“This highly informative and sumptuous book shows how the threat of climate change is also an opportunity to make buildings more beautiful.” —Sunand Prasad, president, Royal Institute of British Architects (RIBA)
Using Natural Finishes: Lime and Clay-based Plasters, Renders and Paints: A Step-by-Step Guideby Adam Weismann, Katy Bryce
With step-by-step instructions for applying lime- and clay-based plasters, renders, and paints, this manual features information on the benefits of natural finishes for personal health, the environment, and buildings. Drawing on traditional methods and materials for using lime and clay finishes on new and historic buildings, the handbook presents design details for
With step-by-step instructions for applying lime- and clay-based plasters, renders, and paints, this manual features information on the benefits of natural finishes for personal health, the environment, and buildings. Drawing on traditional methods and materials for using lime and clay finishes on new and historic buildings, the handbook presents design details for using natural finishes to create healthy and eco-friendly homes and buildings. A comprehensive and up-to-date online resource guide to suppliers, practitioners, and courses is also included.
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Using Natural Finishes
Lime- & Earth-Based Plasters, Renders & Paints
By Adam Weismann, Katy Bryce, Christian Topf
Green Books LtdCopyright © 2008 Adam Weismann and Katy Bryce
All rights reserved.
Buildings and natural finishes
Lime- and earth-based plasters, renders and paints are highly suitable, if not essential, for use in conjunction with sustainable and traditional wall-building materials. Examples include earth (clay), timber, hemp, reed, straw-bale, low-energy bricks and stone. Many of these traditional walling materials have been used successfully for thousands of years and are now enjoying a renaissance, as the foundation for many of the contemporary 'eco-building' methods. A unifying characteristic between these walling materials is that they are all, to a degree, porous. They allow the free passage of moisture into and out of them, and therefore need to 'breathe' to remain healthy. Because of this, they are highly responsive to changes in relative humidity and temperature in the atmosphere. This means that structures made out of these materials will naturally move in response to these differences. All finishes applied to these materials must therefore match the hardness and porosity of the walling substrate below. They must be flexible enough to accommodate natural movements in the building without cracking, and need to have a high degree of vapour permeability to eliminate the potential for trapping moisture within the wall. A case can also be argued for the use of lime- and earth-based plasters, renders and paints (with certain preparatory measures in place) on some of the modern, conventional building materials, such as concrete block and plasterboard. Though their qualities of softness, breathability and flexibility are not essential when used with these conventional materials, lime- and earth-based finishes can bring benefits in the areas of decreased impact on the environment (their production, toxicity and ease of disposal), improved internal air quality, and by improving the look and general 'feel' of the building and the spaces inside.
This section introduces different walling systems that are compatible with lime and earth-based finishes. It also outlines the suitability of applying the various finishes to different substrates.
Earth walls can be constructed in many different ways. Building with earth (clay-rich subsoils) is the most ancient of all building methods. Many variations of earth building have been used throughout the world since man began building shelters. Most earth building methods consist of mixing together a clay-rich subsoil with other ingredients. These are most commonly some form of fibre (straw) to provide tensile strength, and aggregates to provide strength and stability to the mix. Most buildings made out of earth must be raised off the ground at least 450 mm (18"), and should have large roof overhangs to protect the walls from rain.
Many of the earth building methods are currently enjoying a revival because they provide a sustainable way of building. This is because most forms of building with earth have an incredibly low environmental impact, especially when locally resourced materials and simple techniques are used. Earth building materials can be indefinitely recycled, and biodegrade easily. They have tremendous health benefits, such as temperature and humidity regulation, high thermal mass (they can absorb and store large amounts of heat), good toxin and odour absorption, and excellent sound insulation properties. Some of the earth building variations include:
Monolithic earth walls, such as cob, clob, wychert, clom, mud, clat, clay & clunch
A load-bearing walling system – an ancient building technique and material that is used throughout the world, with regional variations, such as 'cob' in the southwest of England. It is composed of clay subsoil, aggregate and straw. These ingredients are mixed together with water to produce a homogenous, malleable and sticky material, which is laid in 'lifts' of 300600 mm (12-24") in one building session. Once the previous one has hardened, consecutive lifts are laid until full wall height has been reached. The cob is built up without forms or shuttering, but is compressed in place by foot or with a garden fork. It is then trimmed with a sharp spade to maintain a plumb line.
Unfired earth blocks, such as adobe, cob block, and clay lump
A load-bearing material used throughout the world for thousands of years. This is a system of creating building blocks out of raw clay-rich subsoil, aggregate and sometimes fibre. The material is either moulded in a plastic state into forms, or compressed by machine in a dry state. The blocks are laid in bonded courses, with either a mud or lime mortar, or simply wetted down and bonded through suction if the blocks are made with dry material.
