Eco House

Homage to Catalonia: A pre-fab straw bale Passive House 'first' for the region, by Oliver Style, ProGETIC

The Larixhaus is the first pre-fabricated straw bale passive house on the Iberian Peninsula. A project that took 7 months from start to finish, this single family home is located in the town of Collsuspina, Catalonia, Spain. Through careful bio-c1imatic design, thermal insulation with straw, an airtight envelope and high-performance windows, the Larixhaus has a projected space heating demand (calculated with PHPP) of 15kWh/m2 .a, approximately 80% less than that required by current Spanish building regulations. The project is a modest, but inspiring example of deep-green energy efficient construction, in preparation for the EU's 2020 deadline, when all newly built homes will need to be 'nearly zero energy'. Oliver Styles reports...

He huffed and he puffed...

Jordi, Itziar and their two daughters live in a small town in the hills above Barcelona, between rolling pine forests and burnt sienna escarpments. Renowned for its cool winters and even colder traditional stone houses, the area has been witness to one of the greener construction projects to have taken place in recent years south of the Pyrenees: the Larixhaus, Spain's first prefabricated straw bale and timber passive house. As straw bale building gains momentum in central Europe, with ground- breaking work from ModCell, White Design, LILAC and the University of Bath, the Larixhaus is a modest but determined example that timber and straw bale construction can move beyond the pigeon-hole of one-off self-builds and contend in the mainstream of beautiful, low-impact, energy efficient architecture. A simple, compact home which, despite the huffing and puffing of the big bad wolf, is set to brave the elements of this Mediterranean mountain region and place nearly zero energy construction on the map in preparation for the EU's 2020 deadline.

Early design: where the first steps count

The client's priority, from the outset, was to bring together a group of professionals with experience in timber and straw bale Passivhaus construction, who could design and build a small home at reasonable cost, where natural, renewable materials were married with a high level of energy efficiency and indoor comfort. In the early design stage, a simple and relatively compact building form was chosen, with 339m2 of thermal envelope enclosing a gross exterior volume of 437m3 over two floors, for a form factor of 0.78. The longest dimension of the building was aligned east-west, to provide maximum day lighting and reduce artificial lighting loads, resulting in a building aspect ratio of 1:1.3.

A location specific climate file was generated with the Meteonorm software and compared with the last 10 years of data from a weather station located 6km from the site, showing good agreement. Shading from nearby mountains was taken into account using a topographical horizon profile. The climate data was then entered into the PassivHaus Planning Package (PHPP) energy simulation and certification tool, for early stage design modelling and analysis.

Contrary to the orientation of all other homes of the street, the building's southern and most highly glazed facade was orientated perfectly south. PHPP modelling provided the required surface area of southern openings to take advantage of free solar heat gains in the winter. A combination of design strategies were modelled and tested for maintaining summer comfort with no active cooling.

To enjoy the spectacular views west to the jagged rock formations of Montserrat and east to the Montseny mountains, bedrooms are located on the ground floor, with a diaphanous kitchen, dining and living room space on the first floor. Wet rooms (bathroom and kitchen) are located in the same vertical plane, to reduce pipe runs and minimise heat losses in the domestic hot water system. To provide full fresh-air ventilation with minimal heat losses in the winter, a whole house ducted heat recovery ventilation system was chosen: careful early planning of duct routing made sure the duct lengths were kept to a minimum, reducing cost and energy losses. Operable windows on the east, north and western facades provide natural ventilation in the summer and sufficient natural light in all habitable rooms.

The skin: timber and straw bale

The timber superstructure and external cladding is PEFC certified, and was laser cut to order and delivered to the Farhaus workshop for prefabrication, 1 5km from site. The straw bales are 1200 mm x 700 mm x 400 mm, positioned vertically in the timber frame structure. The bales were sourced 1 25km away on the Costa Brava. The bales are enclosed on the outside with wood fibre breather board, followed by a 35mm ventilated gap and larch rain-screen cladding, fixed on timber battens. The ventilated wall reduces transmission heat gains in the summer and provides an exit for water vapour in the building structure - an important design consideration to avoid interstitial moisture build-up and condensation damage in straw bale construction. On the inside, the bales are shut in with 22mm formaldehyde-free OSB that acts as the air tight layer. Finally, Fermacell gypsum fibre board, over a service void, provides a dry-lined internal finish. Structural timber that spans the thermal envelope is thermally broken with cork insulation.

