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Energy and Buildings 42 (2010) 13141319

Contents lists available at ScienceDirect

Energy and Buildings

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e n b u i l d

A performance comparison of passive and low-energy buildings

Ardeshir Mahdavi , Eva-Maria Doppelbauer

Department of Building Physics and Building EcologyVienna University of Technology, Austria

a r t i c l e i n f o

Article history:

Received 24 February 2010Accepted 24 February 2010

Keywords:

Passive houseLow-energy houseIndoor climateEnergy performance

a b s t r a c t

This paper compares apartments in two residential blocks in Vienna; onepassive andthe other onelow-energy. Theseblockswere constructedsimultaneouslyin thesame location andwith comparablebuildingconstructionfeaturesand floor plans.The main difference between thetwo blocks (otherthanthe higher

thermal insulation level in the passive building) lies in the ventilation system: passive buildings deploycontrolled ventilation, whereas the low-energybuildingsrely mostlyon user-operated natural (window)ventilation. We measured indoor environmental conditions (indoor air temperature, relative humidity,andCO2 concentration) intwo units of each block over a periodof fivemonths.Additionally,the buildingswere compared in view of operation and embodied energy use, CO2 emissions, and construction costs.

2010 Elsevier B.V. All rights reserved.

1. Introduction

While there is a general consensus that buildings energy useand environmental impact must be reduced, there have beenmany discussions in the recent years as to the proper meansand ways of achieving this. Specifically, the relative advantages

and disadvantages of low-energy, passive, and energy-plus build-ing approaches have been at the center of much debate. Thereare varying definitions for the terms passive house and low-energy house in different countries. In Austria, a building maybe declared as a low-energy house if its annual heating demand(HWB in kWh m2 a1) is equal or less than 17(1+ 2.5/lc), whereinlc denotes the characteristic length (heated volume divided bythe surface area of the building) [1]. Passive houses have not yetbeen conclusively defined, but certain benchmarks have been sug-gested: a passive house is considered to be a very-low-energyhouse with a HWB of less than 10 (1+2.5/lc). Moreover, a passivehouse aims at doing without a dedicated heating system, whichwould require HWB values under 10 kWhm2 a1 and a highlytight envelope (n50

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Table 1

Overview of the selected apartments.

Block Apartment Area [m2] No. of residents Main orientation U-value external walls [W m2 K1] U-value windows [W m2 K1]

Passive PH 1 59.4 1 West 0.13 0.8PH 2 89.5 3 West, North, East 0.13 0.8

Low-energy LH 1 51.6 1 East 0.40 1.34LH 2 84.5 5 West 0.40 1.34

The apartment sizes range from 48 to 120m2

living space. Eachapartment has one to four rooms. Building floor plans differ onlymarginally. We conducted our studies in the passive house andin one the low-energy house blocks. In each block, measure-ments were conducted in one small and one large apartment. Bothselectedapartmentsin the passive house aresituatedon the secondfloor of the building. In case of the low-energy house, the smallerapartmentwaslocatedonthesecondfloorandthelargerapartmenton ground floor (Table 1).

2.2. Construction and building systems

The exterior walls of the passive house buildings consist of18 cm of ferrocement and 30 cm of EPS F-15 insulation. The win-

dows have high-quality three-panel-glazing with well-insulatedframes. The house is equipped with a mixed central and semi-central ventilation system. A central ventilationdevice is situatedina mechanical equipment room in the basement. The central devicehas an outdoor-air filter, ventilators for delivery and return air andefficient waste heatrecovery. Inhabitantscan control theair changerate as well as the temperature. The additional heating of the sup-ply air can be regulated via a thermostat. The required rest heatingenergy and the energy for warm water supply is provided by a dis-trict heating network. An energy-efficient pump is used. The airchange rate can be regulated from off (0.1h1) to eco (0.3 h1),normal (0.45 h1), and party (0.6 h1).

