Clearview stove

see also Burning timber and Combined heat and power

First it is important to get straight whether you want a wood burning stove because -

For the past few years it has been written into the Building Regulations that new and replacement gas boilers have to be of the condensing type because they are potentially more efficient. This is because they can extract the latent heat of evaporation out of the steam before it goes up the chimney or out [...]]]> Gas Boilers

For the past few years it has been written into the Building Regulations that new and replacement gas boilers have to be of the condensing type because they are potentially more efficient. This is because they can extract the latent heat of evaporation out of the steam before it goes up the chimney or out of the flue.

SEDBUK ratings

The actual efficiency is measured by the SEDBUK rating. The government database lists all UK boilers, past and present. However, how efficient they are in practice is a different matter. More on that later.

Boilers can be of two basic types -

• conventional , where the boiler heats the central heating system and also heats up a cylinder full of water ready for baths and washing. After the hot water in the cylinder has been used you have to wait at least half an hour before it has heated up again.
• combination , where the boiler is split into two separate parts – one part heats the central heating and the other part heats up domestic hot water instantaneously and continuously so in theory you could have a never ending shower.

It gets a little more complicated when solar collectors are involved because they are normally used to pre-heat domestic hot water. The boiler then tops up the heat if it is not warm enough. With the conventional boiler setup there is an extra large hot water cylinder with two coils in it instead of one. The second coil is at the very bottom of the tabtank and is connected to the solar panels.

Ariston Primo Twin Coil (Solar)

With the combination boiler the mains cold water feed needs to go through a coil in a tank which is heated from the solar collector so one of the advantages of combination systems (not needing a hot water cylinder) is lost.

It is also important to make sure that the boiler you choose will work properly in this mode. Probably the time to get advice from competent installers.

Efficiencies

The actual efficiency of a condensing boiler depends on whether it is working in condensing mode or not. They only do that when the return water temperature (from the radiators) is 55ºC or lower. Although condensing boilers are very efficient by their nature it can still make a difference of 10% or so if they are not in condensing mode. This is significant and it is quite difficult to guarantee that they are going to be in condensing mode for a number of reasons -

• the boiler may be oversized and not be able to dissipate all its heat so the return water is too hot to allow condensation
• there may be only a few radiators on at any given time so the boilers heat is not dissipated (especially in summer)
• the boiler’s temperature setting may be so high that the return water is too hot
• the pump speed may be too great, causing water to get back to the boiler before it has lost enough of its heat

Enter the modulating boiler which can turn itself down when needed rather than constantly cycling on and off. These boilers tend to modulate from their maximum output down to about 10kW or even 7. This may still not be low enough if your house is very well insulated. A superinsulated house may only be using a couple of kilowatts for heating even in very cold weather (and maybe 3 kW for DHW heating when needed) so the boiler will still be cycling. This is approaching the territory of the Passivhaus where a central heating system is not needed. The situation can be partially remedied by a good plumber who can balance the system well by doing a careful job of adjusting the check valves on radiators in conjunction with adjusting the pump to its lowest setting to function properly (saving electricity). This can take some achieving because most plumbers are quite dismissive about doing this, partly out of a kind of ignorance of the importance of balancing low energy systems and partly because it can be a very lengthy job to cover all the combinations of radiators being on or off and the plumber feels he is perceived to be wasting time.

Positioning of boilers

When possible it is best to situate a boiler next to a service duct . This will work well for the pipe runs, the condensate outlet and the flue, particularly if the duct is next to an external wall.

The building regulations insist that thermostats are fitted on all radiators. For the last thirty years or so these have usually been thermostatic radiator valves (TRVs)

However, using separate zone controls allows you to set different heating regimes for different rooms in the house. These are programmable for both time and [...]]]> Programmable room thermostats and zone controls

The building regulations insist that thermostats are fitted on all radiators. For the last thirty years or so these have usually been thermostatic radiator valves (TRVs)

However, using separate zone controls allows you to set different heating regimes for different rooms in the house. These are programmable for both time and temperature. So for instance you could set the bedrooms for a low temperature during the day and for a higher one when you get up, whereas the living areas could be warm during the day and drop to a lower temperature at night. With this method you would use electric valves to control the radiators rather than TRVs. There are four aspects which are different from a non-zoned system

• The system can remain running all the time but the boiler and pump are only running when a radiator (or cylinder) is calling for heat. This saves on pumping costs. It also means that hot water will always be available.
• you can have a low but fixed night time temperature (more pleasant if you have to get up or work during the night)
• You can over-ride the programmer to increase or decrease the temperature at any time and it will only affect the zone you are in.
• It nearly always removes the need for an optimiser because the warm up period on a morning is predictable, rather relying on the weather.

