In the first part of this article, I looked atthe concept of the Passive House standardand discussed what made a Passive Housebuilding different to a standard building.Having set out important information suchas the certification process and the methodsof calculation required for compliance withthe standard, we can now have a closer lookat the technical aspects of Passive Housedesign and outline the main criteria andsub-criteria which aid designers in achievinga certified Passive House. As with any project the site and

surrounding area will play a major role inwhat is ultimately possible. However, thereare a number of constant factors that canimprove a building’s performance withregard to becoming a Passive House.Provided a design meets the performancecriteria, and is modelled in the PHPP, thedesigner has a high degree of flexibility indesigning a Passive House as they wish. It isworth noting though that deviation fromthe following additional designconsiderations can result in an increase incapital costs due to additional

compensation for avoidable andunnecessary heat losses. (see picture one:Passive House Diagram illustrating thecriteria.)

Orientation and FormA compact building form, with minimumsurface to volume ratios, ensures areduction in thermal bridging and heat loss,whilst a south facing orientation with largeareas of glazing (25-30%) maximises solargains and provides a passive heat source for

Passive HouseTechnical Overview:Part 2Certified Passive House Designer Steff Bell Bsc (Hons) ACIAT explains the passivehouse concept in more detail in the second and final part of his article on the subject.

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14 AT MAY- JUNE 2010

Right: Picture one-Passive House diagram

showing the maincriteria as required inPassive House design

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the building (see picture two above).Natural shading methods such as roofoverhangs, free standing balconies and deepwindow recesses should be considered inthe design to avoid overheating in thesummer. Other forms of shading, popularin both Germany and Austria, includeconcealed roller shutters and exposedmanual shutters.

Preventing Heat Loss:Building FabricSuper InsulationThe building fabric is one of the mostimportant factors of a successful PassiveHouse construction. The insulationsurrounding a Passive House buildingshould be applied without gaps, creating acontinuous thermal envelope around theentire heated area of the building, similar toa giant sleeping bag. These super insulatedwall, roof and floor elements play a largerole in reducing the amount of heat loss,making it possible to heat the buildingwithout a conventional heating system.These components have a recommendedU-value of less than 0.15 W/m²K (sub-criteria) and can vary in size with typical

Passive House wall and roof constructionsbeing up to 350-500mm thick, with typically300mm Insulation or more (see picturethree above).

Air-tightnessIn addition to the super insulation, thebuilding envelope is surrounded by onecontinuous airtight layer with allcomponents and connections fitted in anairtight manner. Air-tightness tapes andseals are applied around all areas wherecomponents meet, with special attentionpaid to the installation of electrical andmechanical services. Air-tightness in aPassive House is verified by conducting apressure test (blowerdoor test) to measurethe amount of air changes the buildingexperiences within one hour. In order toachieve certification, the building’s pressuretest result (measured at 50 Pascal differencebetween inside and outside) must notexceed more than 0.6 ac/h�¹. If thebuilding’s air changes per hour are morethan 0.6 ac/h�¹ then too much heat will belost through leakages and the reducedenergy targets of less than 15kWh/(m²a)will not be reachable (see picture four, five,six).

Thermal bridge free designThe third way in which heat loss can bereduced through the building fabric is toeliminate the thermal bridging effect (alsoknown as cold bridging). Windows anddoor frames are insulated over, and thebuilding is detailed in such a way that areaswhere solid components meet are reduced,or eliminated, so that heat does not have aclear path from the inside of the buildingto the outside. Thermal bridge calculationsare conducted (commonly using the thermor heat programmes) in order to accuratelydetect and eliminate any area that maycreate a thermal bridge. Thermal bridgingwithin a Passive House building isrecommended to be below 0.01 W/(m²K)(sub-criteria) as calculated on an externaldimensions basis.

Passive House WindowsThe windows in a Passive House arerequired to have a much lower U-value thanstandard windows so as to preventexcessive heat loss. The recommendedPassive House window U-value is no higherthan 0.8 W/(m²K) (sub-criteria) and aninstalled U-value of no higher than 0.85W/(m²K) (sub-criteria). These values are

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Right: Picture two– Passive House in

SteingadenGermany (Herz &

Lang project)North-East facades,minimal windows to

the North.

Right: Picture three– Super insulated

walls in MunichGermany (Herz &

Lang project)

Right: Picture four –Internal view of pre-fab wall panels withjoints and insulation

holes taped and sealedfor air-tightness

Right: Picture five –Areas of a house

being tested for air-tightness

Right: Picture six– Pressure test

being conductedin a Passive

House project

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achieved by a combination of triple glazingwith two low-e coatings (typically on glazingsurfaces two and five and filled with argonor krypton gas), warm edge spacers andframes with increased insulation levels (sub-criteria). (see picture seven.)Whilst this reduces heat loss through thewindows, this element of the Passive Housebuilding has a dual purpose and must alsoallow a beneficial amount of solar gains toenter the dwelling. With this in mind thewindow glazing must have a minimum solarenergy transmittance factor of 50% (G-factor = 0.5, sub-criteria) with 62% (G-factor 0.62) being the highest currentlyachievable with Passive House windows.The human body is particularly sensitive totemperature differences within a room, inthe form of radiant temperature asymmetryand temperature stratification. Radianttemperature asymmetry describes asituation where objects simultaneouslyradiate different temperatures towards ourbodies; in this situation a temperaturedifference of 4K or above will causediscomfort. Temperature stratificationdescribes a situation where there is atemperature difference between heights of1.1 to 0.1m within a room, in this case atemperature difference of 2K or above isenough to cause the feeling of discomfort. An advantage of such low heat loss valuesthrough the Passive House windowcomponents is that their internal surface

temperatures will be higher than 17°C (sub-criteria), even during the evenings in winter.The result of this is a dramaticimprovement in a building’s thermalcomfort, especially when sitting next to thewindows, as the improved internaltemperature will eliminate cold drafts dueto temperature stratification, anddiscomfort due to a radiant temperatureasymmetry within a room.

