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Alternative Technology Association
Sydney Central Branch ata.org.au
Hemp lime construction - What you need to know Klara Marosszeky
11 November 2014
Social Housing – The Triangle, Swindon Kevin McLeod
Commercial hemp lime construction – Hemp Technology UK
Hemp’s uses in varying forms as insulation in roofing, flooring and walls l Most common current
use – external walling at min 200mm which is generally rendered externally.
l Internal walling min 150mm thick
l Most architects design for 250mm wall thickness
l R2.7 – 200mm R3.4 250mm
Two storey studio, Kyneton, Vic – Builder Joe D’Alo
1 sq m ‘truth’ window Lime rendered hemp lime walls
Deep Ecology Centre, Maclean
Internal walls unrendered other than around windows and doors Material lends itself to creating nooks
External hemp lime render – no added oxide Other options are clay renders, bagging, limewashing
Clay render, Starseed Nursery Byron Bay
Green Gallery and Heritage Museum, Singapore Botanic Gardens 2013
Modular steel construction, exposed hemp walls
The hemp wall is supported on an L beam and tied into the steel with purlins
Textured finish using weatherboard formwork and colour variations using mineral oxides and coloured river sands
By volume 65% of the building material is hemp hurd Partially processed industrial hemp fibre Bast fibre (outer) and hemp hurd (woody core)
Processed hemp hurd used for construction of walls and for insulation Material needs to be dust free, dry and bast should be removed.
Regional supply –working with small farmers - 11 Kg bags of hemp hurd direct from the farm
11 Kg or 100 L bagged hemp hurd
7.5 batches of Hemp Hurd, Binder and Sand produce a cubic metre of walling material
The Binder and Render – Developed at UNSW and manufactured in Sydney
AHMC Binder 18Kg or 9Kg AHMC Hemp lime Render
The mix The hemp is added first followed by water, the Binder and finally sand
Moist but not wet – just holds together – no slurry – this is a light material
Mixers for hemp lime construction materials
Pan Mixer – 250 l for commercial construction
Planetary, screed or renders mixers – 120l for small builds, owner builds
Options for formwork
Formply with spacers Rammed earth formwork
Kyneton studio External bracing
Forming up fully on one side Using formply or permanent breathable formwork such as MgO board, weatherboards
Combining hemp and other materials The material was originally used in a modern context in the restoration of historic buildings because it adheres to other masonry and according to CAT, Wales removes moisture from damp walls
Retrofit Canberra 40mm thick uninsulated gyprock walls replaced with 120mm hemp lime for acoustic and thermal insulation.
Roofing insulation Hemp and Binder mix without sand, raked in and lightly tamped
Hemp floors in the UK Subfloor insulation or floors slabs finished with a sand lime screed.
Ensuring optimal performance in hemp lime construction l Weather and working with hemp lime construction – 5
degrees C minimum temp
l Storage of materials
l Well made mixes – managing moisture in the mix and in the build, consistency and avoiding balling.
l The slab - engineering and piers, other options – hemp lime should not be used below ground
l Site Drainage
Managing moisture
Core filled bessa blocks Rainforest setting, south facing wall
Ensuring performance
l Framing – timber or steel (protecting the steel) – ensuring a min 50mm in front of studs
l Fixings and fixtures– stainless steel or galvanised screws, conduit, framing for cupboards, fixing to the walls
l To put the roof on first or not
l Building Process - installing the hemp/lime mix - preparation / formwork, moisture, tamping and joints
l Careful curing and finishing after 4 weeks
Curing
l Walls should be protected from extreme winds, rain, and intense direct sunlight in summer for at least week
l Material needs to carbonate rather than dry
l Air circulation and curing l Freshly laid material is
green when forms are first removed and up to 1 week – avoid any knocks
How to reduce waste in the construction process and recycle excess materials
l Clean material that falls to the floor, left over material, or material that needs to removed for some reason can be reintroduced into subsequent fresh mixes at the rate of 10%
l Appropriate equipment – tubs, larger and smaller, renderers hawk and tampers
l If the build is later renovated, the demolished walls could be crumbled and reused with additional Binder
Rectangular tubs – light easy to stack, can be moved on a conveyor
Thermal performance
l Walls built with hemp and lime absorb heat slowly from the air.