Note that in the example shown in the diagram the living and dining areas are open plan so that they can share one programmer. Note also that if you tried having two bedrooms sharing one programmer it would be difficult to get it to work properly because although the timing would work, the temperatures might not match up.

Note also that the cylinder always gets priority of hot water and does not require a two way valve. The differential pressure valve is necessary for when all the radiators are closed and the cylinder calls for heat.

Zoning can save a good deal of energy because only the areas that need heat are getting it. It is particularly useful for rooms where someone has a special requirement such as an elderly relative who may need heating when others don’t, possibly also people who work a lot at night.

There are a couple of drawbacks: some people find the programmers difficult to adjust (although most people can work the over-ride button). Also the equipment is a bit more expensive and the electric valves may be prone to failure if the circulant is not well filtered.

Programmable timer with thermostatic radiator valves

This is probably the commonest arrangement for central heating circuits in the UK but not the best one in terms of energy efficiency or comfort. It ensures that the heating is switched off when you want it off and that no room is warmer than it needs to be. What it does not do is allow you to maintain a minimum temperature (say during the night or during the day when you are out at work) and it does not allow you to vary the temperature up and down in different rooms depending on the time of day unless you are willing to run around adjusting the TRVs all the time.

Stupid

By far the craziest approach is to have TRVs on all the radiators and then to have an electrical thermostat in the hall to override the whole system including the TRVs. This is still not uncommon to see. The problem is that the hall is probably the least representative room in the house for having a thermostat. For instance a gust of cold air from the front door may cause the heating for the whole house to kick in, even if all the other rooms are warm enough. Alternately the thermostat will react to heat nearby and prevent a radiator functioning properly somewhere else.

One of the reasons people still do this is when they want the heating to be able to shut down at night and use a combined thermostat and timer rather than just a timer. The answer then is to turn the thermostat up high and let the TRVs do the temperature control and only use the timer function.

The boiler thermostat

The thermostat built into the boiler determines the temperature of the water that the boiler supplies. The level to set this at is a subject of considerable debate. See the section on  gas boilers

Weather compensation

Boilers need to put out more heat in cold weather. Optimisers are small computers which monitor the outside temperature in order to tell a modulating boiler how low a temperature it can run at to do its job effectively.

Emitters tend to be of two types – traditional radiators and underfloor heating. It’s a bit of a toss-up as to which is better. They have their respective advantages and drawbacks.

Underfloor

LCA of materials

With timber floors and ceilings make sure all materials are woodmarked. Consider using engineered joists (such as Masonite) as these use less material and leak less heat. Plasterboard is showing up as something of a problem for eventual disposal .

Particularly with floors there are two likely [...]]]>

green priorities

LCA of materials

With timber floors and ceilings make sure all materials are woodmarked. Consider using engineered joists (such as Masonite) as these use less material and leak less heat. Plasterboard is showing up as something of a problem for eventual disposal .

Embodied energy

Particularly with floors there are two likely areas where high embodied energy may be a problem

• with the use of concrete floors which use large amounts of energy in the manufacture of cement (but see comments below about thermal mass). Try to use GGBS (Ground Granulated Blast Furnace Slag) cement in a ratio of 50/50 with OPC. This reduces the embodied energy very considerably.
• with the carpeting of floors. A study in the US, where people change their carpets on average every 7 years showed that the amount of energy to make the acrylic and nylon for the carpets accounted for the highest single category of embodied energy use in the house.

Insulation values

Ceilings in roofs

The uppermost ceiling is the point where you can really increase the thickness of the insulation . Unlike wall insulation it does not reduce the volume of the house. You simply build slightly higher. It is the area where the warm air in the house rises to and therefore the place which is most effective to insulate. It also helps to prevent overheating in roof areas in summer.

In many situations, particularly older houses, there is no need to contain the insulation in any way – simply lay it there. (but remember about not causing existing electric wiring to overheat by being surrounded by insulation). It may be that by having higher insulation values in the roof you can allow more window area elsewhere. This depends upon the SAP calculations . How much insulation to have? Well at least 200mm but consider 300mm (or 400mm if you are thinking about Passivhaus standards).

