Introduction to Passivhaus Components
This chapter introduces the Passivhaus concept. As well as providing a brief history of where the Passivhaus idea came from and how it has grown over the years, it aims to explain what Passivhaus means and identify what components are necessary in a building in order for it to be certified a Passivhaus.
The original idea of a Passivhaus came from a conversation between Dr. Wolfgang Feist and Professor Bo Adamson in May 1988. Inspired by super insulated buildings in America their idea was to use the laws of physics to produce extremely efficient, low energy buildings. (Bo Adamson continued to pursue the development with Wolfgang Feist until his retirement).
The first Passivhaus dwellings were completed in Darmstadt, Germany in October, 1991. These were inhabited by four families and the buildings have since been tested regularly to assess the efficiency of the building throughout its lifespan. A Europe wide project, Cost Efficient Passivhaus as European Standard (CEPHEUS) also monitors and scientifically evaluates 250 constructions built to Passivhaus standards. This is to demonstrate the technical feasibility of range of different buildings and designs that implement the standard.
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Dr. Wolfgang Feist went on to found the Passivhaus Institut in Darmstadt in September 1996 as an independent research institution. Employing physicists, mathematicians, and civil, mechanical and environmental engineers they aim to improve the efficiency of energy use through research and development.
The Passivhaus design standard has since become mainstream and widespread and has been applied to residential, commercial, industrial and public buildings all over the world. To date over 17,000 dwellings have been constructed in accordance with Passivhaus principles, with several projects now nearing completion and certification in the UK. (BRE Passivhaus Primer, 2009, p.2)
"I was working as a physicist. I read that the construction industry had experimented with adding insulation to new buildings and that energy consumption had failed to reduce. This offended me - it was counter to the basic laws of physics. I knew that they must be doing something wrong. So I made it my mission to find out what, and to establish what was needed to do it right."
Dr. Wolfgang Feist
'Passivhaus' not 'Passive House'
The following information will explain why the research project refers to the German 'Passivhaus' and not the English 'Passive House' and will clarify the difference between the two.
Passive House refers to a design that maximises the benefits of natural factors to reduce the energy requirements of a building. These passive buildings will usually make use of maximising the passive solar gain by having a highly glazed south face and/or sun rooms, this, as well as being thermally efficient will reduce heating and artificial lighting requirements. Typically these buildings will use natural ventilation systems.
Although Passivhaus does incorporate some features of a Passive design it is different in the way that it is airtight and uses mechanical ventilation technology to preheat fresh incoming air. This is often known as an 'active' approach to heating. This approach allows the designer more flexibility with the heating design. However correct specification of the system and a good quality design is needed in order to achieve the low space heating requirement of 15 kWh/(m2a) to reach the Passivhaus Standard. A drawback to the Passivhaus design is that the mechanical and electrical system will require maintenance. However the building will not be as reliant on orientation, will not lose as much energy through natural ventilation and will not be as prone to overheating as Passive Houses.
Conversely the Passivhaus Institut does use the term 'Passive House' in all its text, this is because it is a direct translation from German which the publications were originally written in.
As with any energy efficient house design, Passivhaus uses insulation throughout the building fabric to keep the building warm in winter and cool in summer. To achieve high R-values and low U-values in the walls, roof and floors a variety of insulating materials can be used. These are likely to be more innovative and/or thicker than that used in traditional houses (hence super-insulation) to significantly reduce the heat transfer through the building envelope.
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Insulation is often described as the most important principle of a Passivhaus design. Insulation needs to be applied continuously around the building envelope without any thermal bridging. This allows the building to retain the heat that is generated within the house on a daily basis from activities - such as cooking, electrical appliances and people's body heat - and also via passive means (solar gain).
The high level of thermal insulation means that heat losses in winter are negligible and during hot periods of summer the interior of the building is protected from the heat. Another important result of the insulation is that the internal surfaces are almost the same temperature as the indoor air temperatures. This avoids damage caused by the humidity of indoor air temperature and gives a comfortable indoor climate.
To achieve the Passivhaus standard all components of the exterior shell of the building should be insulated to achieve a U-value that does not exceed 0.15 W/m2/K.
A disadvantage to using a super-insulated design is that due to the increased thickness of the walls the floor area of the building may be less than compared to traditional construction. This may be resolved if the external dimensions of the building can be enlarged to compensate.