Maximising Heat Gains:Mechanical Ventilationwith Heat Recovery(MVHR)All rooms within the thermal envelope(inside the continuous air-tight layer) areventilated via the use of a mechanicalventilation system with heat recovery. Therequirement for performance of an MVHRsystem, within a Passive House, is aminimum heat recovery efficiency of 75%,with an energy consumption of no more

than 45Wh/(m³) (sub-criteria).The ventilation system provides a steadystream of fresh warm air to the buildingwhich is an absolutely vital requirement asthe building envelope renders the buildingalmost completely airtight. The average airchange rate per person in a Passive Houseis 20-30 m³ h�¹ (sub-criteria) with higher airchange rates in rooms with excess moisture,or odours such as kitchens and bathrooms.The air flow rate of the ventilation systemmust be balanced which is most commonlyachieved by dividing the house into supplyair rooms (such as bedrooms andliving/dining rooms) where fresh air isprovided; overflow areas (such as hallwaysand stairwells/landings) where the air isdrawn across a space; and exhaust air rooms(such as kitchen, bathrooms and laundryrooms) where the stale air leaves thebuilding. (see picture eight)The heat recovery part of the ventilationsystem recycles the heat from the used staleair and uses this to pre-heat the fresh cleanair being pumped in to the building (with aminimum efficiency of 75%). This meansthat in order to reach the Passive Houseinternal design temperature of 20ºC (sub-criteria) the building only requires anadditional 4-5˚C from space heating, as theother 15-16˚C will be recovered from theexhaust air, and channelled back into thebuilding through the supply air. (see picturenine).

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Below: Picture seven – Passivehouse window section diagramwith the low-e coatings applied toglazing surfaces two and five

The ventilation systemprovides a steadystream of fresh warmair to the building

Right: Picture nine –MVHR unit inDunoon Scotland(Scottish PassiveHouse Centre project)

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Space HeatingAn efficient MVHR system ensures that thebuildings space heating requirements arereduced to such a degree that conventionalheating systems are no longer required. Inaddition to the 15-16˚C the building receivesfrom the MVHR unit, solar gains obtainedfrom the large South facing glazing andinternal heat gains, such as body heat, (anaverage person can emit 100W of heatenergy) lighting and household applianceswill easily provide the remaining 4-5˚Crequired to achieve the design temperatureof 20˚C. In some months (mainly summerand autumn) the house will reach thistemperature very quickly and must rely onexternal shading and shock ventilation toprevent the house from over heating bymore than 10% (sub-criteria) at which pointit would become very uncomfortable. Auseful technique for avoiding overheating inthe summer is a summer bypass functionwithin the MVHR unit. This allows thesystem to prevent the heat recoveryfunction therefore reducing the amount ofheat that is brought into the buildingwithout reducing the amount of fresh air. Whilst generally, it is the case that aconventional heating system is not requiredin a Passive House, additional heating maybe required during the very coldest days ofwinter, in order to help the housecomfortably reach the 20˚C designtemperature. This additional heat can be

provided by a range of different methods,the most common of which is via a supplyair post heater, situated after the heatexchange unit, at the beginning of thesupply air duct, inside the buildings thermalenvelope. This source of heat can take theform of electrical heating coils, a small heatpump, solar thermal or even the moreconventional oil burner or natural gas. Forthe purposes of reducing smells fromsinged dust that may be in the ducts, despitethe numerous filters, the supply airtemperature must not exceed 52˚C (sub-criteria).

Lighting and AppliancesLow energy lighting and efficient householdappliances are most commonly used withinPassive Houses in order to achieve theprimary energy consumption criteria of lessthan 120 kWh/(m²a).

ConclusionThis concludes my introduction to thePassive House Standard and the criteriaused in order to achieve a certified PassiveHouse. As discussed in the first part of thisarticle, the Passive House standard in theUK is growing in both popularity andreputation, with an estimated 20– 35 PassiveHouse buildings in existence and a greatdeal more currently under construction orat the planning stage. The biggest driver forchange in any field is the backing of

industry professionals, which only comeswith a greater understanding of the subjectin question. I hope that this article hasenabled people to further understand thepotential the Passive House standard has tooffer the UK and that this will encouragepractices to learn more about the standardand to start using it for buildings in thefuture.If anyone has any questions or would liketo discuss further the possibilities ofimplementing the Passive House standardon future projects please fell free to contactme using the following details:

Steff Bell, Scottish Passive House CentreTel: 0845 3883 716Email: sb@sphc.co.uk

CorrectionIn the previous part of this article(Issue 88) there were two errorsin the labelling of pictures.The image on page 14 requiredthe label ‘Scotland's HighlandHousing Fair, HLM Architects’.The image on page 15 was a Herz& Lang project.

Right: Picture eight – Ventilationbalancing table for an MVHRsystem in a Passive House

The human body isparticularly sensitive totemperature differenceswithin a room

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