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Main structure

see also

timber frame houses in the Field of Dreams, Findhorn Foundation

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.

The Building Regulations have quite a lot to say on traditional forms of construction, especially regarding structural integrity. See the Approved Documents part A

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

Should the planners give you a free hand (which is pretty unlikely) then what you choose in the way of style may well affect the type of superstructure you go for. If you chose to build a dome to live in it would almost certainly be a very different method of construction from a four-square traditional house.The type of construction may well depend on how much you are willing to pay for differing building skills. The obvious choice most people go for is brick walls and tile roof. In theory, builders know how to do this kind of thing. It produces a house which has ridiculously thick walls (if there is a useful amount of insulation incorporated), has high embodied energy, is inflexible and difficult to adapt later. But hey-ho the mortgage companies like it and it doesn’t ruffle any feathers.

Of course there is huge scope to disassociate the style from the construction method. One type of construction can be dressed up as another or ambiguity introduced. Everywhere you can see houses with bits of wood stuck on them to give some sort of timber frame effect or odd bits of stone protruding from the render to give the impression of…..er….god knows what.

The Modern Movement was all about being ‘honest’ about materials and construction so it was ok to use any material that functioned well as long as it was shown to be what it was. Post modernism thought that this was taking things a bit too seriously and so it was ok to stick some fibre glass roman columns on here and there to give a bit of interest and have a bit of a joke.

Lately there has emerged an ‘eco aesthetic’ which emphasises certain natural materials, particularly limited areas of timber boarding, usually ‘cedar’, contrasted with white render, sometimes with a nod to a (usually lower) layer or storey of something rough, earthy or block-like masonry.

Heat 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.

Fabric First (archived version) by the Energy Savings Trust is one of the most useful studies on energy saving for new homes (although it is slightly out of date and presently being updated regarding SAP and the current Building Regulations). The catch phrase about insulation used by Passivhaus designers is – draw the insulation around the house “without taking your pen off the paper”. Meaning that the insulation is continuous with no gaps and no thermal bridges. This is leading to innovative detailing, especially around foundations. With existing buildings it can get very challenging. See, for instance the thermal breaks introduced into existing internal masonry walls at the Under the Sun Passivhaus retrofit in Birmingham.

The Building Regulations cover the lowest legal limits for insulation and the minimum standard for air tightness in Approved Document L1A for new dwellings and L1B for existing ones.

Sound insulation

Since the tightening up of part E (Resistance to the passage of sound) of the Building Regulations in 2010, the type of construction you use might well be influenced by these regs especially if it is not just a detached house. With a detached house it is about isolating sound associated with bathrooms and bedrooms but if it is part of a terrace, or semi detached or flats are involved then it includes acoustic separation between dwellings. Approved Document E is stuffed full of design principles showing what is required. In particular it covers traditional masonry construction, solid concrete and timber frame. The tricky bit is that, as with fan testing, it has to work. There are two ways of making sure it works:

  • acoustic testing before completion of the building (this can be expensive, especially if it fails and retests are required after remediation)
  • using Robust Details
In many cases sound insulation, heat insulation and fire resistance will need to be considered all at the same time because they will be affecting the same parts of the building fabric. Along with this, the structural properties and possibly other aspects of the construction will have to be taken into account.

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 span quite 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.

Whole sections of the English Building Regulations, Approved documents ignore timber construction. For instance in the Approved documents, part A, Structure, most of Section 2 is devoted to restrictions on masonry buildings and there is no mention of timber frame construction

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. Jettying and overhangs 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.

Above is an example of jettying on a Walter Seagal timber frame structure. Along with being an interesting feature it serves the purpose of letting light into the middle of a room without having the window face directly onto neighbouring property. This can be important for planning purposes (overlooking neighbours) and/or fire resistance of the wall.

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.

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:

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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.

‘Breathing’ construction

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, or diffuse 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.

With vapour open construction 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.