l This slow heat penetration means that air in a room will rapidly heat up to a comfortable temperature for occupation, similarly to a lightweight building. (In a heavyweight building this does not happen since the heat transfers rapidly into the thermal mass of the building, causing a slow heat-up response.)
l Research at the Centre for Alternative Technology, Wales has also shown that hemp and lime has the benefit of thermal mass.
l Some of the stored heat energy is released back, thereby maintaining a more stable, temperate environment than a lightweight material.
How hemp walls breathe, eliminating the growth of allergy causing moulds l The composition of the mix l Characteristics of hemp
l Characteristics of the binder and Render
.
Centre of Alternative Technology,, Wales 2009 Dry-lining versus a hemp and lime insulating render for internal thermal renovation of a stone cottage in West Wales " The conditions present in the cottage hemp/lime project at CAT are possibly some of the harshest that might be encountered in the United Kingdom for internal insulation of a solid wall. Even in this harsh environment the hemp lime can be effective at reducing moisture load in the wall.
The moisture spikes in hemp/lime are time dependent and of limited duration. In dry lining the spikes are associated with a build-up of moisture in the wall immediately adjoining the wall/air gap interface. Therefore, moisture spikes in hemp/lime can regarded as a transient issue whereas in dry lining it could lead to long term problems
Dry Lining causes moisture to accumulate in the wall and space behind it which will lead to rot, internal structural damage and increased frost damage. Hemp renders are not only insulating but also dry out the external wall and handle moisture well, suggestive that they reduce the risk of rot, mould and frost damage overtime.
Although from the monitoring it is impossible to discern if the hemp render will dry out the adjacent wall this did occur in three separate WUFI models under differing conditions. This is therefore indicative that the Hemp render will actively dry out an underlying wall”
Hemp agriculture and life cycle
l There are multiple cultivars within the broader categories of fibre and grain varieties
l Breeds based on high fibre biomass landraces are ideal for building materials/ there are dual purpose cultivars.
l Can be grown sustainably without herbicides / chemical fertilisers
l Needs minimally 1/3 water for lucerne
l Can be dryland farmed l Soil remediator l Excellent in crop rotation l Yields are minimally 10 t
fibre per ha (dried). Approx 30% hurd/shiv, 30 % bast / 30% fines
l Planting cycle 4 months to harvest + processing
Ashford, NSW Central West, 2010 Industrial hemp fibre crop at 4 weeks
Mop Crop, Bangalow 2005 Planted at approximately 200 – 250 seeds per sq m
Planting rate 40 Kg seed per ha to encourage growth of wood rather than leaf Leaves fall to the ground as the densely planted plants compete for light
Ashford 2010 4 – 5m plants, minimal irrigation, sustainably farmed Disc mowing, windrowing, drying, tedder raking and round baling
The carbon sink effect of hemp lime construction
l Carbon harvesting capacity of hemp – approx 10 tonnes per ha above ground carbon plus 1 tonne per ha soil carbon
l Small cabin walls only approx 1 - 2 tonnes hurd, 1.5 – 2.5 tonnes Binder, 1.3 – 1.5 tonnes clean river sand. Larger home 4 tonnes hurd
l Binder coats the hemp, the sand fills some of the voids.
l Carbonation – a slow
ongoing curing process achieved through the drawing in of carbon for the atmosphere.
l Carbonation process thought to be approx 50 years
It is claimed that hemp and lime can lock up approximately 110 kg of CO per m3 of wall. 300 year old hemp building, Miasa Mura Japanese National Trust.