Two straw bale roof cassettes, with the bales positioned in the same direction as the walls, provide a thermally efficient roofing system, finished with clay tiles over a ventilated air gap, reducing transmission heat gains and summer overheating. Gravel infill on the intermediate floor adds some thermal inertia, although with the air-tightness and thermal insulation specification, combined with careful design of openings with external blinds, the building's thermal mass (calculated as 84Wh/K for every m2 of Treated Floor Area) was calculated as sufficient for maintaining summer comfort with natural ventilation. Given the site's altitude at 888m, peak summer temperatures are lower than coastal Mediterranean regions, averaging only 20°C in July and August. PH PP modelling showed that with no active cooling and a combination of glazing with a solar factor of 47%, external blinds on southern openings, and natural night ventilation, summer overheating frequency (when the indoor air temperature exceeds 25°C) could be kept below 3%, equivalent to a total of 36 hours in which the indoor ambient temperature rises above 25°C.

Despite not meeting the environmental criteria established between the client and design team, the most cost effective and thermally efficient solution for the floor slab was found to be 130mm of rigid polystyrene under the slab with perimeter insulation of 60mm around the edge of the slab.

The testing of alternative design strategies with PH PP modelling showed that an acceptable balance of heat gains and losses was achieved with triple glazing (with two low-e coatings, argon gas filling and TGI warm spacers), for a centre-pane U-value of 0.65W/m2.K and solar factor of 47%. Soft-wood Farhaus frames provide a U-value = 1.00W/m2.K, with cork insulation to reduce installation thermal bridges. The average installed window U-value is 1.06W/m2.K, not enough for cold' central European climates but sufficient in the Collsuspina climate to meet the comfort and hygiene requirements set by the Passivhaus standard.

The average weighted thermal transmittance of the building envelope is U-value = 0.21 1 W Im2K. The door blower test gave an impressive result of n50 = 0.32ACH (air changes per hour). Cold bridges were eliminated or reduced with modelling and optimisation in the design phase.

The building shell was prefabricated in the Farhaus workshop over a period of 6 weeks. It was divided into 10 separate modules, with the air tight layer and window frames installed and sealed. The modules were transported to site and the basic structure was assembled in two days. Pre-fabrication minimises on-site construction times, providing cost savings and near-zero on-site waste. The Larixhaus' embodied energy and C02 emissions derived from materials are minimised by prioritising natural, non-toxic, renewable materials with minimum processing (certified timber, locally sourced straw, cork, and gypsum fibre board).

Indoor air quality, acoustic comfort and active systems

Healthy indoor air quality is achieved through the use of non-toxic, Iow-VOC, natural materials. Exposed timber inside the home is either untreated or coated with water-based varnish. Healthy materials are combined with whole house ducted heat recovery ventilation to provide efficient, comfortable full fresh air ventilation during the winter. Cool air is brought into the home and pre-heated by outgoing stale air through a Passsivhaus Institut certified Zehnder Comfoair 350 ventilation unit, with an installed sensible heat recovery rate of 79%. PHPP simulations showed an average seasonal COP of 9. Efficient DC fan motors are essential for reducing electricity consumption: calculations showed that, given an average of 4,700 hours of operation per annum, at an air change rate of 0.40 (91 m3/h), the unit will consume only £31 of Spanish electricity each year (where household electricity is the 3rd most expensive in Europe). The ventilation unit is fixed on acoustically insulated mounts and located in the service cupboard by the entrance on the ground floor. Silencers on the indoor air supply and return ducts, combined with adequate duct sizing to control air velocities, means the system has a maximum measured sound pressure level of 33dB(A) in living spaces. The result is a quiet, discrete and efficient comfort ventilation system.

