THE PHILOSOPHY OF NATURAL BUILDING
The natural, ‘green’ building movement in the Western world has helped us to reconnect with our tradition of self-reliant shelter. With cob (a mixture of clay, sand and straw), you take the free building materials from the ground beneath your feet and shape it to make floors, walls, plasters – places of release, relaxation and retreat.
Most of the buildings that we live and work in are soulless, non-sustainable and ugly. You close off your senses when you are inside of them. Earthen architecture feeds your soul. It helps you to feel good. It elevates your daily life. The days of dominating Nature are gone. You need a sustainable architecture that reestablishes your place in Nature where you are constantly reminded of the wonders of the world around you. In earthen architecture, your spirit will soar.
In order to create structures that center on the concept of sustainability, 5 basic traits need to be considered:
- Low Construction Impact: Building, by definition, is initially a destructive act. Land usually has to be at least minimally cleared and reshaped, holes need to be dug, and material resources refashioned to serve the building. A green building minimizes its impact on the building site and the environment at large through careful, conscious design and by utilizing replenishable materials that create a minimum of ecological destruction through their use.
- Resource Efficiency Throughout the Life of the Structure: The impact of a building’s construction is only part of the story. Once a building is built, people move in and use it. This human use requires environmental resources for such things as heating, cooling, water, and electricity. A green building provides these human needs efficiently, conserving resources.
- Long Lasting: Natural resources in the form of building materials, tools, and fuels, as well as human energy and ingenuity, come together to create a building. The longer that building lasts, the longer the time before the environment is asked to give up those resources again to replace the building. Therefore, the longer a building lasts, the greener it is.
- Nontoxicity: To sustain healthy lives, you need to sustain a healthy indoor and outdoor environment. A green building needs to provide a healthy indoor environment while doing nothing to harm the outdoor environment.
- Beauty: A green building or home should be seductive enough for you to long to be there for the many years that it will take to nurture it and it’s own little ecosystem, effectively creating and maintaining the sustainable lifestyle or lifecycle.
THE TIMBERLESS COB LODGE
- It can be built quite easily and quickly and in addition it requires no extra skills
- The materials needed to build it are easily accessible in most regions – such as mud and other additives including straw and dung
- Women, children and the elderly can be a major force in the construction
- The interior temperature is preserved as the mud acts as an insulator
- The absence of any wood in the building materials helps protect the environment reducing the consumption of wood
- This type of home (circular construction with domed roofs) can be traced back to the ancient Nubians which live in the North of Sudan.
- Can be built through traditional community building techniques. Neighbors, family, friends and natural building enthusiasts help in building the house in return for food and drink. This will significantly reduce labor costs.
- Resistant to fire and insects
The Gaudet House c. 1830, Lutcher, Louisiana
French Acadienne house in Lyon, France
Modern cob cottage, Pacific Northwest
OREGON COB TECHNIQUE
“Oregon Cob”, invented by Ianto Evans, Micheal Smith and Linda Smiley, is a technique used to make cob that incorporates a high sand content and long, strong straw with just enough clay to bond the mix together. This cob shrinks very little when it dries. The emphasis on precisely adjusting the mix according to the quality of ingredients produces surprising strength. It allows you to sculpt strong but thin earthen shelves, and interior partitions as thin as two inches.
What Cob Does Best
Cob Insulates from Temperature Change: The rule of thumb is that heat flows through cob at an inch per hour, so a two-foot thick wall takes about 24 hours to transmit the effect of heat or cold all the way through. In reliably sunny conditions, a foot-thick wall can keep a building coolest in the daytime, warmest at night.
Where daytime temperatures are very hight, as in continental climates, cob’s massive walls moderate that heat, absorbing it during the day and then releasing it each night as the temperature drops. Where days are often sunny but nights are cold (even if sunny days are cold), the great thermal mass of cob will soak up and store the sun’s energy, then release it over hours or days.
Cob is a good thermal choice, also, where the air is warm while the ground stays cold, as on hot spring and summer days in most of Canada and in the United States west of the Rockies above the 38th parallel. This is true even if there are hot nights for short periods, provided the constant ground temperature at the site is below about 50ºF.
