Climatic Design for Energy Efficiency in Buildings

V K Mathur, MemberI Chand, Non-member

Necessity and benefits of designing buildings with energy efficiency considerations having been incorporated right fromthe planning stage have been discussed. Climatic classification map of India has been included for identification of theclimate of the building site in question. The requirements of thermal comfort for various climatic zones have beendefined. Method of selection of suitable architectural features like plan form. orientation, location and size of fenestra-tion, shading devices, treatment of building envelop etc, which on incorporation in design of buildings would providethermal comfort with minimum consumption of energy. Simple guidelines for design of buildings, which wouldfunction in conformity with climate, are also presented in the paper.

Keywords: Thermal comfort; Energy efficiency; Tropical summer index (TSI)

V K Mathur and I Chand are with Central Building Research Institute,Roorkee 247 667.

This paper was presented at the 18th National Convention of ArchitecturalEngineers held at Jaipur on October 17-18, 2002.

Vol 84, October 2003 33

INTRODUCTIONProvision of thermal comfort in buildings is an importantconsideration in the design of buildings for efficiency and wellbeing of occupants. Two well known methods for creatingcomfortable environment in the interior of buildings includeadoption of the use of electrically operated mechanicaldevices, such as, air conditioners, heaters, blowers, etc, andnatural systems based on judicious utilization of solar andwind energy. Though the earlier one is more dependable, theestimated electricity consumption on account of heating,ventilating and air conditioning. lighting and water heating isabout 30% that constitutes a significant proportion of the totalelectricity consumption in the country. In the present energyscenario in India where gap between demand and supply ofelectrical energy is continuously increasing, the escalation incost of power and associated environmental concerns havecreated awareness about efficient use of energy in every walkof life. Since, building sector is a major consumer ofelectricity, it is imperative to evolve building designs thatwould utilize solar and wind energy to the fullest possibleextent for ameliorating thermal environment indoors. TheBureau of Energy Efficiency constituted by the Governmentof India in March, 2002, has identified �Energy Efficiency inBuildings and Establishments� and �Energy ConservationBuilding Codes� as the thrust areas of its action plan. The focusof these areas is directed towards improving energy efficiencyin existing buildings and development of codes so that newbuildings be designed and built with energy efficiencyconsiderations having been incorporated right from theplanning stage. This is a testimony to the fact that necessity fordesign of functional and energy efficient buildings has beenvery well recognized and efforts are needed to design buildingsthat would function in conformity with climate and not

against it. Accomplishment of the aforesaid objective involvesthree steps (i) identification of the climate at the building sitein question; (ii) determination of the comfort requirements ofthe relevant climate; and (iii) selection of appropriatearchitectural features including space planning, orientation,location and size of fenestration, shading devices, treatment ofbuilding envelope etc. Extensive studies covering the aforesaidaspects have been carried out at Central Building ResearchInstitute, Roorkee and else where also.

CLIMATIC CLASSIFICATIONClassification of climate in respect of building design meanszoning the country into regions in such a way that thedifference of climate from region to region are reflected in thebuilding design, warranting some special provision for eachregion. Based on this criteria, there are five major climaticzones, (i) hot-dry; (ii) warm-humid; (iii) cold; (iv) temperate;and (v) composite.

Climatic Zone Mean Monthly Mean Monthly

Maximum Relative

Temperature, oC Humidity, %

Hot-Dry above 30 below 55

Warm-Humid above 30 above 55

above 25 above 75

Temperate between 25-30 below 75

Cold below 25 all values

Composite � �

A given station is categorized under a particular zone if itsclimate conforms to that zone for six or more months,otherwise it falls under the composite zone. A map of Indiadepicting various climatic zones is shown in Figure 1. Forexample, in Jaipur, it is cold in January, Temperate duringFebruary, November, December, hot-dry during March toJune and October and warm-humid in July to September.

34 IE (I) Journal�AR

Since, none of the climate persists for six months or more, thestations fall in composite climate zone.