The near-zero heating demand is met by two low-cost 500W wall-mounted electric radiators in each bedroom, and one 450 W electric towel radiator in the bathroom. On the first floor, a 4kW air-tight log stove (that modulates down to 2kW) with a twin-walled concentric chimney flue, provides heat on very cold days, without compromising air tightness. Hot tap water is produced by a compact air-source heat pump unit with a COP of 3.75 (@ air = 15°C and water = 45°C) and a heat store of 300 litres. The air intake of the heat pump is located just above the stale air outlet of the ventilation unit, providing some performance improvement. The clients have set timing controls on the heat pump to make sure it does not activate between 1 1 pm and 8am in the winter, to avoid poor 'performance when outdoor air temperatures are low. Following 3 months of use, there has been sufficient hot water to meet daily demand with this control strategy.

Cooking is done on an induction ceramic cooktop with a re-circulation cooker hood. Artificial lighting is with LEDs and all white goods are A++. If the Spanish government decides, at some point in the future, to reverse the current legislation and encourage the use of renewable energy technologies rather than bending to the pressure of the large energy companies, the clients intend to install a grid-tied photovoltaic array to achieve a net zero energy balance. When the budgets allows, a rain water catchment system will follow suit (pre-installation was done during construction) .

Beyond PHPP and into the real world

A remote monitoring system will be installed in the coming months, providing quantitative data over a 2-year period, monitoring outdoor temperature and humidity, indoor temperature, humidity and C02 levels, together with electrical energy consumption for space heating, ventilation, hot tap water, lighting and equipment. It will be particularly interesting to see the in-use summer performance of the building.

Meanwhile, feedback to date from the clients shows that with outdoor night time temperatures reaching -1°C, indoor temperatures have remained above 20°C with no active heating, as long as there is some sun during the day. For successive days with no sun, they turn on the electric radiators for half an hour at night and in the morning, to maintain comfort. The first test of the log stove ended with Itziar opening the windows as it got too hot in the sitting room (!), confirming the PHPP calculated peak heating load requirement of 11 W 1m2. At least during this first mild winter, the stove seems largely redundant. The specific construction cost of the build has come in at around £1,005/m2, an estimated 14% more costly than building to current regulations in Spain. This gives an approximate simple payback time of just under 9 years, for a building with an expected useful life of 80 years.

So goes the story of the Larixhaus: a green-building homage to Catalonia and its rich history, number 10 in a growing list of healthy, comfortable, Passivhaus constructions south of the Pyrenees.

Oliver Style

Useful links:







Project team:

Client: Jordi Vinade, Itziar Pages

Architecture: Nacho Mart, Maria Molins, Oriol Mart.

Passivhaus design, PHPP analysis, M&E: Oliver Style, Vicenc Fulcara - ­ProGETIC SCP

Contractor: Albert Fargas - FARHAUS

5tructural Engineering: Manuel Garcfa Barbero - Klimark Architectural Consultant: Valentina Maini

Project Spec:

Energy standard: PassivHaus new build Location: ColIsuspina, Barcelona, Spain Treated floor area (PHPP): 92m2 Construction type: timber construction Completion date: December 2013 Completion time: 7 months

Space heating demand (PHPP): 15kWh/(m2a) 5pace cooling demand (PHPP): 3.2kWh/(m2a) Heating load (PHPP): 11W/m2

Cooling load (PHPP): 3.9W/m2

Primary energy requirement (PHPP): 96 kWh/(m2a) Construction costs (gross) [1m2]: 1211/m2

Air tightness (n50): 0.32ACH

Envelope information:

Exterior wall: U-value: 0.127 W/m2.K: in> out

  • 13mm plasterboard (Fermacell)
  • 35mm service void between timber battens at 8%
  • 22mm 05B 4 [air-tight layer]
  • 400mm straw bale insulation [052] between timber joists at 8%. thermally broken with cork insulation [040]
  • 16mm wood fibre breather board (DFP Kronolux) Wind tight membrane and ventilated larch rain screen cladding, flxed on external timber battens

Floor slab: U-value: 0.165 W/m2.K: bottom> top

  • 130mm XP5 insulation [034]
  • 350mm reinforced concrete floor slab
  • 80mm Pavaflex wood fibre insulation [038] between timber joists at 10 %
  • 22mm timber flooring
  • 60mm XP5 insulation [034] around the edge of the floor slab

Roof: U-value: 0.122W/m2.K: bottom> top

  • 15mm timber board (Fir) 22mm 05B 4 [air-tight layer]
  • 400mm straw bale insulation [052] between timber joists at 2% 16mm wood fibre breather board (DFP Kronolux)
  • Timber battens and roof tiles

