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Digest 355October 1990

Concise reviews of building technology el /SfB 81 (R3)

Energy efficiency in dwellingsThis Digest identifies which factors determine the energy requirementsof a dwelling and describes the methods used to assess energy efficiency.The levels required by the Building Regulations for new dwellingsare discussed. Improvements to the energy efficiency of existingbuildings are also considered, taking account of the opportunitiesthat arise when major refurbishments are carried out.

Fig 1 Energy flows into a nd out of a typi cal house

Building Research Establishment................. DEPARTMENT OF THE ENVIRONMENT

Heat losses :windows

walls, loors, roof

ventilation

Technical enquiries to:Building Research EstablishmentGarston, Watford, WD2 7JRTelex 923220 Fax 664010

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355ENERGY EFFICIENCYEnergy used in dwellings accounts for about 30% of all energyconsumed in the United Kingdom and a similar proportion ofenergy-related emissions of carbon dioxide to the atmosphere. Theaverage household devotes 6% of its total annual expenditure todomestic energy.The benefits obtained from using energy depend upon the efficiencywith which it is used. When applied to a national economy, energyefficiency is often expressed in terms of gross domestic productper unit of energy consumed. For individual dwellings it can bemeasured in terms of the amount of energy needed to produce agiven performance. So an energy-efficient house is one which needsless energy than other houses of a similar size to meet the occupantsneeds. This definition allows quantitative comparisons to be madebetween different dwellings and changes to the performance of thedwelling stock to be measured.In assessing energy, efficiency two issues need to be addressed : Firstly, it is necessary to define the levels of service to be achieved.Households vary widely in the way they occupy their homes ,

leading to large variations in the amounts of energy they consume.It is often convenient therefore to standardise needs, based ontypical households with carefully defined living patterns. For spaceheating, indoor temperatures and heating patterns can be definedso that heating energy requirements can be calculated ormeasured . Other uses of energy can be based on averageconsumption derived from measurements and surveys, or byidentifying the appliances used.

The second issue is how to express energy needs once they havebeen calculated. Annual energy cost is often the most appropriatemeasure of energy needs since it allows energy to be assessed onthe same basis as other costs and is most meaningful to theconsumer. Prices can change fairly quickly, however, so energyunits may be a more satisfactory basis for assessing long-termchange if only one fuel is being considered.

TERMS USED TO QUANTIFY ENERGYUseful energy Energy unitsThe energy required to perform a specific function atthe point of application . For space heating it ismeasured at the output of the heating system.Delivered energyThe energy content of fuel delivered to theconsumer. For space heating, it is the energy input tothe heating system.

The JOULE is the basic international unit of energy,abbreviated to J. Because of the joule's small size inrelation to the annual energy requirements of atypical household, the GIGAJOULE (GJ) is used.Other familiar units of energy are the THERM, usedfor gas, and the KILOWATT HOUR which is usedfor electricity.

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Primary energyThe energy content of fuel entering the energyeconomy. It represents the energy content of fuelinput to power stations, oil refineries, processingsolid fuel , etc; energy used during extraction of thefuel itself; losses in distribution networks; and theenergy delivered to users .

1 GJ 109 Joules278 Kilowatt hours9.48 Therms

The WATT is the basic unit of power or rate of flowof energy. It equals 1 Joule per second.

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CALCULATING ENERGY REQUIREMENTSCalculations of energy requirements in dwellings areused because accurate measurements of energyperformance are rarely practicable. To be meaningful,such measurements would need to extend over a wholeyear and to distinguish between different uses ofenergy. Even then, they need interpretation to takeaccount of the behaviour patterns of the householdand the climate. This makes them very expensive.Although actual consumption varies considerablybetween households even when they occupy identicaldwellings, it is practicable to estimate energy needs fora given pattern of usage for both new housing andexisting housing.Domestic energy calculations must take account ofmany factors: climate, including temperature, solar radiation andwind; the physical characteristics of the dwelling and its

heating system; the way in which the house is used, including theextent and duration of space and water heating; internal temperatures and the use of domesticappliances.Figure 1 shows the main flows of energy into and outof a typical house. It is important to note that theheating system is not the only contributor to spaceheating. Even in a house with average standards ofinsulation, solar and internal gains supply a significant

proportion of space heating needs. In very weIlinsulated dwellings, space heating may account foronly a minority of energy use.

