Passivhaus Primer: Designer's Guide A Guide For The Design Team And .

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www.passivhaus.org.ukPassivhaus primer: Designer’s guideA guide for the design teamand local authoritiesBRE is registered with the Passivhaus Institutas an official Certifier for Passivhaus Buildings

2Passivhaus Primer – Designer’s guidePassivhaus Primer – Designer’s Guide:A guide for the design team and local authoritiesForm factorOrientationBuilding formWhere possible a Passivhaus building shouldbe orientated along an east/west principleaxis so that the building faces within 30degrees of due south (in the Northernhemisphere). This allows the building toderive maximum benefit from useful solargains, which are predominantly availableto south facing facades during the wintermonths. With good planning a Passivhausbuilding can also be realised where a southfacing orientation is not possible, althoughthe annual heating demand may increase by30-40% as a result.The compactness of a building is indicatedby the surface area to volume (A/V) ratio.This ratio, between the external surface areaand the internal volume of a building, has aconsiderable influence on the overall energydemand. Buildings with identical U-valuesand air change rates, and orientations couldhave significantly different heating demandssimply as a result of their A/V ratio.The size of a building also influences theA/V ratio. Small buildings with an identicalform have higher A/V ratios than their largercounterparts. It is therefore particularlyimportant to design small detached buildingswith a very compact form, whilst largerbuildings offer the designer greater freedomto explore more complex geometries.Figure 1 illustrates the influence of formand size on the A/V ratio. A favourablecompactness ratio is considered to be onewere the A/V ratio 0.7m²/ m³. In someparts of the UK with poor winter insulationvery small detached dwellings may requireeven lower A/V ratios in order to achieve thePassivhaus specific heating demand.0.60.70.80.91.0A useful variant of the A/V ratio known asthe ‘Form Factor’ describes the relationshipbetween the external surface area (A) andthe internal Treated Floor Area (TFA). Thisallows useful comparisons of the efficiencyof the building form relative to the usefulfloor area. Achieving a heat loss Form Factorsof 3 is a useful bench mark guide whendesigning small Passivhaus buildings.Irregular formsFigures 2 and 3 illustrate the effects ofdesigning more complex forms which resultin an increased surface area for the sameuseful floor area. This change in the A/Vratio impacts on the amount of additionalinsulation required to maintain the sameheating demand:In addition to increasing the insulationrequired to achieve the same overall HeatLoss Parameter (HLP) a building with amore complex form is likely to have a higherproportion of thermal bridges and increasedshading- factors that will have an additionalimpact on the annual energy balance.All of the forms in Figure 4 are able toachieve the Passivhaus standard but a higherperformance specification to the builtfabric is likely, instead of the recommended0.15 W/m²K the fabric may need to achievecirca 0.08-0.10W/m²K.1.21.10.50.40.30.2Figure 1 Surface area to volume ratio (A/V)Surface area increaseof 10%Surface area increaseof 20%Increase ininsulation 20mmIncrease ininsulation 40mmFigure 2 10% greater surface areaFigure 3 20% greater surface areaFigure 4 Forms

Passivhaus Primer – Designer’s guide3U values of fabricand opaque elementsThe Passivhaus standard requires that allthermal elements have a very good U value.Whilst the absolute value adopted for theopaque elements will vary according to thebuildings context (location, form etc), therecommended limits are:–– walls, floors and roofs 0.15 W/m²K–– complete window installation 0.85 W/m²KIn some situations these backstop valueswill need to be improved upon and shouldtherefore be regarded as the maximumacceptable U-values.Construction typesVirtually all construction methods can besuccessfully utilised for Passivhaus design.Masonry (cavity wall and monolithic), timberframe, off-site prefabricated elements,insulated concrete formwork; steel, strawbale and many hybrid constructions havebeen successfully used in Passivhausbuildings.External insulation is widely used inPassivhaus’ due to the reduction in thermalbridging that occurs when the structuralmembers are wrapped in insulation.Figure 5 shows a variety of wall constructionscapable of achieving a U value of 0.15 W/m²K with less than a 450mmbuild-up.Masonry with EIFSPolystyrol rigid foam ICFLightweight elementwooden structural insulatedpanel or fully insulated I-beamICF based on expandedconcretePrefabricated lightweightconcrete elementBlock plank wallPrefabricated polyurethanesandwich elementsFigure 5 Passivhaus wall constructionsHigh-tech vacuuminsulation panelLightweight concrete masonrywith mineral wool insulation