Theoretically any type of window can be used in a Passivhaus construction as long as it meets the Passivhaus standard when put into PHPP. In reality this generally means a using a window that has a whole window U-value (including frame) of around 0.8 kWh/m2/K or less. This therefore means that the window is likely to be triple glazed, because without triple glazing the heat loss is too high and the standard will not be achieved. A benefit is that the surface temperature of the windows will be similar to that of the surrounding internal surfaces so even on a cold day no discomfort will be felt whilst sitting near a window. Triple glazing also has the advantage of reducing sound transmission from outside.
The use of low emissivity coatings on the glazing is also a good idea as it helps control the heat transfer through the window. The coating is a microscopically thin metal or metallic oxide layer deposited directly on the surface of the pane of glass. The low-E coating reduces the infrared radiation from a warm pane of glass to a cooler pane therefore reducing the U-value of the window. For warm climates the coating should be applied to the outer pane(s) of glass to reduce solar gain and for cool climates should be applied to the inner pane(s) to reflect the heat back into the house. Although 10%-15% more expensive than regular windows they can reduce energy loss by 30%-50%.
Having gas filled windows will also reduce the U-value, therefore reducing energy costs. Having the window filled with gasses such as Argon or Krypton minimises the convection currents within the space which reduces the overall heat transfer between the inside and outside of the house. By blocking harmful ultraviolet sunrays the window can reduce the effect of fading on the interior of the building (such as carpets). Having a gas filled window also increases the ability to reduce the amount of frost and condensation build-up and reduces the amount of solar radiation entering the building during summer whilst keeping a higher interior temperature during winter.
Windows used in Passivhaus construction are also likely to use warm edge spacers between the panes of glass and also super insulated frames. These use low conductivity materials to separate panes of glass rather than the conventional aluminium spacer. Using warm edge spacers will increase the thermal performance of the window, reduce condensation and also absorb some noise pollution.
Traditionally Passivhaus buildings are built using either a timber frame method or a solid masonry method with external insulation and render. This is because these are the methods typically used across mainland Europe, where the technique was developed. It is easier to make the building airtight using these methods as you can use vapour barriers to line the construction and air tightness tapes. Passivhaus designers are also likely to be more comfortable with using these methods as they use them more often.
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It has recently been proved however that the Passivhaus standard can be met using a cavity wall construction. This is good news for the UK as British builders and designers will be more familiar with this technique and Passivhaus buildings will be able to fit in with local, surrounding buildings making planning permission more likely to be granted. Using a cavity wall method though will require more attention to air tightness detail as it relies on wet plaster to be the air tightness barrier on walls and junctions with doors, windows, floors and roof. However an advantage of using cavity walls is that it increases the level of thermal mass within the insulated building envelope. Using concrete blocks and concrete ground-floor slab will achieve a greater thermal mass which will act as a heat store, storing heat from passive solar gains and releasing it in cooler periods.
Passivhaus is not only restricted to these methods though, in fact most methods have been tested and work successfully, including; masonry construction, lightweight construction, prefabricated elements, insulating concrete formwork construction, steel construction, and all combinations of the methods above. (BRE Passivhaus Primer, p. 5)
Air tightness is incredibly important in achieving a successful Passivhaus design. Unwanted air leakage can cause several problems within a building. These include; increasing space heating requirements and costs significantly, causing localised discomfort due to draughts, the possibility of moisture build up within the building fabric which may consequently reduce performance and lifespan of the building. It is therefore necessary to achieve an air tightness of 1 m3/(hr.m2) @ 50 Pa or less in a Passivhaus construction to eliminate these problems. Many materials can be used to form a continuous airtight barrier around the building including tapes, grommets, adhesives, wet plaster and/or vapour membranes. It is very important that a suitable strategy is developed within the design stage of the project to ensure construction runs smoothly. When the building is near completion it will receive a 'blower door test' to see if the construction has reached the required standard, careful and quality workmanship will be required in order to achieve the desired result. A good level of air tightness is also necessary for the MVHR unit to work to the best of its ability.