COMFORT REQUIREMENTS OF HOT-DRY ANDWARM-HUMID CLIMATEComfort conditions depend upon air temperature, relativehumidity, wind speed, as well as on clothing, acclimatisation,age, sex, and type of activity of the people. Based on exhaustivestudies carried out on thermal comfort at CBRI Roorkee1, atropical summer index (TSI) representing the combined effectof temperature, relative humidity and wind speed wasevolved. The TSI is defined as the temperature of calm air, at50% relative humidity which imparts same thermal sensationas the given environment. Mathematically, TSI is expressed as

TSI = + − +0 745 0 308 2 0 841. . .t t va w

where ta, dry bulb (globe) temperature, oC ; tw, wet bulbtemperature oC ; v, air speed in m/sec.

The thermal comfort usually lies between TSI values of 25oCand 30oC with maximum per cent of people beingcomfortable at 27 5. oC . On lower side, the coolness ofenvironment is tolerable between 19oC and 25oC (TSI) andbelow 19oC (TSI) it is too cold. This clearly indicates that forachieving comfortable environment indoors, heating upto19oC is necessary in winter, whereas steps need to be taken toachieve indoor conditions conforming to TSI values around27 5. oC in summer. Therefore, in hot-dry climate, emphasis islaid on adopting design techniques that contribute towardsreduction in indoor air temperature or globe temperature and

provision of adequate night ventilation. On the other hand,provision of ample air motion is an important requirement ofbuilding design in warm-humid climate.

DESIGN CONSIDERATIONS FOR ENERGYEFFICIENCY IN BUILDINGSEnergy efficiency in buildings broadly implies three aspects;(i) obviating wastage in energy due to unwanted and non-judicious use of electrically operated gadgets; (ii) developmentof energy efficient appliances; and (iii) optimum utilization ofnon-conventional sources of energy through judiciousplanning and design of buildings. The aspects (i) and (ii)concern with the design, installation and operation ofelectrical appliances whereas aspect (iii) is related toincorporation of appropriate passive features at the initialdesign stage of the buildings. Several theoretical andexperimental studies have demonstrated the usefulness ofthese techniques in respect of ameliorating thermalenvironment indoors. In context with cooling of buildings inhot-dry and warm-humid climates, the passive techniquesmainly aim towards reduction in heat penetration throughbuilding envelope and provision of fenestration for inducingdesired natural ventilation indoors.

Reduction in Heat Penetration through Building EnvelopeSolar radiation incident on building envelope is the mainsource of heat responsible for raising the temperature ofexterior surface of the envelope and also for creatingtemperature gradient across the thickness of the envelope. As aresult, heat is conducted indoors thereby causing a rise in theinterior surface temperature. Hence, reduction in thetemperature of exterior surface is necessary for keeping theindoor surface temperature at a low value. Transparentwindow facing sun also permits direct entry of sun. This alsocontributes to the rise in the temperature of indoor surfaces.Hence, control of direct entry of sun through windows is anessential requirement for preventing the rise in interior surfacetemperature. Based on these considerations, various methodshave been evolved for curtailment of heat flow throughbuilding envelope.Optimum Orientation

It is well known2 that the amount of daily solar radiationincident per unit area on N and S facing walls is much less ascompared to that on the walls facing other directions. Hence,for minimum solar heat gain by the building envelope, it isdesired that the longer axis of building should lie along East-West direction. Further, the effect of orientation of a buildingon heat penetration through envelope also depends on theaspect ratio, ie, length/breadth of the building. For a buildingwith square plan, ie, aspect ratio 1:1 and glass area equallydistributed on all the four walls, the effect of orientation is nil,while for a rectangular building with aspect ratio 2:1, thefabric load is reduced by 30% due to change in orientationfrom worst to best.