Rammed earth & 'pise de terre'
Monolithic earth walls built up between temporary shuttering. A load-bearing material, which is sometimes mixed (slightly moistened) with aggregate, or it can be used in its raw form if the right proportions naturally exist. It is tamped by hand or with a pneumatic tamper, between wood or steel shuttering. The shuttering is moved up until full wall height is reached.
Light clay & light straw clay 'Leichlem' (pronounced 'lie-klem')
A non-load-bearing walling material. Straw is coated with a clay-rich slip which is compacted between temporary shuttering, set within a timber structure.
Wattle & daub
A non-load-bearing walling system of tightly woven sticks (usually a green and flexible wood, such as hazel, willow or maple), set within a timber-framed panel. This lattice (the wattle) is then coated with a 'daub' mixture: a thick clay-rich subsoil mixed with chopped straw or hair, and sometimes animal dung for extra weather resistance and durability. This is squeezed into place between the sticks. The daub can either be finished with a limewash or coated in a lime plaster/render for extra protection. It can be used for exterior walls or interior partition walls.
All lime- and earth-based finishes are ideally suited for all types of raw earth construction. Cement and synthetic paints must not be used.
Straw-bale wall construction
Straw-bale wall construction originated in the USA around the end of the nineteenth century. It coincided with the development of baling machines. The straw bales are used like large building blocks, stacked on top of one another in staggered courses. They are usually speared onto pins – often sharp rods of hazel or steel. This ties them into the foundation and provides structural stability for the walls. There are many different methods of building with straw bales, but these can be broadly categorized into (a) load-bearing, where the bales take the full weight of the roof, and (b) non-load-bearing, where the bales are set within a timber structure and used as a wall infill between the posts. As with walls made out of earth, it is essential that the bales are built onto a raised plinth so that they do not come into contact with water. They also require large roof overhangs to direct moisture away from the wall face.
Straw-bale construction generally has a very low environmental impact, especially when materials are sourced locally. The use of large amounts of plant materials in buildings has the added advantage of being able to create 'carbon sinks'. This is due to the absorption of CO2 from the atmosphere as the plant grows, and turns this CO2 into oxygen. The more plant material that can be used in building construction, the more potential there is for decreasing the damaging levels of CO2 currently present in the atmosphere. Straw-bale walls are Rammed earth Light clay highly breathable and have excellent insulative properties. This gives straw-bale buildings the potential to provide a very healthy living environment. Straw bales are fully biodegradable and will last for hundreds of years if protected with breathable coatings and utilised within a well designed building.
Straw-bale walls must be protected from moisture, and need to be sealed to keep out draughts and protect them from hungry animals.
Straw is a breathable material, and hence must not be coated with any nonbreathable renders, plasters and paints, such as cement and synthetic paints. These could lead to moisture getting trapped in the bales, which will eventually cause them to rot. Lime- and earth-based finishes are ideal. Straw-bale walls that are exposed to extreme conditions, such as on the weather-facing wall, may require additional protection from the elements such as timber cladding.
Lath & plaster
Lath and plaster is made up of thin, narrow strips of wood which are attached to wood battens, joists or studding and then plastered. This system can be used as an external and internal wall structure when set within a timber frame. It can also be used to create ceilings. Alternatively it can be used as a system on solid walls which are unable to receive plaster/render directly, such as impervious materials or damp substrates. The substrate is battened and then lathed with a suitable air gap for adequate air circulation. The wood laths can be split by hand (riven lath), or mechanically sawn. Straight-grained wood is necessary, and hand-split laths are considered to be stronger. Sweet chestnut, oak and Scots pine are the most suitable woods to use. The laths can vary in size, but the optimum size is 30mm (1¼") wide, and 5-6mm (¼") thick, at lengths up to 1500mm (4' 6"). They must be attached with non-ferrous or galvanized nails to prevent rusting.
The laths are fixed parallel to each other and spaced at regular intervals of approximately 10mm (3/8") apart. The vertical wood supports are spaced at 300mm (12") centres, and break joints are provided every 10-12 laths. The ends of the laths must be butted at a distance of approximately 3mm (1/8") to allow for potential swelling of the wood, as it absorbs moisture from the wet plaster. A well-haired lime mortar (not earth) is then applied onto the lath. The spacings between the laths allow the mortar to squeeze through the gaps, creating a 'hooking' action to attach the mortar solidly in place.
Lath and plaster can have a low environmental impact as long as sustainably harvested wood is used, and especially when hand-splitting (riven laths) methods of production are employed. They are non-toxic, breathable and fully biodegradable.
