Strength and stability

The strength of timber varies with species and is generally greater with dense hardwoods than less dense softwoods. Strength is also affected by defects in timber such as knots, shakes, wane and slope of the grain of the wood. The strength and stability of a structural timber frame depends on the combination of elements, such as columns and beams and the rigidity of the connections. Simple calculations will help to establish the size of columns and beams. Connections tend to follow a tradition of using a combination of nails, screws and bolts. Factory produced assemblies will be joined by mechanical connections.

Durability and freedom from maintenance

The durability of timber is determined by careful specification and selection of seasoned timber together with careful detailing to avoid the possibility of rot. As a general rule, hardwood timbers are more durable than softwood timbers. Softwood will usually require some form of surface protection to increase its durability, such as paint, stain or impregnation with a preservative. Some hardwoods, such as oak, can be left to weather naturally without surface protection. Careful detailing can also help to avoid the possibility of wet or dry rot, as discussed later.

Fire safety

There is a misconception that timber framed buildings are not safe in a fire; this is incorrect. Structural fire safety is provided by passive protection, relating to the sizing, spacing and protection of timber. The reaction of timber in a fire has been extensively tested over the years, thus the behaviour is well known and buildings can be designed accordingly. Timber will char when exposed to flame, forming a protective layer and helping to preserve structural integrity. Large section timber beams and columns may be left without additional protection; however, the smaller section timber members of low rise timber framed houses will need to be protected. As a general rule, 30 minutes of fire protection can be achieved from components made of solid timber and glulam, by protecting it with a 12.5 mm (or 15 mm) thick layer of gypsum plasterboard. Additional fire resistance; for example, 60 minutes, can be achieved by using 25 mm thick gypsum plasterboard. For exposed timber structures, the fire resistance will depend on the charring characteristics of the timber components and the protection of fixings and joints. Exposed timber frames can be designed to incorporate sacrificial timber so that the timber exposed to flame will char and protect the inner, loadbearing material, from damage. The biggest threat to timber frame construction occurs during construction and refurbishment work, when unprotected elements are more susceptible to fire.

Properties of timber

Up to two‐thirds of the weight of growing wood is due to water in the cells of the wood. When the tree is felled and the wood is cut into timber, this water begins to evaporate, and the wood gradually shrinks as water leaves the cell walls. As the shrinkage in timber is not uniform, the timber may lose shape and it is said to warp. It is essential that before timber is used in buildings, either it should be stacked for a sufficient time in the open air to allow most of the water in it to dry out or it should be artificially dried in a kiln. If unseasoned or wet timber is used in building, it will dry out and shrink, causing twisting of doors and windows and cracking at joints with plastered walls. This means that only seasoned timber should be used and that the timber should be protected from moisture on the building site, when stored and as work proceeds. The process of allowing, or causing, newly cut wood to dry out is called seasoning, and timber that is ready for use in building is said to have been properly seasoned.

Conversion of wood into timber

The method of converting wood to timber affects the timber in two ways: i) by the change of shape of the timber during seasoning; and ii) in the texture and differences in colour on the surface of the wood. Because the spring wood is less dense than the summer wood, the shrinkage caused when the wood is seasoned (dried) occurs mainly along the line of the annual rings. Circumferential shrinkage is greater than the radial shrinkage. Because of this, the shrinkage of one piece of timber cut from a log may be quite different from that cut from another part of the log. This can be illustrated by showing what happens to the planks of a log converted by the ‘through and through’ cut method shown in Figure 5.1. When the planks have been thoroughly seasoned, their deformation due to shrinkage can be compared by putting them together in the order in which they were cut from the log, as in Figure 5.1.

If the face of a timber is cut on a radius of the circle of the log, the cells of the medullary rays may be exposed where the cut is made. With many woods, this produces very pleasing texture and colour on the surface of the wood and it is said that the ‘figure’ of the wood has been exposed (Figure 5.1). Radial cutting of boards as shown is very expensive and is employed only for high‐class cabinet‐making and panelling timbers where the wood will be used decoratively.

