Table of Contents
The main types of walls
Regardless of the type of wall construction, all walls have certain functions to fulfil and certain challenges to contend with. From a green perspective, three of the most important aspects of walls are their insulation values, the embodied energythe total amount of energy it takes to make a material (or a building). See more on embodied energy which goes into constructing them and the sustainability of the materials used. This is mainly covered here:
- Timber frame and 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
- Walter SegalThe architect who devised a simple timber frame self build system (often simply known as 'Segal self build'. see more on the Segal method
- Solid timber
- Ziegel blocks
- Steel frame
- Insulating concrete forms
- Concrete blocks
- Hemp / Lime
- Rammed Earth
- Straw Bale
There are certain issues which crop up with many of the wall types:
External wall insulation
There are two main reasons you might consider external insulation on walls:
- you may want to insulate an existing building externally
- to avoid losing space internally
- to avoid disturbing internal surfaces or features
- to use the walls as a heat store/buffer
- to avoid the ‘cooling fin’ effect caused where internal walls connect to external ones.
- you may want to insulate a new building externally so that the mass of the walls acts as a heat store/buffer to achieve a decrement delayThis relates to the lag time that insulation itself takes to heat up or cool down. It introduces a delay into the effect of the insulation. This can help level out peaks and troughs of temperature. See the section on Decrement Delay
For several reasons it usually makes sense to insulate on the outside, providing planning permissionthe legal basis for being allowed to do some form of development such as building a house. (not to be confused with 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 is all about whether the building is properly constructed). see more on Planning Permission can be obtained and providing it does not cause too much disruption
- Space is not lost within the dwelling
- the 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 is on the inside of the building
- Internal decorations and finishes are not interfered with
- The insulation is not compromised at junctions with internal walls
The above plan shows how an internal wall[for the purposes of part C of the Approved Documents] - Any wall that does not have a separating function. connecting with an external solid wall causes a direct pathway for heat to escape out. External insulation prevents this but internal insulation has little effect. Of course 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. wall insulation isolates internal walls from external ones but 50mm gives a very low level of insulation in the first place.
External wall insulation is not straight forward. You may need planning permission to add insulation to the outside of a building. Contact the local planning department at the council. Considering the work involved in insulating the outside of a building it makes sense to add as much insulation as is feasible while you are doing the job. 100mm would seem like a minimum, 200mm sensible and 300mm would mean you might be able to achieve 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. standard. This of course will most likely entail the repositioning of gutters, fall pipes and possibly gullies. It could necessitate extending the roof downwards and outwards to cover the extra wall thickness and it may mean some fairly nifty detailing round openings, particularly windows. This may also have implications for the eaves level in relation to the tops of windows. You also need to give some design consideration to how the roof or loft insulation meets the new wall insulation to avoid 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. Avoiding condensation and allowing traditional walls to breathe are also crucial.
So far very few already existing houses in the UK have been upgraded with external insulation though on the continent there is plenty of activity in this area.
One of the few books on the subject is The Complete Guide to External Wall Insulation by Christopher Pearson.
Problems with fitting wall insulation
These problems can apply to new build and retrofit.
There are five main types of situation where poor fitting can happen:
- poor jointing
- gaps on the face of the insulation
- areas missing
- slump and settlement
- insulation causing bridging of cavities
Poor jointing and gaps on the face
this is a particular problem where solid sheets of insulation such as polystyrene are used. In some cases it only takes a gap of a few millimetres between sheets to completely spoil the efficacy of the insulation. Take for example a traditional cavity wall which gets insulated using expanded polystyrene sheets and imagine that the sheets fit closely but have slight gaps because they were not cut perfectly or mortar snots hold them away from the inner leaf.
As can be seen, the inner leaf is subjected to a stream of cold outside air and has virtually no insulation. Something similar happens with mineral fibre, especially at joints and edges. If a number of such cavities link up then this can form a huge source of escaping heat.
this tends to happen when there are areas which are inaccessible or where a spray in product such as warmcell is being used and it is physically impossible to reach the area. This can easily happen at the eaves or in complex areas where there are extra noggins. This problem needs addressing at the design stage.
Slump and settlement
this can occasionally happen with insulation such as Warmcell, polystyrene bead and any other poured or pumped insulation. In the case of Warmcell in floors it should be packed in quite tightly so that if it settles slightly then it is still in contact with the lower surface of the floor itself.
Bridging of cavities
if the wall design includes cavities then it is most important that they don’t get accidentally bridged by using the wrong kind of insulation. This can happen particularly with mineral wool being badly fitted and the result is that moisture may find its way over from the outer leaf to the inner one.