Reed mat consists of sturdy lengths of reed, bound together with a zinc-coated wire. Reed mat usually comes in rolled bundles of 10 metres (11 yards) in length, 2 metres (6' 6") high and 8 mm (5/8") thick. These mats are attached (similar to lath and plaster technique) horizontally onto wood uprights with sturdy staples. They can be used to create internal partition walls and ceilings, and are well suited for creating curved surfaces, due to their flexibility.
Reed mat can also be cut into smaller sections (with secateurs or a jigsaw) and used to prepare wall substrates for plastering or rendering. Examples include covering differential materials within a wall, such as lintels and timber uprights, or sections of repair. Reed mat provides a good alternative to lath and plaster as it is more economical, but it does not provide as solid a backing. Reed mats are perfectly suited to receiving lime and earth plaster/renders.
Reed mat has a low environmental impact during its production, and is easily biodegraded with the exception of the minimal amounts of zinc-plated wire that binds the reed together. It is also highly breathable.
Masonry buildings are constructed out of individual building units which are laid in and bound together by mortar of varying types.
They must always be built up in staggered courses with no vertical joints, in order to tie the wall structure together.
There are many different types of stone used in building. This is a reflection of the varied geology across the world. Broadly speaking, stones can be categorized into the softer, more permeable stones, such as the sedimentary rocks of limestone and sandstone, and the harder, less permeable stones, such as the igneous rocks of granite, gabbro and basalt. Even within these categories, however, there are regional differences in strength, porosity and weathering between stones of the same type. For example, there are some varieties of hard limestone and sandstone, and some porous, soft granites.
Locally sourced, naturally occurring field stone has a low environmental impact, and can be eternally reused as long as suitably soft bedding mortars, such as lime, are used. Soft mortars can be easily chipped off without damaging the stone. The environmental impact of quarried stone depends on the methods of extraction employed, as well as the distance the quarried material is transported. Quarries can also disturb the natural environment.
On its own, stone is a poor insulator, but has high thermal mass. It is non-toxic and generally breathable, with the exception of very hard, impervious types. The determining factor for making a decision on which plaster/render is suitable is down to the hardness and porosity of the stone. Softer, more permeable stones will partner well with earth plasters and the weaker building limes, whereas a harder, less permeable stone will be better suited to the stronger, hydraulic building limes. Some less permeable stones will require a priming coat of some sort before they are suitably placed to receive a lime or earth plaster/render. However, the priming coat should not inhibit the vapour permeability of the stone.
Traditional bricks are made from clay, which is fired at high temperatures. The firing process drives off the water and creates an irreversible chemical reaction such that the clay will not be returned to a plastic state with the addition of water. These bricks remain relatively soft and very permeable. Many modern bricks are made from sand or flint mixed with lime. They are not fired, but are moulded under steam pressure. This method produces a much harder brick, with strength and permeability characteristics more in line with concrete (a reduced permeability).
As with stone, traditional porous bricks will work best with the softer, highly breathable finishes of earth and the weaker building limes, whereas modern bricks are more compatible with the stronger, hydraulic limes. They may require a priming coat before receiving a first coat of lime plaster/render. Producing bricks is a very energy-intensive process (especially modern bricks), but they can be recycled many times over as long as a soft lime bedding material is used. Bricks will also last for a very long time if laid and finished with suitable mortars, such as lime or earth.
Fired clay honeycomb insulating blocks
These consist of a fired clay block with a honeycomb cross-section. They are suitable for use for both external and internal load-bearing walls, and have been widely used on the continent for many years. The honeycomb structure of the block means that it is only necessary to create a single skin, because the block itself provides excellent insulation properties. They can replace conventional cavity wall construction. Mortar is only necessary for the horizontal joint, due to a tongue-and-groove vertical edge which locks the abutting blocks firmly together. They are ideally suited for use with earth and lime mortars and finishes, and make highly breathable buildings.
In their most basic form, concrete blocks consists of cement, sand and aggregates. Concrete blocks are relatively cheap to produce, and are one of the most widely used construction materials for external walls in the UK and throughout most of the world. Other types of concrete block include lightweight concrete blocks, such as breeze block. There are also aerated concrete blocks, which are made from cement, sand and lime. They both have better insulating qualities than the standard blocks. Concrete can also be cast on site in shuttering. Cement finishes are most commonly used with concrete materials, but earth and lime finishes can be used, with suitable preparatory measures, to enhance the look, feel and internal air quality of a building.
Excerpted from Using Natural Finishes by Adam Weismann, Katy Bryce, Christian Topf. Copyright © 2008 Adam Weismann and Katy Bryce. Excerpted by permission of Green Books Ltd.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.
Meet the Author
Adam Weismann and Katy Bryce work with lime and clay on a daily basis through their company, Cob in Cornwall. They learned these skills through restoring ancient vernacular buildings in Cornwall, and then began to apply the traditional techniques and materials to contemporary eco builds. They are the authors of Building with Cob.
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