Methven Strawbale House - Under Construction

With the spirit of building with locally available materials, the owners were keen on using strawbales from nearby wheat fields to insulate their walls. The plastered strawbale walls are a key feature of the house interior and exterior and complement feature timber elements. The house is also aiming for Homestar 7, and performs well in terms of energy efficiency. 
For the Construction, we acknowledge Stu Bell Construction and Everhomes.

​HOUSE PLAN​
​The building extends as a long simple form to ensure that all bedrooms and living spaces are facing North. There is a feature tall ceiling in the dining living room with access to an outdoor living space either East or West, depending on which way the wind blows. The floor plan accommodates a family of four plus guests as the owners are originally from overseas.
 
House Plan Insulated zone = 182.47m2
Garage, carport and sunroom =105.78 m2 

STRUCTURE

• ​Poly-block concrete foundation (with underfloor heating).


• Timber frame walls and truss roof with some sloped frame roofs.


• The wall framing is unique with the wide double stud frames to suit the 350mm wide strawbales. There is a continuous insulated ply box beam around the top perimeter which the functions as a top plate and lintels. This is used to compress and support the strawbale walls.

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INSULATION
Insulated slab, 350mm thick strawbale walls and 140mm timber stud gable end wall, double glazed thermally broken NK windows and doors, recessed to ensure the thermal part of the window is in line with the thermal part of the wall (unlike many buildings built to code minimum).
Insulation R Value:
Roof: 7
Strawbale walls: 6.63
Slab: 2.6
Windows: 0.71
Skylights: 0.5

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CARBON STORAGE
Twice per year in Canterbury grains are harvested. Strawbales for houses are the bi-product from wheat or other grain production harvested in February. The straw is dry, bailed and stored under cover ready for the house build. 
Overall, strawbales are sequestering carbon. Andrew Alcorn calculated the embodied CO₂ emissions of straw bales in Aotearoa as being -210Kg/mᶟ (2010, p.54). See this link to a paper on "Counting straw: the capacity of New Zealand’s grain growing sector to supply straw for construction" by Min Hall, AUT

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STRAWBALE CONSTRUCTION PROCESS
​After the structure is complete, and the roofing is on, the strawbales are packed into the large wall openings between the double stud frames. It is important the strawbales are kept dry from farm, to barn, to building site. Some strawbales must be re-sized with special techniques to fit into the gaps. The opening space is slightly smaller than the stacked strawbales so there is a tight compressed fit. Before the final strawbale is fitted into place, jacks and ropes pull the bales down, these are released and the bales compress against the ply box beam.

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SYSTEMS AND AIRTIGHTNESS
The building has an internal airtightness membrane and a heat recovery ventilation system. This ensures that moisture is removed, and introduced fresh air is pre-warmed. Airtightness layers also improve the thermal performance of the house. The ceiling airtight membrane is Intello, a product by Pro clima. The earthen clay plaster on the interior walls of the strawbales forms the wall airtight barrier. 

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​The building is heated with underfloor heating in the slab, and the house is oriented towards the sun for passive solar design, with eaves overhang to prevent overheating in the summer, but to allow the sun in during the winter.
The building will undergo a Blower door Test to determine the amount of air changes per hour occurring through the building envelope. The house is targeting Homestar 7 which for this particular house requires only 1 air change per hour.
​(Learn about blower door tests here).
The house is expected to use 52.7 kWh/m2 per annum (ECCHO model). 

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EXTERIOR
​The main claddings are direct earthen plastered strawbale walls with lime plaster and silicate paint finish coats.  Suitable walls for earthen and lime plasters are determined using the Risk Matrix included within Appendix E of NZS4299:2020 'Earth Buildings not requiring Specific Engineering Design'.

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Any walls outside of the constraints (eave width, weather exposure etc.) of the Risk Matrix require conventional cladding over the strawbale walls. To this house, the wind exposed gable end walls have vertical timber cladding is installed over the strawbale to maximise weather protection. ​

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INTERIOR
​The interior plastered strawbale walls have three layers of a clay-based plaster. The total thickness is 40mm which provides a smooth surface over the strawbales. This clay plaster not only looks beautiful but also naturally moderates humidity in the room, maintaining optimum humidity levels for occupant comfort.  Clay occurs naturally in a variety of colours, but if the local clay is not suitable for the design of the house pigments that can be added to the plaster to give colour variation. They can also be painted over although pigment in the plaster itself looks better. Artwork can be painted on to the plaster or embedded or carved within it. It is possible to form beautiful curves and even complex geometric patterns with plaster.

The majority of the floor will be an ‘earthen floor’ overlay. The insulated concrete floors will have a 15 to 20mm overlay of earthen plaster, much like the plaster applied to strawbale walls. This is polished and finished with three coats of linseed or tongue oil. The earth floor provides thermal mass whilst also providing a softer more comfortable surface to live on than concrete.

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