4Passivhaus Primer – Designer’s guideThermal bridgesGeometric junctions and connectionsbetween elements typically provide athermally conductive bypass route for heatloss and must be reduced or eliminatedwherever possible. The diagram top leftshows some common areas which thermalbridges can occur. Careful constructiondetailing is required to ensure the junctionsdo not create unnecessary heat loss paths.The use of external insulation provides amajor advantage in reducing thermal bridgesat geometric junctions. Strategic placementof insulation in and around junction detailshelps to reduce connection heat loss paths.To avoid unnecessary heat loss a Passivhausshould be thermally bridge free, in practicethis means that any linear ( two dimensional)thermal bridges should have a psi (Ψ) valueof 0.01 W/mK. In a Passivhaus the heatloss areas and thermal bridges are calculatedrelative to the external boundary layer, andwith good detailing it is possible to achievenegative psi values in some cases. A negativepsi value implies that a junction is so wellinsulated that the two dimensional heatflow through the junction is less than therespective one dimensional heat flows.Point (three dimensional) thermal bridgescan occur at the corners of buildings andwhenever a column or point fixing creates athree dimensional heat flow path through athermal element. In Passivhaus design thesebridges are usually designed out and can beignored unless they contribute to significantheat losses.The Passivhaus Planning Package (PHPP)and certification process requires thequantified input of all significant thermalbridges. Where standard Passivhaus detailsare used a number of reference sources areavailable for psi value determination. Theuse of bespoke construction details willnecessitate accurate modelling of individualthermal bridges using specialist software.Two and three dimensional thermal bridgingcalculations can be carried out by BRE toconfirm the Psi and Chi values of any junctionand expert advice given on ways to improveconstruction detailing.Once built thermal bridges may be identifiedthrough the use of infra-red thermography,however at this stage it is usually too late todo anything about them! The first strategyin thermal bridge free detailing is to identifyall of the possible thermal bridges at theoutset and design them out systematically.With experience it is usually possible to tellwhether a thermally bridge free design hasbeen achieved before numerical calculationsare carried out.In Figure 6 you can see the cold bridging atthe wall and roof junctions and around thewindows (image shown is not a Passivhaus)Figure 6 Cold bridging

Passivhaus Primer – Designer’s guideAirtightnessIn order to reduce the heating demandand prevent warm moisture laden airfrom entering the fabric the building musthave very good airtightness levels. Goodairtightness levels can only be achieved byusing an air tight membrane or barrier withineach of the building elements. Dependingon the type of construction being used theair tight barrier may be formed by eithera parging coat (masonry) a vapour barriermembrane or by using OSB-3 board or othertimber sheet products of suitable thicknessand airtightness. (See Figures 7 and 8.)Where this layer is interrupted - by a windowfor example - a suitable proprietary tape isapplied to connect the air-tight membraneor barrier layer to the window to ensurecontinuity of the air tight layer. The airtightlayer should be clearly defined and specifiedat the Detailed Design stage and illustratedon all production drawings.The air tightness of a Passivhaus building isdefined by an n50 test measurement whichcombines both under and over pressurisationtests. The resultant air leakage at 50 Pascalspressure must be no greater than 0.6 airchanges per hour (0.6 ac/h @50 Pa). Then50 test differs from the UK q50 test whichmeasures the rate of air infiltration of theexposed building fabric irrespective of thebuildings volume (m3 /m2h @ 50 Pa). Sincethe testing conventions also differ it isnot possible to make direct comparisonsbetween n50 and q50 readings.Delivering such rigorous standards in practiceis facilitated by good design at the outset.Correctly siting the air tight barrier within theconstruction build up reduces the need forrepeated service penetrations (for cabling,pipe work etc). Where such penetrationsare unavoidable proprietary gaskets andgrommets are available to maintain an airtight seal. The airtightness of the building isgreatly affected by the workmanship of thesite operatives, so air tightness details mustbe buildable on site. It is imperative that thecontractors are aware of the impacts of theirtrades on maintaining the airtight barrierand its importance in achieving the overallPassivhaus standard.It is strongly recommended that at leasttwo airtightness tests are built in to theconstruction schedule. The first test shouldbe commissioned whilst the air-barrier is stillexposed, ideally at the end of first fix stagewhen the joinery has been installed. A finaltest can then be carried out at completion byan ATTMA registered tester for inclusion inthe PHI certification dossier.Figure 7 Airtight membraneFigure 8 OSB-3 with joints taped5

