Airtightness for NZ's First Straw Bale Passive House

Jessica Eyers of Hiberna recently designed this three-bedroom, straw bale house, which her husband Ben built. The home is the Southern Hemisphere's* first Straw Bale certified Passive House. The build utilises natural, low-tech, locally sourced materials and pairs them with high-tech scientific design and construction principles.

The structure comprises a post and box beam wall system with straw used as in-fill. The airtightness layer is achieved using a combination of plaster on the walls and pro clima INTELLO Plus Intelligent Air Barrier in the ceiling. A special tape, Contega PV, is used to connect the pro clima INTELLO PLUS to the plaster and the plaster to the windows and was very effective in this application. Ventilation is supplied using a Zehnder Q350 heat recovery ventilation unit.

During their research into the heat conductivity of straw bales, they discovered that the orientation of the bale has a great impact on the insulation value. Bales on edge make for a slimmer wall but overall the U-value is greater.

A 3Kw PV system was installed in the roof which will meet the home's electricity needs as well as powering an electric car. Hot water is supplied from a heat pump while the home is heated by two 1KW radiators fed via a scavenging coil in the water cylinder.

Ben initially planned to make the windows himself but eventually decided to source the triple glazed timber windows from a friend in Germany and now imports these as a side business. Polystyrene was used to insulate the window jambs to help achieve the Passive House standard. Jessica will specify timber fibre insulation in the future to fit better with Hiberna’s natural ethos.

The airtightness was quite a challenge due to the two-storey structure and the plaster airtight layer and they ended up just squeezing in at 0.59 AC/hour.

The couple have learned a lot from this build and look forward to many more builds like this for clients in the future

*There are about 15 certified Passive Houses on the database that include straw in their composition. The only other home pro clima is aware of that uses just bales and plaster (no membranes, linings, or cavities) for the walls is in the UK.

INTELLO® is a registered trademark of pro clima

Intelligent Airtightness Membrane (INTELLO PLUS)

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The pro clima airtightness system prevents uncontrolled air infiltration or leakage through the building envelope, and assists the drying capacity of the building through its unique moisture control properties.

Suitable for all construction and where there is risk of interstitial condensation
The protection of insulation from air movement
Preventing uncontrollable air flow though the building envelope
An optional continuous cavity for electrical and plumbing services can be created on the inside face of the external wall by fixing battens horizontally over the INTELLO membrane before installing the wall lining
The main advantage of a service cavity is to shift the plumbing and wiring out of the insulation layer
A service cavity also reduces the penetrations through the airtightness membrane allowing easy and fast installation of services
This also eliminates pressure points on inner lining (eg. plasterboard) from insulation expansion

The NZBC requires only that airtightness be considered, not measured. INTELLO directly assists with meeting the following requirements:

Clause E3.1 INTERNAL MOISTURE: Buildings must safeguard people against illness, injury, or loss of amenity that could result from the accumulation of internal moisture
Clause E3.2 INTERNAL MOISTURE: Buildings must avoid the likelihood of fungal growth or the accumulation of contaminants on linings and other building elements
Clause E2 EXTERNAL MOISTURE: Concealed spaces and cavities in buildings must be constructed in a way that prevents external moisture being accumulated or transferred and causing condensation, fungal growth, or degradation of building elements
Clause E2 EXTERNAL MOISTURE: Building elements must make due allowance for the following: (a) the consequences of failure, (c) variation in the properties of materials and in the characteristics of the site
Clause E2 EXTERNAL MOISTURE: Interior linings with all joints stopped require barriers to airflow
Clause H1 : ENERGY EFFICIENCY: The building must be constructed to limit uncontrollable airflow
Clause H1 : ENERGY EFFICIENCY: Account must be taken of physical conditions likely to affect energy performance of buildings; (c) Airtightness of the building envelope.
Clause B2 : DURABILITY: Performance B2.3.1(a), not less than 50 years, B2.3.1(b), 15 years and B2.3.2

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