Surface finishes for timber

Timber can be left unfinished or a protective decorative finish can be applied to the surface. Unfinished timber is sometimes used on buildings for aesthetic effect and to reduce the environmental impact and maintenance costs of applying finishes to the timber at regular intervals. Care is required in the selection and detailing of suitable timbers for a given location and degree of exposure. There are three types of applied surface finish for wood: paint, varnish and stains (see also Chapter 10). Paint and varnish are protective and decorative finishes that afford some protection against water and provide a decorative finish that can easily be cleaned. Paints are opaque and hide the surfaces of the wood, whereas varnishes are sufficiently transparent for the texture and grain of the wood to show. There is a wide range of stains available, from those that leave a definite film on the surface to those that penetrate the surface. Stains range from gloss through semi‐gloss to matt finish. The purpose of this finish is to give a selected uniform colour to wood without masking the grain and texture of the wood. Most stains contain a preservative to inhibit fungal surface growth. These stains are most effective on rough sawn timbers.

Decay in timber

Decay in timber is caused by fungal decay and insect infestation, both of which can be prevented through careful detailing and specification as well as through careful work both off and on the construction site. Insect attack will depend upon location, moisture content and temperature. Any one of a number of wood‐destroying fungi may attack timber that is persistently wet and has a moisture content of over 20%. Fungal decay is classified as being ‘dry’ or ‘wet’ rot.

Modified wood products (MWPs)

It is possible to modify the properties of timber through impregnation, chemical treatment and/or heat treatment. This allows cheaper, less durable timbers to be used and modified into highly durable products that are less sensitive to moisture, UV, dimensional change, fungal and insect attack. The timbers most used include pine, poplar, eucalyptus and other fast growing varieties. Modified timber products may be used for flooring in wet areas and externally for cladding buildings. There are many process and products on the market, but the main approaches are:

• Impregnation: is achieved by using a liquid catalyst and vacuum pressure impregnation, with a drying temperature above 100 °C.
• Thermal modification: involves heating the timber to temperatures over 200 °C in an oxygen‐free environment. The lack of oxygen prevents the wood from burning. The heat treatment causes a chemical reaction within the timber, which ‘bakes’ the sugars and tannins in the wood. This makes the wood darker and harder and also makes it inedible to microbes and insects. This tends to result in a better performing product compared to pressure‐treated timber.
• Chemical modification: involves a variety of processes, the most well known being a process called acetylation. The wood is impregnated with acetic anhydride and reacts at high temperature. The process turns free hydroxyls, which absorb and release moisture and which are naturally present in the wood, into acetyl groups. This reduces the timber’s ability to absorb water, which makes it more dimensionally stable and very durable. Silicon‐based compounds are also used to similar effect.

Wood structural panels

Wood structural panels are precision engineered panels, manufactured under factory conditions. Wood structural panels comprise chipboard, MDF, plywood and orientated strand board (OSB). Plywood and OSB have similar characteristics in terms of strength and stiffness, but their composition is different. As a general rule, all of these products need to be protected from moisture during construction, and also after construction, to prevent expansion and damage from moisture/water ingress. Wood structural panels must be correctly installed and kept dry and ventilated. As a general rule, they should not be used in areas of high humidity.

Structurally insulated panels (SIPS)

Structurally insulated panels (SIPS) have been developed around the concept of composite (or sandwich) panels. The SIPS are composed of two faces, usually OSB/3, that ‘sandwich’ a rigid core of expanded polystyrene (EPS) or polyurethane (PU). This forms a lightweight panel that is easy to erect on site. The panels offer very good thermal insulation and airtightness in addition to inherent structural properties. SIPS offer a simple, dry form of construction that include the insulation and structure in one panel; rather then building the structure then adding the insulation. Panels tend to be around 140 to 175 mm thick and are manufactured in standard widths from 200 mm to 1200 mm, and lengths of up to 7.5 m. Bespoke widths can be made to order. The panels are used in floor, wall and roof construction, usually in conjunction with timber I beams and joists. A variety of proprietary products are available, some of which are designed specifically as cladding panels. SIPS may be used to achieve Passivhaus standards.

