Table of Contents
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 :
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 screenthis is a (usually thin) outer cladding on a wall which prevents rain, snow, etc getting at the structure of the wall behind. see more on rain screen.
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.
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. PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. 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 bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a thermal bridge. see more on thermal bridging which could lead to heat escaping at that point.
Fabric First 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 SAPStandard Assessment Procedure - the method used in the building regulations for calculating the energy use of a house. see Part L and 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.
Since the tightening up of part E (Resistance to the passage of sound) of the Building RegulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see 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 DocumentThese are a part of the Building Regulations which ensure, if you follow them, that your plans will be automatically approved. The full set of the documents is available here 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'The Robust Details Certification Scheme is for separating walls and floors in new build joined houses, bungalows and flats in England and Wales'. It covers sound insulation requirements for part E of the Building Regulations. If the robust details are properly executed then there is automatic approval without testing. see Robust Details
Choosing a method of construction such as timber frame can dramatically lower the embodied energythe total amount of energy it takes to make a material (or a building). See more on 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 massthis is about how much heat something can absorb - so it involves its specific heat capacity and its volume. It can be useful for levelling out the peaks and troughs of temperature within a house. See the page on 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 SIPSStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on 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.
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 bridgingthis is a pathway where heat can easily escape (or get in) through some part of the structure. It is usually caused by some element of structure such as a steel lintel or wooden studwork. Also known as a thermal bridge. see more on 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 concretethis is concrete cast on site in its final position rather than being fabricated somewhere else. This can of course act like foundations and you can change to timber for higher storeys.
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.
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[for the purposes of part B of the Approved Documents] - A space enclosed by elements of a building (including a suspended ceiling) or contained within an element, but not a room, cupboard, circulation space, protected shaft or space within a flue, chute, duct, pipe or conduit. 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:
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 pointWarm air contains (invisible) water vapour. If you cool the air down you will reach a point where this vapour turns to liquid water. This is called the 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 CC – Site preparation and resistance to contaminants and moisture. see more on 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.
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 sarkinga waterproof layer in a roof which acts as backup protection in case the main layer fails. Usually a membrane or board and usually situated between roof laths and joists. board or a building paperstrong kraft paper used to prevent thermal bypass in wall insulation.
Note that timber frame panels (such as SIPS) which have OSBOriented Strand Board 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 AECBthe Sustainable Building Association article on low energy houses at the Greenoak developments.
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.
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 DocumentsThese are a part of the Building Regulations which ensure, if you follow them, that your plans will be automatically approved. The full set of the documents is available here part LThe Building Regulations, part L is the section which covers energy conservation for new buildings (with part L1A covering new buildings and part L1B covering existing ones) 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.
The Building Regulations part A covers the structure of a building. The Approved DocumentsThese are a part of the Building RegulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) which ensure, if you follow them, that your plans will be automatically approved. The full set of the documents is available here part Apart A of the Building Regulations Approved Documents relates to Structure. See an abridged version which covers house building go into a lot of detail for traditional masonry buildings but almost none for timber frame, steel frame, earth building SIPsStructural Insulated Panels - prefabricated (usually in a factory) timber panels often forming part of an integrated building system and aimed at fast site erection. see more on SIPs etc. For these you will need to consult a structural engineer (while SIPs structures are usually handled by the manufacturer)
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 the Approved Documents Part B (Fire Safety)
The regulations include a section called “Part C – Site preparation and resistance to contaminants and moisture” which covers site remediation along with protection from nasties which might affect the construction and occupants such as damp, rain, radon etc. There is an abridged version of the Approved DocumentThese are a part of the Building RegulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) which ensure, if you follow them, that your plans will be automatically approved. The full set of the documents is available here specially for houses.