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Wall construction

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What’s the most eco-friendly wall?

Well there are several angles on that. Some of the most important aspects of walls are their insulation values, their air-tightness, the embodied energy and the sustainability of the materials used. Thermal mass may be a factor.

Obviously walls have lots of other functions such as

  • structural, such as supporting themselves, floors and the roof    see more
    Most of this is taken care of in the Building Regulations, especially Part A. Particularly with masonry construction, the Approved Documents are a good guideline for design. With timber frame, steel frame and SIPS, a structural engineer will probably need to be involved
  • keeping out the weather     see more
    Much of the traditional design in the UK has been focused on keeping out driving rain, particularly on the West coast and particularly in Scotland. Driving rain can force itself through joints in the mortar of masonry walls and affect the internal surfaces of walls. This was the reason for the advent of cavity walls (nothing to do with insulation – cavity walls do almost nothing to increase the insulation of a wall unless the cavity is filled with an insulating material such as mineral fibre. And then it is no longer really a cavity wall)
  • keeping out intruders
  • keeping out vermin        see more
    This is sometimes more serious than it sounds. In the UK we are generally used to masonry walls with very few penetrations. Air bricks are designed so that mice cannot get through. (The rule of thumb is that if you can poke a pencil through a hole then a small mouse could get through, and grow and breed).

    With timber frame housing, there is the possibility that rodents might find a way in and steadily chew their way round the structure. Particularly in autumn, rats, mice and grey squirrels seek warm quarters for over-wintering and they can steadily gnaw their way past timber, wood fibre insulation and plasterboard. If they can smell food on an air current passing through the structure then this is an added incentive for them to spend long hours chewing away. Not only do they do damage to the structure, they will get in if possible to get food and at the same time create channels for drafts. Rodents spend a lot of time gnawing in order to keep their teeth in good condition.They may destroy polythene vapour barriers (which can then cause condensation and damp) and may gnaw through plastic plumbing pipes if they encounter them. With plastic plumbing they tend to stop chewing when they encounter water. This tends to create a very small leak which can be an enormous problem if it goes un-noticed because it will rot timber and possible allow dry rot to develop.

    rodent damage

    teeth marks of a rat chewing through a push fit plumbing pipe

  • sound insulation        see more
    Party walls are covered in the Building Regulations, Part E and with traditional masonry construction the sound insulation is usually achieved by the density of the wall (and making sure that there are no weak points such as perpends which have not been properly filled with mortar.

    However, with timber construction, party walls can still achieve the required degree of sound insulation mainly by isolating elements rather than adding mass.

  • visual privacy
  • hanging things on walls     see more
    Fixing things to walls (such as shelves, pictures, furniture etc.) depends on the structure of the wall. Solid masonry walls can usually carry huge loads if the correct fixings are used and it does not matter much where the fixings go (although things can occasionally become difficult if the fixing falls on a mortar joint.

    With timber frame walls it matters whether the fixing is going into timber frame studwork behind, or is simply going onto the plasterboard (or wood fibre layer). If it’s possible to get the fixings to coincide with the studwork then it works well using long screws but if the fixing position falls between the studs then some kind of expanding anchor has to be used and these tend to be expensive and fiddly.

  • services     see more
    Walls tend to carry a huge amount of cables and pipes. It is now quite common (in timber frame construction) to allow a 50 mm. service void behind the wall surface to cope with these service runs. See Service Ducts

And that is apart from the visual and cultural aspects.

However the push towards zero carbon house building, set by the EU to come into operation by 2021, will see levels of wall insulation and air tightness being increased to something like Passivhaus standard.  See pages on:

The main types of walls


Insulating walls – the best methods

The Building Regulations cover the minimum wall insulation required in The Approved Documents L1A (for new dwellings) and L1B (for existing ones)

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 delay

Existing buildings

For several reasons it usually makes sense to insulate on the outside, providing planning permission can be obtained and providing it does not cause too much disruption

  • Space is not lost within the dwelling
  • The 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

However if it is not possible to insulate on the outside then applying it to the inside is fine but it has two drawbacks

  • The size of the rooms will be reduced, possibly by something like 250 mm (if  Passivhaus standard is to be achieved)
  • Internal walls which connect with external ones will also need insulating on both sides for about a metre back from the external one. See below.

The above plan shows how an internal wall 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 wall insulation isolates internal walls from external ones but 50mm gives a very low level of insulation in the first place. Below is one way to avoid this problem by returning the insulation about a metre back up the internal wall.

insulation returned

Another way is to cut a vertical gap (of say 20 to 30 cm) between the internal and external walls where they meet. This will probably have structural implications as the walls support each other at the joint. There is a picture of such a junction at Under the Sun


Complications

External wall insulation is not straight forward. You may need planning permission to add insulation to the outside of an existing 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 Passivhaus 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 bridging. Avoiding condensation and allowing traditional walls to breathe are also crucial.

Another complication concerns vapour barriers. With many older buildings there can be disastrous results if vapour barriers are not incorporated properly. See Vapour Barriers.

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.

green advertising

green advertising

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.

Areas missing

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 Documents  Part C page 37

Thermal bypass

This is when air can blow through the insulation itself and carry heat away. It typically happens with fibre insulation which has no external seal such as building paper or sarking board up against it. It can also occur due to stack effect.

See also Vapour Barriers

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.


DPCs

DPCs 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 DPM 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.

DPCs need incorporating in the following places:

In walls

DPCs

around window and door openings to prevent the frames coming in contact with the damp outer leaf of masonry.

