土木工程-建筑-外文翻译-外文文献-英文文献-环境变化影响下的建筑学.doc

Architecture in a Climate of Change （H）
Page52-Page62
Low energy techniques for housing
It would appear that,for the industrialised countries,the best chance of rescue lies with the built environment because buildings in use or in the course of erection are the biggest single indirect source of carbon emissions generated by burning fossil fuels,accounting for over 50 per cent of total emissions.If you add the transport costs generated by buildings the UK government estimate is 75 per cent.It is the built environment which is the sector that can most easily accommodate fairly rapid change without pain.In fact,upgrading buildings, especially the lower end of the housing stock,creates a cluster of interlocking virtuous circles.
Construction systems
Having considered the challenge presented by global warming and the opportunities to generate fossil-free energy,it is now time to consider how the demand side of the energy equation can respond to that challenge.The built environment is the greatest sectoral consumer of energy and,within that sector,housing is in pole position accounting for 28 per cent of all UK carbon dioxide (CO2) emissions.
In the UK housing has traditionally been of masonry and since the early 1920s this has largely been of cavity construction.The purpose was to ensure that a saturated external leaf would have no physical contact with the inner leaf apart from wall ties and that water would be discharged through weep holes at the damp-proof course level.Since the introduction of thermal regulations,initially deemed necessary to conserve energy rather than the planet,it has been common practice to introduce insulation into the cavity.For a long time it was mandatory to preserve a space within the cavity and a long rearguard battle was fought by the traditionalists to preserve this‘sacred space’.Defeat was finally conceded when some extensive research by the Building Research Establishment found that there was no greater risk of damp penetration with filled cavities and in fact damp through condensation was reduced.
Solid masonry walls with external insulation are common practice in continental Europe and are beginning to make an appearance in the UK.In Cornwall the Penwith Housing Association has built apartments of this construction on the sea front, perhaps the most challenging of situations.
The advantages of masonry construction are:
● It is a tried and tested technology familiar to house building companies of all sizes.
● It is durable and generally risk free as regards catastrophic failure–though not entirely.A few years ago the entire outer leaf of a university building in Plymouth collapsed due to the fact that the wall ties had corroded.
● Exposed brickwork is a low maintenance system; maintenance demands rise considerably if it receives a rendered finish.
● From the energy efficiency point of view,masonry homes have a relatively high thermal mass which is considerably improved if there are high density masonry internal walls and concrete floors.
Framed construction
Volume house builders are increasingly resorting to timber-framed construction with a brick outer skin,making them appear identical to full masonry construction.The attraction is the speed of erection especially when elements are fabricated off site. However,there is an unfortunate history behind this system due to shortcomings in quality control.This can apply to timber which has not been adequately cured or seasoned.Framed buildings need to have a vapour barrier to walls as well as roofs. With timber framing it is difficult to avoid piercing the barrier.There can also be problems achieving internal fixings.For the purist,the ultimate criticism is that it is illogical to have a framed building clad in masonry when it cries out for a panel,boarded,slate or tile hung external finish.
Pressed steel frames for homes are now being vigorously promoted by the steel industry.The selling point is again speed of erection but with the added benefit of a guaranteed quality in terms of strength and durability of the material.
From the energy point of view,framed buildings can accommodate high levels of insulation but have relatively poor thermal mass unless this is provided by floors and internal walls.
Innovative techniques
Permanent Insulation Formwork Systems (PIFS) are beginning to make an appearance in Britain.The principle behind PIFS is the use of precision moulded interlocking hollow blocks made from an insulation material,usually expanded polystyrene.They can be rapidly assembled on site and then filled with pump grade concrete.When the concrete has set the result is a highly insulated wall ready for the installation of services and internal and exterior finishes.They can achieve a U-value as low as 0.11 W/m2K.Above three storeys the addition of steel reinforcement is necessary.
The advantages of this system are:
● Design flexibility; almost any plan shape is possible.
● Ease and speed of erection;skill requirements are modest which is why it has proved popular with the self-build sector.Experienced erectors can achieve 5 m2 per man hour for erection and placement of concrete.
● The finished product has high structural strength together with considerable thermal mass and high insulation value.
Solar design
Passive solar design
Since the sun drives every aspect of the climate it is logical to describe the techniques adopted in buildings to take advantage of this fact as‘solar design’. The most basic response is referred to as‘passive solar design’.In this case buildings are designed to take full advantage of solar gain without any intermediate operations.
Access to solar radiation is determined by a number of conditions:
● the sun’s position relative to the principal facades of the building(solar altitude and azimuth);
● site orientation and slope;
● existing obstructions on the site;
● potential for overshadowing from obstructions outside the site boundary.
One of the methods by which solar access can be evaluated is the use of some form of sun chart.Most often used is the stereographic sun chart in which a series of radiating lines and concentric circles allow the position of nearby obstructions to insolation,such as other buildings,to be plotted.On the same chart a series of sun path trajectories are also drawn(usually one arc for the 21st day of each month); also marked are the times of the day.The intersection of the obstructions’outlines and the solar trajectories indicate times of transition between sunlight and shade. Normally a different chart is constructed for use at different latitudes (at about two degree intervals).
Sunlight and shade patterns cast by the proposed building itself should also be considered.Graphical and computer prediction techniques may be employed as well as techniques such as the testing of physical models with a heliodon.
Computer modelling of shadows cast by the sun from any position is offered by Integrated Environmental Solutions (IES) with its‘Suncast’program.This is a user-friendly program which should be well within normal undergraduate competence.
The spacing between buildings is important if overshading is to be avoided during winter months when the benefit of solar heat gain reaches its peak.On sloping sites there is a critical relationship between the angle of slope and the level of overshading.For example, if overshading is to be avoided at a latitude of 50N,rows of houses on a 10 north-facing slope must be more than twice as far apart than on
10 south-facing slope.