With timber frame construction the outer sheathing layer should have high vapour permeability

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. An interesting example of problems experienced in achieving air tightness is in an AECB article on low energy houses at the Greenoak developments.

With a lot of traditional heavy masonry construction the situation becomes more complex, especially when driving rain can be forced deep into the wall thickness. What can happen then is that a layer of very damp material somewhere near the centre of the wall thickness can exert vapour pressure in two directions: towards the external surface (fine, this drives out moisture) and towards the internal surface (not good if this moisture meets a vapour barrier near the internal wall surface and condenses out, forming wet patches).

This problem can be compounded if internal insulation has been applied which has effectively moved the position at which a dew point occurs closer to the internal surface.

Two opposite extremes

To help understand the difference between when the internal surface of an external wall needs to be ‘breathable’ or moisture open and when it can be completely sealed it helps to compare two extreme cases

  • a modern, light-weight highly insulated wall with a rain screen
  • a heavy traditional stone wall, prone to driven rain penetration, and with newly applied internal insulation

With the light-weight timber frame wall above, water vapour cannot get into the wall from the room side because of the vapour barrier (which might for instance be polythene or OSB) and rain cannot get in from the outside because of the rain screen which is backed up by sarking board. If a tiny amount should get in by accident (say due to a puncture in the vapour barrier) then it will diffuse out through the sarking board. So in this diagram, any moisture is constantly moving outwards from left to right. The dew point will be somewhere near the outer surface and moisture can then escape outwards from there via the vented space. The accepted rule is that the vapour permeability of the outer face of the wall (in this case the sarking board)  should be at least 5 times that of the internal face of the wall (in this case the internal lining and the vapour barrier combined). This makes sure that vapour is always travelling in one direction, – outwards.

The opposite to the timber frame construction is the heavy masonry wall above, which has been insulated internally. Driving rain can periodically enter the wall. This will cause a vapour pressure in multiple directions, forcing some of the vapour towards the inside surface. Because the wall has been well insulated, the area next to the insulation (marked as ‘previous plaster finish’) will be cold and below the dew point so water vapour will tend to condense out there if it cannot get through the membrane and the insulation. This will then lead to the insulation and the inner wall surface getting wet.

This is where an ‘intelligent’ membrane such as Intello comes in. It will allow that trapped moisture to very gradually escape through the internal surface of the wall into the room. The sort of time spans over which this happens can be several years and the process is very complicated because it depends on many factors such as:

  • how much rain gets into the wall and how often
  • how thick the wall is and what it is made of
  • how mobile the water is in the wall in terms of factors like capillary action
  • the temperatures involved which are influenced by the amount of insulation

There are of course all sorts of intermediate examples of varying types of construction.

This sort of situation requires specialist professional advice and even then it is an imprecise science which is developing slowly. An excellent study is Assessing risks in insulation retrofits using hygrothermal software tools

However, apart from the excellent work being done by some researchers such as mentioned above, the subject is bogged down in a degree of confusion and has assumed a quasi-mystical status particularly with the ‘Polythene is the work of the devil’ brigade.  see more