355

Traditionally, calculation methods have usuallyconcentrated on the heat losses and ignored heat gains.This meant that they were suitable for estimating themaximum loads to be met by heating systems but werepoor at estimating annual energy needs. The BR EDomestic Energy Model (BRED EM) has beendeveloped to make realistic estimates of annual needssimply and conveniently. BREDEM is based onexperience gained from measurements made in a largenumber of occupied dwellings and the results ofresearch into many aspects of dwelling design whichrelate to energy use. Information Paper IP13/88describes a worksheet version of BREDEM which canbe operated using either a hand-held calculator or apersonal computer. It includes standardised values forinternal temperatures, solar and internal gains, hotwater usage and lighting and appliance usage .Altogether, it provides a robust basis for assessing theenergy efficiency of dwellings and the benefits derivingfrom energy efficiency measures.Figure 2 shows the results of some BREDEMcalculations on a semi-detached house, illustrating howenergy costs vary with insulation levels. It shows thatspace heating costs are dominant where the house ispoorly insulated but are greatly reduced with betterinsulation.

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355FACTORS AFFECTING ENERGY EFFICIENCYHeat lossesWhen a building is heated, indoor temperature ishigher than outdoor temperature and heat is lost byconduction through the fabric of the building. Thefabric heat loss rate is quantified in terms of heat flowin watts per degree of temperature difference betweenindoors and outdoors (W /K).Heat is also lost through the replacement of heated airby fresh air drawn from outdoors. The amount of airinvolved is the product of the internal volume of thebuilding and the rate at which air is replaced. This ratemust include both adventitious ventilation, orinfiltration , as well as deliberate ventilation by the useof windows, extract fans etc. The ventilation heat lossrate, like fabric heat loss rate, is quantified in W /K.Together, fabric and ventilation heat loss rates makeup the specific heat loss rate; this is a good measure ofthe thermal performance of the building and gives onebasis for comparing its performance with otherbuildings of similar size.Heating system efficiencyThe efficiency of a heating system is the ratio of theenergy it produces to that it consumes; it is usuallyexpressed as a percentage. It can vary over a widerange and can therefore have a strong effect on theamount offuel consumed (see Table 1). The benefitsof good insulation can be offset by an inefficientheating system while, conversely, a hard-to-heatbuilding can benefit greatly from the installation of ahighly efficient system. A calculation of running costallows investment in an improved system to becompared with investment in better insulation and themost cost-effective combination of measures to beselected.The controls fitted to a heating system are alsoimportant because they affect both the efficiency ofthe system itself and the extent to which it matches therequirements of the occupants.Table 1 shows the annual efficiencies that can beexpected from a range of commonly used heatingsystems. The figures given are generally below therated efficiencies of appliances under test conditions atfull load, reflecting the fact that for much of the timethey are operating at low loads.Internal heat gainsMany of the heat gains in dwellings are inevitable byproducts of activities such as cooking, water heatingand the use of electrical appliances. In these cases,energy efficiency considerations dictate that theappliances themselves should be made more efficient.This will, of course, reduce the heat gains and therebygive a marginal increase in the need for space heating.It will, though, have a beneficial effect overall since by