Cross‐laminated timber (CLT) panels

Cross‐laminated timber (CLT) panels are engineered timber products using kiln dried small sections of timber. Panels are manufactured in a similar way to glue‐laminated timber beams in a quality controlled factory environment. Small sections of timber are bonded together in layers with permanent adhesives to form large panels. Although the glues tend to have a high environmental impact, the glue content is very low, accounting for around 0.6% of the panel. Layers of timber (lamellas) are bonded perpendicularly to one another (longitudinal and transverse layers), hence the term cross‐laminated (or ‘crosslam’ or ‘X‐LAM’). The cross‐lamination gives the panel strength in two directions (dimensions) and helps to ensure the product retains its dimensional integrity when subjected to changing levels of humidity. Panels comprise a minimum of three layers of timber, rising to a maximum of seven layers, depending on the desired structural properties and overall thickness. The timber most commonly used is spruce, but other softwoods such as larch and some of the fir family are also used. The majority of the timber is kiln‐dried C24, with a small amount of C16 grade timber. The moisture content should be 12% (plus or minus 2%). Panel depth depends upon the number of layers used, ranging from 60 mm to around 340 mm.

Panels are manufactured in a range of sizes from 1.2 m to 3.5 m wide, typically 2.4 m wide for floor panels and 2.95 m for wall panels. Length may be up to 22 m, but more usually around 13 m to facilitate ease of transportation. The CLT panels are transported to site, craned into position and secured using screws and/or brackets. Panels should be kept dry on site, and if they do get wet (e.g. from a rain shower), they should be allowed to dry out prior to fixing and/or covering with another material. The size of the panels tends to be restricted by transportation to the site and access restrictions. This provides the potential for fast, dry onsite assembly.

Three grades of surface finish (visual quality) are available, depending on building function, whether or not the panels will be covered or left fair face and the desired aesthetic. The quality classifications are:

1. Visible quality AB. Panels comprise a mixture of A and B quality lamellas that are planed and sanded on one side to give a visual quality suitable for residential buildings, offices and schools. The inherent finish can be left as supplied or a decorative finish (wash or varnish) may be applied.
2. Visible quality BC. One side of the panel is planed and sanded. Typically used in industrial and commercial buildings where the visible quality may not be a major concern.
3. Non‐visible quality C. This is the lowest visual quality and therefore the panels are commonly used in situations where they will be covered by, for example, a roof covering, plasterboard or render.

CLT panels are vapour permeable, which means that they can be used as part of a ‘breathing wall’ assemblage. They have a low environmental impact (carbon is stored within the product during its service life) and when the raw material is sourced from ecological, economical and socially responsible forests, they make a positive contribution to a green supply chain. The thermal conductivity is around 0.13 W/mK, which is similar to a lightweight concrete block. CLT has very good fire resistance, retaining its loadbearing capacity in a fire. The surface will char when exposed to flame and hence protect the inner core. The product can also positively contribute to acoustic insulation and airtightness, subject to appropriate detailing and fixing.

CLT panels are used in the construction of floors, walls and roofs. The smooth finish of the panels provides an inherent surface finish that does not require further treatment (unless required, such as a coloured wash). The panels are easy to fix into for first and second fix, using self‐tapping wood screws. CLT panels are used in the construction of high‐rise timber framed buildings, because of their strength and light weight.