Thermal bridging is where heat from inside the building will follow the path with the least resistance (the lower the resistance the easier it is for heat to flow) to the exterior of the building. If there is an element with much higher conductivity than the material surrounding it (e.g. steel surrounded by concrete) then heat will escape from the building through this element. The path will not necessarily be perpendicular to the surfaces.
In any building it is preferable to avoid any thermal bridging as they significantly increase heat losses and also decrease interior surface temperatures which can result in areas of high humidity in parts of the construction. The most common places where thermal bridges are found are junctions between; wall and ground floor slab, wall and windows, wall and floors, and wall and roof.
It is integral to keep thermal bridging to an absolute minimum in a Passivhaus design in order to keep heat from escaping from the building. If the thermal bridge coefficient, which is a measure of extra heat losses through a thermal bridge, is less than 0.01 W/(mK) then the detail is said to be thermal bridge free, a necessary in a Passivhaus design. As Passivhaus' have higher internal surface temperatures than traditional builds, critical humidity cannot occur at any place and the additional heat losses will be negligible.
In order to keep the design 'thermal bridge free' it is important to make sure insulation around the building envelope is continuous, to use well designed and highly insulated window frames and doors, and to make use of materials such as porous bricks with low thermal conductivity for the first row of bricks between the junction of the interior masonry wall and ground floor slab.
Although Passivhaus standards do not explicitly address the issue of thermal bypass and wind tightness it is important to consider this problem when designing a highly energy efficient building. Thermal bypass happens by way of natural convection or forced convection (such as wind) or as a combination of the two. Even in an airtight building air can sometimes be permitted to move through or around the insulation, this will, in effect, bypass the thermal properties of the insulation and reduce its performance.
To avoid thermal bypass, it should be ensured that the outer leaf of the building will act as a wind and rain barrier, paying particular attention to continuity at both structural openings and service penetrations. Cavities, where possible, should be fully filled with insulation without any gaps (this is not possible in timber frame construction as ventilated cavities are required). This should ensure that air is not permitted to move through insulation, around insulation, over the face of insulation or behind insulation therefore preventing thermal bypass.
Mechanical Ventilation with Heat Recovery - MVHR
All buildings occupied by humans need a supply of fresh air to ensure health and comfort. As the Passivhaus technique relies on being airtight, it is necessary to use a mechanical ventilation system to introduce fresh air to the building and dispose of stale air. If there was no mechanical ventilation the occupant would have to open the windows once every three hours, for five to ten minutes, to receive adequate ventilation (BRE Passivhaus Primer, p.5). This would be unfeasible and cause intolerable heat losses. An air change rate of 0.3 times the building volume per hour is deemed to be the optimal level to give a healthy, comfortable environment with nominal heat loss. The system works by removing stale air from rooms that generate a lot of pollution and/or humidity such as kitchens, bathrooms and toilets. Fresh air from outside is supplied to living rooms, bedrooms and workrooms to replace the stale air that has been removed. A high level of indoor air quality is achieved by the system supplying exactly as much fresh air is needed for comfort; air from inside the building is never re-circulated as this would be counterproductive. Any unwanted odours, moisture and carbon dioxide generated by occupants is removed and replaced with fresh air.
Theoretically this could be provided by an exhaust fan ventilation system with fresh air vents. For a Passivhaus however the heat losses from this would be unacceptable, so the system must use heat recovery to become more energy efficient. Heat from exhaust air is transferred to the fresh air by way of a counter flow heat exchanger driven by two efficient fans. This works by flowing the extracted air through a channel where it delivers its heat to plates, the cooled exhaust air is then removed from the building. Fresh air is pulled over the exchanger plates through different channels where it absorbs the heat from the plates. The warmed air is then supplied to the building. If the exchanger is long enough almost 100% of the heat can be recovered, in practice systems are available that can supply 75% - 95% heat recovery.
The efficiency of ventilation systems can also be increased through the use of earth buried ducts. This allows fresh air to absorb heat from the ground during winter and to be cooled during summer. This is because air temperatures are lower than ground temperatures during winter and higher than ground temperatures during summer.
Windows in a Passivhaus can still be opened whenever the occupant feels the need, maybe to cool the house on a hot day or to hear the sounds of nature). It is likely that the user will want to do this less often than in a traditional building, as they are receiving excellent inside air quality from their MVHR system.