Shading of WindowsLouvers, overhangs or awnings provided on windows help

Figure 1 Map of India depicting different climatic zones

Vol 84, October 2003 35

control direct entry of sun into the room especially duringsummer months. Optimum dimensions of the louver dependon the duration of sunshine on the window facade. Windowsof the same dimensions but oriented differently should havedifferent dimensions of louvers to be effective. A simple boxtype louver3 may be suitable on an eastern facade, a slightlymore complicated vertical and horizontal louver system onthe southern facade and an egg crate type on the westernfacade. The northern facade receives only very early morningor late afternoon sunshine and hence no elaborate systems areneeded and only rain shade is sufficient. It is reported4 thatoverhang with optimum dimensions can produce cooling loadreduction of 12.7% in summer without causing any sufficientchange in sunshine hours received in winter. It is worthmentioning that an overshadowing of the windows must beavoided as it reduces availability of daylight indoors, which inturn results in increased consumption of energy for artificiallighting.

Exterior Surface Solar Reflectance

Surface colour of the external wall affects both the percentageof solar radiation absorbed by the external surface and also thelong wave radiation emission. Hence, the heat flux transmittedinto the building is considerably reduced when externalsurface is painted with a colour with minimum absorption ofsolar radiation and high emission in long wave region. Suchdata5 for a few materials are given in Table 1. Simulationstudies6 conducted at Lawrence Berkley Laboratory USAindicate that by changing the overall albedo of a city from anexisting value of about 0.2 to a white washed of 0.4 may resultin saving of electrical energy by 40% to 50%.

Roof and Wall Insulation

Provision of insulation on walls and roof of a buildingincreases their thermal resistance and curtails conductive heatflow through the building envelope. Recommended thicknessesof some of the insulating materials for roofs of unconditionedand conditioned buildings are given in Table 2. Introductionof air cavity in a wall also increases its thermal resistance.Studies7 on estimation of thermal properties of such a wallrevealed that the overall heat transmission co-efficient U valueof a 27.5 cm brick cavity wall (11.25 cm brick + 5.0 cm air gap+ 11.25 cm brick) is 1.63 W/m2 K while that of a 22.5 cm

solid brick wall with 1.25 cm cement plaster on both the sideU value is 2.26 W/m2 K. Here, it is worth emphasizing thatthe thermal performance of the above cavity wall is slightlybetter than that of a 35 cm solid brick wall.

Energy Efficient Windows

Window is a critical component in the design of energyefficient buildings. The most effective way of window designto conserve energy is by optimising the window size andlocation. Windows on East and West facades should beavoided as these are the worst orientations from the heat gainpoint of view. In air conditioned buildings, windows areconsiderably less insulating than other parts of the envelope ofthe structure. It is observed that for a single glazed windowsystem the U value is 5.22 W/m2 K which is less than thedesired value. The U value is considerably less (3 W/m2 K) fora window system consisting of a double glazing with an air gapof 12 mm-18 mm. Adoption of such a system reduces heat gainby at least 10%.

GUIDELINES FOR INDUCEMENT OF AIRMOTION INDOORS8

1. For achieving maximum benefit from natural wind,buildings need not necessarily be orientedperpendicular to the prevailing outdoor wind; thesemay be oriented at any convenient angle between 0o

and 30o without losing any beneficial aspect ofbreeze. If the prevailing wind is from East or West,buildings can be oriented at 45o to the incident windfor diminishing the solar heat gain withoutsignificantly affecting the air motion indoors.

2. Atleast one window should be provided on windwardwall and the other on leeward wall.

3. Maximum air movement at a particular plane isachieved by keeping the sill height at 85% of theheight of the plane.

4. In rooms of normal size having identical windows onopposite walls, the average indoor air speed increasesrapidly by increasing the width of window up toabout 2/3 of the wall width; beyond that the increase

Table 1 Reflectivity and emissivity of different coatings

Material Reflectivity Emissivity(Solar Radiation) (Long wave Radiation)

Aluminum foil bright 0.95 0.05

Aluminum paint 0.50 0.50

White wash new 0.88 0.90

Grey colour light 0.60 0.90

Grey colour dark 0.30 0.90

Red brick 0.40 0.90

Glass 0.08 0.90

Table 2 Recommended thickness of insulation for roof

Insulating Density Thermal Thickness in cm for Material kg/m3 Conduc- Uncondi- Condi-