All CLT panels must be kept dry during storage on site and during installation. They must be positioned above the level of the damp‐proof course (dpc) or damp proof membrane (dpm). CLT panels will require some form of protection, such as external cladding or render, if used for the external leaf of a wall. However, the panels are more commonly used for the inner leaf of walls and for internal partition walls. When used to form a floor, it is possible to achieve clear spans of around 8 m. Given the structural properties of CLT panels, it is possible to construct comparatively slim floors compared to more traditional approaches.

Engineered timber beams

Engineered timber beams are usually provided as an I section. The horizontal sections of the I are called flanges and the vertical element is the web. They are manufactured using wood composites and therefore can utilize young, easy to cut, sustainably sourced trees. Compared to traditional timber beams and joists, engineered timber beams are much lighter and use about one‐sixth of the material for the same structural strength. They are also dimensionally more precise and stable, as they do not dry and shrink as much as solid timber once the building is occupied. Engineered timber beams are used for floor joists (I‐joists) to the longer spanning I‐beams (e.g. for roof structures). They are also becoming common components of a range of prefabricated timber assemblies, such as wall, floor and roof panels. Manufacturers provide typical examples of spans and some provide interactive clear span tables for typical applications, such as floors.

Engineered bamboo products

Bamboo is a member of the grass family. As a fast growing and sustainable material, it offers benefits over slower growing timber. Bamboo exhibits similar characteristics to timber, and has long been used in construction; especially in countries where the product is abundant. The bamboo canes are referred to as culms. In its natural state, the irregularities in the bamboo culm make it challenging to connect individual sections for building. Engineered products offer greater potential for construction. The bamboo culm is processed in a factory to form a laminate composite. The culm is split, planed, bleached, caramelized and then glued and pressed to form a laminated sheet product. Engineered bamboo appears to have comparable structural properties to timber. Research and development is ongoing.

Stability of timber framed buildings

The stability of a timber framed wall depends on a sound foundation on which a stable structure can be constructed. Figure 5.2 is an illustration of the base of a timber framed wall set on a brick upstand raised from a strip foundation. The 150 × 50 mm timber sole plate is bedded on a horizontal dpc; shot fired nail with sufficient anchorage is used to locate the sole plate, in exposed positions 13 mm bolts at 2 m centres built into the foundation to anchor the plate against wind uplift. As an alternative, the bolts may be shaped so that the bottom flange is built into the wall, run up on the inside face of the wall with a top flange turned over the top of the plate. Angle brackets may also be used to secure the sole plate.

The upstand kerb of a concrete raft foundation serves as a base for the timber wall, with the anchor bolts set into the concrete kerbs and turned over the top of the sole plate. The vertical 150 × 50 mm studs are nailed to the sole plate at 400–600 mm centres with double studs at angles to facilitate fixing finishes (illustrated in Figure 5.3). Photograph 5.2 shows the sole plate, a packing piece of timber and the timber stud panel fixed to the floor.

A timber stud wall consists of small section timbers fixed vertically between horizontal timber head and sole plates, as illustrated in Figure 5.4 and Photograph 5.3. The vertical stud members are usually spaced at centres of 400–600 mm to support the anticipated loads and to provide fixing for external and internal linings. The horizontal noggins fixed between studs are used to stiffen the studs against movement that might otherwise cause finishes to crack. A face of plywood sheeting is often applied to both sides of the insulated stud panel to provide considerable lateral stability.

A timber stud wall has poor structural stability along its length because of the non‐rigid nailed connection of the studs to the head and sole plate, which will not strongly resist racking deformation. It must be braced (stiffened) against racking. As an internal wall or partition, a timber stud frame may be braced by diagonal timbers or by being wedged between solid brick or block walls. As an external wall, a timber stud frame may be braced between division walls and braced at angles where one wall butts to another, as illustrated in Figure 5.4. Diagonally fixed boarding or plywood sheathing fixed externally as a background for finishes, braces an external stud frame wall.