The Building Regulations on cavity walls are in 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 CC – Site preparation and resistance to contaminants and moisture. see more on part C page 37
Thermal mass There is a quite complicated discussion going on amongst building technologists about the thermal mass of buildings and its affect on storing solar energy and thermal stability generally. This partly concerns the Decrement delay value. To maximise the thermal mass without having overly thick external walls it makes sense to construct the internal walls (and possibly floors) of high density material. This is now beginning to enter into the building regulations.
DPCsDamp Proof Course - a strip of (usually) plastic built into walls to prevent damp rising or penetrating. see more are used to isolate all the dry inner surfaces of a building from any outer surfaces which might absorb water and be damp. The Building Regulations cover this subject here. While some outer surfaces are designed to be totally water resistant (such as roof coverings) many other areas can absorb water quite readily e.g.-
- external leaves of masonry walls such as stone, brick and blockwork. Also cement render if it is not in perfect condition.
- solid concrete floors which rest on the ground
- external timber cladding
To prevent moisture absorbtion there needs to be a thin layer or membrane, usually plastic, inserted between the damp outer surface and the dry inner one.
The most common situations that these membranes are used are:
at the base of walls to prevent damp rising from the ground. The damp proof course (DPC) is a way of preventing damp from the ground creeping up into the walls and causing problems. It is mandatory in the building regulations to include some way of doing this. With typical masonry building such as brick and block it is normal to lay a strip of plastic in the wall to stop damp rising up through the masonry. The DPC, as it is known, is usually placed at least 150mm above ground level in the outer leaf of a cavity wall so that it is normally above snow level and splashes from rainfall, or above the level of damp debris which might accumulate on the ground outside.
If the ground level varies then the level of the DPC in the brick courses will probably vary to follow changes in the floor level. The DPC in the inner leaf may be at a different height, usually just below the level of floor joists so that they are protected. If there is a solid ground floor with a DPMdamp proof membrane - a sheet of (usually) plastic used to prevent dampness rising up through a floor or in through an underground wall. see more under it then the DPC and DPM are lap jointed.
Historically, DPCs only came into major use in the early part of the 20thC. Before plastics were invented they were of lead, slate, engineering brick, bitumen, zinc etc. and in older buildings these may have corroded or been breached and be in need of repair.
around window and door openings to prevent the frames coming in contact with the damp outer leaf of masonry.
at the head of an exposed wall. Parapets must prevent water getting to the insulation and internal wall below.
Existing buildings may suffer from a lack of DPCDamp Proof Course - a strip of (usually) plastic built into walls to prevent damp rising or penetrating. see more if they are more than about a hundred years old (the 1875 Public Health act began to introduce them although it took nearly 25 years before they were universally used) or they may be breached (damaged). Using a damp meter on an internal wall is not an infallible way of diagnosing rising damp: walls may be damp because of rain penetration or because of condensation. Rising damp will normally be in a strip along the bottom of the wall and be within the lowest metre. It will probably show efflorescence along the topmost edge where salts from the ground have been deposited as the water dries out.
An isolated patch is more likely to be rain or condensation
There are numerous ways of treating rising damp and many companies who undertake it. The main thing is to get a written guarantee from them.
- inserting a physical barrier such as a polythene DPC or lead sheet. This can be an excellent method but can be very expensive and disruptive. It entails sawing out short sections (usually less than 1m at a time) of the wall with a diamond chain saw, inserting a length of DPC, filling any remaining gap with mortar and then moving on to the next section
- chemical treatment. This usually works quite well on plain brick walling but may not be effective in stone walls, particularly those which are thick, randomly coursed or have rubble infill because the chemical doesn’t reach the places it needs to.
- electro-osmosis. This is usually effective and relies on applying a very small electric current to electrodes fixed at regular intervals in the wall and connecting them to an anode in the ground. The science of how this works is complex but it effectively neutralizes the forces which help moisture to wick up a wall.
- clay tubes and damp proofing bricks. These are inserted at intervals in the wall and allow air to evaporate damp before it rises up the wall. There are basically two types of design -
tubes which go right through the wall (this relies on there being a vented space under the floor)
bricks such as the Holland type which allow air to circulate within the wall before being expelled.
DPMsdamp proof membrane - a sheet of (usually) plastic used to prevent dampness rising up through a floor or in through an underground wall. see more are usually large sheets of plastic which are laid under a concrete floor which is directly on the ground, to prevent moisture coming up through from the ground. However they may also be used on the walls of damp basedments to provide a barrier. See Basements
These are used to catch any water which might be dripping down the inside of a cavity and deflect it out of the wall through weep holes. This can for instance occur at lintels
They are also used in situations where what is an external wall above turns into an internal wall below such as when a single storey[for the purposes of part B (fire) of the Approved Documents to the Building Regulations] this means a. any gallery[for the purposes of part B of the Approved Documents] - A raised area or platform around the sides or at the back of a room which provides extra space. Habitable room A room used, or intended to be used, for dwellinghouse[for the purposes of part B of the Approved Documents] -
A unit of residential accommodation occupied (whether or not as a sole or main residence):
a. by a single person or by people living together as a family
b. by not more than six residents living together as a single household, including a household where care is provided for residents. (See also paragraphs 0.22 and 0.23.)