Cob Complements Passive Solar: Cob is useful in passive solar buildings as storage for sun-heat, especially when used for floors and interior walls, and with heavy, natural planters. Cob is of moderate density, with a specific gravity of 1.2 to 1.9, and provides much better insulation than rock, brick, or concrete.
Organic Shapes Are Easy: For creating buildings with irregular, curved, organic shapes, cob is ideal. Curvilinear, thick-walled spaces feel larger than rectilinear ones of the same measured area. Spaces can thus be designed tiny, fitting snugly around the uses they protect and responding to personal spatial needs rather than creating use-neutral containers. Curvilinear buildings require less space and thus less heating and cooling, less maintenance, and fewer resources.
Corners Aren’t Necessary: Where cold winds are a problem, as in the Great Plains, cob’s rounded aerodynamic shapes reduce infiltration of cold air. Whereas boxy shapes create areas of extra-high and extra-low pressure surrounding the building-sucking warm air out and forcing cold air in through the tiniest cracks-more rounded buildings lack corners to protrude into the cold wind. Interior corners are the coldest parts of a building, as outdoor winds speed up there, moving heat away faster.
Cob Rewards Time and Creativity, Not Money: Cob is a good choice if you are short of money but rich in time. Cob is simple enough to learn mostly by practicing. Very little can go dangerously wrong; almost nothing is irreversible. At worst, even if time is wasted, money is not lost.
Cob Insulates from Sound: Cob is a poor transmitter of sound and is useful where outdoor noise-from highways, railroads, flight paths, factories-is a problem. An earthen house in Toronto, build in 1827 with 20-inch walls, is still lived in. The current owner states that one great advantage is sound protection from the railroad that passes only fifty feet away. Not only can cob keep noise out, it can keep noise in, for example from a machine shop or music practice studio. Additionally, cob is well suited to surface modeling for sound absorption.
Cob Works Where Other Earthen Techniques Don’t: Cob is often useful where other earthen techniques are undesirable, for instance, in place of adobe in cool damp regions, instead of rammed earth if machinery is too expensive or unpleasant to work with, or where materials for wattle are scarce.
Cob Is Safe for Inexperienced Builders: There is no need for mechanical tools or power equipment on the site. The building process is safe because the building has no heavy components, no sharp parts, no toxic chemicals. Mud pies are not at all intimidating to people new to construction. Materials are familiar and almost impossible to misuse or waste.
Cob is Democratic: Because cob is built by the handful (and it fits any size of hand), it is making construction accessible to women, children, the elderly, and the frail. Other building techniques in the modern Western world, it seems, were designed to require the strength of energetic young men. Concrete blocks, sheets of plywood, bags of cement all come with the assumption that you are active, healthy, muscular and with a good back. By contrast, cob fits almost anyone’s physical strength. You can use any size of unit, any weight of loaf to build with. A mix can be any amount of material that you can roll. There are no heavy loads to lift.
Cob is Fire-Resistant: In forest fire zones and arid areas, local governments often mandate nonflammable roof materials. With cob and an earthen roof, the whole structure is non-flammable. In the tremendous bush fires in 1994 that almost surrounded Sydney, Australia, the only surviving building in one entire neighborhood had an earthen roof.
Cob is Wind-Resistant: In hurricane, cyclone, and tornado regions, the solidity of cob is protective-like hiding in a basement. Windows should be shuttered and their sizes kept smaller in such conditions, and you should either use a heavyweight earthen roof or anchor your roof very securely in the walls.
Remote Building Sites Aren’t a Problem: Cob is a good solution on remote sites where it is difficult to import building materials or where lumber and other processed building materials are expensive.
Cob is Durable: Well-maintained traditional cob buildings have lasted for centuries, possibly millennia, without major repairs. Wood-framed houses built today often need major repairs in ten to fifteen years.
Preparing the site
A significant amount of water should be stored next to the work site and the site cleared from rocks, trees and grass.
Making the cob
Will form the walls and inner roof.
To start, the clay must be light and contain no less than 30% sand. Water is added with additives such as straw or dung to reduce the elasticity of the clay as well as reducing its weight. The mixture needs to be stirred continuously which can take some time until the correct consistency is achieved.
