CLAY BUILDING MATERIALS
|sculpted cob bench at Black Ankle Vineyards|
Clay building materials are classified as "thermal mass", which means they act as a battery or fly-wheel for heat energy...absorbing heat energy when the environment around is warmer than the clay and releasing heat energy when the environment around is cooler. Clay walls work well as solar heat storage walls (trombe walls) for passive solar designs. Clay materials do not have insulating value, so should be used where high thermal mass, not insulators, provide the best thermal efficiency. (For more information on where to use clay-based materials, see http://buildnaturally.blogspot.com/2010/12/strawbale-vs-cob.html.)
ADOBE refers to blocks, bricks, formed from a wet mixture of clay, sand and long straw or other fibers. Enough water is added to the mixture so that the clay is slightly soupy, the consistency of sour cream, allowing the mixture to be poured into molds without leaving any voids. The top is smoothed over, the forms are removed, and the bricks are placed in the sun to dry. The sun-baking hardens the clay (more than if they dried in the shade, but not quite as hard as firing). When dry, the bricks are mortared together using a clay mixture similar to the original adobe recipe.
COB is sometimes called "sculptural adobe", because it uses the same ingredients as adobe bricks (clay, sand and straw or other fiber), but it is formed, or sculpted, in place...building a monolithic wall system from the bottom up. The main difference between cob & adobe, is that the walls are built while the clay is wet. Wet clay has a limited capacity to support weight before it begins to flow like a liquid, so walls need to dry in place as you build in order to be strong enough to support additional weight above. The dryer your cob mixture, the quicker you can continue to build vertically; the wetter the mixture, the longer you need to wait for it to dry before continuing to build. Cob walls are generally built in a battered form, which means they slope inward as you go up, getting thinner and thinner at the top.
RAMMED EARTH uses a mixture of clay and sand tamped into formwork to create a monolithic wall system. The compression of the tamping mimics natural geologic forces that form sedimentary rock...so rammed earth construction resembles hand-formed sedimentary stone. Rammed earth is denser than adobe or cob, so is stronger and has higher thermal mass per volume of material. Tamping is traditionally done by hand, though modern rammed earth often uses pneumatic machines. It takes surprisingly little effort to hand tamp! Because the stickiness of the clay platelets is achieved through force, the mixture is not wet, as cob or adobe are...the mixture is more damp and crumbly.
COMPRESSED EARTH BLOCKS are blocks made from rammed earth, often using a hydraulic machine that extrudes the blocks. The blocks are often shaped to interlock with each other so they can be assembled without the use of mortar.
|stacked strawbales with bamboo pins|
Straw is best used where you benefit from high insulation. For example if you heat a space in Winter, you want insulating walls to keep the heat inside, saving on energy costs. The variation depends on the quality of installation. Straw is cellulosic, like wood, so it will rot if persistently over 20% moisture content. However, like wood, it can be easily detailed to ensure that it remains dry & durable...bottom line is that if you can build in your area with wood, then you can build with straw. (For more information on how to build durably with strawbales in any climate, see http://buildnaturally.blogspot.com/2011/01/yes-you-can-build-with-strawbale-in-wet.html.)
LOADBEARING STRAWBALE refers to a building where the weight of the roof and any floors are held up by walls made of strawbale. The bales are stacked like bricks and pinned together for stability. A box beam runs across the top of the strawbale walls to distribute any roof or floor loads evenly onto the strawbales. To ensure structural stability, structural strawbale walls should have a ratio of height to width that does not exceed 5.6 to 1 (ie, the height should not be more than 5.6 times the width of your bales) and the ratio of unsupported length to bale width should not exceed 13 to 1 (ie, an un-buttressed wall should not be more than 13 times the width of your bales).
LIGHT CLAY STRAW/SLIP STRAW is a centuries-old material used for insulation between half-timber (fachwerk) buildings. (The name light clay straw is a translation from the German term "leichtlehm", which means "light clay".) Light clay straw is made by coating loose straw fibers with a light coating of clay slip and then packing the coated straw tightly between formwork. The clay dries and binds the straw into a tight, insulating wall, rated at about R-1.5 per inch. Once dried, the walls are often plastered on both sides.
|cordwood & bottle wall with lime mortar|
WATTLE & DAUB is an ancient technique where a sticky clay mixture is pushed though and around woven lattice work. The lattice, or wattle, is formed by weaving thin wood or bamboo around vertical stakes. The daub material is made from a binder (usually clay or cow manure), aggregate (such as sand), and a reinforcing fiber (such as straw). Wattle and daub walls can be quite thin, since their strength is reinforced by the internal lattice work. They are suitable for interior walls, or where exterior walls provide enclosure but do not require high thermal mass or insulating properties.
CORDWOOD construction uses short, debarked pieces of wood plus a mortar to build a monolithic masonry wall system. The wood is stacked, like firewood, so that the cut ends because the face of the wall. Each piece of lumber is cut to a uniform length (usually around 16" to 24"), which then determines the thickness of the constructed wall. The mortar used between the wood can be lime-based or clay-based cob. Cordwood walls perform with a combination of insulating and thermal mass properties. The R-value is around R-1.5 per inch thickness. Cordwood can be appropriate for exterior walls and loadbearing walls. This method of construction should only be used where wood is in abundance, especially where log-ends or salvage wood are available for repurposing.