Windows / doors

  • Farhaus, Fargas window frames
  • 50ft-wood laminated wooden window frames (90mm)

Uframe = 1.00W/m2.K

Average U- value window = 1.06W/m2.K

  • Triple glazing. with two low-e coatings and argon gas fllling; 33.2/16argon/4/16argon/4; TGI warm spacer

U-value glazing = 0.65W/m2.K

g-value = 47%

Entrance door - Farhaus: Fargas door

  • Triple glazed entrance door with identical speciflcation as windows

Udoor = 1.00 W/m2.K


  • Zehnder ComfoAir 350 Luxe
  • PHI certifled ducted whole house mechanical ventilation with sensible heat recovery
  • Distribution in HDPE 90mm pipes

Heating system

  • Ground floor: electric radiators
  • First floor: Rika Passiv blomass stove

Domestic hot water system

  • Theodoor Aerotermo 300 Plus
  • 3.6kW thermal compact air source heat pump unit, with backup electric immersion heater.

Oliver is a certified PassivHaus designer/energy consultant and co-founder of ProGETIC, an engineering practice based in Barcelona, Spain. He specialises in passive design and building performance optimisation through energy model­ling. He has an MsC with distinction in architecture, with the Centre of Alternative Technology and University of East London. He enjoys working closely with designers, developers, engineers and free thinkers who want to build or renovate beautiful, healthy, comfortable buildings with low running costs. - OSTYLE@PROGETIC.COM

Style, O. (2014) ‘Homage to Catalonia: A pre-fab straw bale passive house 'first' for the region’, Green Building Magazine, vol. 23, Spring 2014, pp. 20-24.


Eco houses a good or bad idea?

What is an Eco house?

Architect Paul McAlister

Architect Paul McAlister

The name is banded around and is ultimately misunderstood by the general public. It has also become a generic term encompassing all things ecological and sustainable in terms of building and living in an environmentally friendly home. There is actually no simple definition of an eco house and it could be said that any house which incorporates technology, such as renewable energy solutions, may be considered an eco house to a degree. Another ‘Green’ concept would be to build a house using some overtly ‘green’ material such as sheep’s wool insulation or straw bales for insulation of the external walls. The commitment to build in these greener materials indicates a strong statement in terms of the ‘greenness’ of the build but without a holistic approach to the dwelling the green opportunity of building a new dwelling may be lost.

The global problem of build-up in CO2 gases in the atmosphere and the knock on effect of global warming, and damage to the ecosystem, is a manmade phenomenon which needs to be addressed for a sustainable future.

In the UK 30% of CO2 is produced as a result of the energy requirement of the housing stock, which means the homes, that we all live in, produce a significant amount of all CO2. Governments have recognised this and have a target, enforceable by building regulations, for all new homes to be carbon neutral by 2016. This would be achieved by a series of incremental increases in the energy efficiency of dwellings and the increased use of renewable technologies.

The energy efficiency of dwellings is therefore the one area in which the individual may play a major part in reducing their ‘carbon footprint’ making a contribution to reduce CO2 emissions.

An added benefit of an energy efficient home is the actual running costs of the house itself. An initial capital investment providing a super-insulated envelope for the building and suitable means of energy efficient ventilation may have a payback period of 5-10 years, depending on specification. After this period the house is saving money for the occupiers, as it is possible to design passive houses that need no additional heat source except in severe weather conditions.

If we identify energy efficiency as one of the key elements in environmentally friendly building design then it would seem appropriate to focus on this area, as a key element in the design and specification, were the correct choice of building materials and construction techniques will make a significant contribution to the home. The use of renewable energy sources also has a role to play and the investment of relatively common technology, such as solar panels for hot water heating, should be considered for any new home striving to be energy efficient.

The other element of Eco build that makes an environmental impact is reducing water supply needs by recycling of rainwater from roofs. This water is stored and reused for washing and flushing toilets whilst the waste produced from the home may be filtered by reed beds.