Marking the Foundations
On a at clean surface an appropriate point for the center of the building is chosen and an iron peg is put in the ground to mark it. A piece of rope or tape is tied at one end to this peg and at the other end to a peg that is used for marking. This device can now be used to mark the position of the foundations. The peg is used to mark a circle on the ground with a radius. A second circle is drawn in the interior to mark the distance of the wall. Once the lines have been drawn chalk or lime can be used to embolden them.
Building the Foundations
The 50 cm wide area for the foundations is dug to a depth of 50 – 70 cm. This small depth is enough due to the light weight of the building that will sit above these foundations. Accuracy is important when digging the foundations and a measuring device such as a tape measure should be constantly used to check the distance. The depth of the foundations will vary depending on the quality of the soil; if the soil is very sandy a deeper depth is required.
Once the hole is dug, the bottom 40 cm (if the hole is 50 cm deep) is laid with a combination of rocks, gravel and sand.
The second stage is to build the flood protection layer which is built 50 cm above the surface also using stone, they are bound with a small amount of clay binder to act as cement.
Plastering the base
The bottom 2 ft of the house are plastered with a lime mix for flood protection and also to stop the damp coming up through the soil.
ALTERNATIVES TO PORTLAND CEMENT
Cement, or some form of cementing material is an essential ingredient in most forms of building construction. Cement is the vital binding agent in concretes, mortars and renders, and is used for the production of walling blocks and roofing tiles.
Since its invention in the first half of the 19th century, Portland cement has become the most widely available cementitious material. Its dominance over alternative cements has been in part, due to successful, aggressive marketing. This is despite its clear technical disadvantages for certain applications. In addition Portland cement is relatively expensive to produce and is often in short supply in many developing countries. Typically, a rural African labourer may need to work for up to two weeks to earn enough money to buy one bag of cement. In comparison alternative cements can be produced locally, on a small scale and at much lower cost.
Alternative cements are not capable of replacing Portland cement totally, but they can be used in the many construction applications where they have advantages. These are as mortars, renders and non-structural concretes. Alternative cements are not normally considered suitable for structural applications such as reinforced concrete beams and columns.
The most common of these so-called ‘alternative’ binders is lime, to which other materials, known as pozzolanas, can be added to enhance strength and water resistance. Other binders such as gypsum, sulphur, bitumen, mud and animal dung have also been used.
Binding systems from history
The simplest, and possibly the earliest, binding material used was wet mud, and there are records of its use in ancient Egypt. Another example of a binder from the distant past is the use of naturally occurring bitumen by the Babylonians and Assyrians in their brick and alabaster (gypsum plaster) constructions.
Lime was known to the Greeks and was widely used by the Romans. The Roman architect and engineer Vitruvius published the first specification for the use of lime in building in his celebrated work De Architectura. The Romans also knew how to make a lime-pozzolana cement by adding materials such as volcanic ash or powdered bricks, tiles and pottery to lime.
That lime is an appropriate and durable binding material, especially when mixed with pozzolana, is well proven. The Pont du Gard at Nimes in France, a Roman aqueduct built in AD 18 with hydraulic lime-based mortar, is still waterproof; the excellence of the mortar is attributed to the selection of the materials and to the time spent tamping the mix into place during construction.
The rebuilding of the Eddystone lighthouse in the English Channel by John Smeaton in 1756 is a more recent development in ‘lime technology’. It was achieved through Smeaton recognizing the hydraulic properties of lime that result from the burning of a clayey limestone. To make the highly water-resistant mortar needed for bonding the courses of stone, he thoroughly mixed this already hydraulic lime with an equal proportion of imported Italian pozzolana (so adding extra ‘hydraulicity’ to the mortar).
Why to use alternative cements
Major advantages of alternatives to Portland cement are that they are usually cheaper to produce, needing much lower or even negligible capital inputs to get started, and requiring far less imported technology and equipment. They can also be produced on a small scale to supply a local market resulting in greatly reduced transportation costs and a much greater degree of local accountability in the supply of building materials.
From an environmental angle lime-pozzolana cements can be produced with lower energy input than either lime by itself or Portland cement – giving a half to one third consumption in use compared with Portland cement and about one fifth compared with lime by itself. Low energy consumption is particularly prevalent with naturally occurring pozzolanas, or those from waste materials, which might need little additional processing other than drying. The use of clay as a binder, of course, results in negligible energy consumption in production.