Ground floors

There are two kinds of ground floors in terms of insulation:- those that rest on the ground and those that are suspended. The latter should be considered as similar to walls because air flows beneath them and they need similar levels of insulation, at least 200mm but more likely 300mm if you are considering Passivhaus standards. (air tightness is also very important).

Floors which rest on the ground are quite interesting because building technologists are still not quite sure what to make of them and research is rather limited and inconclusive. There is one thing certain which is that perimeter walls around such ground floors need insulating to prevent heat going downwards and then escaping outwards. What is not certain is what happens to heat that goes downwards from the centre of a building, because although earth is a poor insulator there is an almost infinite amount of it (infinitely thick insulation – which gets warmer the deeper you get). In fact with a building which has a large floor area compared to its perimeter there is an argument that the ground beneath it could usefully act as a heat store to stabilize temperatures within the building and that putting insulation into the floor would have a detrimental effect. In practical terms, given that houses generally have relatively small footprint areas compared with their perimeter walls then it seems sensible to use a layer of insulation beneath the floor which connects with an extension of the vertical wall insulation going down into the ground. However, opinion on this matter may change in the future in favour of having no horizontal floor insulation but extending the wall insulation much deeper to create a large thermal store.

Floor insulation which is in contact with damp ground needs to be inert and to be able to withstand the pressures of the structure above it. The high density polystyrene and polyurethane flooring insulations are ideal for this although they are high in embodied energy. Timber I beams are much better insulators than the traditional timber joist. Over the last decade or so timber I beams have gained popularity because of the way that engineered timber can create more efficient structural members. They use finger jointed timber flanges and OSB webs which make them lighter, stronger and more efficient in timber use. They generally do not need strutting and the slenderness of the web means there is much less thermal bridging.

The thermal mass of floors

Although the use of concrete in floor construction represents high embodied energy there may be an argument for it, especially with houses where the bedrooms are on the ground floor and living rooms on the first. Part of the Passivhaus principle is the capture of solar energy through glazing (either normal windows or a type of conservatory area). This warmth tends to rise and is easier to capture in a dense thermal store at first floor level than into the ground floor. In other words the living room, dining room, kitchen floors store the heat and the bedrooms below are cooler. Also the acoustic insulation of the concrete becomes beneficial to those sleeping below. See more on Passive solar design

structural

From a green standpoint the main thing to remember is that timber should be woodmarked and sourced as locally as possible, preferably in the UK.

Structural timber has the advantage over most other structural materials of being renewable and also a relatively good thermal insulation material.

Structural timber is usually of the following types

• normal softwood sections such as floor and ceiling joists as specified in the building regulations.
• larger sections of softwood or hardwood forming beams, trusses etc. and usually calculated by a structural engineer.
• laminated structural timber
• engineered timber such as I beams.

The normal softwood sections are usually graded as C16 or C24 (respectively replacing SC3 and SC4) and this will be stated in the approved building regulation drawings. It is often available from UK plantations

Larger sections of softwood are also available from the UK and it is often possible to get them from local sawmills.

The larger hardwood sections may be more of a problem to source sustainably and in the case of tropical hardwoods, only the woodmark can be relied upon. (However it is seldom that tropical hardwoods are used structurally in housing). There is a considerable amount of timber produced sustainably in the UK which is not woodmarked mainly because it is produced in such small quantities that the certification procedure would not be warranted. E.g. there is a constant supply of hedgerow ash and to a lesser extent oak and other species which is not woodmarked but which gets replaced. There are also organisations such as Woodlots which may be of help in sourcing local timber.

Laminated timber (sometimes known as Glulam) is generally well sourced environmentally. However, due to the poor understanding of timber building culture in the UK it has been marketed mainly towards large structures such as offices, swimming pools, theaters etc rather than the housebuilding market, so it can be difficult to find merchants who are supplying off-the-peg structural members. The Glued Laminated Timber Association has a list of member companies. Also try Panel Agency Limited and Lamisell

Engineered timber such as Masonite beams represents a huge step forward in timber technology. Compared with traditional beams and joists Masonite sections are, for the same structural strength, much lighter, much more dimensionally regular and use considerably less timber. They can span up to about 7m. The dimensional stability with traditional beams and joists can be a major problem if they are not supplied at a moisture content of 12%, as shrinkage can cause considerable movement. It is not unusual to hear of 20mm movement over a two storey timber structure in the first year . They also help with the insulation because the webs, being much thinner, cause minimal thermal bridging. They do however run at about twice the price of solid timber sections with merchants such as Arnold Laver quoting around £4.80/m for 220 x 38 I beams. (supplied in 12m lengths).