Finally the choice of material to construct the eco home brings more choices to be made in terms of Green materials. The concept of embodied energy, the energy required to produce the material, becomes a deciding factor and also the amount of CO2 used in the production of the material. The use of timber as a building material, from sustainable and managed forests, is an obvious ‘green’ material as the trees themselves absorb CO2 when growing. The use of recycled or natural material also has environmentally green credentials. The judgement may come to personal preference or the financial implications of some of the less mass produced products may make their use prohibitive.

In the end market forces and government legislation will determine changes in building design. There is a strong argument to future-proof new dwellings for the lifetime of the home occupier and for generations to come. This investment will ensure a sustainable future for our housing stock and makes the Eco house concept one that becomes the prudent benchmark of new homes.

Paul McAlister

Taking Control Of Air tightness

According to the Energy Saving Trust's Chief Executive Philip Sellwood, almost a third of new homes are still failing to meet energy efficiency guidelines. He told the BBC " ... the Government's 'Code for Sustainable Homes' is not being adequately enforced, giving cause for real concern. Our building regulations in the UK are among some of the toughest in Europe, but they are extremely poorly enforced as far as energy efficiency goes".

David Arendell, MD of roof ventilation specialist Klober feels the situation in respect of building air tightness gives grounds for even greater concern. He commented, "In the light of the EST's comments on energy efficiency, it is fair to assume that the level of understanding of how best to achieve air tight construction remains poor.

This is despite the fact that the phrase 'Build tight, ventilate right' has become synonymous with the strategy to achieve low energy buildings. If we don't understand how best to achieve the right balance of air tightness and controlled ventilation, we run the risk of perpetuating condensation problems within the roof space and building fabric. With every upgrade in insulation standards, so the risk increases.

Delays in consultation on Approved Documents Land G have prompted deferment in CSH 2010 until the end of the year, but the clock is undoubtedly ticking towards an ultimate target whereby all new homes achieve CSH Level 6 (effectively zero carbon). However, with house builders having lobbied consistently for tighter definition of how 'zero carbon' can be achieved, the Zero Carbon Task Group was set up.

There is some evidence to support such calls for redefinition. Research carried out in 2007 by the Richard Hodkinson Consultancy, for example, showed that 'PassivHaus' (a Europe-wide Standard with stringent air tightness requirements managed by the BRE and the Energy Saving Trust) would not actually meet CSH 3.

CSH assessment uses the Standard Assessment Procedure (SAP) test to calculate energy performance, and for a number of years there have been questions over the efficacy of the test, especially in relation to more thermally efficient buildings.

In terms of roof design, the requirement already exists for new public sector housing to meet CSH 3. The impetus towards 'zero carbon' will be reinforced when the equivalent of CSH 3 is incorporated into Building Regulations for England and Wales (some authorities indeed have already adopted this requirement). In Scotland, where many elements of the Code have already been incorporated into Building Standards, similar improvements are planned.

Of the nine categories within the CSH method of assessment, that for 'energy and C02 emissions' is by far the most significant. This is true for both the allocation of credits within each category and the final point’s allocations that result from use of weighting factors. 29 credits are available for energy and C02 emissions which, when weighted contributes 36.4% to the total available performance.

The right balance between air tightness and ventilation can certainly be struck without significant addition to building costs. Material choice however, can greatly influence a building's long-term air tightness. Sheet membrane air barriers coupled with sealants, for example, are more effective than sealants alone, counteracting the effects of buildings (particularly timber frame) drying out.

Housing designers can now benefit from Accredited Construction Details (ACDs), Enhanced Construction details (ECDs) and, in Scotland, the Scottish Ecological Design Association Guide for both warm and cold roof construction. Examples of wall/ceiling ACDs include a junction of ceiling level air barrier with masonry inner leaf and warm roof with room in the roof. Accredited detail Sheet MCI RE 02, for example, shows a warm roof detail at the eaves in a non-habitable loft using Klober Permo forte vapor permeable underlay and appropriate tapes (with an alternative pre-taped option).

For non-residential construction, air tightness is just as important, despite the absence of any CSH equivalent. Roofing materials such as zinc, for example, require airtight construction if the metal's underside is unventilated. At the recently build Abergwynfi primary school near Neath, built to achieve a BREEAM 'Excellent' rating, zinc was used on a series of circular roofs. A Klober Wallint air barrier was installed with sealing tape to meet the specified air tightness performance.