6Passivhaus Primer – Designer’s guideGlazing, solar gainsand shadingIn order to benefit from the useful solargains a Passivhaus requires the glazing to beoptimised on the south façade with reducedglazing on the North façade. HistoricallyPassivhaus buildings in continental Europehad very large areas of South facing glazingoften in excess of 50% of the façade area.With good design and modern glazingsystems it is possible to reduce the glazedarea of the South façade to approximately25-35% allowing more conventional glazingratios to be adopted where this is a planningrequirement.It is desirable to position habitable roomssuch as living rooms, dining rooms, children’sbedrooms etc. on the south façade as thelarger windows will provide a pleasantambiance with good daylight factors.Conversely, rooms where a view out andgood daylight factors are not so importantsuch as WCs, bathrooms, store rooms andbuilding services can be placed on the northfaçade (Figure 9).Triple glazing and doorsIn more temperate climates such as partsof Southern European it is possible toachieve the Passivhaus standard using goodquality double glazing. In the UK howeverPassivhaus buildings must use triple glazedwindows; there are two main reasons forthis:1 To reduce unwanted heat losses throughthe window2 To increase the surface temperature ofthe inner pane thereby reducing radiantthe sensation of cold “draughts” from theglass and the possibility of mould growth.Glazing suitable for use in a Passivhausbuilding should have been independentlycertified by the Passivhaus institute inorder to verify that a standard glazing unit(1.24 x 1.48m) has a whole window UWvalue of 0.80 W/m²K and can achieveU value 0.85 W/m²K once installed.Glazed components of doors must achieve asimilar glazing specification and the installedU value of a Passivhaus door should be 0.80 W/m²K.Airtightness is a critical aspect of allPassivhaus glazing and doors and is oftenoverlooked. It is critical that multiplecontinuous airtight seals are used inconjunction with a robust gearing systemto ensure that air leakage when tested atQ(100 Pa) 2.25 m3 /hm.North facingSouth facingFigure 9Solar gains and glazingSolar gains make up a significantcomponent of the free heat gains availableto a Passivhaus during the heating season(Figure 10). For this reason ProfessorWolfgang Feist stated that “as a side effectthese windows themselves become radiatorsfor the room”!To make optimum use of the usefulsolar gains in winter in addition to goodorientation the glazing must have lowinstalled U values ( 0.85 W/m²K) to reduceheat losses and good solar transmittance(g-values 0.5).Conversely too much glazing can lead toan overheating risk in summer, so goodseasonal shading is important in Passivhausdesign. Careful attention should be givento shading from the high summer sunangle particularly on South, West and Eastfacades. The efficacy of seasonal andpermanent shading devices can be testedin the Passivhaus Planning Package as partof an overheating reduction strategy. Formaximum performance the glazing andshading specification should be fine-tunedon each façade of a Passivhaus building.