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no means all of the gains will contribute usefully toheating, many of them occurring at times when no heatis needed. Moreover, electrical appliancespredominantly use full tariff electricity which is likelyto be much more expensive than the fuel used forspace heating.Heat gains from sunshine are not the result of otherenergy use and should, therefore, be made as large aspossible when space heating is required . The benefitthat can be derived from solar heat gains is determinedby the design of the dwelling as well as the climate ofits location. Passive solar design sets out to make thebest use of sunshine through controlling the form andfabric of the building.Although heat gains need to be taken into accountwhen calculating annual energy needs , they should notbe considered when sizing a heating system formaximum demand as there will be times when heatingis required and gains are a low level.Fuel costThe cost of the energy required to heat and light thedwelling and to supply it with hot water and power forthe appliances is what matters to the householder.Clearly the unit cost of the fuel affects this directly andis as important as the physical factors that affect theneed for energy. This issue is partly obscured from thelayman by the different units employed by the fuelindustries as their basis for charging. Table 2 shows thefamiliar units converted to a common basis to allowcomparison of costs.Electric storage heaters are designed to takeadvantage of off-peak tariffs. It is usual to designsystems based on storage heaters so that they are ableto meet 90% of heating needs by using electricity atthe off-peak rate. In practice, the proportion ofelectricity actually consumed at off-peak rate can varyquite widely according to the way the heating is used ,but experience has shown the 90% figure to be a goodguide for systems designed and used for whole househeating. Systems used for heating only part of a housegenerally require a greater proportion at the on-peakrate.Sometimes the most effective improvement that can bemade to energy efficiency is a change of fuel or tariff.For example, a change of electricity tariff to the offpeak rate can often make a large reduction to spaceand water heating costs. The benefit from fuelchanging must also be considered in the context of theprice stability of the fuels in question. In the pastdecade, oil has varied from being one of the mostcostly to one of the cheapest fuels available. Coalprices have been more stable but vary considerablywith region, being lower in mining areas.

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1J

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CALCULATION OF BUILDING HEAT LOSSESU-valueThe rate at which heat is lost through an element of abuilding expressed in W/(m2K).Fabric heat loss rateThe rate at which heat is lost through all theenclosing elements of a building, such as the walls,roof and windows. It can be calculated by adding upthe products of the areas and V-values of eachindividual element:

NFabric heat loss (FHL) = L Ai Vi W/Ki = I

Ventilation heat loss rateThe rate at which heat is lost through thereplacement of air in the building by fresh air drawnfrom outdoors. It may be calculated from the volumeof air replaced and its specific heat and density:Ventilation heat loss (YHL) = n Vcrp W/Kwhere n is the number of air changes per hour :sV is the enclosed volume of the building (m)

cr is the specific heat of air in J/kg 1

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355TH E BUILDING REGULATIONSBuilding Regulations have had provisions for thermalinsulation since they were first established. Provisionswere also included in the model bye laws whichpredated the Regulations and have influenced insulationof most dwellings built since 1945.The 1965 Regulations for thermal insulation werepitched at a level which could be met by a standardbrick-brick or brick-block cavity wall and 20 mm ofglass-fibre quilt in the roof. Those levels remained inforce until concern about energy conservation grew inthe early 1970s in response to the international 'oilcrisis'. Regulations were formally revised in 1976including the specification of better U-values and aprovision for limiting the total area of windows. Furtherimprovements were introduced in 1982 and 1990 withsignificant increases in insulation on each occasion. The1990 revision is notable because it introduces groundfloor insulation for the first time in the UK. It alsoallows much more flexibility than in previousRegulations , making it possible for builders tocompensate for reduced insulation in one element byincreasing it in another. Table 3 shows the U-valuesrequired by the successive revisions of the Regulations.Figure 3 shows how the changes in the Regulations haveaffected specific heat loss rate and energy costs in atypical house.Construction methods have evolved to take account ofthe changing Regulations. The 1976 revision could bemet by the adoption of lightweight concrete blocks forthe inner leaves of cavity walls. The 1982 revisionincreased the use of insulation within the cavity itselfbut also led to the development of high performanceinsulating blocks which allowed the cavity to be keptclear. The 1990 revision may result in the deve lopmentof a range of new techniques although satisfactorymethods already exist. For example, ground floorinsulation has not been widely used to date and themost economic solutions wi ll evolve to meet thegrowing market for such insulation. The ApprovedDocument for Part L of the Building Regulationsgives a range of solutions which meet the requirements.

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Detailed practical guidance on insulation fornew dwellings is given in the BR E report:ThermaL insuLation: avoiding risks.

