Kensington Hemp Extension

Under construction

Location:

Kensington, Melbourne, Victoria

Type of Build:

Extension and separate workshop

Owner:

Grant Randle and Shelley Hosking

Designer:

Barbara Weimar Architect

Builder:

Grant Randle - Owner Builder

Size:

80 sq. new;  total 115 sq. including retrofit of part of existing house; plus 15 sq. workshop/brewery

Cost:

$430,000 (est)

Date of construction:

Commenced: September 2014

Purpose of Build:

To provide a new kitchen, dining and living room and a link, between the old weatherboard home and the new extension, containing a music room. The design uses high windows to bring northern light into the extension which is to the south of the existing building.

Choice of Material:

Grant and Shelley wanted to use a zero carbon material in their extension and researched several alternatives including using straw bale, but straw bale would have meant thick walls, taking up valuable floor space on an inner city site. As well as choosing to build with hemp for the walls and as the ceiling insulation Grant and Shelley have chosen to use other low carbon, natural materials, choosing timber frame and beams over steel. Grant and Shelley came across hemp building while researching sustainable design on the internet. They chose hemp for its thermal effusivity resulting from the unique combination of thermal mass, thermal resistance and moisture absorption properties. They were also attracted to the simplicity of the basic materials, the robustness compared with strawbale and the possibility to self build with limited skills.

Inside under construction

Climate:

Zone 6 - Mild Temperate

Local Council:

Melbourne City Council

Obtaining Approval:

The building application was initially lodged with a drawing note referencing HCA (Hempcrete Australia) material “installed as per manufacturer’s recommendations”. It did not explicitly mention the NCC approval. When the binder was changed to the Australian Hemp Masonry Company (AHMC) binder, an engineers report was provided to the private building surveyor. Melbourne City council had no issues with the hemp walls during the planning approval stage and the building surveyor has had no concerns to date.

Construction:

The walls, insulation, MgO Board walls and kitchen installation are being carried out by the owner. A builder was contracted to do the concrete slab, timber frame and roof.

Footings:

Concrete raft slab on ground with expanded clay aggregate balls (Clinka) underneath and EPS (expanded polystyrene) around slab edge to achieve an R1.8 rating for the entire slab.

Floor:

Polished concrete in living and dining room; Cork and linoleum in kitchen.

Walls:

300mm hemp walls, using Australian Hemp Masonry Company (AHMC) binder, cast around 90mm pine frame. Render to be decided.

Windows:

European Timber Windows, triple glazed in the highlights and double glazed elsewhere.

Ceiling:

10mm Magnesium Oxide board on timber battens at 450mm centres, with hemp insulation 150mm thick, using an AHMC ceiling mix of 5.5kg of hemp to 4.5kg lime, and no sand.

Hemp/lime as ceiling insulation. Photo:Shelley Hosking

Roof:

Corrugate Zincalum with silver batts mounted under rafters to provide a thermal siphon effect to limit thermal load from the attic. Pitched roof chosen to provide solar panel mounting without an ugly frame, easier hemp placement at the top of the wall and thermal barrier between roof and ceiling.

Heating:

Wall mounted hydronic heating.

Cooling:

Passive ventilation. Design includes high opening windows to assist with ventilation.

Wall construction:

Grant and Shelley, with family and helpers built the walls using a 120 litre pan mixer purchased from through e-bay. Prior to commencing construction Grant and Shelley had attended a 3 day hemp building workshop with Hempcrete.

Formwork:

Geoplast plastic formwork and OSB (oriented strand board) reinforced external with timber. The 6mm OSB is not stiff enough for formwork on its own. Grant and Shelley only used the 6mm OSB as they inadvertently received it with the frame for bracing. The sheet bracing was replaced with steel strap, and the surplus used as formwork. They have also purchased some concreter’s form ply for formwork.  

Geoplast and OSB formwork

Section Diagram:

Technical issues:

Grant and Shelley found it difficult to get information on hemp building.

Advice/Recommendations:

If using plastic formwork design the house to fit the formwork. Try to avoid double and triple studs in the timber frame by adding timber spacers between the studs.

For the big lintels and beams where they only had 50mm cover, they used permanent formwork of wood wool board from Knauf (Heradesign), rather than risk the hemp falling off.

For window and door openings, Grant and Shelley suggest extending lintels 300mm past the opening so that supporting double and triple studs can be fully encased in hemp.

They also suggest using timber bracing a they had in a workshop they attended..

For more helpful tips see Grant and Shelley’s blog below.

Contact:

Read more about Grant and Shelley’s hemp build at hempcretehome.blogspot.com