With current Building Regulation requirements stipulating air tightness of only 7m3/hr per m2 compared with CSH 3 at 3m3, techniques used to achieve it must undoubtedly change. CPD presentations and literature on the subject are to be welcomed. 'The Code for sustainable Homes and air tightness in roofs' is a CPD presentation from Klober examining how to 'build tight and ventilate right' within the realms of practical pitched roofing construction. Supported by ‘Taking control of air leakage' tightness it is a welcome source of information on a subject for which information is otherwise lacking.

By David Arendell, MD of Klober

A Passive house, what’s in a name ?

The term Passive house comes from Swedish and German research into creating low energy houses, in terms of running costs. The concept was taken further in Germany with actual working models constructed that became known as Passivhaus or Passive House. The Passivhaus Standard is a holistic view of how a house may be constructed with the primary aim being energy efficient living. The Passivhaus Institut in Germany has set criteria for achieving the Passivhaus standard which, in effect, calculates the dwellings energy consumption.     The key to linking the name ‘Passive’ with the function of the building is to remember that the house is Passive in terms of its impact on the environment and its active energy requirement. To achieve this, these houses must incorporate features that other houses do not have at present.

This Passive House technology is now readily available, but is often poorly understood. It is also perceived as much more expensive to implement. The additional cost figures vary at between 10-15% more expensive that conventional building to the current building regulations. When taking a view as to the extra cost of achieving Passive standards it must be remembered that the energy saving year on year will pay-back the difference in of 5-10 years, after which time the dwelling is saving the occupants money. This future proofing of the house, in terms of energy use means that the houses inherent value would be maintained over the future years.

How do we achieve a Passive house? It is an area that needs professional advice and a ‘whole house’ approach as there are a number of permutations in achieving the standard.  The technology is also advancing making the choice of materials and technology reliant on the latest market innovations.

In order to put a marker down we have itemised the main principles and elements to make a passive house function. These ‘Passive house’ standards  are intended for the  European climate and would not be suitable for more extreme climates.

Characteristics of a Passive house

A ‘whole house’ approach is needed to the design a passive house.  The technology available to achieve the considerable energy efficiencies is advancing, making the choice of materials and technology reliant on the latest innovations in the market. The skill of the designer is to best employ the available materials and systems to achieve this low energy standard.  

The ‘Passive house’ must have all of the following features to function in the typical European climate.

Super Insulation and elimination of cold bridges

The most important aspect of low energy housing is the most simple to comprehend. It is a super-insulated envelope ( walls, floors, roof, windows and doors ) to the dwelling. It requires very high levels of insulation and special detailing of openings and junctions to ensure there are no ‘cold bridges’ to allow for the transmittance of heat through the building fabric. The windows for a passive house also have to have exceptional insulation standards in keeping with the other elements. It is often a misconception that the employment of renewable energies will produce a passive or low energy house but without the fundamental insulation element the use of renewable energy technology can be wasted.

Air-tightness of building fabric

In conventional houses up to 20% of energy may be lost through air infiltration or drafts that occur commonly at junctions of floors, doors and windows. For this reason passive houses are extremely well sealed in terms of air-tightness and this aids retention of heat. In current building an air pressure test is carried out on new dwellings. The building regulations are relatively easy to obtain, in terms of workmanship. The passive house standard is more onerous and relies on more careful consideration and detailing of the building fabric. Good on site construction techniques and supervision is recommended.

Circulation of fresh air

The need for active ventilation to passive houses is recognised due to the air tightness and this requirement has been developed as an advantage. A mechanical ventilation system is employed to maintain air quality. The rate of air change can be optimised and carefully controlled at about 0.4 air changes per hour. Passive houses have active ventilation to all rooms in the dwelling. The air in rooms that produce active heat, and odours, such as kitchen, bathrooms and utility room is extracted. Rooms that require heating such as bedrooms and living spaces have fresh air vented to them.  Overall the balance of air is maintained within the house by air movement below internal doors and an open plan house. The reason for this ventilation is to provide fresh air to the house and to avoid the build up of water vapour or condensation in the house due to the air-tightness. It should also be remembered that at any time a window can be opened for additional ventilation although in winter opening a window for ventilation would cause heat loss.

