The Naturally Air Conditioned House

Neal Mortensen Environmental Architecture Pty Ltd

SUMMARY

This paper outlines the development of energy efficient design from ancient Greece to the present time. It describes the valuable sources of appropriate technology which were developed intuitively prior to the industrial revolution, and how present day research into energy efficient solutions to the problems of our built environment is coming up with techniques known to pre-industrial man. The elements that influence energy efficient design will be discussed and conclusions drawn regarding their appropriate use. Evidence will be presented which shows how high mass, when correctly used, can be our greatest ally for producing environmentally sensitive buildings.

1.0 INTRODUCTION

Because of global warming and deterioration of the ozone layer there is a growing need for building services engineers and other related professions to design buildings which not only provide comfort for the occupants but also minimise the consumption of fossil fuels and resultant greenhouse gas emissions, and the production of CFCs in the process of heating and cooling buildings. Buildings contribute substantially to the depletion of the planet’s resources and the reduction of environmental quality. The building industry, both in the construction and occupation stages, is a major contributor to the total greenhouse gas emissions from industry as a whole.

The challenge that passive solar design presents to building services engineers, is to work with architects from the outset to reduce the need for mechanical systems. Today’s knowledge makes it possible for building professionals to develop solutions for the built environment which can enable the conservation and enhancement of our natural environment and ecological systems.

To achieve this we need to consider the building itself as a heating and cooling system utilising existing natural, climatic and environmental energies to obtain an acceptable level of comfort and health. The design team can explore many options of the site and building layout, types of windows and shading, insulation and thermal mass levels both within the building and external to the structure in order to tap the energy required for year-round comfort. If backup energy is required, surely its selection should be based on minimal environmental cost rather than the immediate and lowest financial cost. With the growing acceptance of environmental/life cycle costing we are likely to see the demand for environmental sensitive buildings coming from our clients.

This paper while describing general principles of solar design, will focus on the use of high mass in passive solar designed buildings.

2.0 HISTORICAL BACKGROUND

Many principles of passive solar design were promoted 2,400 years ago by Socrates who recognised the comfort value of orientating rooms to allow low angled winter sun to enter, while keeping out high angled summer sun.

Through indigenous architecture man has adapted to different climates with structures varying from tents, grass huts, mud walls & roofs to troglodytes in Turkey (houses carved in rocks) and subterranean dwellings in China. All of these structures make appropriate use of materials to modify the thermal environment of the occupants in the most energy efficient way.

In the process of solving climatic problems, specific roof forms evolved: flat mud roofs in hot zones, vaulted roofs in hot dry areas, inclined roofs in temperate areas with dry summers and steeper pitches in wet temperate and cooler areas. The profile of the dome provided the maximum surface area for cooling by wind and night time sky radiation while developing minimum possible surfaces temperatures during the day.

In 1100 AD the Pueblo Indians of America developed thermally comfortable dwellings in a hot arid climate with high day time temperatures and very low night time temperatures. They achieved this by building with materials which have a high heat capacity such as adobe mud and stone. This provided a “heat sink” absorbing heat from the sun during the day which reradiated into the dwelling during the night. By building on top of one another and side by side they achieved maximum volume with minimum surface area exposed to the outside heat. They further minimised heat gain with small window openings placed high in the walls and painted the external walls a light colour to reflect the heat.

Up to the time of the Industrial Revolution, man, when constructing houses, intuitively used appropriate building materials and correct orientation to the sun. The Industrial Revolution brought with it the belief that man could dominate nature. As a result there was a lack of interest in the design and utilisation of natural principles and systems.

By the early 1900’s, successful attempts were made to harness solar energy to drive various machines, but this could not compete with the cheaper fossil fuel. We are only now paying for the real price of fossil fuels and their impact of global warming.

In 1930, American architects began experimenting with the greenhouse effect as a means of house heating. They found that although it was possible to collect large amounts of heat during the day (to a point of discomfort), the heat lost at night exceeded the daily heat gain. Many solar houses were sold but as they were using light weight structures, the problem of excessive heat gained during the day and how to prevent the heat loss at night was never fully solved.

In 1944, the Australian Commonwealth Experimental Building Station was formed in order to produce standards for more efficient buildings with improved thermal comfort. Most of our basic principles of passive solar design date back to the work of these scientists.