Because of its small mass, a timber frame wall has poor lateral stability against forces such as wind, which tend to overturn the wall. For stability along the length of the wall, connected external and internal walls or partitions will serve as buttresses. For stability up the height of the wall, timber upper floors and the roof connected to the wall will serve as buttresses. Buttressing to timber walls that run parallel to the span of floor joists and roof frames is provided by steel straps that are fixed across floor joists and roof rafters and fixed to timber walls in the same way that straps are used to buttress solid walls as previously described.

The usual method of supporting and fixing the upper floor joists to the timber wall frame is by using separate room height wall frames. The heads of the ground floor frames provide support for the floor joists on top of which the upper floor wall frame is fixed, as illustrated in Figure 5.5. The roof rafters are notched and fixed to the head of the upper floor wall frame. As an alternative, a system of storey height wall frames may be used with the top of the head of the lower frame in line with the top of the floor joists that are supported by a timber plate nailed to the studs, as illustrated in Figure 5.6. The upper frame is formed on the lower frame. The advantage of this system is that there is continuity of the wall frame and the disadvantage is that there is a less secure connection.

Figure 5.7 is an illustration of a two‐storey house with timber walls, floor and roof with a brick outer leaf, with the insulation filling the cavity rather than contained within the timber stud panels. It is rare to see timber frames constructed in this manner, but there is no reason why it cannot be done. Most timber frame panels are prefabricated, therefore site assembly is much quicker if the units come with insulation installed.

Timber or tile clad timber frame

The traditional weather envelope for timber walls is timber weatherboarding nailed horizontally across the stud frame. The weatherboards are shaped to overlap to shed water. Some typical sections of boarding are illustrated in Figure 5.8. The wedge section, feather edge boarding, is either fixed to a simple overlap or rebated to lie flat against the studs as illustrated. The shaped chamfered and rebated and tongued and grooved shiplap boarding is used for appearance, particularly when the boarding is to be painted. To minimise the possibility of boards twisting, it is usual practice to use boards of narrow widths, such as 100 and 150 mm.

As protection against rain and wind penetrating the weatherboarding, it is usual to fix sheets of breather paper behind the weatherboarding. Breather paper serves to act as a barrier to water and at the same time allows the release of moisture vapour under pressure to move through the sheet. Instead of nailing weatherboarding directly to the studs of the wall frame, it is usual to fix either diagonally fixed boarding or sheets of plywood across the external faces of the stud frame. The boarding and ply sheets serve as a brace to the frame and as a sheath to seal the frame against weather. Figure 5.9 is an illustration of weatherboarding fixed to plywood sheathing with insulation fixed between studs.

Around openings to windows and doors, the weatherboarding and ply sheath may be butted to the back of window and door frames fixed to project beyond the stud frames for the purpose. At the head of the opening, the head of the frame may be reduced in depth so that the boarding runs down over the face of the frame. The weatherboarding butts up to the underside of a projecting cill. For extra protection, sheet lead may be fixed behind the weatherboarding and nailed and welted to window and door frames. At external angles, the weatherboarding may be mitred or finished square edged. An effective weathering is to fix a strip of lead behind the weatherboarding to form a sort of secret gutter.

Tile, slate hanging or rendering may be used to provide more durable protection, especially in exposed positions. In the UK, it has been common practice in speculative house construction to provide the weather protection with a brick outer leaf, which provides protection to the timber in most situations. A sensible argument for this form of construction could be speed of erection and completion of building work by combining the rapid framing of a timber wall, floor and roof structure that could be completed and covered in a matter of a few days, with a brick outer leaf and speedy installation of electrical, water and heating services and dry linings.

Photographs show full‐scale models of timber frame construction. The details (Figures) show the individual elements of these units. Although the timber frame provides the structural unit, the cladding can be any material that will provide the required protection from the elements and necessary security. As with all construction, attention needs to be paid to the control of airtightness. Vapour control and airtightness membranes should be lapped and taped to ensure airtightness at all points of the structure (Figure 5.11).