There are five 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. This is completely spurious. It’s not natural to live in a house anyway. People should live up trees or in caves. Bicycles are not natural etc. The word ‘natural’ becomes a catchall for other vague unstated feelings and associations. If a house is properly ventilated then the air will always be of high quality and dampness will be removed at source. In fact air in a house is only as good as the air being drawn in from outside and this may be of low quality, particularly in cities where traffic is dense. The extremely cramped living conditions during the early part of the industrial revolution in many UK cities led to high levels of tuberculosis which were associated with generally poor public health including poor ventilation. The sanatoria where TB was treated stressed the need for lots of cold fresh air (and sunlight) and this may have gone into the public consciousness and become confused with good ventilation.
  • Vapour permeable construction allows all those nasty toxic gasses in a house to escape. This is also almost completely spurious. The actual amount of vapour which passes out through a vapour permeable structure is negligible compared with the removal of vapour by proper ventilation. Even with Passivhaus design which limits the air changes per hour to a very low level, this still shifts many times as much vapour as compared with that which ‘breathes’ out through the structure. Passivhaus ventilation is designed to provide high quality ventilation without wasting any of the energy associated with drafts.
  • Polythene vapour barriers are difficult to install without getting punctured and joints between adjacent sheets of vapour barrier may not be sealed properly. This is slightly firmer ground because polythene can get damaged easily and it can be difficult to see whether edges are properly sealed. However what is actually being talked about is not so much vapour barriers as air tightness and this can indeed be a huge problem. When even small holes are made in an air tight barrier then warm air will whisle out through those holes and carry its moisture to the nearest cool surface in the structure and deposit it there. The confusion here is that the polythene is being relied on for two purposes: to provide both a vapour barrier and an airtight membrane. The argument goes that an internal lining such as OSB gets damaged less than a sheet of polythene. This might be so but might well not and damage to the OSB  (say by an electrician) which is not mended is just as much of a problem as damage to polythene. This is entirely down to quality of design and workmanship. Vapour control membranes of all types can be poorly fixed, especially where taping is concerned. There is a strong argument to use a solid vapour barrier such as coated OSB (supported on all joints) on the internal side of walls because it is much easier to seal the edges round boards than to be sure of sealing the edges round membranes. If polythene is used then the edges and joints should be double folded and fixed by trapping them with continuous timber strips nailed or screwed to a firm background. Panels need to be taped or sealed on the edges.
  • Polythene vapour barriers prevent the buffering of indoor moisture levels. This is about the ability of walls and other surfaces to temporarily absorb moisture from the air and give it back out later. The problem might occur, for instance, while someone takes a shower, during cooking or drying clothes or during short periods of very damp weather. The relative humidity increases suddenly and there is the risk of localized condensation on indoor surfaces. This in turn can lead to mould growth, increases in mites, bacteria etc. Certain building materials (particularly clay plaster) can rapidly absorb this extra moisture and desorb it later. However if there is a polythene vapour barrier close to the internal surface then buffering may be limited.This is a relatively recent field of research which has been studied mainly in Germany by the ‘Building Biology’ movement (which doesn’t really have an equivalent in the UK) and there is a good article, Humidity buffering by absorbent materials in walls by The Technical University of Denmark. One of the problems with understanding moisture buffering is the complexity of the dynamics. Moisture is mainly absorbed quickly in the very outer surface of the material, eg. in the plaster or timber boarding, say in the outer few millimetres. It may have travelled from its source and be spread around the house, depending on air movement. The frequency and duration of high humidity events has to be taken into consideration.There is an interesting discussion on the subject in the Green Building Forum which shows how complex and misunderstood it all is.
  • Polythene has high embodied energy. Ah yes, top marks. The embodied energy of polythene is about 83 MJ/Kg which is very high. Given that a detached house of say 150m² might easily need 250m² of vapour barrier, using a thickness of 0.25 mm then this adds up to 62 litres of polythene. This weighs about 60 kg. and represents about 5000 KJ of energy.

Much of the ‘Polythene is the work of the devil’ arguement is similar to the anti-PVC rhetoric mentioned here

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

Thermal bridges

This is the term for small areas of a building’s fabric which have low insulation value and penetrate the main insulation. Some, like steel lintels can be quite obvious but others such as stud work have, until recently, been ignored in thermal calculations. They are increasingly being incorporated into the Building Regulations. There is a good study of bridging at the Low Carbon Housing Learning Zone and the Approved Documents part L has a section called Accredited Construction Details for Part L which gives consideration of thermal performance at junctions and the continuity of air barriers etc.

Building Regulations

Structure

The Building Regulations part A covers the structure of a building. This Approved Document goes into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)

Fire safety

With most forms of construction there will be implications concerning fire safety. These are covered in the Building Regulations and you can see examples of how to conform with these in Part B (Fire Safety)

Site preparation and resistance to contaminants

This section,  Part C, covers site remediation along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc.

 

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