Heat recovery from circulated air.

The circulation of air is handled with mechanical ventilation which has the crucial element of heat recovery, commonly known as MVHR ( mechanical ventilation with heat recovery). The clever aspect of the system is that it literally ‘transfers’ heat from the extracted stale air to the incoming fresh air (which is ducted from the outside). The efficiency of heat transfer is over 85% and the majority of the heat from extracted air is therefore not lost to the building envelope, as with conventional extraction.  The system of heat transference to the cold incoming air means that the houses environment may be regulated and maintained at a higher temperature, for most of the time, without the need for an additional energy source except the low running cost of the MVHR. This system is often the least understood in terms of convincing the public of the benefits of passive houses and perhaps most off-putting to the public. Negative images of ‘air conditioning’ units humming on the ceiling are brought to mind. In fact this is not ‘air conditioning’ of the conventional type. The MVHR runs on less energy needed for a 100 watt light bulb and functions continuously, the unit may be located in utility rooms and have health benefits in that the air is filtered before entering the house. A significant amount of heat is produced with common domestic processes such as cooking, showering and the use of various electrical appliances.  Human bodies give off a significant amount of heat (People, on average, emit heat energy equivalent to 100 Watts) and this is significant to a passive house as the energy loss through the fabric of the building is so low. )To put this in perspective a 100 watt light bulb should be capable of maintaining the space heating of a 10 metres square room. Two 100 watt light bulbs would therefore have the capability of heating to a higher temperature the same room.)

Ventilation system used for active space heating.

The MVHR system is required to maintain air quality. The MVHR also acts as ‘air source’ space heating for the house in that the heat recovered from extracted air is transferred to incoming air and circulated throughout the house. This benefit along with a super-insulated envelope means that passive houses are able to dispense with conventional heating system as they would quickly overheat the building. Some additional source of heating is recommended as this would be required when the house is not occupied (no passive heat is generated ) or in severe weather conditions. Most Passive buildings do include a system to provide supplemental space heating. This may be distributed through the low-volume ( MVHR ) mechanical ventilation system. One recommendation is to have a dual purpose electrically operated 800 to 1,500 Watt heating and/or cooling element integrated with the fresh air supply duct of the heat exchanger ventilation system. It is important that the (MVHR) ducting is adequately sized to allow for this element of active space heating and the volume of air required for space heating ventilation.

Energy source for active space heating to the passive house.

The air-heating element associated with the fresh air intake air to the MVHR units can be heated by a small heat pump, by direct solar thermal energy, or simply by a natural gas or oil burner. In some cases a micro-heat pump is used to extract additional heat from the exhaust ventilation air, using it to heat either the incoming air or the hot water storage tank.

In some instances small wood-burning stoves can also be used in space heating and the water tank. Care is required to ensure that the room in which the stove is located does not overheat. The ventilation system would also need to be configured to circulate this hot air around a house that may not be open-plan.

Triple glazed high specification windows

A requirement of the super-insulation of passive houses is that the windows frame and glazing must be of a suitable insulation quality to stop excessive heat loss. Conventional double glazed windows are unable to meet the thermal values required and for this reason triple glazing is required along with specially designed window frames, with thermal breaks, that have a low thermal conductivity. It should be noted that the window glass may be coated and an insulating gas used in the sealed system to achieve the desired values.

Passive solar gain / orientation

One last feature of passive houses is the orientation of living spaces in terms of making use of solar gain. The capture of passive solar energy by glazing within the fabric of the building may be used to heat the Thermal mass of the building during the day and this heat is then released or radiated during the night stabilising the temperature of the house. The use of concrete floors to act as a thermal store for the suns heat during the day is a simple way of retaining heat and for this reason the glazing to the south may be sized to take advantage of this. The passive houses we are designing are not ‘Passive Solar Houses’ and overheating due to thermal gain can be a problem in sunnier climates and potentially lead to overheating of Passive houses.

Some characteristics of Passive Houses

The air is fresh, and very clean. Note that for the parameters tested, and provided the filters are maintained, quality air is provided. 0.3 air changes per hour are recommended, otherwise the air can become "stale" (excess CO2, flushing of indoor air pollutants) and any greater, excessively dry (less than 40% humidity). The use of a mechanical venting system also implies higher positive ion values. This can be counteracted somewhat via opening the window for a very brief time, plants and indoor fountains. However, it should be noted that failure to exchange air with the outside during occupied periods is not advisable.