PASSIVE SOLAR ENERGYSolar radiation can be a significant source of heateven in an existing house in the UK climate. In a newbuilding, the designer can deliberately set out tominimise heating requirements by controlling thearea and orientation of windows and by the use ofspecial architectural features such as conservatoriesand roof-space collectors. This is often referred to aspassive solar design.At the simplest level, this means ensuring that glazingis predominantly on south-facing aspects and limitingovershading from ad jacent buildings. This can often

r- oeat- lossSolid600 wall onnualenergy-500 -:.: -400 r- -Q)

r - r -ononE t-300.Q).cu;;: r - -uQ)c- -en 200 r- -

100 t-

o 1930 1965 1976 1982 1990Fig 3 Impact of Building Regul ations on heat loss andannu al space hea ting energy requirements

- 60

- 50

- 40

- 30

- 20

-1 0

o

Based on a sem i-detached hOLi se of loor area 80m 2 andpresLiming the same sTandard of heating over the period

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>-Q)cQ)"iii:JCC..

Q;z

Table 3 U-values required by the Building RegulationsRoofs Walls Floors

1965 1.42 1.70 1.42*1976 0.60 1.00 1.00*1982 0.35 0.60 0.60*1990 0.25 0.45 0.45*** applies to exposed floors only** applies to all floors including those in contact with the ground

be done without incurring significant extra cost. It isbeneficial when applied to the layout of a new estatedevelopment where there are often opportunities forinfluencing orientation and positioning of dwellings.Passive solar designs which have very large areas ofglazing need to be carefully analysed to ensure thatoverheating does not occur in summer. Overheatingmay be avoided by limiting the amount of glazing andby the appropriate use of shading, ventilation andthermal mass.

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355IMPROVING INSULATION IN EXISTING BUILDINGSMany existing dwellings were built before even basicthermal insulation provisions were introduced andmany more before the standards of 1976 and 1982 wereestablished. Consequently there are considerableopportunities for improving energy efficiency inexisting dwellings. Those opportunities depend uponhow individual buildings are constructed both for thepractical difficulty of carrying out the improvementsand their cost-effectiveness.Pitched roofs with accessible lofts are usually easy toimprove and highly cost-effective. Many suchimprovements have already been carried out , oftensupported by the Government's Home InsulationScheme. Flat roofs are more difficult to insulate as thespace between ceiling and roof deck is ofteninaccessible. However, it is possible to install insulationabove the roof deck using 'warm deck' or 'inverted'construction. This is likely to be most cost-effectivewhen renewal of the roofing felt is needed.The practicality of insulating external walls dependsparticularly on whether they have cavities which can befilled with insulating material. Cavity insulation canyield a large and cost-effective improvement to energyefficiency. However, it should be undertaken only by acompetent installer working to the relevant BritishStandards or covered by a British Board of Agrementcertificate. The solid brick walls of older homes (builtbefore about 1935) may also be insulated but usually athigher cost than for cavity walls. Insu lation appliedexternally is particularly appropriate when the existingwall is in poor repair or an external render needsrenewing. In such cases the marginal cost of insulatingat the same time as re-rendering may be low enough tomake the insulation cost-effective, at the same timegreatly reducing the risk of the condensation which isoften found in solid-walled dwellings. Insulation mayalso be applied internally either using compositeinsulation boards or by placing insulation behind aplasterboard dry lining. A vapour barrier is needed onthe warm side of the insulation to prevent moisturereaching the cold surface behind the insulation.Windows account for a substantial proportion of totalheat loss in a typical dwelling. Double glazing willusually reduce the loss through windows by about half;low-emissivity glass and inert gas fillings give furtherreductions. Although replacement windows are notusually cost-effective purely on the basis of energysavings, it makes sense to specify double glazing ifwindows are being replaced for other reasons.In flats, windows can form a large proportio n ofexternal surface area and, consequently, be thedominant factor determining space heating needs. Insuch cases, double glazing may result in much lowerdemands being placed on the heating system andoverall savings being achieved when the heatingsystem has to be replaced at the same time.