Trees can obviously obstruct sunlight.However,if they are deciduous,they perform the dual function of permitting solar penetration during the winter whilst providing a degree of shading in the summer.
Again spacing between trees and buildings is critical.
Passive solar design can be divided into three broad categories:
● direct gain;
● indirect gain;
● attached sunspace or conservatory.
Each of the three categories relies in a different way on the‘greenhouse effect’ as a means of absorbing and retaining heat.The greenhouse effect in buildings is that process which is mimicked by global environmental warming.In buildings,the incident solar radiation is transmitted by facade glazing to the interior where it is absorbed by the internal surfaces causing warming.However,re-emission of heat back through the glazing is blocked by the fact that the radiation is of a much longer
wavelength than the incoming radiation.This is because the re-emission is from surfaces at a much lower temperature and the glazing reflects back such radiation to the interior.
Direct gain
Direct gain is the design technique in which one attempts to concentrate the majority of the building’s glazing on the sun-facing facade.Solar radiation is admitted directly into the space concerned.Two examples 30 years apart are the author’s house in Sheffield,designed in 1967 and the Hockerton Project of 1998 by Robert and Brenda Vale.The main design characteristics are:
● Apertures through which sunlight is admitted should be on the solar side of the building, within about 30 of south for the northern hemisphere.
● Windows facing west may pose a summer overheating risk.
● Windows should be at least double glazed with low emissivity glass (Low E) as now required by the UK Building Regulations.
● The main occupied living spaces should be located on the solar side of the building.
● The floor should be of a high thermal mass to absorb the heat and provide thermal inertia,which reduces temperature fluctuations inside the building.
● As regards the benefits of thermal mass,for the normal daily cycle of heat absorption and emission,it is only about the first 100 mm of thickness which is involved in the storage process.Thickness greater than this provides marginal improvements in performance but can be useful in some longer-term storage options.
● In the case of solid floors,insulation should be beneath the slab.
● A vapour barrier should always be on the warm side of any insulation.
● Thick carpets should be avoided over the main sunlit and heatabsorbing portion of the floor if it serves as a thermal store.However,with suspended timber floors a carpet is an advantage in excluding draughts from a ventilated underfloor zone.
During the day and into the evening the warmed floor should slowly release its heat, and the time period over which it happens makes it a very suitable match to domestic circumstances when the main demand for heat is in the early evening.
As far as the glazing is concerned,the following features are recommended:
● Use of external shutters and/or internal insulating panels might be considered to reduce night-time heat loss.
● To reduce the potential of overheating in the summer,shading may be provided by designing deep eaves or external louvres. Internal blinds are the most common technique but have the disadvantage of absorbing radiant heat thus adding to the internal temperature.
● Heat reflecting or absorbing glass may be used to limit overheating.The downside is that it also reduces heat gain at times of the year when it is beneficial.
● Light shelves can help reduce summer overheating whilst improving daylight distribution.
Direct gain is also possible through the glazing located between the building interior and attached sunspace or conservatory;it also takes place through upper level windows of clerestory designs.In each of these cases some consideration is required concerning the nature and position of the absorbing surfaces.
In the UK climate and latitude as a general rule of thumb room depth should not be more than two and a half times the window head height and the glazing area should be between about 25 and 35 per cent of the floor area.
Indirect gain
In this form of design a heat absorbing element is inserted between the incident solar radiation and the space to be heated;thus the heat is transferred in an indirect way.This often consists of a wall placed behind glazing facing towards the sun,and this thermal storage wall controls the flow of heat into the building.The main elements
● High thermal mass element positioned between sun and internal spaces,the heat absorbed slowly conducts across the wall and is liberated to the interior some time later.
● Materials and thickness of the wall are chosen to modify the heat flow.In homes the flow can be delayed so that it arrives in the evening matched to occupancy periods. Typical thicknesses of the thermal wall are 20–30 cm.
● Glazing on the outer side of the thermal wall is used to provide some insulation against heat loss and help retain the solar gain by making use of the greenhouse effect.
● The area of the thermal storage wall element should be about 15–20 per cent of the floor area of the space into which it emits heat.
● In order to derive more immediate heat benefit,air can be circulated from the building through the air gap between wall and glazing and back into the room.In this modified form this element is usually referred to as a Trombe wall. Heat reflecting blinds should be inserted between the glazing and the thermal wall to limit heat build-up in summer.
In countries which receive inconsistent levels of solar radiation throughout the day because of climatic factors (such as in the UK),the option to circulate air is likely to be of greater benefit than awaiting its arrival after passage through the thermal storage wall.
At times of excess heat gain the system can provide alternative benefits with the air circulation vented directly to the exterior carrying away its heat,at the same time drawing in outside air to the building from cooler external spaces.
Indirect gain options are often viewed as being the least aesthetically pleasing of the passive solar options,partly because of the restrictions on position and view out from remaining windows,and partly as a result of the implied dark surface finishes of the absorbing surfaces.
As a result,this category of the three prime solar design technologies is not as widely used as its efficiency and effectiveness would suggest.
Attached sunspace/conservatory
This has become a popular feature in both new housing and as an addition to existing homes.It can function as an extension of living space,a solar heat store,a preheater for ventilation air or simply an adjunct greenhouse for plants.On balance it is considered that conservatories are a net contributor to global warming since they are often heated.Ideally the sunspace should be capable of being isolated from the main building to reduce heat loss in winter and excessive gain in summer.The area of glazing in the sunspace should be 20–30 per cent of the area of the room to which it is attached.The most adventurous sunspace so far encountered is in the Hockerton housing development which will feature later.
Ideally the summer heat gain should be used to charge a seasonal th