Hybrid joists. Another engineered product is the joist with metal webs and timber flanges such as Ecojoist made by Gang-Nail or Easi-joist by Wolf Systems. The main advantage here is the ability to thread quite large service runs through the metal web (not only near the ends (as is the case with traditional joists and timber I beams) but anywhere along the length.

European Standards

On 1st April 2010 the new CEN Eurocode standards for structural timber came into force in place of the old BS standard. These are -

• BS EN 1995-1-1 Eurocode 5: Design of timber structures. Part 1-1 General – Common rules and rules for buildings
• BS EN 1995-1-2 Eurocode 5: Design of timber structures Part 1.2 General – Structural fire design

TRADA have published span tables in a new softback book called Eurocode 5 Span Tables: For Solid Timber Members in Floors, Ceilings and Roofs for Dwellings and various companies do online calculation software. However for practical purposes the self builder will still find the span tables in the old (archived) Approved Documents from 1992 to be useful in determining sizes for floor, ceiling and roof joists, binders, rafters and purlins. There is extremely little difference between the old span tables and the new ones. The slight discrepancy is mainly in spans shorter that 2.4m. See more on the Eurocode News site.

Radon

The Building Regulations require that radon gas cannot enter a building via the ground floor. > England and Wales. Scotland. N. Ireland

sound insulation

The acoustic insulation of floors is a mixed bag as far as green building techniques is concerned. The sound attenuation, measured in decibels, can basically be achieved in two ways, by mass or by isolation of the floor surface from the surrounding structure (or both). Each achieves a slightly different result – for instance a heavy concrete floor will absorb sounds such as speech and music but may still transmit impact sounds such as chairs being moved around. A timber floating floor, on the other hand may not transmit impact sounds so much but will not be so good with airborne sound.

Heavy, dense insulating floors

Heavy floors such as concrete beam and block can give good sound insulation and this can be beneficial in three ways (apart from simply sound insulation)

• it can be part of the thermal mass of the building which helps with thermal stability. See Passivhaus standards and passive solar design
• it can work well with underfloor heating by setting the heating pipes in a screed on the concrete floor
• it can fit very well with the idea of Flexible design because if a house needs to be modified at some point to provide an upstairs flat then sound and fire insulation will be required by the building regulations and this can easily be achieved with a concrete floor.

However the heavyweight approach can have some drawbacks, depending on the design of the house.

• concrete floors are high in embodied energy and difficult to recycle
• they require heavy structural walls to support them which involves more embodied energy (or structural columns at the corners – which might not play well with thermal bridging). No good trying to use timber to support concrete floors. Heavy masonry walls cause extra thickness to a wall which may already be very thick due to insulation. The exception to all this might be when there is a party wall between semis or in a terrace. In that case there is already a heavyweight wall which is effectively internal and can support heavy floors.
• the high thermal mass approach doesn’t fit well with intermittent heating

Lightweight floating floors

If you want to provide acoustic insulation with a lightweight timber floor then you have to float the floor finish on the structure. This involves placing a layer of resilient material under the floor and around its edges so that it has no solid contact with either the floor joists or the walls or the skirting boards etc. There are various systems on the market.

concrete floors

concrete floor on columns (concrete or steel)

Although the embodied energy of concrete is high, especially if it contains reinforcing steel, there may be a strong argument for using it in floors when high thermal mass is a priority, for instance in conjunction with passive solar design or when acoustic separation or fire resistance is required. Also a concrete slab may be the best way of providing foundations on certain types of uneven ground.

If you use concrete at first floor level or above it does of course mean that you cannot use a timber structure for the walls. You need to use a material which is stronger in compression such as concrete or steel columns or masonry walls.

If you use concrete or steel columns there is a potential problem with cold bridging if the columns are within the thickness of the walls and also where the columns have contact with the ground. Columns will need externally cladding with insulation.

If the floor spans between load bearing walls then cavity wall insulation or external insulation will be needed to cover the blockwork and the edge of the floor.

The insulation of concrete ground floors in Radon areas, both new and retrofit, has been addressed quite neatly by Eco-slab who make an EPS product which creates voids beneath the insulation. These can be vented. See detail drawing.