Stimulated by the energy crisis, the next burst of activity came in the mid 70’s in America and Australia where experimentation with active and passive solar systems and earth covered buildings occurred. The active systems had separate absorber, heat store and distribution systems along with pumps and controls. Although more expensive than a direct gain passive system, a high degree of control was possible.

The passive systems concentrated on managing the heat trapped inside the house by the greenhouse effect. The problem of heat storage was resolved by using the mass of the building itself to store the heat, and in very cold climates insulation was placed on the outside of the external mass walls. The windows were double glazed, with moveable insulated shutters or curtains being the only method to control the movement of heat. The whole house was a complete passive system. Both systems had different implications for planning. The active systems enabled the heating of rooms which were remote from direct solar gain. The passive systems required as many rooms as possible to have access to direct solar gain.

These solar houses generally worked on a diurnal offset cycle, where daytime temperatures moderate evening temperatures in winter and vice versa in summer. Earth-covered buildings on the other hand could be designed to work on a seasonal cycle where the summer temperatures within the ground warm the building in the winter season and winter temperatures cool the building in the summer season. Up to one metre of soil was placed on the roof thus locating the floor level four metres below the ground.

In the early 80’s, the CSIRO Building Research developed the five star design rating program. Their guidelines considered the most economic way to construct an energy efficient house. The design principles are based on the correct use of common building materials i.e. glass, mass and insulation. With proper solar orientation, shading, ventilation controls and zoned planning, it is possible to achieve comfort conditions with minimum back up heating and no back up cooling. All this can be achieved without adding to the normal construction costs or imposing any particular aesthetic.

In 1984, commercially available “dry-compressed earth block” was introduced into Australia. A cement stabilised version of these blocks was later developed and successfully used as solid internal and external walls of passive solar houses. The superior thermal performance of these houses surprised even the most experienced solar house designers. Concurrently a sophisticated rammed earth wall system was developed in Western Australia giving similar thermal performances.

In 1990, there was a transition of earth wall construction from small scale one and two storey buildings to four and five storey commercial buildings. The development of designed mixes in Commercial Engineered Aggregate Construction ( C.E.A.C.) introduced a high quality control which could perform in a fast track construction program. The largest project being the 100 room M$25 Kooralbyn Hotel Resort in Queensland.

In 1992, the Australian Conservation Foundation instigated the ACF Green Home, Guidelines were set to achieve energy efficient and environmentally friendly homes by using materials which have a low environmental impact, which minimise energy use and greenhouse gas emissions, and minimise loads on the infrastructure such as stormwater, sewerage and water supply.

By complying with the guidelines one is eligible for special finance packages through the ACF Green Bonds scheme. It is the first time in Australia that the availability of finance has been linked to the purchase of energy efficient houses.

Universities throughout Australia are continuing to research and test solar technology and energy efficient buildings. “Solarch” at the University of N.S.W., the ECO Foundation at Sydney University, The Natural Energy and Environment Group of the R.A.LA. to name a few, are educating and lobbying for design concepts based on the need to restore, maintain and enrich our natural environment.

3.0 “ENERGY EFFICIENT DESIGN” AND “SOLAR DESIGN”

Solar and energy efficient designs are at last attracting the interest of the home buyer, and to maintain a permanent position in the market place the designs need to be attractive. Designers cannot afford to decrease existing comfort levels or increase personal inconvenience. It is a mistake for house designers to consider energy efficiency as mere add on technical solution. Energy efficient design has to become part of the expression of architecture and a way of life. Following is a discussion of the elements that deserve consideration when designing an energy efficient house.

3.01 The Site

Energy efficient design starts with the site. The first criteria is to ensure that winter sunshine is not blocked by existing or future development, trees or escarpments. It is an advantage if the longest side of the block faces north as this will give maximum winter sun exposure but this is not essential. As long as winter sun does reach the site it does not matter which way the site faces, the house can be designed to be fully solar. Likewise, it does not matter in which direction the view faces. The view and the winter sun do not have to be on the same side and the living room does not have to face the street. In exposed cold climates a southerly view can cause problems, it may require well sealed and double glazed windows and should be kept to a minimum area. In milder coastal climates southerly views can be easily accommodated and windows used to capture summer breezes. Westerly views have an undeservedly bad reputation. Provided they are properly shaded with a wide low verandah, the view of a setting sun can be very rewarding.