Pozzolanic cements additionally have numerous other technical advantages to the user:
- Improved workability
- Improved water retention/reduced bleeding
- Improved sulphate resistance
- Improved resistance to alkali – aggregate reaction
- Lower heat of hydration
Social advantages of alternative cements to Portland cement include the potential for affordable quality housing.
The technical and economic advantages of alternative cements are not lost on architects and engineers from developed countries. Increasingly, architects, are becoming aware of the brittleness associated with Portland cement mortars, for example, and are now specifying blended lime/Portland cement mortars instead. As well as re-discovering the ‘lost arts’ of using alternative binders, recent research has enabled the properties of alternative binders to be thoroughly investigated and catalogued. A body of experience has built up on the appropriate application of traditional binders such as clay, lime and pozzolanas, not only in the repair and conservation of historic buildings, monuments and structures but also in adventurous and innovative new build applications.
In some developing countries traditional binders are still slighted, probably because they might be associated with poverty or considered to be low status materials. Their performance and technical specifications might, completely unjustifiably, also be considered inferior to Portland cement, they might not be widely produced or available, or the skills to produce and use them might well have disappeared. A good case can be made for disseminating the developed country experience to the South more widely. This would increase interest and awareness of alternative binders, allow producers and users to gain skills and confidence and determine the rightful place of alternative binders in appropriate building for sustainable development.
Types of alternative cements
There are two forms of lime: quicklime and hydrated lime.
Quicklime is produced by heating rock or stone containing calcium carbonate (limestone, marble, chalk, shells, etc.) to a temperature of around 1000°C for several hours in a process known as ‘calcining’ or sometimes simply ‘burning’. It is an unstable and slightly hazardous product and therefore is normally ‘hydrated’ or ‘slaked’, by adding water, becoming not only more stable but also easier and safer to handle.
To produce dry powdered hydrated lime just sufficient water is added for the quicklime lumps to break down to a fine powder. This material would have a ‘shelf life’ of only a number of weeks, depending on storage conditions. ‘Old’ hydrated lime would have partially carbonated and become a less effective binder.
However, if quicklime is hydrated with a large excess of water and well agitated, it forms a milky suspension known as milk of lime. Allowing the solids to settle, and drawing off the excess water, forms a paste-like residue, termed lime putty, which is the form of lime which can be used in building applications to best effect. This will keep almost indefinitely and, in fact, improves with age. In most countries, though, lime is most widely available as a powder, due to its widespread utilisation in process and treatment industries rather than in construction. Lime putty, which needs a stiff bag or container for transportation, is more rarely produced.
Limes with high calcium content, often called ‘fat’ or ‘white’ limes are desirable for most industries, although the construction industry can use limes containing impurities. For instance, limestones containing a proportion of clay are often seen as an advantage in building as they produce hydraulic limes which will set under water and will produce stronger mortars.
In the construction industry, lime, in its hydrated or putty form, is mixed with aggregate and water to produce concrete or mortar in the usual manner. Lime putties generally produce mortars and renders of excellent quality and consistency.
Plain lime-sand mortars are quite weak; any early adhesive strength results from drying out, and longer term hardening occurs through the action of the air’s carbon dioxide on the lime.
Traditionally lime renders and plasters were often mixed with animal hair to improve cohesion. Today the addition of gypsum or Portland cement and/or pozzolanas to increase durability and give faster setting times is usual.
Pozzolanas are materials which, although not cementitious in themselves, will combine chemically with lime in the presence of water to form a strong cementing material. They include:
- Volcanic ash
- Power station fly ash (usually known as pfa)
- Burnt clays
- Ash from some burnt plant materials
- Silicious earths (such as diatomite)
In some countries (e.g. India and Kenya), pozzolanas are mixed with Portland cement and sold as blended cement, which in many respects is similar to Portland cement. In other countries (e.g. Cuba) lime/pozzolana/Portland cement blends are sold as an alternative to Portland cement. Lime-pozzolana cement by itself can make an excellent cementing material for low-rise construction or mass concrete and in some countries (e.g. Indonesia) is still produced extensively.