Particularly with in-situ concrete it is possible to create complex floor shapes with long spans and with stair openings pretty much wherever you want them simply by placing more reinforcing steel where necessary. The drawback with casting in-situ concrete is the need for shuttering and the time it has to be left in place after the concrete is poured. Beam and block is more suited to regular right angled floor plans and is fast to install. Hollow beams can allow for running services as they can be drilled and cut in certain places. There is also the potential to use the hollow space to circulate air for heating and cooling which utilises the thermal mass of the concrete.

Underfloor heating is well suited to concrete floors. It can either be layed in the screed which covers the beam and block type floor or it can be cast directly into in-situ concrete. In both cases the pipes are well protected from future accidental damage. Concrete floors provide very good fire resistance and acoustic insulation (providing they have a surface which reduces noise from impact.

Self Levelling Concrete

Although concrete is a material which should be used as little as possible because of its extremely high embodied energy, there are times when there is little alternative. One of these is laying ground floors in old buildings which have no existing floor except earth, or a floor in very poor condition. At this point, self levelling concrete, which is a recent innovation, may be very attractive to the self builder. As its name implies, it is very easy to level, requiring no levelling boards, and no tamping. In most cases the sequence is as follows

• Dig out the old earth and floor down to a suitable level leaving it clean and firm.
• Lay a layer of high density polyurethane or polystyrene foam board insulation.
• Lay a polythene DPM
• Pour and spread the concrete (which comes as a ready mix). The spreading can be done with a large rake such as is used for raking tarmac. It is then levelled with a special board with two handles. (This is not particularly hard work)

Above is a picture of a barn conversion where previously there had been various broken bits of concrete floor. The DPM has been laid on the sheets of insulation and the joints taped.

The floor after pouring. No further screeding should be necessary.

At present this type of concrete seems to be available only from Lafarge in their Agilia range

timber floors

The traditional way of designing and building timber floors into masonry walls tends to suffer from lack of insulation and lack of draft proofing.

Especially as timber joists dry out and shrink, gaps appear round them. Although each gap is small (maybe say only a milimeter shrinkage for timber which starts out damp and finishes at say 12%) this would add up to something like 250 sq.mm. per joist and be a total of something in the region of 130 sq. cm. for a small two storey house. That’s a hole 11cm. x 11cm.! Almost like leaving a cat flap open. You can get a lot of cold air in through that sort of gap over a freezing winter. A better way is to fix the joists to the walls using joist hangers, making sure that the perpends behind the joist ends are filled with mortar.

With the ground floor thermal bridging is decreased if you use timber I beam joists as their thinner web conducts less heat.

Another method is to build in an airtight strip, probably of polythene, (shown in red) so that it carries across the floor thickness. This is a method used in timber frame construction as well as masonry. For instance in Walter Segal timber frame construction a strip of polythene can be trapped between the posts and beams as they are being bolted together. This is then connected to the internal air barrier above and below the floor. It is important that a secure fixing is formed between the polythene and the wall lining at points A and B. It should be fixed with tape or an adhesive and then mechanically trapped.

In the case of masonry, if you want to build the joists into an internal blockwork leaf, the polythene in the form of a long strip (red) can be taken up round the back of the leaf at the floor level and then be fixed to the blockwork with a strip of expanded metal (blue) which is plastered over to form an airtight connection. The polythene can be taken right round the room on external walls including walls that are parallel to joists. This has the double advantage of sealing the ends of the joists and also the unplastered joints of blockwork between joists.

Above is the recently completed passivhaus at Denby Dale. Deliberately designed to fit in with the traditional stone buildings of the area it was also constructed where possible of components available locally, automatic blinds to the sun space being one of the exceptions. The company behind this development, the Green Building Store have [...]]]>

Above is the recently completed passivhaus at Denby Dale. Deliberately designed to fit in with the traditional stone buildings of the area it was also constructed where possible of components available locally, automatic blinds to the sun space being one of the exceptions. The company behind this development, the Green Building Store have a series of on line videos explaining various aspects of the construction.

The Passivhaus standard has been developed to provide a practical methodology for designing and building houses which require very little energy for heating, so little in fact that they require no central heating system, even in the coldest parts of Europe. Note that Passivhaus is about energy only and does not cover other ecological aspects in the way that, say Code for Sustainable Homes does.