3.02 Solar Principles

For the winter cycle you have to ensure that as many rooms as possible have large glazed areas extending down to floor level and facing within 20?of solar north. This will allow winter sun to warm the house by the greenhouse effect. The incoming short-wave radiation from the sun passes through the glass, raises the temperature of the air and internal mass and is then re-radiated as long-wave radiation, which this time is reflected by the glass back into the room. For the summer cycle all of these glazed areas require overhanging eaves or angled louvre blades calculated to screen out the hot summer sun.

On the east and west sides, generally the more winter sun you allow in, the more problem you will have with summer sun penetration. Openings on these sides require special consideration and shading techniques with either fixed or moveable devices or appropriate landscaping. In mild climates like Sydney the eastern morning sun can be a delight for up 10 months of the year but the western side needs to be well protected from summer afternoon heat with adjustable awnings or sunscreens. These would require adjustment only twice a year to allow winter sun penetration and exclusion of summer heat. The southern side needs to be closed off from winter winds, but needs sufficient openings to allow cross ventilation on summer evenings.

3.03 Landscaping

Outside the building, the thermal environment can be greatly improved with judicious planting of shrubs and deciduous trees. Deciduous trees provide shade in summer but allow winter sunshine in. Dense shrubbery provides shelter from undesirable winter winds. External paving can play a major role in modifying the ambient temperatures when shaded and dampened in summer and alternatively, when dry and exposed to the sun in winter. Landscaping is the most under utilised area of domestic architecture. Dollar for dollar it is probably the cheapest and most effective way of improving year round comfort and energy efficiency. An uninsulated tin shed can be made habitable in the middle of a hot summer day if covered by a thick foliaged vine.

3.04 Mass

Mass is used in solar designed houses to stabilise the internal temperatures. This can operate in two ways.

Firstly, its ability to improve the environmental temperature. The use of internal mass takes advantage of the fact that small changes in mean radiant temperatures have a far greater effect on the occupant than similar changes in air temperatures. The higher the density of the material the more heat energy it can absorb before it raises its own temperature in the summer, and the more heat energy it can re-radiate before its surface temperature drops in the winter.

Secondly, its ability to delay the time it takes for heat to pass through the external mass walls. Again the higher the density the longer the delay. The ideal time lag is six months. This will give summer warmth inside the house in winter and winter coolness in summer. This can only be achieved with well designed earth covered buildings. The other desirable time lag period is 10 to 12 hours. In summer this will delay the impact of day time heat until evening when H can be removed by the cooler night time ventilation, and in winter the external walls receiving sun during the day will re-radiate this heat into the interior during the night. Based on the phenomena of time lag in the earth, the most efficient thermal stabiliser in a solar house is the concrete slab on the ground followed by the internal mass walls.

3.05 Cross Ventilation

The thermal importance of ventilation is twofold. In summer, because of lower night time temperatures, cross ventilation on a summer evening will remove the stored heat of the day from the mass inside the house. This allows the mass walls and concrete floors to absorb heat from the occupant by radiation and conduction.

The other function of cross ventilation is to cool the occupant directly with cool air passing over their skin thereby losing heat to the air by evaporation. To achieve this every room needs openings from at least two directions and ceiling fans as a backup when there are no breezes. Casement windows opening in different directions can scoop in the slightest of breezes, while highlights extract the rising hot air. Another purpose of ventilation is to remove odours, stale and moisture laden air. In winter it is desirable to keep this to the minimum required by health standards. Living plants inside the house can also improve the air quality through their transpiration.

3.06 Envelope Shapes

The walls in a passive solar house receive the majority of their heat from the air. If the house has a low flat plaster or concrete ceiling the rising hot air will stratify and heat energy will pass into the walls. If however there is a boarded cathedral ceiling, rising warm air escapes through the boarded joints reducing the amount of heat absorbed by the walls. Environmental temperature variation of 3-4 can occur between rooms having the same floor areas and solar access but with different ceiling configurations.

3.07 Insulation

The function of insulation is to resist the flow of heat from one side of the building to the other. The amount of resistance required varies with different climatic zones. A well designed passive solar house will only require insulation in the roof and in selected cavity walls. For raking ceilings the critical parts in the roof are the ridge and wall junctions. It is worthwhile providing an extra layer of insulation at these points. In pitched roofs with horizontal ceilings, foil backed fibrous insulation under metal roofs and double sided foil sarking under tiled roofs resist heat gain in summer, while fibreglass batts or polystyrene sheets at the ceiling level prevent heat loss in winter. Ridge ventilation in conjunction with eaves ventilation in summer adds to the quality of the roof insulation as well as removing any condensation.