Other energy efficiency improvements to existingdwellings include draughtproofing and the insulationof hot water storage cylinders. There is alsoconsiderable potential for improving the efficiency ofexisting heating systems. This has already occurred ona large scale through the replacement of open fires bycentral heating systems. Many existing gas centralheating boilers are now in need of replacement andimprovements in efficiency can be obtained simply byreplacing them with their modern counterparts.Condensing boilers offer greater opportunities forsavings although they are more expensive. Typicallythey operate at an annual average efficiency of 85% ,compared with about 70% for a standard type. Sincemany of the boilers to be replaced will have beeninstalled in the early 1970s or before, they have evenlower efficiencies and it is possible to reduce fuelconsumption by up to one third. This improvement isgreatest where demand for heat is highest, ie in large,badly-insulated properties, and is often the most costeffective measure available. Clearly, the bestopportunity for installing a condensing boiler is whenthe existing boiler needs replacement: at that time onlythe marginal cost of the condensing boiler over thestandard type needs to be considered. Replacement ofa boiler with many years of useful service ahead of it isless likely to be economic.Practical guidance on improving the energy efficiencyof existing dwellings can be found in A designer'smanual for the energy efficient refurbishment ofhousing. The cost-effectiveness of some commonmeasures is shown in Table 4.

Table 4 Energy efficiency in existing dwellingsMeasure

Hot watercylinder jacketLoft insulation

Condensingboilers

Typical payback period Applications andcomments

6 to 12 months All dwellings withhot water storage] to 3 years All dwellings with

accessible lofts2 to 4 years At replacement of

worn-out boilersDraughtproofing 2 to]O years All dwellings - subjectwindows and to need to maintaindoors adequate ventilationCavity wall 4 to 7 years Dwellings with cavityinsulation walls not subjected to

severe driving rainDouble glazing ] 0 to 12 years Cost effective when

windows need replacing

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355ENERGY EFFICIENCY GOOD PRACTICE IN HOUSINGNew buildingsThe Building Regulations set minimum standardsfor new buildings based upon cost-effectivenesscalculated using typical fuel and building costs.Individual circumstances may justify higherstandards of energy efficiency, for example, when: fuel costs are higher in a particular location higher than usual temperatures are needed unusual methods of construction are used.In some cases, it is possible to achieve significantimprovements at little or no extra cost, eithe rthrough simple passive solar design or because extrainsulation costs are offset by lower heating systemcosts.

FURTHER READING

Existing buildingsThere are many opportunities for improving theenergy efficiency of existing buildings. Some may beapplied without disruption to the occupants of thedwelling and are cost-effective whenever they areapplied. These include: pitched roof insulation hot water cylinder insulation cavity wall insulation draughtproofingFurther opportunities arise when buildings arerefurbished or components are replaced: double glazing when windows are replaced solid wall insulation when re-rendering or rainscreening ground floor insulation when floors are renewed high efficiency boilers and controls when boilersare replaced or central heating is installed.Good practice will require consideration of all theseopportunities bu t may result in some of them beingrejected in particular circumstances.

ANDERSON, B R. Energy assessment for dwellings using BREDEM worksheets.BRE Information Paper IP13/88.The Building Regulations 1985.1990 edition. Approved Document Ll . HMSO, 1989.Thermal insulation: avoiding risks. BRE Report BR143, BRE, 1989.A designer 's manual for the energy efficient refurbishment of housing. BS r, 1989.Other BRE Digests108 Standard U-values140 Doubl e glazing and double wi ndows206 Ventilation requirements210 Principles of natural ventilation306 Domestic draughtp roofing; ventilation considera tions312 Flat roof design: the technical opt ions319 Domestic draughtp roofin g: mate rials, costs and benefits324 Flat roof design: thermal insulation339 Condensing boilers

Printed in the UK and published by Bui ldi ng Research Establ ishment.Price Group 3. Also available by subscription. Current prices from: Crown copyright 1990BREBookshop, Building Research Establishment, Garston, Watford , WD2 7JR (Tel : 0923 664444).Full details of all rece nt issues of BRE publications are given in BRE News sent free to subscribers.8 Pr inted in the UK for HMSO. Dd.82761 03, 10/90, e1 60, 38938. ISBN 0 85125 467 5