The basis of the standard is about achieving very low energy use. The actual standard is that

• the total energy demand for space heating and cooling is less than 15 kWh/m 2 /yr
• the total primary energy use for all appliances, domestic hot water and space heating and cooling is less than 120 kWh/m 2 /yr

(the floor area is measured to the inside face of external walls and ignores internal wall thicknesses and certain areas which are not considered habitable such as stairs and landings)

The standard is achieved by a combination of -

• very high insulation in walls, ceilings and floors – typically 300mm thick (with very little thermal bridging) with doors and windows having high values of insulation built in. Glazing is typically triple with low E coatings and argon filled. See more on Passivhaus window design
• low air infiltration – below 0.6 air changes per hour; enough to provide very high air quality without unnecessarily cooling the building.
• compact form which ensures a minimum of heat loss through the fabric
• passive solar collection which is incorporated by having most of the glazing to the south and designing windows with high solar heat-gain coefficients.
• heat recovery ventilation which is designed so that the minimum of air necessary to maintain high quality ventilation is achieved, (1m³/m² of floor area/hr). An air to air heat exchanger recovers most of the heat from the outgoing air (usually over 80%). Incoming air first goes through the heat exchanger and then an in-line booster heater which lifts the temperature up to room temperature. It is then distributed by ductwork to the various rooms in the house. more +/-»

The Passivhaus Institute has an excellent web site called Passipedia which explains all the fundamental principles of the standard

Once you design to this level of energy efficiency no central heating system is necessary. Any heating which may be required is added to the incoming ventilation air either directly by an electric heater or (thanks to recent developments) via a ground source heat pump which has its heat output into the ventilation air. This combination of energy saving measures has a built in logic to it.

vent outlets ensure there is never a direct draft

The rate of air change is minimal but sufficient to guarantee a high level of air quality. It is also a sufficiently low rate to ensure that air is never moving at a speed of more than one tenth of a metre per second, the threshold at which air movement becomes noticeable. This allows a maximum heat input of 10W/m2 of floor area which is typically provided by a small heating coil or resistance heater situated right after the MVHR heat exchanger. So for instance a 150m2 house would have an extra heat input on top of solar and internal gains) of 1.5kW under the severest conditions. For more details on air quality see Mark Siddall’s AECB article Passivhaus ventilation  It’s not a lot of hot air

Although on-site generation of energy from sources such as photovoltaics is not an integral part of PassivHaus design there is no reason it cannot be incorporated additionally, possibly at a later date (PV ready).

There is a short primer on Passivhaus on the BRE website

Comparison with SAP and CSH

Passivhaus does not prescribe what type of energy is used (except the bit about ‘primary’), or whether it is carbon based. Only how much energy is used. This contrasts with the UK building regulations which have built in factors in the SAP calculation which take into account the amount of carbon emissions.

The Code for Sustainable Homes covers much more than just energy so comparing it with the Passivhaus standard is like comparing apples and oranges. However the energy saving element of a Passivhaus is roughly equivalent to CSH level 4. If you were then to add PV energy generation  and some of the other features which count in CSH then you could reach level 6 (carbon neutral). The standard assessment procedure (SAP) which is the part of the building regulations used to calculate energy efficiency has its counterpart in the Passivhouse Planning Package.

There is some debate going on as to whether it would be better to adopt the PassivHaus approach rather than the SAP approach to energy calculation. A detailed comparison of the two is available on the AECB web site. It should be borne in mind that the continental approach to ecological building has tended to separate energy calculations from the ecological and health impact of building materials rather than rolling them into one as the CSH tends to do.

Design and certification

To achieve this level of low energy consumption there is a combination of computer aided design (via a large spreadsheet)  and a certification procedure, all based on the experience of the  thousand or so existing passivhauses which have been built already, mainly across Europe. (There are possibly another 6000 which are to the PassivHaus level but not certified). At present both the BRE and the AECB have taken on the responsibility for administering this process in the UK and information is available on the BRE website and the computer design software (the Passivhaus planning package – PHPP) can be downloaded from there and also from the AECB website

There is a well developed certification procedure for building components such as doors, windows etc. to gain Passivhaus ratings so that these ratings can be fed into the spreadsheet which produces the final energy use values for a house.

Although the standards are very high and very strict there is (in theory) no restriction on the types of materials used or the design of components such as windows and doors, just so long as the final standards are met. Manufacturers are queuing up to get their products approved. To date – September 2009 – only one house in the UK has actually been certified to Passivhaus standard although several others have probably achieved it.

Cost

Experience in Germany, Sweden and Austria, where most of the development has been done shows that initially the cost of individual Passivhauses is quite high but that as builders become more conversant with the standards and more local manufacturers produce components then the extra price comes down to about 4 to 6 percent above normal building rates.