Internal plasterboard linings to the timber framed walls, the soffit of the first floor and the ceiling will provide a sufficient period of fire resistance to meet the requirements for a two‐floor house. The requirement for barriers in external cavity walls to small houses applies only to the junction of a cavity and a wall separating buildings.

Prefabricated timber frames

There are three types of prefabricated timber frame systems that can be used: stick build, platform frame and balloon frame. Choice is often determined by the desired amount of work to be carried out on the building site, which has to be balanced against the cost of the timber frame. For example, some wall panels are delivered to site complete with sheathing and insulation, whereas other manufacturers provide the frame only, with the sheathing and insulation to be fixed on site.

Platform frame

The advantage of using frames that are fabricated either on or off site complete with outer and inner finishes is speed of erection. Where a number of houses are to be built, it is possible to complete a building in a matter of days. The systems most used are either platform or storey frames. The term ‘platform’ frame equally applies to light steel frame house construction made from cold‐formed steel sections.

The platform frame system of construction employs prefabrication frames that are floor to ceiling level high, with the sole of the lower stud frame bearing on the foundation and the head of the frame supporting first floor joists, as illustrated in Figure 5.13. The first floor can then be used as a working platform from which the upper frames are set on top of the lower. The wall frames or panels may be the full width of the front and rear walls of narrow terrace houses or made in two or more panels. The first floor joists and roof provide sufficient bracing up the height and the separating wall will brace across the width of the wall. The wall frames may be prefabricated as stud frames sheathed with plywood or made complete with finishes on both sides.

Platform frame is currently the most common form of timber frame construction. The panel sizes will fit easily onto standard road transport and can be easily lifted into position. Photographs show platform construction used to assemble a multi‐storey timber frame building.

Structurally insulated panels are becoming more common and can be designed to achieve passive standards (see Photographs 5.14a–c).

Balloon frames

Storey frames (‘balloon frames’) are made the height of a storey, floor to floor, so that the top of the head of a frame is level with the top of the floor joists. A bearer fixed to the stud frame supports the joists. This arrangement provides continuity of the stud framing up the height of the wall at the expense of some loss of secure anchor of floor joists to wall. A balloon wall frame is fabricated as one continuous panel, the height of the two floors of small houses, as illustrated in Figure 5.14. The advantage of the balloon frame is speed of fabrication and erection, and the least number of joints between frames that have to be covered and weathered externally. The term ‘balloon frames’ also applies to steel frame housing (light steel, cold‐formed sections).

Functional requirements specific to timber framed buildings

Fire safety

Specifying a minimum period of fire resistance for the elements of structure restricts the premature failure of the structural stability of a building. A timber framed wall covered with plasterboard internally satisfies the requirement for houses of up to two storeys. To prevent the spread of fire between buildings, limits to the size of ‘unprotected areas’ of walls and finishes to roofs close to boundaries are set out in the Building Regulations. By reference to the boundaries of the site, the control will limit the spread of fire. Unprotected areas are those parts of external walls that may contribute to spread of fire and include glazed windows, doors and those parts of a wall that may have less than a notional fire resistance. Limits are set on the use of roof coverings that will not provide adequate protection against the spread of fire across their surface to adjacent buildings.

The passage of fire through connecting voids in timber frame construction is as significant as it is in masonry (loadbearing) construction. The connecting external and party wall cavity barriers must be sealed to prevent the passage of smoke and fire. The addition of edge seals reduces the passage of heat and improves airtightness.

Figure 5.15 shows a tee barrier used to improve the thermal insulating properties of the cavity, reduce the passage of sound and prevent the passage of fire within a specified period.