Passivhaus Primer – Designer’s guideA central part of the Passivhaus principle is tomake use of solar gains in winter to reducethe heating demand. This can mean thatthere is a potential for overheating in thesummer, particularly where south facingglazing has been maximised. Thereforeit may be necessary to incorporate someexternal shading to reduce the amount ofdirect solar gains in the summer. The diagramon the previous page shows a simple externalshading system which utilises extended eavesfor the first floor and a thermally brokenbrise-soleil for the ground floor in order toreduce high angled solar gains in summer.Correct positioning of fixed shading deviceswill allow maximum use of direct solar gainsfrom the lower angled winter sun when it ismost needed.It is a requirement for Passivhaus certificationthat temperatures exceeding 25 C cannotoccur in a building for more than 10%of the occupied year. For a dwelling theoccupied year is considered to be 365 daysa year but for a school this period might bemuch shorter. In the light of climate changepredictions designers are recommendedto achieve a figure of 5% overheatingfrequency or less (using current day data)and to make provision for additionalseasonal shading devices to combat futureoverheating risks.A number of further strategies areavailable to reduce overheating risk inPassivhaus design. These include the useof conventional cross ventilation and nightpurge ventilation. Mechanical optionsinclude using the Heat Recovery Ventilationsystem in by pass mode with or withoutadditional ground or brine loop pre-coolingoptions. Thermal mass may also be usedwhere appropriate to attenuate some ofthe diurnal temperature variations inducedby unwanted solar gains however attentionshould be given avoid over reliance on thisconcept since it may be contra indicatedduring periods of prolonged overheating.40Solar gainwindowssouthSolarFree heatWindowsNorth30Specific heat demand (kWh/m2.yr)Summer overheating insFigure 10 Useful solar gains as a percentageof free heat gainsDenby Dale Green Building Store7

8Passivhaus Primer – Designer’s guideMechanical ventilation with heatrecovery (MVHR)Ventilation losses account for a significantcomponent of the total heat losses in a lowenergy building. The only way to furtherreduce these loses whilst maintaining indoorair quality is to recover some of the heatlost from the outgoing air. By using naturalventilation through open windows in winterall of the heat from the warm air leavingthe building is lost. Furthermore naturalventilation in winter is often undesirable ascan involve involves cold drafts and in somecases noise and environmental pollutionentering the building.Currently the only way to recover ventilationheat losses and provide consistently goodair quality in a sufficiently energy efficientmanner is by using Mechanical Ventilationwith Heat Recovery (MVHR). An MVHRsystem works by extracting air from certainrooms and supplying fresh air to others.The air that is extracted is warm indoorair from wet rooms such as kitchens andbathrooms - this air then passes through aheat exchanger which gives up the warmthfrom that air to the incoming fresh outdoorair. The incoming and exhaust air massesremain separate throughout and the “prewarmed” fresh outdoor air is then suppliedto bedrooms, living rooms, dining rooms etc. Matrix Bau LtdThis is a very efficient and controlled meansof providing fresh air to the habitable spaces.In most situations a small heater coil or frostprotection unit is used in the MVHR unit toprevent frost occurring within the unit itselfduring critical winter conditions.Only ventilation units which have beencertified by the Passivhaus Institute andhave a heat recovery efficiency of 75%(calculated according to the PassivhausInstitut methodology) and a specific fanpower of 0.45 Wh/m3 should be specified.Detailed installation criteria are in place tostrictly limit any unwanted noise transferfrom the MVHR unit and sound transferbetween rooms.HeatingAs the heating demand is so low aconventional heating system consistingof a centralised boiler feeding a series ofradiators or under-floor heating coils in eachroom becomes unnecessary. In all but thecoldest temperatures a Passivhaus buildingwill be capable of maintaining an internaltemperature of 20 C solely by relying on theheat given off by appliances, occupants andsolar gain. During the very coldest weeks ofthe year a small amount of supplementaryheating may be required and this can beprovided in the form of a post-air heatingunit in the MVHR ventilation system and/or small towel radiators or under floorheating in the bathrooms. Care should betaken when specifying the type and controlsused for any back up heating as a poorlydesigned system could result in unwantedover-heating.