Min Max tivity tioned tionedW/mK Buildings Buildings

Cellularconcrete 450 600 0.081 5.0 10.0

Foamconcrete 320 400 0.070 5.0 10.0

Light weightbricks 400 450 0.081 5.0 10.0

Thermocole 16 20 0.041 2.5 10.0

36 IE (I) Journal�AR

is in much smaller proportion than the increase of thewindow width (Figure 2(a)).

5. The average indoor wind speed in the working zone ismaximum when window height is 1.1 m. Furtherincrease in window height promotes air motion at thetop level of window, but does not contributeadditional benefits as regards air motion in theoccupancy zone in buildings.

6. For a total fenestration area (inlet plus outlet) of 20%to 30% of floor area, the average indoor wind velocityis around 27% of outdoor velocity. Further increase inwindow size increases the velocity but not in the sameproportion. In fact, even under ideal conditions the

maximum average indoor wind velocity does notexceed 40% of the outdoor velocity (Figure 2(b)).

7. In regions having fairly constant wind direction, thesize of the inlet should be kept within 30% to 50% ofthe total area of fenestration and building should beoriented perpendicular to the incident wind. Since,inlets smaller than outlets are more sensitive tochange in wind direction, openings of equal sizes arepreferred in the regions having frequent changes inwind direction.

8. In case of room with only one wall exposed to outside,provision of two windows is preferred to that of asingle window.

9. Windows located diagonally opposite to each other,with the windward window near the upstreamcorner, give better performance than other windowarrangements for most of the building orientations.

10. Horizontal louver, ie, a sunshade, atop a windowdeflects the incident wind upward and reduces airmotion in the zone of occupancy. A horizontal slotbetween the wall and horizontal louver preventsupward deflection of air in the interior of rooms.Provision of L type louver increases the air motion inthe room provided that the vertical projection doesnot obstruct the incident wind (Figures 3(a) and 3(h)).

Figure 2(a) Effect of window width on average indoor wind speed

Figure 2(b) Effect of fenestration area on average indoor wind speed

(a) effect of horizontal louver; (b) effect of a slot between wall andlouver; (c) effect of roof overhang; (d) partition upto floor level;

(a)

(b)

(c)

(d)

Vol 84, October 2003 37

11. Provision of horizontal sashes inclined at an angle of

45o in the appropriate direction helps to promote theair motion inside rooms. Sashes projecting outwardare more effective than those projecting inwards.

12. Air movement at working plane 0.4 m above the floorcan be enhanced by 30% using a pelmet type winddeflector (Figure 4).

13. Roof overhangs help air motion in the working zoneinside buildings.

14. Verandah open on three sides is preferable since itcauses an increase in the room air motion for most ofthe orientations of building with respect to theincident wind.

15. A partition placed parallel to the incident wind, haslittle influence on the pattern of air flow but whenlocated perpendicular to the main flow, the samepartition creates a wind shadow. Provision of apartition with spacing of 0.3 m underneath, helpsaugmenting air motion near floor level in the leewardcompartment of wide span buildings.

16. Air motion in a building unit having windowstangential to the incident wind is accelerated whenanother unit is located at end-on position ondownstream side (Figures 5(a) and 5(b)).

17. Air motion in two wings oriented parallel to theprevailing breeze is promoted by connecting themwith a block on the downstream side (Figures 6(a) and6(b)).

Figure 4 Pelmet type wind deflector

Figure 3 Design considerations for energy efficiency in buildings

(e) partition with a spacing underneath; (f) no partition; (g) partitionparallel to the main flow; and (h) partition perpendicular to the main flow

(a) isolated building unit; and (b) two units located at end-on position

Figure 5 Air motion in a building unit

(e)

(f)

(g)

(h)

18. Air motion in a building is not affected byconstructing another building of equal or smallerheight on the leeward side, but it is slightly reduced ifthe leeward building is taller than the windward block(Figures 7(a), 7(b) and 7(c)).