Resistance to the passage of heat

Timber is a comparatively good insulator. However, the sections of a timber frame do not by themselves generally afford sufficient insulation and a layer of some insulating material has to be incorporated in the construction. The layer of insulation is fixed either between the vertical studs of the frame or on the outside or inside of the framing. The disadvantage of fixing the insulation between the studs is that there may be a great deal of wasteful cutting of insulation boards to fit them between studs; and to the extent that the U‐value of the timber stud is less than that of the insulation material, there will be a small degree of thermal bridge across the studs. The advantage of fixing the insulation across the outer face of the timber frame is simplicity in fixing and the least amount of cutting. Also, the void space between the studs will augment insulation and provide space in which to conceal service pipes and cables. The disadvantage of external insulation is that the weathering finish such as weatherboarding has to be fixed to vertical battens screwed or nailed through the insulation to the studs. Unless the insulation is one of the rigid boards, it may be difficult to make a fixing for battens sufficiently firm to nail the battens to. Internal insulation is usually in the form of one of the insulation boards that combine insulation with a plasterboard finish.

Inorganic materials are most used for insulation between the studs, because there is no advantage in using the more expensive organic materials. Rolls of loosely felted fibres or compressed semi‐rigid batts or slabs of glass fibre or rockwool are used. The material in the form of rolls is hung between the studs, where it is suspended by top fixing and a loose friction fit between studs, which generally maintain the insulating material in position for the comparatively small floor heights of domestic buildings. The friction fit of semi‐rigid slabs or batts between studs is generally sufficient to maintain them, close butted, in position. For insulating lining to the outside face of studs, one of the organic insulants such as XPS or PIR provides the advantage of least thickness of insulating material for a given resistance to the transfer of heat. The more expensive organic insulants, in the form of boards, are fixed across the face of studs for ease of fixing and to save wasteful cutting. A vapour check should be fixed on to, or next to, the warm inside faces of insulants against penetration of moisture vapour. Organic insulants, such as XPS, which are substantially impervious to moisture vapour, can serve as a vapour check, particularly when rebated edge boards are used and the boards are close butted together.

Vapour check

A high level of insulation required for walls may well encourage moisture vapour held by warm inside air, particularly in bathrooms and kitchens, to find its way due to moisture vapour pressure into a timber framed wall and condense to water on the cold side of the insulation. Condensate may damage the timber frame so as a barrier to warm moist air, there should be some form of vapour check fixed on the warm side of the insulation. Closed cell insulating materials such as XPS, in the form of rigid boards, are impermeable to moisture vapour and will act as a vapour check. The boards should either be closely butted together or supplied with rebated or tongued and grooved edges so that they fit tightly and serve as an efficient vapour check.

Where insulation materials that are pervious to moisture vapour, such as mineral fibre, are used for insulation between studs, a vapour check of polythene sheet must be fixed across the warm side of the insulation. The polythene sheet should be lapped at joints and continued up to unite with any vapour check in the roof and should, as far as practical, not be punctured by service pipes.

The vapour check is now also being incorporated as an airtightness control membrane (see Photographs ). To ensure airtightness and vapour control, timber framed buildings are now externally wrapped and sealed internally.

To prevent overheating of electrical cables that run through insulation, the cables should be de‐rated by a factor of 0.75 by using larger cables than specified, which will generate less heat. So that cables are not run through insulation, it is wise to fix the inside dry lining to timber frames that are filled with insulation onto timber battens nailed across the frame, so that there is a void space in which cables can be safely run.

Resistance to the passage of sound

The small mass of a timber framed wall affords little resistance to airborne sound, but does not readily conduct impact sound. The thermal insulation necessary for the conservation of heat will give some reduction in airborne sound and the use of a brick or block outer leaf will appreciably reduce the intrusion of airborne sound from outside the property. Where sound pollution is a problem, acoustic plasterboard may be used to reduce sound transmission. The acoustic plasterboard is designed to reduce sound transmission and is fixed to studwork in a similar manner to plasterboard. To ensure that the acoustic board is effective, the joints should be staggered and sealed, holes should not be cut into the board and services should not pass through the board.