In case of the low-energy house, the exterior walls consist of18cm of ferrocement and 11cm insulation. The apartments are

equipped in this case merelywith a basic ventilation system, whichprovides a minimal air change rate. Thus, the apartments must beventilated mainly via manual operation of the windows. The basicventilation system works as follows: a supply-air device is installedabove each window. Working as a ventilation slot, this device auto-matically opens and closes depending on the measured humiditylevelsof theindoor air. Inthe open position,a ventilator inthe bath-room draws the air from the interior of the apartment and directsit to the outside. Gaps below door panels allow for the air to flowfrom one room to another. Outdoor air entering into the roomsis warmed as it is mixed with indoor air at the ceiling level. Thebuildings heating energy is supplied by a district heating network.

2.3. Monitored indoor conditions

Indoor air temperature, relative humidity, and CO2 concen-tration levels were measured continuously (every 515min) viasensors in each apartment situated in the living rooms (LR) and thesleeping rooms (SR). The measurements extended over a period offive months (from early February to the end of June 2009). Thus,data for both colder and warmer outside conditions could be col-lected.

2.4. Additional data

Additional data was collected pertaining to metered energy use,construction costs, embodied energy assumptions, and CO2 emis-sion assumptions pertaining to the energy mix deployed. Upon

conclusionof the measurements, the inhabitants were interviewed

to assesstheir satisfaction withthe apartments energy systems, airquality, and the ventilation systems.

3. Results and discussion

3.1. Histograms

Fig. 1 shows the frequency distribution of measured CO2 con-centrations (in all observed apartments over the entire monitoringperiod). It reveals that CO2 concentrations lie within reasonableranges in all apartments as only a small fraction of the recordeddata lies above the Pettenkofer threshold of 1000ppm. Nonethe-less, PH 1 has overall slightly lower CO2 concentrations than LH 1and instancesof high concentrationsin PH 2 aremuchless frequent

than in LH 2. The differences are more discernable if the data arevisualized in a cumulative manner. As Fig. 2 (cumulative depictionof CO2 concentrations for all apartments in April) shows, CO2 con-centrations in the low-occupancy apartments (PH 1 and LH 1) aregenerally lower than those in the other apartments. Nonetheless,passive apartment PH 2 can be shown to perform slightly betterthan LH 2, particularly in the colder periods of the year (see Fig. 3)and most significantly in the sleeping rooms during the night.

Fig. 1. CO2 concentration in all apartments.

Fig. 2. CO2 concentration (cumulative) in sleeping room of all apartments in April.
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Fig. 3. CO2 concentration (cumulative), sleeping room of 3 apartments (February).

Fig. 4 shows the measured indoor air relative humidity values(RH). These values are within a reasonable range most of the time.The RH values in LH 1 varymorewidely that those measuredin theother apartments. Moreover, the multiple-occupancy apartmentsPH 2 and LH 2 show higher RH values as compared to the smallerapartments. Fig. 5 shows the measured indoor air temperature val-ues. It suggests a similar distribution pattern in all apartments,with the exception of LH 1, which displays a shift toward highertemperatures. This specific circumstance may be a consequence ofuser behavior, i.e. absence of night-time window ventilation andday-time window shading [3].

Fig. 4. Relative humidity values in all apartments.

Fig. 5. Indoor air temperatures in all apartments.

Table 2

Fraction of time (in %) within comfort zones.

Month Room Apartment

PH 1 LH 1 PH 2 LH 2

February SR 3.9 1.0 5.0LR 0.2 39.7 1.5

March SR 62.2 21.6 67.4LR 91.0 73.2 22.5

April SR 100.0 94.3 84.0 99.3LR 100.0 45.9 100.0 87.8

May SR 60.9 64.8 41.6 61.7LR 81.7 40.3 77.6 58.2

June SR 60.5 62.9 70.0 86.4LR 90.7 35.7 97.4 79.4

Entire period SR, LR 65.1 60.6 56.9

April, May, June SR, LR 82.3 57.3 78.4 78.8

3.2. Psychrometric charts

To assessthe thermal comfort conditions inside the apartments,

we calculated the fraction of time (in%), when measured indoor airtemperature and relative humidity values were inside the comfortzone. The extent of the comfort zone was obtained as follows. Foreach month the mean outdoor temperature was calculated. Thisvalue was used to compute the neutrality temperature and theextent of the comfort zones on psychrometric charts according toRef. [4].

