's heating demand is only 14 kWh/m2/a and is fairly airtight with only 0.6 air changes per hour. It has a heat recover ventilation system that saves opening windows in the cooler months. The building has a comfortable temperature all year around between 18 and 25 degrees with very little heating required and due to energy modelling during the design process, the windows are just the right sizes for views, natural light and passive heating with the ideal amount of eaves for shading. 

The house is featured on Grand Designs NZ, Season 10: Click here

​Designer: Elizabeth Norris
Builder: Everhomes
Engineers: eZED Engineers

Certified Passive House with natural materials

It is the third strawbale house in New Zealand to become a Certified Passive House.

A strawbale house differs in terms of the materials used to obtain Passive House Certification which normally include a lot more synthetic material. Instead the clay and lime plasters form the airtight layer of the walls with synthetic airtight membranes and tapes only required for ceilings and junctions. The hygroscopic (internal moisture management) properties of the natural plaster systems, work in conjunction with the Mechanical Heat Recovery Ventilation system to ensure the house remains at optimal humidity levels maximising occupant comfort year round.
 
You can view the technical information with building assembly R values on the international passive house database here: Project 8114

See above a before and after image of a window to wall junction. The blue contega tape stuck to the timber window is about to have the mesh side plastered into the reveal. The finish plaster hides away all the tapes and meshes that make the building more airtight and stronger. Image to the right shows the Intello airtight membrane on the ceiling. ​

​Perfect temperatures all year around

The building is designed with North orientation, windows and shading in mind, North facing rooms with just the right size windows and eaves. During the design process It was energy modelled then tweaked in numerous ways to bring the energy demand below 15 kWh/m2/a. This means, there is just one small heat pump in the living room and a few panel heaters spread about the house that will be turned on only in the heart of winter.  It is warm in winter and just right in summer with barely any heating input and no air conditioning. Another important factor to a passive house is loads of insulation in all directions. This house has Terra Lana wool insulation in the roof and floor and strawbales in the walls. The timber frame windows are triple glazed. Take a look at the Methven Strawable house blog to see what strawbale walls look like before they are plastered. 

Homestar 10 certificate

Attaining a Homestar 10 certificate is not an easy achievement, however with the Norris house performing so well thermally, one third of the requirements were fully met. The other areas that needed to be proved were an Embodied Carbon calculation and various other points were awarded for eco-friendly living such as good Wels start ratings for water efficiency, energy efficient appliances, non toxic materials, native plants, fruite trees, garden beds, compost toilets, elecrtric vehicle charging, bike storage and more. It was a long list. Elizabeth is our in-house Homestar assessor and Passive house designer, so she had the experience to work through everything and achieve a high result. 

Tadelakt plastered walls

The beautiful wall finishes in a strawbale house are what make it so enticing. Tadelakt is a Moroccan plaster technique with lime plaster as the main material. It's smoother than clay plaster and works well in combination with tiles, clay plastered walls and timber ceilings or floors. It was used as a feature in the bathrooms, kitchen and one side of the stairway.

Tadelakt plastering is far more time consuming than clay plastering. It requires multiple steps including a final olive oil soap polished into the wall. This is what makes it so ideal for wetter areas. Pigments are mixed with the white hydraulic lime that has been soaked and made into a putty texture. This is then plastered over a base coat surface. 

Earthen floor overlay

Earthen floors offer an affordable continuous floor surface that aesthetically suits clay plastered walls. The earthen floors look similar to concrete floors but are warmer underfoot and slightly softer. Any colour can be added to the clay / sand mix, as well as textures. The floor is sealed with oils. 

The material is a plaster mix made with mostly clay and sand, which is spread over a membrane and plywood structural surface (in other instances it can be over a concrete floor). In the Norris house it was applied in two coats with a coarse base coat and a thinner finer finish coat that had a blue pigment added to make the floor look like a grey / blue stone. Depending on the applicator, it can be a fairly dry mix or a wet mix. Layers require drying times, sped up with dehumidifiers, fans and heaters are recommended to speed up the process. Once the floor has dried, three coats of linseed or tung oil are applied to create a smooth hard surface.