|Photo: Nathan Devine|
Culburra Beach, NSW South Coast
Type of Build:
Single storey detached residence.
Kirstie Wulf and Ben Schueler
Kirstie Wulf, Shelter Building Design
123m2 building, 55m2 decks
Date of construction:
April 2012 - January 2015
Purpose of Build:
To provide a comfortable, natural, low energy holiday home with room for guests.
Choice of Material:
Kirstie was looking into natural building materials suitable for construction by an owner builder. She was investigating light earth (also known as light clay straw) when she found an article in Owner Builder magazine on hemp and liked the materials combination of insulation and thermal mass, the
Zone 6 – Mild Temperate
Although Shoalhaven Council had not come across hemp walls before the Council did not have an issue with them, but was quite picky about the wording of the Engineer’s report on the hemp walls and how it referred to the specific sections of the BCA. This held up Council approval by a few months. Council inspectors had no issue with the build during construction.
The build was carried out by Kirstie and Ben as owner builders. The excavation, concrete slab, frame and frame erection, roof and plumbing and electrical were contracted out and Kirstie and Ben and generous friends constructed the walls and the internals and fit out.
Waffle Pod concrete slab on ground.
Polished concrete slab. The slab had 3% onyx oxide added making it a very dark grey. The top of the slab was lightly ground back to expose the mixed river stone in the slab in a “salt and pepper” look. Ceramic tiles were used in the bathrooms and laundry.
Made off site by Truss-T –Frames at Bellambi, near Wollongong. The frame was erected by local builder, Rob Cheadle.
200mm hemp masonry walls externally, constructed using AHMC binder, cast around a centrally placed 90mm structural pine frame. 10mm of AHMC hemp/lime render inside and out. Internal walls Magnesium oxide board finished with
Rylock thermally improved aluminium double glazed sliding doors and windows. Temporary formwork only used around windows and doors. External window reveals were rendered and sealed to the window with colour matched Sikaflex. Internally the Rylock windows came with an attached timber reveal. The revel was sized and the windows placed so that the render finished flush with the timber window reveal, avoiding the need for further trim and creating a minimalist look.
Raked ceilings using hoop pine plywood, left unfinished with a 9mm black shadowline between sheets. Higgins sheep’s wool insulation installed between rafters, R3.5 in kitchen/living/dining room and R3 in bedrooms and bathroom. Air cell R1.5 combined sarking, reflective foil and insulation installed on top of rafters under roof sheeting.
Colorbond custom orb (corrugated profile) in shale grey.
A gas bayonette has been installed but due to the house’s solar passive design and good insulation, to date the house has not required any heating in winter.
Natural ventilation only. Doors and windows were placed to catch summer afternoon sea breezes and are left open on summer nights to cool down the thermal mass in the concrete slab.
Kirstie attended a hemp building workshop with Klara Maroszekky from the AHMC and a workshop with Steve Allin (from Ireland) prior to commencing the build. The workshops both focussed on the mixing and placing of the hemp material, but lacked information on construction detailing to be used with hemp walls.
OSB (oriented strand board) recycled from large packing crates, was used as formwork. The formwork was temporarily attached to the frame with coach bolts and spacers made from rigid conduit. The spacer holes were filled with hemp mix after the formwork was removed.
Hemping up under the rafters was awkward but not impossible. Challenges were also faced with placing formwork and hemp in to small spaces, particularly in an area between two roofs. The frame had been constructed without any direction to the frame builders apart from the specification of 600mm centres for the studs. In a number of locations the presence of triple studs, particularly next to doors, meant that the hemp had nothing to key into and in some spots it pulled away from the frame as it dried. This was resolved by placing crews with large plastic washers through the wall into the frame pulling the hemp wall back to the frame. Dealing with long and thick lintels over large windows also posed some challenges, which with hindsight could have been avoided by using permanent formwork, but such a decision needed to have been made prior to the manufacture of the frame to allow for the extra thickness in the openings of the permanent formwork. Difficulties were also experienced with the application of the render, particularly on the MgO board.
In the design of the house the ease and ability to make the hemp walls needs to be taken into consideration. Similarly the house frame needs to be designed to work with the need for the hemp walls to key in around the frame. It is recommended to test several renders and colours before committing to a particular render.