Table 2 summarizesthe results (percentage of timewithin com-fort zone) for individual months as well as the total monitoringperiod. Note that for these results different times of the day wereconsidered depending on the room usage. The relevant times ofthe day for the living rooms was considered to be from 8:00 to22:00, whereas the relevant times for the sleeping room was from22:00 to 8:00. Since we could not obtain results for LH 1 in the

months of February andMarch, two differentpercentages werecal-culated. The first one covers the whole monitoring period and thesecond one covers the period from April to June. PH 1, PH 2, andLH 2 display comparable results that are significantly better thanthose from LH 1, which shows very high temperatures (inMay and

June temperatures even rose above 30 C). This specific situationmaybe explainedby considering the orientation, window size, andthe shading circumstances. Thewindow size turnsout to be a majorfactor in particular when we compare temperatures in the sleep-ing and living rooms of LH 1 (Figs. 6 and 7), which have the sameorientation and shading conditions. However, the living room win-dowsize (inrelationto floor area) is significantlylarger than that ofthe sleeping room. As Fig. 7 shows, the temperature in the sleepingroom of LH 1 does not rise above 27 C in June. (Note that this dif-ference prevails during both day and night). Table 2 also suggestsa very low fraction of time in comfort zone for all apartments inFebruary. The adaptive comfort theory suggests in this case com-fort temperatures significantly lower than those maintained by theoccupants.

3.3. PMV and PPD

Calculating the PMV (predicted mean vote) and PPD (predictedpercentage of dissatisfied) is an alternative approach in assessingthermal comfort [5]. Our underlying assumptions for the calcula-tion of PMV and PPD are summarized in Table 3. For the purpose ofthe calculations, mean radiant temperature values were assumedto be equal to the (measured) room air temperatures. Table 4 sum-

marizes PPDvalues foreach month andeach room as well as forthe

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Fig. 6. Indoor conditions, living room, LH 1, June.

Fig. 7. Indoor conditions, sleeping room, LH 1, sleeping room, June.

Table 3

Assumptions for the calculation of PMV and PPD.

Room Hours Winte r (Feb, Mar, Apr) S ummer (May, Jun)

Clo Met v[m s1] Clo Met v [m s1]

SR 22:008:00 2.5 0.8 0 1.5 0.8 0LR 8:0022:00 1.0 1.3 0.15 0.5 1.3 0.15

Table 4

Predicted percentage of dissatisfied [%].

Month Room Apartment

PH 1 LH 1 PH 2 LH 2

February SR 16.2 16.1 20.0LR 5.6 5.4 12.4

March SR 17.5 22.0 16.8LR 5.3 5.5 8.2

April SR 28.7 27.2 37.1 12.9LR 7.9 21.5 9.4 13.3

May SR 17.4 11.8 20.8 11.0LR 6.4 17.4 6.5 9.3

June SR 22.5 13.2 19.4 13.6LR 5.8 17.9 5.8 7.1

Entire period SR, LR 13.3 14.8 12.5

April, May, June SR, LR 14.8 18.2 16.5 11.2

Fig. 8. PMV values, PH 1, living room.

Fig. 9. PMV values, PH 1, sleeping room.

wholeperiod. Thetableimplies that theresults for the livingrooms

are considerably better than the results for the sleeping roomsexcept for LH 1. Generally, all apartments perform rather well.Figs. 8 and 9 show PMV values for PH 1 during the entire mea-

surement period for the livingand sleeping rooms. Nearly all livingroom data lies within a range of0.5 to +0.5. The correspondingvalues for the sleeping room suggest warmer conditions. The otherthree apartments display similar conditions except LH 1, whichshows higher ranges in both rooms. The reason for this is againthe window size, orientation and shading situation. In summary,PMV values in living rooms are fairly reasonable in all apartments,whereas sleeping rooms display higher PMV values. It is possiblethat the actual clo-values maintained by inhabitants were lowerthan our assumptions (see Table 3).