Pozzolanas can also be mixed with lime and/or Portland cement at the construction site but care must be taken to ensure the pozzolana is of a consistent quality and that the materials are thoroughly mixed.
Gypsum is a not an uncommon mineral, and needs only a low temperature, of around 150°C, to convert it into a very useful binding material, known as hemi-hydrate or plaster of Paris.
On its own, plaster of Paris sets very rapidly when mixed with water. To give time for it to be applied, around 5% of lime and 0.8% of a retarding material (such as the keratin glue-like extracts from boiling fish bones or animal hoof and horn) are added to the plaster.
Retarded plaster of Paris can be used on its own or mixed with up to three parts of clean, sharp sand. Hydrated lime can be added to increase its strength and water resistance. Gypsum plasters can be reinforced with various fibrous materials from reeds to chopped glass fibers.
Gypsum plaster is not wholly resistant to moist conditions and so is normally used internally, except in the drier Mediterranean and Middle Eastern countries where it has traditionally been used as an external render.
A number of other alternative binders have been used in a number of applications, which generally relate to soil stabilization, waterproofing, or the application of a waterproofing or wear resistant coating to vulnerable earth based constructions. Such binders include tars and bitumens (as by-products from petro-chemical industries), sodium silicate (produced from the heat activated reaction between silica and sodium hydroxide), casein (milk whey), oils and fats, molasses, and certain locally specific plant-based materials such as gum arabic, other specific resins and the sap, latexes and juices from specific trees and other plants.
MUD PLASTERS AND RENDERS
The traditional use of mud plasters and renders to coat and protect walls dates back a very long time and is found in almost all regions of the world. Finishing a house with mud plaster when the house itself has been built with earth is a natural, complementary technique, but mud plasters can also be used for buildings of stone and fired brick provided they incorporate an earth-based mortar for the joints.
Earth-based plasters often use earth in combination with other natural materials such as wheat straw or cow dung, or with mineral additives such as bitumen, to improve the basic qualities of the earth by acting as stabilizers, hardeners, and waterproofers. Even without additives, however, mud plasters and renders can give excellent results provided that they are made and applied with skill and care, and maintained regularly. Today, with low-cost mass housing a priority and with the increasing interest in the preservation of architectural heritage, the need for plastering materials which are efficient and economical has awakened a new interest in earth. Earth-based plasters are completely compatible with traditional materials and building techniques, and the almost universal availability of suitable earth for building gives them a distinct advantage over some modern synthetic plasters.
Figure 1: Walls built using traditional methods, such as cob, are very suitable for mud renderings.
The need for a plaster and the type of plaster that should be used depends particularly on the method of construction and quality of construction. The provision of adequate footings, basements, eaves, and overhangs to a roof can in certain circumstances eliminate the need for a plaster coating altogether. As plastering can amount to 15 to 20 per cent of the total cost of a house, its benefits need to be considered relative to alternative options.
In general, except in the case of highly exposed walls in areas of heavy rain, a plaster should protect against wind, rain, knocks and abrasion, and should improve the thermal insulation and appearance of a wall. At the same time it has to be easy to apply without requiring expensive and elaborate tools, and must be affordable. All types of mud plasters, but especially those on external surfaces, need to offer erosion resistance, impermeability to moisture, and impact resistance, and be well bonded to the wall.
The main cause of erosion is heavy rain, and high winds driving the rain hard onto walls at an angle will increase erosion further still. Heavy rain, even for a short time, is much more damaging than prolonged light rain. A knowledge of local weather patterns and an analysis of meteorological data can give an indication of erosion risk and hence appropriate plastering materials and methods.
The durability of mud plasters depends on their ability to withstand the impact of humans and animals by bumping, scratching, or scraping. Impact resistance is closely linked with the quality of the plaster, which is determined by its density, methods of application, number of coats used, and maintenance practices. The texture of the plaster is also important.
The bonding of earth plasters to walls is very important. When plastering a stone or earth wall the composition of the mix as well as its application are both crucial in producing a good bond (the join between the two materials). The plaster and the wall itself should ideally be compatible so that shear forces are transmitted between them and not terminated at the bond. Good bonding reduces the incidence of cracking caused by changes in ambient temperature and humidity. The plaster must be applied in coats of recommended thickness to prevent excessive strain at the bond.