9Passivhaus Primer – Designer’s guidePrimary energy appliancesThe primary energy demand for heating,ventilation, hot water and domesticelectricity is limited to 120 kWh/(m²a).Primary Energy is defined as “Energy asfound in the natural environment priorto any conversion process i.e. the energycontent of raw unprocessed fuels at thepoint of extraction and renewable energyresources”. Primary energy takes in toaccount the losses incurred during raw fuelextraction and processing losses, as well asthe generation, transmission/transportationlosses. Producing and delivering one kWh ofelectricity requires approximately 2.5 timesmore energy than delivering one kWh ofnatural gas. Primary energy is strictly limitedin Passivhaus buildings to ensure that thereis not a considerable primary energy penalty(and associated carbon emissions) if electricresistance heating is specified throughout. Inorder to achieve the overall Primary Energytarget highly energy efficient appliances(A rated washing machines, dishwashersetc.) and equipment (fans, pumps, lightingetc.) must be specified.It is worth noting that energy generatedby Photovoltaic (PV) systems may not becounted against the Primary Energy target inthe PHPP methodology. This is deliberatelyimplemented to prevent poor standards ofenergy efficiency being offset by the use ofrenewable energy.Specification and contractWhen undertaking a Passivhaus projectit is prudent to ensure that all of the keyPassivhaus requirements are written intothe appropriate contract documentation.This will help ensure that all parties areaware of the requirements and designtargets set for the project. Specificcontractual clauses should be written tocover performance targets, the quality ofmaterials and any substitution clauses aswell as the workmanship required to meetthe Passivhaus standard. An example ofthis would be the airtightness target whichis highly dependent upon the quality ofconstruction site workmanship.Passivhaus CertificationrequirementsIn order to have a building certified as aPassivhaus it needs to meet the followingperformance criteria:Building energy performanceSpecific heatingdemandor Specific Peak load 15kWh/m2.yr 10 W/m2Specific coolingdemand 15kWh/m2.yrPrimary energydemand 120kWh/m2.yrElemental performance requirementsAirtightness 0.6 ac/h (n50)Window U value 0.80 W/m2KWindow installedU value 0.85 W/m2KServices performanceMVHR heat recoveryefficiencyMVHR electricalefficiency 75%* 0.45 Wh/m3Thermal and acoustic comfort criteriaOverheatingfrequency 25 C 10% ofyearMaximum sound fromMVHR unit35 dB(A)Maximum transfersound in occupiedrooms25 dB(A)* Note MVHR efficiency must becalculated according to Passivhausstandards not manufacturer’s rating

10Passivhaus Primer – Designer’s guideWarm climatesThe Passivhaus standard was initiallydeveloped for mid and northern Europeanclimates but the concept has been proven towork extremely well in hot climates as well.High levels of airtightness and insulationwork equally well in protecting buildingsfrom overheating provided there is adequatesolar shading in place.The Passivhaus Institute published a detailedstudy of Passivhaus performance in SouthernEuropean climates and found the following:–– Double glazing is acceptable in moretemperate climates–– Thermal mass and moisture absorbing(hygrothermal) materials gain inimportance–– Movable external shading is essential–– Maybe need for active cooling and/ordehumidifying–– Any additional cooling demand 15 kWh/(m²a)–– The ground can be used as a heat or coldbuffer for tempering the supply airWhere the frequency of internaltemperatures above 25 C exceeds 10% ofthe year additional measures are required toprotect against summer overheating. Crossventilation through open windows andnight purge ventilation strategies may alsobe incorporated as part of the Passivhauscooling concept when appropriate. Wheresuch strategies are not possible Passivhauspermits 15 kWh/(m²a) of additional coolingenergy to be used. Such a small cooling loadhas proven to be sufficient in almost all casesbecause the Passivhaus concept is highlyeffective in reducing unwanted heat gains.Certified Passivhaus, Santa Fe, CaliforniaSouthern hemisphereThe Passivhaus concept has also beenproven to work equally well in the Southernhemisphere. The main difference is the needfor north facing glazing rather than southfacing glazing as is required in the NorthernHemisphere. Additionally, depending on thelatitude and climate, it is likely that externalshading will be needed on the north andwest facing façades to prevent overheating.A mirroring tool is available from Passipediawhich allows Southern Hemisphere climatedata to be correctly entered in to the PHPPmodel.