3.4. Evaluation by inhabitants

Given the small number of interviewees (five inhabitants ofthe passive house and six inhabitants of the low-energy house),the results are of limited representative value. Thus, they are nottreated here in detail. Nonetheless, they do imply that the inhab-itants in both building types are generally satisfied with indoorconditions. Moreover, the level of acceptance regarding the ven-tilation system in the passive house is quite high. High indoor airtemperatures in the summer period represent a problem for someresidents, whereby the inhabitants of south-facing apartments areparticularly affected. PH inhabitants had some issues with too lowhumidity values in winter, whereas LH inhabitants complained tothe contrary.None of the interviewees showedany signs of thesickbuilding syndrome.

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Table 5

Energy use data for 2008 (totals for the apartments and per net floor area).

Type of energy Unit Apartment

PH 1 PH 2 LH 1 LH 2

Heating [kWh] 498.9 1358.7 1153.8 3922.2[kWhm2] 8.4 15.2 22.4 46.4

Electrical (apartment component) [kWh] 669.4 2085 1388.8 4281.3[kWhm2] 11.28 23.30 26.94 50.67

Electrical (general component, entire block) [kWh] 22786.3 7441.4[kWhm2] 9.48 3.18

Electrical (total) [kWh m2] 20.76 32.78 30.12 53.85

Table 6

Total CO2 emissions in kg (calculated for 2008).

Type of energy Apartment

PH 1 PH 2 LH 1 LH 2

District heating 112 306 260 882Electrical power 728 1731 916 2685

Total 840 2037 1176 3567

Table 7Additional embodied energy and CO2 emissionsassociated with the construction ofpassive buildings.

Embodied energy [kWh m2] CO2 emission [kg m2]

Insulation 47.7 25.7Windows 6.1 1.6Ventilation system 15.0 3.2

Total 68.8 30.5

3.5. Energy use

Energy use data (heating and electrical) for the four observedapartments (2008) is given in Table 5. The heating energy use of

passive houses is significantly lower, due to better insulation andlower ventilation heat recovery. As to the electrical energy use,a distinction can be made between general areas of each house(including the houses ventilation system) and the apartments. Thegeneral electrical energy use is higher in the passive house dueto the energy requirements of the ventilation system. However,in the present case, passive house apartments require less electri-cal energy. The reason for this is not obvious. Contributing factorscould be user behavior, more effective daylight usage, and higherefficiency devices and appliances. Considering both componentstogether, the electrical energy use of the low-energy apartmentswas, in the present case, somewhat higher than the passive houseapartments.

3.6. CO2

emissions

Table 6 includes computed CO2 emissions associated with theapartments energy use (heating and electrical). It can be assertedthat the passive house apartments produce significantly less CO2emissions than the low-energy apartments. Note that all valuesof Table 6 may be said to be rather low, given the nature of the

energymix in this specific case (heatingenergyis provided througha district heating network).

3.7. Embodied energy

Another aspect of energy consumption is the embodied energyof a building. Embodied energy is the energy that was used duringthe production process of a product includingraw material extrac-tion, transport, manufacture and deconstruction. We estimated the

additional embodied energy and CO2 emissions due to additionalinsulation, better windowsand additional ventilation system of thepassive house (Table 7) based on data obtained from [6] and [7].Subsequently we estimated the amortization period due to pas-sive buildings lower energy demand and CO2 emissions duringoperation time. Table 8 shows that the amortization time for theadditional (embodied) energy investment in passive apartments isaround 13 years with respect to embodied energy and 25 yearsregarding CO2 emissions.

3.8. Costs

The initial costs for the construction of the passive house blockwas in the present case approximately 5% higher than the com-

parable low-energy house block. The major contributing factors tothe passive buildings higher construction costs are the ventilationsystem, the higher performing windows, and the additional ther-mal insulation (particularly exterior walls). On the other hand, thepassive buildings energy use during the operation time is lower.A simple payback analysis (using energy price information givenin Refs. [8] and [9]) resulted in an amortization period of roughly818 years for the additional construction investment in the pas-

Table 9

Amortization time estimation for additional initial investments for passivebuildings.