Testing the performance
A simple soil test which will show whether the soil is suitable is to plaster an area of wall and to observe the development of cracks on drying. A number of different compositions can be tried to find the one which produces the least cracks and satisfies the need for hardness and water resistance in that particular situation.
Simulation tests in the laboratory, such as the spray erosion test, can only be indicative because factors such as changes in scale, influence of true climatic conditions, building usage and maintenance practices are not easily replicated. One of the most realistic simulation methods is to expose small test-walls to natural weather conditions; this has been done in Australia, the United States, Senegal, and France, for example. This test is a good indicator of the durability of different plasters and allows a realistic comparison between plasters with different compositions and methods of application. The main drawback with this test is the length of time needed to obtain meaningful results, and building projects cannot always afford to wait that long.
The composition of traditional mud plasters varies from place to place and is an important factor in determining durability. The clay content is particularly significant, because if it is too low the plaster will lack strength and cohesion, and if it is too high there will be a risk of cracking due to shrinkage, which will weaken the bond to the wall. A suitable clay content is usually around 10 to 15 per cent, but values outside this range could also be suitable depending on the type of clay. Soils with unstable or swelling clays must be used with great care. The sand-to-silt ratio is also very important in determining
the quality of a plaster. Traditionally, clay plasters were often applied in one coat both internally and externally. If applied in two coats, the first can contain more clay, even if cracks develop, while the second, containing more sand, is applied in a thinner layer. The second coat will help to close the micro-cracks in the first, provided the surface has been lightly dampened before plastering. Finally, lime distemper or whitewash can be applied to give some additional weatherproofing. This will need to be re-applied periodically.
Clay renders are commonly improved by adding natural fibers such as cereal straw, pine needles, bark, and wood shavings. Long straw is chopped into short lengths (2 to 5cm) for easier mixing: the function of the fibers is to resist cracks and facilitate the drying process. They also make the plaster less dense and improve its insulation properties. The amount of fibers required will vary depending on soil characteristics and can be from 35 to 70kg per cubic meter for straw; 50kg per cubic meter is a typical figure. In India, paddy straw (blusa) is added at a rate of 6 per cent by weight, or 60 to 65kg per cubic meter. The straw is soaked for several days in water to facilitate a rotting process, and the complete mixing process can take 10 to 15 days.
Another traditional practice is the addition of cow dung, which improves the cohesion and plasticity of soils of low clay content. Sometimes the dung is applied to mud plaster which is partially dry to help stop the development of cracks. A traditional waterproofing in India, known as Gohber leaping, consists of one part cow dung and five parts earth by weight, made into a fine paste with water and applied to fill up surface cracks. Another practice is the addition of horse urine, which acts as a hardener and improves impermeability and impact resistance.
Improving the composition
It is possible to improve the quality of mud plasters by:
• controlling the quantity of the sand fraction in the soil; to no less than three parts sand to one of clay, for example. This helps reduce cracks without compromising cohesion. A shrinkage of more than a quarter of an inch (0.64cm) over the 2-foot (0.6m) length of the box in the shrink box test indicates a soil liable to significant cracking.
• stabilizing the plaster by adding cement, lime, bitumen, or some other binder in small quantities. Possible limitations include the cost of the stabilizer and lack of skill in its proper use.
Bitumen cutback plaster is prepared by mixing hot bitumen with kerosene in a 5:1 ratio, and then combining one part of that mixture with 20 parts of previously fermented soil and wheat straw. Water is added and the whole mixed together thoroughly. This type of plaster is applied in two layers, and the second is applied only after the first has dried.
Lime-soil plaster can be made with one part hydrated lime mixed with two parts of clayey soil and 3 to 6 parts sand, the optimum amount of sand depending on the clay content of the soil. The quality of the plaster depends a lot on the quality of the lime available and the type of soil.
Another proprietary plaster is ‘dagga-cement’, a mix of two parts sand to one part clayey soil to 0.2 parts cement by volume. This produces a good weather-resistant plastering mix.
