All About Green Building Materials
The materials used in the construction of an eco-house are key to its green credentials. There are various green options available, but it is important to consider loadbearing issues when making your decision. Ask your supplier to identify which type of block is suitable for exterior and interior loadbearing walls, as some may be loadbearing, but still unsuitable in an exterior wall. One of the best ways to help the environment is to source materials locally; transport costs are reduced and you may be able to check production methods and the sustainability of sources.
Strengthening the Structure
Laying a block flat, rather than on its edge, can make a significant difference to a wall’s strength. Some compressed earth blocks, for example, have a minimum compressive strength of 435psi on their edge, rising to 2,465psi when they are laid flat. On their edge, most loadbearing concrete blocks have a minimum compressive strength of 1,015psi.
Combining conventional bricks and blocks with green bricks and blocks in the same structure is not recommended. While conventional bricks and concrete blocks are often combined, green blocks should be used consistently. In addition, as green blocks can vary in size dramatically, complete walls should be built with cut blocks rather than alternating sizes.
Types of Green Building Materials
This is an unfired clay brick, commonly used in cobwork. They can be used for non-loadbearing walls or infills in lumber-frame constructions.
The base material of these blocks consists of fibers from the hemp plant mixed with sand and lime.
Compressed earth block
To make this block, clay, aggregates and water are pressed into a mold and dried. Traditional blocks of this nature, such as adobe blocks, were sun-dried, but modern versions are mechanically compressed.
Extruded earth block
This block is molded, or "extruded," into a continuous length in a machine, and then cut into smaller blocks.
This sun-dried block is made from mud held together by straw, and is normally used to repair existing cob buildings.
Clay plasterboard block
As the name suggests, this type of block has been made from clay and recycled drywall. It uses a thin-joint mortar system and may be laid on its edge or on the flat for greater loadbearing strength.
Fired aerated clay block
Although these bricks are fired, their mode of manufacture uses a relatively small amount of energy, lowering their "embodied" carbon dioxide content.
Made from the dry stalks of cereal plants, such as barley and wheat, straw is an excellent insulator, and consequently a good material for blocks.
The most common finishes for green materials are lime-based. Lime mortars and stuccos are considered greener than modern cement and gypsum-based stuccos because, although their manufacture gives off carbon dioxide, it is reabsorbed as the lime sets, making it a carbon-neutral product. Lime is also recyclable and biodegradable.
It is important to ascertain the type of mortar needed for a particular green block. While it will usually be lime-based, the particular type and strength of a mortar may be crucial.
This mortar is so called because it does not harden underwater. It is produced by heating a pure form of limestone to a very high temperature, burning off carbon dioxide and leaving quicklime. This is then mixed with rainwater to form lime putty — a process known as "slaking."
Lime putty: Left to mature for a number of months, lime putty is the raw material in stuccos and mortars that are completely lime-based. It is mixed with sand to create mortar.
Non-hydraulic hydrated lime: Sold in bagged powder form, this has had less water added to it during production than lime putty. It is considered inferior to mature lime putty.
This mortar is produced by heating up a less pure form of limestone than that used for non-hydraulic hydrated lime. The impurities found in the mix include materials such as clay. The manufacturing process for hydraulic lime means that it dries to a more hardened finish than non-hydraulic lime. It is breathable, but is much less flexible than non-hydraulic lime.
The manufacturing process for hydraulic lime means that it dries to a more hardened finish than non-hydraulic lime. It is breathable, but is much less flexible than non-hydraulic lime.
Sometimes small amounts of portland cement are added to lime, to hasten the setting processs. Purists do not do this, due to the possibility of segregation occuring as the mixture dries. Animal hair — typically horse or goat — can be added, as can modern, synthetic products. Minerals called pozzolans, which allow the mortar to harden quicker, are also used.
Horse hair: For stuccos, horse or goat hair may be introduced to the mix. This lessens the chances of cracking in mortar that is still drying, or mortar that is prone to flexing.
When mixing lime and water, be sure to add the lime to the water, and not the other way around. This is particularly important when making lime putty from quicklime, as there can be a risk of explosion. Lime is a skin, eye and respiratory irritant, so wear protective clothing.
The following examples are greener alternatives to the more conventional types of building board. They are made from natural products, so they are eco-friendly and suited for use in conjunction with the other materials shown on these pages. All these products are 100-percent biodegradable, avoiding the disposal problems of drywall, which is often left in landfill. However, recycling options are being developed, such as clay drywall blocks.
The primary component of this board is clay, often bound together with reed and hessian. It offers a direct alternative to gypsum-based drywall. Clay board is heavier and thicker than drywall, and is best cut using a saw or jigsaw.
Manufactured from straw, and free of formaldehyde, straw board can be used for flooring, or wall applications.
This is a drywall alternative made from natural reeds laid side-by-side and bound together to form a rigid board structure.
Reed on a roll
A more flexible version of reed board, this is ideal for ceiling applications and walls on which the studwork may be undulating. This makes it ideal for use in restoration work on walls that have bowed over time.
The forerunner to drywall, wooden laths are nailed to studwork and used as a base for the application of lime plaster. They are extremely eco-friendly as they are produced from a sustainable resource. Sweet chestnut and oak laths are typically handmade using traditional methods.
Both sweet chestnut and oak laths have a rough key because they are made by hand. This makes them ideal for use on ceilings. In contrast, larch laths are machine-made and square-edged, which makes them better suited for use on walls.
Storing Green Materials
Care must be taken when storing green materials. Most do not store well outside so should be kept inside. If they must be stored in the open air, they should be covered accordingly. Although some blocks can form part of a stucco exterior wall, they will be susceptible to rain damage prior to stucco. It is important to note that most green blocks and bricks cannot be used below the damp-proof course level.
Copyright 2009 Dorling Kindersley Limited
Text copyright 2009 Julian Cassell and Peter Parham