Unit PH 1 PH 2 LH 1 LH 2

District heating kWh m2 a1 8.40 15.20 22.40 46.40E lectri cal energy kWh m2 a1 20.76 32.78 30.12 53.85District heating D m2 a1 0.78 1.42 2.09 4.32Electrical energy D m2 a1 3.60 5.69 5.23 9.35Total energy costs D m2 a1 4.39 7.11 7.32 13.67Additional initial costs D m2 52.08 52.08Energy cost savings D m2 a1 2.93 6.56Amortization time years 17.8 7.9

Table 8

Amortization periods pertaining to passive buildings additional embodied energy and CO2 emissions.

Apartment Energy CO2 Energy CO2PH 1 PH 2

Additional investment 68.8 kWh m2 30.5kgm2 68.8kWhm2 30.5kgm2

Annual reduction 23.4 kWh m2 5.7kgm2 52.3kWhm2 17.1kgm2

Amortization [years] 2.9 5.4 1.3 1.8

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sive buildings (see Table 9). Note that this calculation does notaddress rising energy prices or other energy sources, which couldhave resulted in shorter amortization times.

4. Conclusion

A comparison of passive house apartments with low-energyapartments in Vienna was conducted based on monitored indoorenvironmental conditions, user evaluation, metered energy use,calculated embodied energy and CO2 emissions, as well as con-struction costs data.

The results suggest that both passive and low-energy apart-ments performedwell in view of thermal conditions and indoor airquality, even though the performance of passive apartments wasslightlybetter.Specifically, it canbe asserted that the useof a venti-lation system clearly contributed to lower levels of carbon dioxideconcentrations in particular during cold periods and especially inmultiple-occupancy apartments. The inhabitants of both buildingswere generally satisfied with indoor conditions and building sys-tems.

Passive house apartments were shown to consume approx-imately 65% less heating energy and 35% less electrical energyas compared to low-energy apartments. Moreover, passive apart-

ments CO2 emissions (estimated based on metered operationenergy usage) were approximately 2540% less than low-energyhouses. Considering the construction implications of passive houseapartments in view of higher embodied energy andCO2 emissions,relatively short amortizationtimes werecalculated (approximatelyone to five years). The initial costs penalty associated with theconstruction of passive apartments (as compared to low-energyapartments) was 5%, resulting in an estimated payback period of818 years. The latter estimation must be seen in the context of a

twofold uncertainty:given a differentsource of energy thanthe onerelevant in the present case (district heating) or in case of drasticfuture energy price increase scenarios, the payback period couldbe significantly shorter. On the other hand, ifin contrast to thepresent case studythe electrical energy use of passive buildingswould have be higher than the low-energy buildings, the paybacktime would have been longer.

In summary, our study suggests that passive buildingsascompared to low-energy buildingsuse significantly less heatingenergy and offer slightly better indoor conditions. Thereby, therequired additional expenditure of resources (as represented byembodied energy) and environmental impact (as represented byCO2 emissions) are offset in a rather short period. Moreover, therequired addition construction cost does not appear to be eitherexcessive or prohibitive.

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[2] Passivhaus Institut Darmstadt, URL: www.passiv.de (visited in August 2009).[3] Wagner, W., Prein A., Felberbauer K.-P., Energietechnische und baubiologis-

che Begleituntersuchung der Bauprojekte: Berichtsteil PassivauswohnanlageDreherstrasse, Zwischenbericht, AEEInstitut fr nachhaltige Technologien,

Gleisdorf, Austria (2008).[4] S. Szokolay, Introduction to Architectural Science: The Basis of SustainableDesign, Architectural Press, Oxford, Great Britain, 2004.

[5] P.O. Fanger, Thermal Comfort, McGraw-Hill Inc., US, 1970.[6] M. Schuss, Life-Cycle-Analyse vonPassivhusern,TechnischeUniversitaetWien,

Austria, 2004.[7] T. Waltjen, kologischer Bauteilkatalog. Bewertete gngige Konstruktionen,

Springer Verlag Wien, Austria, 1999.[8] IWO-Austria, http://www.iwo-austria.at/index.php?id=126 (visited in January

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