Dwellinghouse does not include a flat or a building containing a flat. purposes (including; for the purposes of Part B, a kitchen, but not a bathroom). if its area is more than half that of the space into which it projects; and b. a roof, unless it is accessible only for maintenance and repair. extension goes onto a two storey house.
They prevent dampness in the upper, exposed part of the wall from moving down into the part which is an internal wall in the extension. Of course the same would apply if it were all new build. This kind of detailing takes quite a bit of thinking through because of the changing courses of brickwork and how the flashings relate to the roof tiles or slates etc. There are some useful diagrams on the web site of Cavity Trays
Note also that the internal / external wall shown in brown is potentially a serious thermal bridge for heat to escape from the inner leaf of the extension inside the house to the cold outer area above it. It may need to be of insulating blockwork, especially where the change occurs.
The difference between cladding and rain screens is not large but it is important. Cladding tends be fixed directly to the face of a building whereas rain screens tend to have an air gap between themselves and the wall. With cladding (such as tiles fixed directly to blockwork) there is a chance that rain water might penetrate joints between the tiles and then get drawn back into the wall proper. With a rain screen, any water that might get driven by wind back behind the screen then drips out down the back of the screen to the ground.
The other difference is that a rain screen has an air gap which allows the face material to dry out quickly (important for wood) and this gap also allows moisture from the wall itself to dry out easily (important for a breathing walla bit of a misnomer. A better term is 'vapour open' because it is not air which is moving through the wall but water vapour. See more ). On a timber structure there is usually a vapour permeable membrane behind the battensNarrow strips of wood. Roof battens are narrow strips used to fix slates and tiles. Also used for tile and slate hanging on walls and rain screens so that if any rain should blow right through to the wall surface it will not get through into the wall. Rain screens can be quite thin and this is an advantage if the thickness of a wall is becoming a problem due to the large amount of insulation being used. Rain screens may have several advantages over a self finish or applied finish
- they are good at preventing driving rain entering a wall
- they may be useful for cladding external insulation
- they work well with a breathing wall
- they can even up an irregular finish (such as cob, straw bale, low quality brickwork/block-work etc.)
- they can form a protective finish against impact where there is no such surface behind they can give a decorative appearance they can cover external services such as wiring
Materials used for rain screens on housing include
- timber (note that there are some quite complicated rules in the Building Regulations about the proximity of combustible external wall materials to neighbouring buildings).
- vertical board on board, board on batten, batten on board
- horizontal shiplap
- shakes and shingles
- fibre cement sheets
The best fixings for rain screens are of stainless steel. Although slightly dearer they don’t cause staining and allow for easy removal of the screen for alteration, repair or reuse.
An excellent set of articles about the use of timber cladding in Scotland is on the Scottish government web site. They are particularly good because they address the subject of timber cladding in an area with the harshest climates in the UK and also because they draw on Norwegian timber cladding experience. See Timber Cladding in Scotland
There is a “Desktop study for benchmarking experimental cladding designs” done by the BREBuilding Research Establishment. for the Forestry Commission, which looks at the potential for using home grown Sitka spruce for cladding. This document also includes some of the best detailing for horizontal boarding
Oak shingles are surprisingly little used in the UK. Below is an example on an eco extension to a former chapel at the Bridge of Lyon, Aberfeldy.
The original reason for cavity walls was to prevent rain driving in through joints in the masonry (not to give insulation – they give hardly any). Rain screensthis 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 screens can do this better. Metal wall ties in masonry walls can be a major problem in themselves; they can introduce a considerable amount of 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 between the leaves of cavity walls and conduct water from the outer leaf to the inner one if they are not installed properly. Alternatives are plastic ties and the recently developed basalt tie – the Ancon TeploTie
The reason for cladding houses in a brick outer skin have more to do with a failure of modern design to take hold than with anything to do with structure: a retreat to some kind of ‘traditionalism’ which is costly in terms of space and energy.
Unless you are building in some situation where space limits are not a problem (possibly a rural area) or costs are not an issue then the above adds up to quite a strong argument in favour of making as much of the wall thickness as possible out of insulation. This in turn suggests using timber frame systems such as SIPS or post and beamSubstantial, usually horizontal structural member. construction for the external walls.