Inside temperature is homogeneous; it is impossible to have single rooms (e.g. the sleeping rooms) at a different temperature from the rest of the house. Bedroom windows can be opened slightly to alleviate this when necessary.

The temperature changes only very slowly - with ventilation and heating systems switched off, a passive house typically loses less than 0.5 °C (1 °F) per day (in winter), stabilising at around 15 °C (59 °F) in the central European climate.

Opening windows or doors for a short time has only a very limited effect; after the windows are closed, the air very quickly returns to the "normal" temperature.

The air inside Passive Houses, due to the lack of ventilating cold air, can be drier than in 'Standard' Houses. This may be counteracted by slowing the rate of ventilation to allow water vapour to build up within the house as required to comfort levels.

Appendix 1.   Technical energy targets for passive house in Western Europe Climate.

The Passive House standard for central Europe requires that the building fulfils the following requirement,

The building must not use more than 15 kWh/m² per year heating and cooling energy.

 Total energy consumption (energy for heating, hot water and electricity) must not be more than 42 kWh/m² per year

Total primary energy (source energy for electricity and etc.) consumption (primary energy for heating, hot water and electricity) must not be more than 120 kWh/m² per year


 With the building de-pressurised to 50 Pa (N/m²) below atmospheric pressure by a blower door, the building must not leak more air than 0.6 times the house volume per hour (n50 = 0.6 / hour).

 Further, the specific heat load for the heating source at design temperature is recommended, but not required, to be less than 10 W/m².

Have ‘Zero Carbon’ Homes come of age?

Recent spikes in energy prices and concerns over the imminent threat of global warming are making opting for zero carbon housing an increasingly more attractive alternative. What exactly is a zero carbon home?

Basically, a "zero carbon" home is one which generates more energy than it uses over a set period of time. A building's "carbon footprint" is calculated over the full life of a property, taking account of carbon dioxide generated in its construction and subsequent use. This means building a highly efficient, super-insulated home from sustainable, environmentally friendly materials to minimize CO2 (carbon dioxide) use. These homes should ideally be self-sufficient in all of their service demands; operating independently of public utilities, such as the national grid, sewage treatment and water services. They may employ sustainable technologies such as rainwater harvesting and grey-water treatment. A "zero carbon" build is usually achieved by offsetting any CO2 produced through the use of small scale renewable energy generation. This means including technology such as properly sited wind turbines, solar panels, extensive glazing to take advantage of natural light and passive solar gain, photovoltaic cells and geothermal heat pumps. Excess production of "zero carbon" renewable energy can even be sold back to the national grid, resulting in zero net carbon emissions.

Why would I want to build one?

There are many benefits to embarking on a zero carbon build, the most obvious being its running costs. These buildings must be highly efficient, with air tightness and especially insulation standards far in excess of current building regulations. To reduce heat lose, a heat recovery ventilation system is required, to help circulate clean, fresh air, avoiding the formation of mould, also benefitting allergy sufferers. If implemented properly, the combination of energy efficiency, micro-generation of renewable power and super insulation in a zero carbon home will lead to massive reductions in utility bills, and ultimately, if energy is sold back to the grid, no bills at all.

Is it expensive?

This is really a matter of opinion; for a new build, initial costs are indeed significantly higher. However, taking into account whole life costing of the building and the savings to be made on utility bills, this may be no more expensive than a traditional, less efficient build. Furthermore, increases in the number of people investing in green building technology and eco-housing are making it an increasingly more affordable option, lowering initial costs.

Would there be grants available?

This is also a matter of future-proofing your home - Gordon Brown has already pledged that by 2016, all new homes will be "zero carbon" and has indicated that further legislation is in the pipeline. As an incentive, all zero carbon homes built up until 2012 to a value of £500,000 will be exempt from stamp duty. Zero carbon homes over that value will see a £15,000 reduction in stamp duty fees. Achieving such high levels of energy efficiency can be a complex process; however an architect will be able to provide you with advice on how to construct your zero carbon home.