DPCs around openings

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 DPC 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 SmartDry type which allow air to circulate within the wall before being expelled.

Condensation can also be mistaken for rising damp and, particularly for older houses which have wall insulation added, an understanding of vapour barriers is required. See Vapour Barriers

DPMs

DPMs 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

Cavity trays   

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 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.


Rain screens

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 wall). On a timber structure there is usually a vapour permeable membrane behind the battens 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
  • slates
  • tiles
  • 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

P1000532

There is a “Desktop study for benchmarking experimental cladding designsdone by the BRE  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.

more information on this house at the Gaia Architects web site There’s a useful book on timber rain screens and cladding generally called External Timber Cladding

Were traditional masonry cavity walls ever a good idea?

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 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 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 beam construction for the external walls.

Fire resistance of rain screens

The type of rain screen material you can use may be limited by the fire resistance required by the wall. Especially if it is close to a boundary, the material may need to be non combustible in which case timber or plastic will almost certainly be ruled out. See External fire spread.

Renders

External wall renders (or harling as it is called in Scotland) has traditionally been of a sand cement mixture. Small size aggregate 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 three 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
  • impervious renders can prevent moisture from diffusing out of the wall and trap it inside. This can cause dampness on the internal surface, which may be confused with rising damp. See Vapour Barriers

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

Lime render

Historically lime was often used as a render and there are frequent cases of problems where areas of walls have been repaired with cement renders. This is a fairly specialist subject. See the SPAB page on the subject


Internal walls and linings

There are several ways in which internal walls impact on environmental issues –

Other factors to consider are

  • fire resistance (if it is a separating wall) 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 (see Wikipedia on gypsum recycling) and plasterboard walls are relatively 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 phase change (eutectic) wax substances (PCMs) which increase the effective thermal mass very considerably. See Knauf Comfortboard. 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 (see Air Tightness). This can occur if the joints with the adjoining external walls, floor and ceiling 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 unless it goes for recycling.

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. Another factor is thermal mass; the newer boards help maintain thermal stability.

Suppliers

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

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. There is an abridged version of the Approved Document specially for houses.

Urea formaldehyde insulation

There is a small section in the regulations concerning the use of urea formaldehyde insulation in cavity walls.

Resistance to the passage of sound

The degree of sound insulation required within and between houses and flats is covered in Part E of the Approved Documents.

Combustion appliances and fuel storage has a detailed section in the Approved Documents (part J) and this makes reference to several aspects of the design and construction of houses including:


see also Vapour barriers

 

5 comments to Wall construction

  • Jack

    I have a utility room which is a single corse brick wall, I get consesation on the bottom two rows of brick on the North wall. if I were to build in internal wall on this north wall not using brick as I would loss too much space, what other method would be the best way to build a wall using plasterboard

  • Dave

    Hi I’m building 2= 3 bedroom town house’s I’ve built the inner leaf on both at the same time both have floor joists and rsj’s in place, the stone for the outer skin is going to 12 weeks, I’m wondering if its possible to go all the way to roof high as long as all the floor joists/rsj’s and fixings are all in place, can you advise me on this many thanks

  • bobthebuilder

    Hi Alan,

    if you use a properly designed rain screen then the external walls should be OK. Any driving rain which might occasionally get through the rain screen should run down the inside surface of it and drain out to the outside so it will not wet the existing wall or new insulation. Air will circulate behind the rainscreen so this will allow any moisture in the wall to escape. See the section on rain screens above.

    Hope that helps

    Bob

  • Alan Chesterman

    Your guide page on walls and external insulation points to importance of avoiding condensation and allowing traditional walls to breathe.

    I have a 60s house with solid wall and plasterboard dry lining on wood battens. Sub floor is insulated under floor boards, attic space insulated above ceilings. Attic and roof spaces are both cold in winter and freely ventilated to space behind dry lining making internal walls cold. All the internal walls, other than where insulated in the odd place, are ventilated to the attic and sub floor.

    I want insulate solid wall externally, but unless cold air flow through dry lining space is reduced, and this space insulated, the benefits of ext insulation will be greatly reduced. However, I am very concerned that moisture coming in either from rain driven onto solid wall, or by condensation forming in solid wall could damage the joists and floor and the cost of repairs and disruption wiping out any energy savings many fold.

    If a rain screen / wood cladding with ventilation space were applied to solid wall, will this prevent water getting into solid wall, allowing ventilation behind dry lining to be reduced and or this space insulated e.g with blown fibre? What is your advice?

    Thanks, Alan Chesterman.

  • Dave Hart

    Hello, I have a mid 1960`s built timber frame bungalow which has a wood wool cement panel to the outside walls which has been screeded & pebble dashed, the wood wool cement panel is approx 30mm thick with a 15mm screed/pebble dash finish. The cavity between the inner plasterboard inside the property & the inside of the woodwool cement panel I estimate at 155mm or 6 inches. I am going to fill the cavity with sheep wool from the inside of the property by drilling hand size holes through the plasterboard & stuffing the cavity with sheep wool. Do you see any problems with this? Can you tell me what would be the thermal efficiency of my walls c/ Combining the 6 inches of sheep wool with the 30mm of woodwoll cement panel? I am finding it difficult to understand how thermal qualities are measured? Any assistance would be welcome to help me understand this? Dave Hart. Moorings,Shore road,Blairmore,Argyll,PA238TL. Tel;01369840849.

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