PLASTERS & PAINT
|durable clay plaster|
CLAY-BASED PLASTERS use sticky clay binders to adhere to walls. Clay is sourced from the earth, often found locally in the soils nearby the building site, meaning the materials are literally dirt cheap and non-toxic. Since clay increases in volume when wet, and then shrinks when dry, sand is added to control cracking (since sand is the same volume wet or dry) and to strengthen the plaster. Clay creates a breathable finish with the natural capacity to regulate moisture and temperature in the surrounding air. Clay plasters are simple to make and extremely forgiving to use, even for beginners.
LIME PLASTER can be confusing, because the term 'lime' can refer to various chemically different (but related) materials. (Not to mention the citrus fruit!) Lime has been used for thousands of years as a fabulous binder in mortars, plasters, and paints. It wasn't until the post-World War II housing boom that quick-setting cement products eclipsed lime in construction. Lime does cure more slowly than cement, but it holds many advantages as a workable, self-healing, breathable, nearly carbon neutral material, making it much more suited to natural building. First, because it has a lower environmental impact. (Cement production creates 1.25 pounds of CO2 for each pound of cement produced, whereas lime is nearly carbon neutral.) Lime plasters are suitable finishes over nearly every natural substrate, and create a breathable (vapor permeable) surface that is weather-proof. I use it for exterior plasters on strawbale as well as in shower enclosures.
TADELAKT is a Moroccan technique for finishing lime plaster that creates a smooth, sensuous, nearly waterproof surface. The traditional application involves polishing olive oil soap into a lime plastered surface, while the the surface is still "green" or plastic, using smooth stones. The polishing process tightens the pours in the plaster.
NATURAL PAINTS describe a variety of paints for a range of applications, all of which are made from natural, non-toxic, non-off-gassing ingredients. Natural paints start with a binder, such as clay, casein (the protein found in milk), egg, beeswax, lime, and others. The binder provides the glue that allows the pigment or color to stick to the surface you are painting. Pigments are generally mineral- or plant-based, so also natural and non-toxic. Not all natural paints are appropriate for every application...for example, clay paints need absorbent surfaces (like unpainted drywall or clay walls), lime paints & washes can be used outside where weather resistance is important, egg yolk paints work on furniture, etc.
FLOORS & FOUNDATIONS
|installing finish layer of an earthen floor|
RUBBLE TRENCH FOOTER is simply a continuous trench footer around the structural perimeter, dug as deeply as the ground freezing point in winter. The trench is lined with filter fabric and filled with stone. A structural (usually concrete) grade beam (a beam that rests on the ground) is poured on top of the stone-filled trench, and distributes the structural loads of the building across the surface area of the trench below. This type of foundation uniquely provides both structural bearing as well as water drainage in a single foundation system. Drainage is important with most foundation systems, since water is the single largest culprit for foundation failure. Liquid water can erode the ground bearing around a foundation footer, and frozen water expands when it freezes, which causes the ground around your foundation to also expand and contract with freeze-thaw seasonal cycles. When installed correctly, a rubble trench results in a resource-efficient, high-performing, eco-friendly, and low-cost foundation footer.
ADOBE FLOORS/EARTHEN FLOORS are made with the same ingredients as adobe and cob: clay, sand, and straw. The clay is the binder that holds everything together. The sand is the aggregate, that makes the floor strong and helps to control shrinking (and thus cracking) as the floor dries. And the straw provides tensile strength, which knits the material together and also helps to prevent cracking. Earthen floors work well where a high-thermal mass (heat absorbing) material is beneficial, such as for passive solar designs. The adobe is installed in one, two, or three layers, depending on the finesse of finish and on construction timing. And is installed over a moisture barrier, several inches of sand or stone to create a capillary break (prevent water from rising into the floor), and usually some insulation to allow for temperature control inside the space. The surface is sealed when the clay is completely dry, usually with linseed oil or beeswax (or both), to create a beautiful, durable, and wear-resistant floor surface.
DRY LAID STONE refers to stone walls that are built without mortar. Eliminating the mortar heightens the importance of fitting each stone to the ones below. No mortar also means no wicking points for moisture to enter the stone wall.
|living roof in Pennsylvania|
THATCH is a highly insulating roofing system made by lashing bundles of reeds or straw (or some other appropriate, locally sourced plant) to battens on the roof. Quality thatch is superbly durable, lasting decades, with the ridge sometimes requiring repair or re-thatching each decade or so. There is a perception that thatch roofs are highly flammable (most because of movies, I think), but in fact, the surface can burn, but then it chars and slows the flame (I've read it described as trying to burn a closed book). Thatch is appropriate in any climate, especially where a highly insulating roof helps keep heat inside during Winter, but should be used on roofs with a good pitch on them (to help shed water).
LIVING ROOFS/GREEN ROOFS are roofing systems that allow plants to thrive on the surface of a rooftop, without access to groundwater...so creating an ecosystem that relies on rainwater alone). Plants used are generally rock garden plants that thrive in your particular climate, and specifically often include succulents, like sedum plants. The two main benefits of planting your roof are absorption of rainwater (especially useful in urban and suburban areas) and a net cooling of the roof surface (to help keep your building cool in summer). Living roofs can also work in any climate, especially where a cooling roof will help keep you nice and cool in Summer. Installation of a living roof is easiest when the pitch of the roof is shallower than 30-degrees.