19. Air motion in a shielded building is less than that inan unobstructed building. To minimise the shieldingeffect, the distance between the two rows should beabout 8 H for semidetached houses and 10 H for along rows of houses. However, the shielding effect isdiminished by raising the height of the shieldedbuilding.

20. Hedges and shrubs deflect the air away from the inletopenings and cause a reduction in air motion indoors.These elements should not be planted at a distance lessthan 8 m from the building because the induced airmotion is reduced to minimum in that case. However,

(a) (b)

38 IE (I) Journal�AR

air motion on the leeward part of the building can beenhanced by planting a low hedge at distance of 2 mfrom the building (Figure 8).

21. Trees with large foliage mass having trunk bare ofbranches up to the top level of window, deflect theoutdoor wind downward and promote air motion inthe occupancy zone inside the buildings.

22. Ventilation conditions indoors can be ameliorated byconstructing buildings on earth mound having a slantsurface with a slope of 10o on upstream side.

23. A non-conventional system of ventilation, commonlycalled as wind tower, helps to induce air motion inrooms devoid of windows on two exposed walls. Thewind tower consists of a vertical wind carrying shaftwith a wind scooping attachment atop thereof. On itsvertical sides, the shaft is provided with severalopenings, which connect the tower to the differentrooms intended to be ventilated. Openings in roomsare also provided on walls other than the one facingthe tower. Such an arrangement of openings facilitatescross ventilation in the rooms. The impingement ofwind on the face of the tower causes development ofpositive pressure thereon. As the wind flows aroundthe building, separation of flow takes place at thewindward edges and negative pressure is created overall the leeward faces of the building. Thus, a pressuredifference exists between the tower inlet and openings

(a) better arrangement; and (b) parallel wings

Figure 6 Air motion in two parallel wings

(a) (b)

(a) air flow in isolated building; (b) air flow in a shielded building; and(c) air flow in a taller shielded building

Figure 7 Effect of air motion in buildings

(a)

(b)

(c)

Figure 8 Effect of hedges and struts on air motion

Figure 9 Model of a typical wind tower

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located on the leeward side of the rooms.Consequently, flow of wind occurs from tower inletto the room openings. In the process, the windentering through the wind tower sweeps the roomarea and finally exits through the room openingthereby ventilating the room (Figure 9).

CONCLUSION

Design techniques for cooling of buildings have beendescribed. It has been established that adoption of some simplepassive features like optimum orientation, adequate shading ofwindows, reflective coatings on exterior surfaces, greenerycover over the building, roof and wall insulation, energyefficient window system, judicious provision of windows forample natural ventilation etc results in significant saving in theenergy consumed while creating comfortable environmentindoors.

REFERENCES1. I Chand and P K Bhargava. �The Climatic Data Hand Book.� Tata McGrawHill, New Delhi, 1999.

2. �Orientation of Buildings.� Building Digest, CBRI, no 74, Roorkee, 1963.

3. �Shading Devices for Glass Openings in Air Conditioned Buildings.� BuildingDigest, CBRI, no 119, Roorkee, 1976.

4. S Raeissi and M Taheri. �Optimum Overhang Dimensions for EnergySaving.� Building and Environment, vol 33, no 5, 1998, pp 293-302.

5. M S Sodha, et al. �Solar Passive Buildings.� Pergamon Press, 1986, p 126.

6. H Akabari, S Bretz, D M Kurn and J Hanford. �Peak Power and CoolingEnergy Savings of High Albedo Roofs.� Energy and Buildings, vol 25, 1997,pp 117-126.

7. �Thermal Data of Building Fabrics and its Application in Building Design.�Building Digest, CBRI, no 52, Roorkee.

8. I Chand and P K Bhargava. �Guidelines for Designing Airy Buildings.�Building Digest, CBRI, no 121, 1976.