Using a MVHR unit also provides another benefit as it also gives the opportunity to heat the building by heating the supply air. This will be discussed further in the heating section.
Passive Solar Gain
A Passivhaus construction will normally incorporate passive solar building techniques. This means making buildings compact in shape, where possible, to reduce their surface area and orientating windows to face the equator, (i.e. facing south in the northern hemisphere and north in the southern hemisphere) to maximise passive solar gain. A shading system on the facade of the building is also likely to be a necessity, so that in the summer months direct sunlight is blocked out in order that the building does not overheat.
It is also recommended that a Passivhaus design does incorporate an element of internal thermal mass even if the structure is constructed from lightweight materials. This thermal mass will reduce temperatures during the peak of summer, maintain stable temperatures through winter and prevent the possibility of overheating in both spring and autumn when the sun is not high enough in the sky for the solar shading to become effective.
Energy requirements are reduced by maximising passive solar gain as heat transferred to the building will be trapped inside the super insulated shell. Day lighting requirements can also be reduced by use of an optimised design.
Passivhaus design aims to maximise the potential of internal gains and therefore minimise the need for a separate heating system. In any building lots of heat is generated on a daily basis, not including that coming from space heating systems. This is from humans, pets, appliances and lighting, basically anything that needs energy to work. The design of a Passivhaus is such that it keeps this heat inside the dwelling so that none of the energy is wasted. Obviously this is a great advantage as it reduces space heating costs, but the designs are often so efficient that there is risk of overheating. It is therefore advised that low energy lighting and appliances are used throughout the building to minimise internal gains, this will also reduce running costs.
Undoubtedly in some climates extra heating will be needed on very cold days, or cooling on very hot days. It is therefore suggested that an integrated compact unit is used which can supply additional heating (or cooling) for the supply air in the ventilation system (MVHR) and also a domestic hot water boiler. There are various solutions that can be chosen for heat generation including; use of a small heat pump (which may be used in conjunction with photovoltaic cells), use of a small condensing burner (with natural gas) or use of a small combustion unit for biomass fuel. Implementing renewable energy technologies is not a core requirement of the Passivhaus standard, although using renewable energy will further reduce running costs and carbon dioxide emissions. This may become necessary in the future with the push towards 'zero carbon'.
PHPP - Passivhaus Planning (Design) Package
When designing a building to Passivhaus standard it is important to use PHPP software to ensure that the building will meet Passivhaus standards once built. Simply put, the software is used for calculating the energy consumption levels of the proposed construction. It can be compared to other programmes such as SAP or NHER but PHPP goes into more detail than these programmes and is specifically designed for use with super-insulated buildings with mechanical ventilation systems. Although PHPP is a bit more complicated to use than these other systems it provides much more accurate results and feedback with respect to passive solar design, heat recovery and ventilation, thermal bridging and the impact of thermal mass.
It was first published in 1998 by the Passivhaus Intitut (PHI) Germany and has been continually developed and updated since then. Subsequent releases of PHPP were made in 1999, 2002, 2003, 2004, with the most recent edition published in 2007. (Clarke & Reason 2008, p.10) It consists of a CD which contains the PHPP tool, a spreadsheet that runs on Microsoft excel, and a 200 page paper manual. The manual not only tells the user how to use the software but also gives advice to architect or engineer using the software on how to optimize their design. PHPP is continuously becoming more accurate as it is always being validated and refined based on measurements from more than 300 projects being compared with calculation results and also new scientific research studies and results.
energy calculations (incl. R or U-values)
design of window specifications
design of the indoor air quality ventilation system
sizing of the heating load
sizing of the cooling load
forecasting for summer comfort
sizing of the heating and domestic hot water (DHW) systems
calculations of auxiliary electricity, primary energy requirements of such (circulation pumps, etc.), as well as projection of CO2 emissions
verifying calculation proofs of KfW and EnEV (Europe)
Climate Data Sheet: Climate regions may be selected from over 200 locations in Europe and North America. User-defined data can also be used.
... and a lot more tools useful in the design of passive houses, e.g. a calculation tool to determine internal heat loads, data tables for primary energy factors, etc.
A comprehensive handbook, not only introducing PHPP use, but also highlighting crucial topics to be considered in Passive House design.
(Wolfgang Feist, 2007)