The most passive of all cooling strategies is the use of thermal mass, the capacity of a body to store heat. The ability of thermally massive materials to retain heat creates a thermal flywheel effect – that is, the mass will absorb heat when its surroundings are hotter than the mass, and release heat later, after its surroundings have cooled. Thus thermal mass within a building has the effect of dampening interior temperature swings relative to exterior temperatures.
Thermal mass can be heated directly, by the sun striking it, or indirectly, by contact with warm air. Of the two mechanisms, direct solar gain is by far the most effective.
The higher a material’s thermal mass, the more heat it will hold and the longer it will take to release that heat; thus the more significantly it will modulate interior temperature. Materials with relatively high thermal mass include rammed earth, cob, brick, ceramic tile and water. Materials with lower thermal mass include wood, air and insulation materials.
Thermal mass wall systems such as cob can be used effectively for passive cooling (and night-time heating) in certain climates. Thermal mass was used extensively by solar building design pioneers in the 1970’s.
Passive ventilation provides cooling effects by displacing warm air with cooler air – known as ‘stack effect’ – and to a lesser degree through the sensation of air movement (wind). John Weale notes, “The stack effect is very reliable. If any wind is available, it is usually more powerful than the stack effect in low-rise structures. However, it is very difficult to count on the wind, or even predict where it comes from, hence we focus on stack effect over the more powerful but unreliable wind-catching approaches”.
Passive ventilation cooling consists of strategically locating window and other openings to capitalize on localized wind patterns and/or naturally-occurring pressure differentials within and around a building. A principal passive ventilation mechanism is stack-effect – the tendency of warm air to rise. Stack effect can be used to good effect by placing openings high in a building, such as the top of a stairwell or solar chimney. As hot air rises and exits, the negative pressure created by the exhausting air draws in cooler air from openings lower in the building.
Stack effect and pressure differentials will vary in their effectiveness at cooling a building based on a number of variables:
- The difference in height between openings
- The horizontal distance between openings
- The number and sizes of openings
- The relative placement and distribution of openings within the building and/or within rooms
- The type of window operation (casement, double-hung, awning, hopper, etc.)
- Climatic variables such as diurnal temperature swing
- Microclimatic and building conditions such as sunlight and shading
- How effectively the openings are regulated (opened and closed); their operation can be either manual or automated (sensor-based)
Passive ventilation design is not a precise science but an art informed by experience, intuition, and observation.
PASSIVE SOLAR DESIGN
The sun can be a friend or an enemy in buildings. Poor climatic design of buildings, all too often seen in ‘modern’ architecture, causes many buildings to overheat, even in temperate or cold climates where such problems traditionally never existed. The power of the sun should be understood and respected by good designers of well-designed, passive solar buildings in which the free energy of the sun is used to power the building but not allowed to interfere with the comfort and economy of the building’s occupants.
The five things a designer needs to know for a good passive solar design are:
- how strong the sun at the site is at different times of the year
- where the sun will be at different times of the year in relation to the site
- how much of the sun’s heat a building will need, or not need, at different times of the year to enable the building occupants to be comfortable
- how much storage capacity the building should have in relation to the available solar gain at the site to meet those needs
- what the additional requirements are for controlling the heat gain from direct solar radiation, convection or conduction in a design and how they can be met by envelope performance, building form and ventilation
There are a number of factors that influence the incidence, or strength, of solar radiation at the site including:
- the latitude of the site
- the altitude and azimuth of the site
- how much shade will be given by any obstacles that exist between the building and the site
- the weather above the site
The angle with which the sun strikes at a location is represented by the terms ‘altitude’ and ‘azimuth’. Altitude is the vertical angle in the sky (sometimes referred to as height); azimuth is the horizontal direction from which it comes (also referred to as bearing). Altitude angles range from 0º (horizontal) to 90º (vertical:directly overhead) Azimuth is generally measured clockwise from north so that due east is 90º, south 180º and west 270º (or-90º).
Because the Earth revolves around the sun once a year, we have four seasons. The Earth’s axis remains in a constant alignment in its rotation so twice a year the incoming solar radiation is perpendicular to the tropics of Cancer and Capricorn. The changing values of azimuth and altitude angles are predominantly a reflection of the changes in the relative positions of Earth and sun. These are governed by:
- the rotation of the Earth around the sun
- the rotation of the Earth about its axis
All passive solar features involve the transmission of solar radiation through a protective glazing layer(s) on the sun side of a building, into a building space where it is absorbed and stored by thermal mass.