Passivhaus Primer – Designer’s guideOur servicesConsultancyTrainingAll members of BRE’s Passivhaus teamhave completed the Certified PassivhausDesigner training and are able toprovide expert advice at all stages of thedevelopment, including:BRE is registered with the Passivhaus Institutas an official training centre for Passivhaustraining courses–– Design Concept and Strategy–– Low energy design advice–– Construction techniques–– Toolbox talks for on-site inductions andmanagement–– Generating Passivhaus informationpacks for specific buildings–– Thermal modelling of constructiondetailsCertificationBRE is registered with the Passivhaus Institutas an official Certifier for Passivhaus buildings–– Pre assessments–– Full Building Certification–– Reduced rate certification via BREsPassivhaus Certification Scheme (PCS) forqualified CEPH DesignersTesting–– Airtightness detailing and testing to n50and q50 testing methods–– Airtightness compliance reportsfor Passivhaus Certification and UKcompliance–– IR Thermography–– Co-heating testing–– Monitoring and testing for ‘as built’performance–– Post occupancy surveysCertified Passivhaus(CEPH) DesignerA fast track training programme leadingto the examination required to become afully Certified European Passivhaus (CEPH)Designer. This qualification is recognised asthe industry standard for those intendingto work professionally as a Passivhausdesigner in the UK and abroad. BREs CEPHDesigner training course has one of thehighest pass rates in Europe which has beenacknowledged by Dr. Wolfgang Feist.One-day PassivhausIntroduction and WorkshopAn introductory course and workshop is aimedat all those with an interest in low-energydesign, construction and the Passivhausstandard. It introduces the delegates to thePassivhaus principles, walking them throughbest practice construction techniques whilsthighlighting the outline specification andcertification criteria. The course also introducesthe delegates to the Passivhaus PlanningPackage (PHPP).For further informationPassivhausBREBucknalls LaneWatfordHertfordshireWD25 9XXE passivhaus@bre.co.ukW www.passivhaus.org.ukR 44 (0) 845 873 5552T www.twitter.com/PassivhausUKBRE are registered with the Passivhaus Institutas certifiers and designers for the Passivhausand EnerPHit Standards.Passive HouseCertifierPassive House Institute accreditedDESIGNERCERTIFIEDPASSIVE HOUSEDESIGNER Matrix Bau Ltd11

BRE TrustThe BRE Trust uses profits made by BREGroup to fund new research and educationprogrammes, that will help it meet its goal of‘building a better world’.The BRE Trust is a registered charity in England & Wales:No. 1092193, and Scotland: No. SC039320.AcknowledgementsAuthors: Rob McLeodKym MeadMark StandenWith thanks to: Toby RollasonOther Primers in this series:Passivhaus Primer: IntroductionPassivhaus Primer: Contractor’s GuidePassivhaus Primer: Airtightness GuidePassivhausBREBucknalls LaneWatfordHertfordshireWD25 9XXE passivhaus@bre.co.ukW www.passivhaus.org.ukR 44 (0) 845 873 5552

glazing on the North façade. Historically Passivhaus buildings in continental Europe had very large areas of South facing glazing often in excess of 50% of the façade area. With good design and modern glazing systems it is possible to reduce the glazed area of the South façade to approximately 25-35% allowing more conventional glazing

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