All brick cavity walls except those facing north require insulation. This can be achieved with double sided foil in mild climates and polystyrene sheet in colder climates. In extreme cold climates it is worthwhile laying a polystyrene sheet foam insulation along the edge of the concrete slab to a depth of 1200mm. There is no point in placing insulation under a concrete slab on the ground or a suspended timber floor as it isolates the house from summer cooling by the earth.

Other forms of insulation include double glazing and curtains with sealed pelmets. Two layers of light weight fabric are more efficient than one heavy curtain. Skylights in summer can be easily insulated with reflective horizontal curtains but in winter double glazing and air seals are the only means to prevent heat loss. In addition, light colours on external surfaces reflect summer heat thereby contributing to the quality of insulation.

3.08 Infrastructure Services

No matter how small the contribution to reduction in energy use by a single development, when combined with other similar developments there can be a large reduction in costs to the community. The demand on electricity will be reduced as more energy efficient houses fill the housing stock. The introduction of community based site management where groups of developments aim to provide much of their own infrastructure services will reduce development costs and benefit the local environment. For example, stormwater run off can be reduced by absorbent paving, underground holding tanks and rain water collection tanks. Electricity can be reduced by using gas boosted solar hot water services and, with further advances in technology, solar cells for generating electricity. Along with waste recycling and permaculture they can all contribute to reducing the stress humanity is placing on the planet. This growing awareness at the individual and community level can then develop into a national and global awareness that the care of our planet is a top priority.

4.0 EARTH WALL STRUCTURES

Earth walled construction, in a wide variety of forms is the oldest and most widely used form of construction technology in the world today. More than 50% of the world’s population today live in earth wall buildings*, the majority being in third world countries. (Mc Henry, Paul G. Jr 1984)

In recent years in Australia we have seen earth walls used in the following building types: public, educational, tourist resort, hotel, medium density residential and hundreds of individual houses. The main reason for today’s interest in earthed wall buildings is because their inherent mass combined with passive solar design, can provide non-mechanical, natural air-conditioned levels of comfort.

A significant development in this technology has been the manufacture of cement stabilised compressed earth block. Soils used in these blocks are a clay based weathered rock. The blocks are stabilised with as little as 5% off white cement and compressed to give a mass of 2,200kg/m3. Houses built from these blocks when compared to houses built from extruded double brick structures have between 50-80% more effective thermal mass and when compared to solid, dry pressed, clay-fired brick have between 30-50% more effective thermal mass. (Baggs. D.W. 1992)

Houses constructed with stabilised earth walls have maintained comfort conditions in a wide variety of climatic zones. They take maximum advantage of the fact that small changes in mean radiant temperatures have a far greater effect on the comfort of the occupants than similar changes in air temperatures. In a high mass building the environmental or experiential temperature can be 3-4 below the recorded air temperature in summer peaks and 2-3 higher than recorded air temperatures in winter lows.

The external walls have a low resistive insulation but a high thermal lag (9-11 hrs), thereby providing a diurnal “thermal flywheel” effect. Therefore it is wise to expose as much of the external skin as possible to sun in winter and shade in summer. In winter this will provide warmth to the inside at night and in summer when cooled by the night air will provide coolness during the day. It is our experience that contrary to theoretical calculations, the low resistive insulation in the external walls does not create a thermal comfort problem.

The internal compressed earth walls compound this “thermal flywheel” effect by their capacity to retain heat energy in winter and their ability to absorb heat energy in summer with only small changes in surface temperatures. Keeping in mind that a high quality 250mm thick compressed earth wall is equal in mass to 375mm solid, dry pressed clay fired brick.

Using the above techniques along with normal passive solar orientation, slab on ground construction and insulated roofs, you can expect to achieve temperature differentials of 10-15 between internal and ambient conditions without any additional heating or cooling. One can also expect that the internal environmental temperatures will not go much below 14 even in occasional ambient sub zero conditions.