Passivhaus refurbishment

One of the most thoroughly thought through renovations of an old house is ‘Under the Sun‘ in Birmingham. This has been successfully brought up to Passivhaus standard

Under the Sun. Click the image for more info.

Volfgang Feist

Volfgang, a physicist, is the originator of the Passivhaus concept along with Bo Adamson, and there is an interview with him by energy consultant Peter Warm on the background and principles involved. It is in 6 parts.

Part 1 on the background to it

Part 2 on principles

Part 3 on renewables

Part 4 on the pace of implementation

Part 5 on the history of Passivhaus development

Part 6 on technical aspects

The concept is moving into the mainstream now with developments such as Camden Council’s new council house scheme for 55 new homes to be built by Willmott Dixon and designed by architects Rick Mather

Method of construction

How you design and build the structural elements of a house depends on many factors. Although not in particular order, because they can all dynamically affect each other, the first few do tend to set the scene, especially regarding what the planners will allow.

Planning conditions

Planners [...]]]>

Method of construction

How you design and build the structural elements of a house depends on many factors. Although not in particular order, because they can all dynamically affect each other, the first few do tend to set the scene, especially regarding what the planners will allow.

Traditional considerations :

Planning conditions

Planners may well insist on a particular style of building, including the materials used. In or near sensitive areas such as green belt, national parks, areas of outstanding natural beauty, conservation areas etc. they may lay down very strict rules which will go a long way to determining the type of superstructure. For instance they may ask for stone walls and stone flags on the roof. This may impact on plan layout, foundations, roof design, arrangement of insulation etc. and this would produce a very different building from something with a timber frame structure with lightweight cladding or rain screen.

Style

Assuming the planners give you a free hand (and neighbours don’t object too much) then what you choose in the way of style may well affect the type of superstructure. If you chose to build a dome to live in it would be a very different method of construction from a four-square traditional house.

Insulation

With the coming of higher insulation levels it is no longer a matter of bunging in a couple of inches of rock wool here and there. Passivhaus standards rely on something like 300mm. of insulation around the outside of everything without ANY gaps. This affect the superstructure enormously. For instance where a balcony meets a wall or where an unheated garage roof meets a heated house wall there must be no thermal bridging which could lead to heat escaping at that point.

Embodied energy

Choosing a method of construction such as timber frame can dramatically lower the embodied energy locked up in the building. Timber locks up carbon through at least the life time of a building whereas concrete blocks use a lot of energy in the making

Building mass (and thermal mass)

This is quite a tricky one and there are two pairs of arguments

• Quick warm-up time vs. thermal stability (partly a lifestyle issue)
• Insulation on the inside vs. insulation on the outside (this particularly affects existing buildings)

These two affect the type of superstructure enormously and the subject is discussed here

Speed of construction

Many people are working to a tight deadline or dread the possibility of getting into long delays due to poor weather conditions. This is where a SIPS structure, which can be weather tight in a few days contrasts with traditional bricks and mortar, which, even with ideal weather takes months for the plaster and masonry to dry out.

Floor spans and plan regularity

The size of rooms can partially dictate the method of construction. Traditional timber joists spanquite easily up to about 5m. Timber I joists span up to about 8m. if you don’t mind them being 450mm. deep. Anything above that will require laminated timber or steel joists and this will probably affect the type of wall they rest on. For instance steel beams and joists may well need to rest on steel columns or masonry rather than on timber.
If walls do not rise vertically and regularly from foundation to roof then extra structural members may be needed. Irregular floor plans can easily be created using steelwork but there is more cost and there can be difficulties preventing cold bridging

Adaptability of design

Along with the notion of Lifetime Homes and the way some types of construction are easier to modify and adapt than others, then forms of superstructure such as timber frame prove to be much more flexible than heavyweight masonry. It’s easier to remove walls because there is not so much weight resting on them above. It’s also physically easier to move walls around.

Jettying and overhangs

These can be very attractive and useful features on a building but are very difficult to achieve using masonry construction because of the weight. With timber and steel it is easy.

Site profile and ground conditions

It is not a good idea to use timber where there is a risk of sustained dampness so if you want to bury any rooms of a house then go for masonry or in situ concrete. This can of course act like foundations and you can change to timber for higher storeys.