Masonry heaters are passive solar energy under human control. With this simple technology, you get to decide when the sun shines in your house. Passive solar heating is great and affords an option but there’s no accounting for long weeks without blue skies in a cold winter in Arkansas.
Also known as masonry stoves, kachelofens, Russian fireplaces, Finnish fireplaces, Swedish stoves, tile stoves, contra-flow fireplaces, radiant fireplaces and mass-storage fireplaces. Inside, masonry stoves burn hotter than metal wood stoves and their winding maze of flue (baffles) warms the surrounding masonry, which then emits heat for 18 to 24 hours. The temperature can reach 2000 degrees inside some masonry heaters (vs 700 inside a metal stove), yet they stay comfortable to the touch on the surface. At these high internal temperatures, the hydrocarbon gases ignite, leaving very minimal pollution. When burning wood, about 30% of the generated heat is supplied by the wood solids and 70% of its heat is contained in released gases.
If the volatile gases are not fully combusted, they escape as wasted heat and polluting particulate emissions. Igniting and then drawing the heat out of the combustion gases turns almost every ounce of wood into energy. A slow burning, low temperature, low oxygen fire produces tar and hydrocarbons, a fast, hot, air-fed fire burns the pollutants up. Add a storage battery (the masonry) and you have a very efficient, non-polluting heating system. A metal stove gives out its heat rapidly, thus never allowing the inside combustion temperatures to achieve the 1100 degree F plus needed to ignite all the gases.
Because the stored heat radiates slowly from the masonry, it is only necessary to light a fire once a day in most circumstances. In really cold conditions, you might need to light two fires a day. Metal wood stoves must be tended to continually and fluctuate from peak high temperatures, to no heat, when the fire is out. If you tamp down the flue on a metal wood stove you increase the emissions of pollutants as the combustion of the wood is incomplete. A masonry heater always burns wood at the highest heat, if you desire less heat, you simply use less wood. In a well insulated home, a masonry heater will use 1/5th (or much) less wood, then a home heated with a metal wood stove. All well-designed masonry heaters easily outperform the highest rated EPA metal wood stoves. And like a wood stove, a masonry heater can exhaust through a metal flue pipe.
The masonry stove has been around in many different forms in almost all ancient northern cultures, from the 7200 year old Kang bed stove in China to the Hypocaust in ancient Rome. In northern Europe, 500-600 years ago, a long-lived cold spell caused local wood to become scarce and masonry heaters became common due to their efficiency. In the past hundred years dirty coal, then oil replaced the masonry heater. Wood is a renewable resource and absorbs CO2 as it grows. Burning releases about the same amount of carbon monoxide, carbon dioxide and methane as would occur if the wood were decomposing naturally on the forest floor.* Yet wood is a sustainable energy source, only when proper wood lot management is employed and when its energy is extracted efficiently and cleanly.
Directing the hot flue exhaust through a series of baffles heats up the surrounding masonry. The baffles can meander in numerous directions. Some stove’s baffles take the exhaust side to side, some up and down, some front to back, and vice versa. There is always a source of air coming in the base of the heater to feed the fire. Yet, flues that are too long and convoluted might restrict the draft through the system, as each change of direction creates resistance to the gas flow and decreases the suction of the chimney draft.
In a building of 1500 sqft or less you can be quite comfortable and energy self-sufficient with a little passive solar design and one high-quality masonry heater. Such a building could be heated with less than two cords of wood annually in Arkansas for less than $400 (or for free with some extra effort).
It all comes down to personal priorities, of course. We at the N.A.T.U.R.E. Center do mind being dependent on some distant land or corporation; we do mind being subjected to the ups and downs of energy markets; we do mind being at the whim of the politics and military conflicts that arise around energy; we do mind having a heating system that is useless if a storm cuts power; we do mind having cold feet and nowhere in the building to get warm. This is why we crave being free from dependence and love the idea of getting energy free from the Earth and the Sun.