So far there have been no humidity problems in these buildings. It is thought that the moisture in the air is absorbed by the bricks and then released later when the room air is drier. This may explain why the actual thermal performance of these buildings often exceeds theoretical (calculated) performance. Is the latent heat of water changing the thermal conductivity of wall? There is never any condensation on these walls. Hence where is the dew point? These questions can only be answered with further research.

These buildings have been erected in climatic zones varying from Threadbow, Gulf of Carpentaria, Sydney, Mudgee, Canberra, Toowoomba and Cummeragunga. All buildings provide a thermally comfortable environment requiring a minimum of backup heating and no back up cooling.

As an environmental bonus this material has extremely low C02 contribution during its production. A 250mm thick compressed earth wall has a contained energy content (total energy required to manufacture transport and construct the element in the building) that is 23 times less than equivalent 270mm double skin clay fired brick wall. (Baggs, W.D. 1992)

4.01 Case Studies

Temperature readings taken at Mudgee in winter have shown that the ambient temperatures dropped as low as -8 while internal air temperatures in an earth walled house stayed as high as 14 (comfort conditions when wearing thick clothing). This includes rooms that did not receive direct winter sun.

Temperature readings taken in summer on a properly near the Gulf of Carpentaria have shown that the ambient temperatures rose as high as 45 while the internal air temperature did not exceed 30 (the real environmental temperature would be closer to 26-27).

Data collected in a house at the Hunter Valley N.S.W. has shown that when outside temperatures ranged from 2 to 42, the temperatures inside the house during waking hours ranged from 17-30 with the lowest reading being 13.

A house at Mona Vale, Sydney with single glazed windows and doors and containing over 200 tons of thermal mass in the walls and suspended concrete slab achieved an internal temperature range of 18 -27 during waking hours with the lowest recorded temperature being 14 while the ambient temperature range was 7 to 42. It should be noted that these temperature readings were taken during the first 13 months following completion of construction when the house was unoccupied (owner working overseas). As the house was unoccupied and the curtains had not been installed there was no manual thermal management (i.e. closing curtains on winter nights and on hot summer days or opening windows on summer nights to flush out heat absorbed in the walls during the day).

Furthermore, there was more south glazing than north glazing and the majority of this south glazing was in the stair well along with 5mm of glazed roof. It was originally intended close the stair well off from the living areas of the house, but this has proved to be unnecessary.

Following a period of three years occupation, the family of six reported that they found the Mona Vale house to be a very relaxed house to live in during all seasons. The only backup heating used is the kitchen floorheat which they have on during the last 4 weeks of winter. In summer they open the windows in the evening for cross ventilation. They have never found the need to use ceiling fans even on hot humid days. Curtains are drawn for privacy reasons only.

It is to be noted that because earth walls have a calculated low thermal resistive value NONE of the above buildings can qualify for the FIVE STAR DESIGN RATING offered by the CSIRO regardless of how well they perform in a passive solar building. Building regulations which require a minimum insulation in walls is another threat to acceptance of walls whose thermal performance depends on thermal lag and not on thermal resistance.

5.0 CONCLUSION

Passive solar building design with a high mass content is a concept which can potentially eliminate all mechanical systems for space heating and cooling in all forms of residential buildings.

The use of high mass earth walls in passive solar buildings accrue environmental benefits not provided by ordinary building systems.

Through lack of research in this area there is a danger of regulations shutting out our most environmentally sensitive building materials

The major challenge for building services engineers is to incorporate “passive solar technology” into the larger, internal-load-dominated buildings. If this challenge is accepted ft will be the key direction for buildings of the 21st century.

REFERENCES:

Mc Henry, Paul G. Jr Adobe and Rammed Earth Buildings Design and Construction. Wiley Interscience, New York, 1984.

Baggs, D.W. : “Future Architectural Trends with fewer building services”. Journal of the Geotecture International Association Vol.9, No.2, p.37-40, (1992)

BIBLIOGRAPHY:

Ballinger, J.A., Prasad, D.K. and Rudder, D.: Energy Efficient Australian Housing, 2nd ed., Australian Government Publishing Services Canberra, (1992)

The AIA Research Corporation Washington, DC: Solar Dwelling Design Concepts, The US Department of Housing and Urban Development Office of Policy Development and Research, (1976)

Olgyay, V.: Design with Climate, Princeton University Press , Princeton, New Jersey, (1967)

Originally posted @ Sharing Sustainable Solutions

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