Skills

If you are intending to do some or all of the building work yourself, then it is probably best to stick with the skills you have and this will influence the type of superstructure you choose.
If, on the other hand, you are intending to manage and use specialist sub contractors then the situation is almost reversed. There are plenty of excellent companies who specialize in timber frame, SIPS, steel erection, specialist glazing etc. and your job will be making sure that they are well co-ordinated and the site is well managed.

Local climate

The west coast of the UK is wetter and windier than the east cost and this is reflected in the Building Regulations map of exposure. There are varying degrees of protection needed for walls to prevent driving rain forcing its way into the house through small building cracks in the fabric. Much of the traditional thinking relied on 50mm. cavity walls where any rain which got forced in through tiny gaps in the mortar would run down the inside face of the outer leaf of the wall. (this of course relies on there being no bits of mortar bridging the wall ties, otherwise moisture finds its way across to the inner leaf). Enter cavity insulation! Filling the cavity can create routes for moisture to get from the outer to the inner leaf. This is especially true if workmanship is not up to a high standard or where an external leaf of stonework has a rough inner surface which is difficult to keep clear of mortar droppings.

An alternative approach is using a rain screen which places a cavity on the outer face of the wall.

Another approach is to utilize a totally waterproof render on the outer face of the wall. Modern render systems are streets ahead of the traditional sand/cement renders which were prone to craze, crack and often fall off.
This all has a bearing on the type of superstructure you choose, especially on the overall thickness of the wall.

Vapour barriers and air tightness

The method of construction you use may well be influenced by the way vapour barriers (or vapour checks) and air tightness are handled. These two subjects may be closely inter-related when it comes to low energy construction in a way that was not true for traditional building techniques. It can be very difficult to achieve high levels of air tightness unless the whole building process is rethought. 

If there is no vapour barrier, or a badly damaged one this is what happens:

fig 1.

Vapour barriers prevent water vapour inside a building from slowly permeating into the thickness of the structure where it might cool down below its dew point and condense out, forming damp areas. It is important to design and fit vapour barriers or vapour checks correctly otherwise considerable damage can result especially if the dampness forms in moisture prone insulation or around timber. The insulation might get degraded and the timber might rot.

This applies to all the external surfaces such as walls, ceilings and floors. The Building Regulations (part C pages 28 – 40) cover the risk of condensation and building professionals use software to predict the risk and design against it.

Vapour barriers and air tightness should not be confused. No matter how well designed a vapour barrier might be it will not prevent condensation forming in an air leaky building fabric. The escaping warm air will carry moisture with it which will condense out when it gets below its dew point. See Air Tightness

Conventional wisdom has been to place an impermeable vapour barrier around the inside surfaces of the whole building to prevent any moisture getting into the shell and this is the main thrust of the present Building Regulations.

Recent developments

Over the last couple of decades there has been a move towards vapour permeable or vapour open (also misleadingly called breathing wall) design.

The principle is that a certain, controlled amount of moisture is allowed to migrate out via a calculated vapour check through the external fabric (walls, floor, roof) of the house and it includes the idea that a vapour barrier should not be totally relied upon because it may get damaged either during the building process or later on in the building’s life.

Moisture can escape out of the wall much faster than it can enter from the inside of the house. This is particularly important for timber frame construction where the outer sheathing layer should have high vapour permeability. This can be achieved using a vapour permeable sarking board or a building paper.

Note that timber frame panels (such as SIPS) which have OSB on both faces of the panel do not achieve this kind of performance unless they also have some form of extra vapour barrier (such as polythene) on the inner face. If there is any accidental damage to the inside face of the OSB then vapour can get into the internal space, will migrate over to the inside face of the outer panel and condense out and cause damage.

An excellent article on the subject is Breathability: The Key to Building Performance by Neil May.

However the subject is bogged down in a degree of confusion and has assumed a quasi-magical status.

There are six arguments used for favouring vapour permeable construction over impermeable vapour barriers and four of them are inaccurate or spurious -

• It’s not natural to live in a place surrounded by a polythene bag more +/-»
• Vapour permeable construction allows all those nasty toxic gasses in a house to escape more +/-»
• Polythene vapour barriers are difficult to install without getting punctured and joints between adjacent sheets of vapour barrier may not be sealed properly more +/-»
• Polythene has high embodied energy more +/-»
• ‘Breathing’ walls allow any bits of moisture which might get trapped in the wall to find their way out more +/-»
• Polythene vapour barriers prevent the buffering of indoor moisture levels more +/-»

See also Ventilation because this is linked to the level of air tightness achieved.