Fire resistance of rain screens
External wall renders (or harling as it is called in Scotland) has traditionally been of a sand cement mixture. Small size aggregategravel, crushed stone and other coarse material used in concrete or as hardcore may have been added by throwing it at the render while still wet (as with roughcast, Canterbury spa and a host of others).
The purpose of these renders has been threefold
- to provide a coating which was impermeable to driving rain
- to provide a visual contrast with other areas of the wall surface such as brickwork
- to improve the appearance of areas of cheap or poor walling such as low quality brickwork or blockwork
they have often suffered from two failings
- the mixture of the material was too strong in cement: render needs to have a weak mixture
- the base wall structure would tend to move very slightly, often due to changing temperatures and this would cause the render to craze and let in water
Modern renders are of a much higher quality and incorporate reinforcement (usually a glass fibre matting) see for instance the range of renders by Sto
Internal walls and linings
There are several ways in which internal walls impact on environmental issues -
- the sustainability of the materials – see Building materials and life cycle analysis / Plasterboard.
- risk of reducing air tightness
- the embodied energy of the materials
- indoor pollution – see Avoiding pollution
- their permanence or adaptability – see Flexible design
- their thermal mass
Other factors to consider are
- fire resistance (if it is a separating wall[for the purposes of part B (fire) of the Approved Documents] - A wall or part of a wall which is common to adjoining buildings, and constructed to meet the requirements of regulation B3(2) [for the purposes of part C (sound) of the Approved Documents] - Wall that separates adjoining dwelling-houses, flats or rooms for residential purposes.) and spread of flame
- sound resistance
- running of services
One of the deciding factors with internal wall construction is whether, and to what extent the wall needs to be load bearing. The qualities of high thermal mass and acoustic insulation tend to work together with heavyweight materials. Of the traditional methods for internal wall building, both concrete block and brick rate quite poorly in terms of embodied energy, sustainability and adaptability though they both score well on fire, thermal mass and sound reduction.
Plasterboard on stud is quite good now that old plasterboard is becoming recyclable and plasterboard walls are easy to alter and remove, providing the floor level is consistent between rooms. Plasterboard is low in thermal mass (and not so good for sound insulation unless it is filled with acoustic mineral fibre). BASF has been developing gypsum based wall boards which incorporate microcapsules of eutectic wax substances (PCMs) which increase the effective thermal mass very considerably. However they do not seem to be on the market in the UK yet. Plasterboard can achieve high fire resistance if extra layers are added.
With stud walling there may sometimes be a risk of compromising air tightness, especially with timber frame construction. This can occur if the joints with the adjoining external walls, floor and ceiling[for the purposes of part B of the Approved Documents] - A part of a building which encloses and is exposed overhead in a room, protected shaft or circulation space. (The soffit of a rooflight is included as part of the surface of the ceiling, but not the frame. An upstand below a rooflight would be considered as a wall.) are not properly sealed and the joint also hides a break in the external construction thus allowing cold air to move straight into a ‘cooling fin’ within the house.
Alternatives to plasterboard
There have been several fairly recent developments in dry lining boards for internal walls. Traditional plasterboard is being challenged by other variations of gypsum based boards and also by clay based lining boards. Lining boards usually have to have a degree of fire resistance both structurally and with regard to the spread of flame (Building Regulations: Linings and Structure) and this limits the materials that can be used.
The problems with plasterboard have been to do with mining and processing the gypsum and also with the disposal of plasterboard because in normal land fill situations the calcium sulphate reacts with various acids also present in the land fill to produce sulphuric acid. Because of this, plasterboard must now be disposed of in special ‘toxic box’ sites designated by the local authority.
An added factor driving the change is that plasterboard has so little strength before it is fixed and damaged boards tend to litter many a building site so some of the newer boards tend to be less brittle and also tend to have a better finish. Most of the newer boards come from northern Europe and are still being imported which adds to the embodied energy and cost.
Sasmox is 85% Gypsum and 15% wood fibre which gives it more strength and better fixing properties. It has its own proprietary joint filler and does not need plastering. It comes from Finland.
Heraklith wood wool is a mixture of wood strands and cement which behaves well in fire and is also acoustically insulating. Made in Germany.
Fermacell is a gypsum/celulose lining board made to a high ecological standard in Germany. To quote their literature – “Essentially, Fermacell has a very simple homogenous composition: 80% gypsum (recycled) and 20% cellulose fibres derived from recycled papers, mixed with water. In other words, it’s a 100% recycled material. There are no additives or preservatives used whatsoever. The entire process is also fully recycling, which means that all by-products are fed back into the production cycle.”
Claytec manufacture a clay based lining board
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.
see also Vapour barriers