Section 5: Walls

5.1 This section gives guidance for four situations:

a. internal and external walls exposed to moisture from the ground (see paragraphs 5.4 to 5.6);

b. external walls exposed to precipitation from the outside, covering:

i. solid external walls (see paragraphs 5.8 to 5.11);

ii. cavity external walls (see paragraphs 5.12 to 5.15);

iii. framed external walls (see paragraph 5.17);

iv. cracking of walls (see paragraph 5.18);

v. impervious cladding systems (see paragraphs 5.19 to 5.28);

vi. the joint between window and door frames and external walls and door thresholds (see paragraphs 5.29 to 5.33);

c. the risk of interstitial condensation in any type of wall (see paragraphs 5.34 to 5.35);

d. the risk of surface condensation or mould growth on any type of wall (see paragraph 5.36).

A wall includes piers, columns and parapets. It also includes chimneys if they are attached to the building. It does not include windows, doors and similar openings, but does include the joint
between their frames and the wall. In the following, the term ‘precipitation’ includes the effects of spray blown from the sea or any other body of water adjacent to the building.

5.2 Walls should:

a. resist the passage of moisture from the ground to the inside of the building; and

b. not be damaged by moisture from the ground and not carry moisture from the ground to any part which would be damaged by it, and, if the wall is an external wall:

c. resist the penetration of precipitation to components of the structure that might be damaged by moisture; and

d. resist the penetration of precipitation to the inside of the building; and

e. be designed and constructed so that their structural and thermal performance are not adversely affected by interstitial condensation; and

f. not promote surface condensation or mould growth, given reasonable occupancy conditions.

5.3 Consideration should be given to whether provisions 5.2(a) and (d) need apply to a building used wholly for:

a. storing goods, provided that any persons who are habitually employed in the building are engaged only in taking in, caring for or taking out the goods; or

b. a purpose such that the provision would not serve to increase protection to the health or safety of any persons habitually employed in the building.

INTERNAL AND ExTERNAL WALLS (MOISTURE FROM THE GROUND)

5.4 Any internal or external wall will meet the requirement if a damp proof course is provided.

Technical solution

5.5 An internal or external wall will meet the requirement if it is built as follows (unless it is subject to groundwater pressure, in which case see the Alternative approach – paragraph 5.6):

a. damp-proof course of bituminous material, polyethylene, engineering bricks or slates in cement mortar or any other material that will prevent the passage of moisture. The damp proof course should be continuous with any damp-proof membrane in the floors; and

b. if the wall is an external wall, the damp-proof course should be at least 150mm above the level of the adjoining ground (see Diagram 8), unless the design is such that a part of the building will protect the wall; and

c. if the wall is an external cavity wall, (see Diagram 9a) the cavity should be taken down at least 225mm below the level of the lowest damp-proof course, or a damp-proof tray should be provided so as to prevent precipitation passing into the inner leaf (see Diagram 9b), with weep holes every 900mm to assist in the transfer of moisture through the external leaf. Where the damp-proof tray does not extend the full length of the exposed wall, i.e. above an opening, stop ends and at least two weep holes should be provided.

Diagram 8 Damp proof courses

Diagram 9 Protecting inner leaf

Alternative approach

5.6 The requirement can also be met by following the relevant recommendations of Clauses 4 and 5 of BS 8215:1991101. BS 8102:1990102 includes recommendations for walls subject to groundwater pressure including basement walls.

External walls (moisture from the outside)

5.7 As well as giving protection against moisture from the ground, an external wall should give protection against precipitation. This protection can be given by a solid wall of sufficient thickness (see paragraphs 5.8 to 5.11), or by a cavity wall (see paragraphs 5.12 to 5.18), or by an impervious or weather-resisting cladding (see paragraphs 5.19 to 5.28).

Solid external walls

5.8 Any solid wall will meet the requirement if it will hold moisture arising from rain and snow until it can be released in a dry period without penetrating to the inside of the building, or causing damage to the building. The wall thickness will depend on the type of brick and block and on the severity of wind-driven rain. A method of describing the exposure to wind-driven rain is given in BS 8104:1992; see also BS 5628-3:2001.

Technical solution

5.9 A solid external wall in conditions of very severe exposure should be protected by external impervious cladding, but in conditions of severe exposure may be built as follows:

a. brickwork or stonework at least 328mm thick, dense aggregate concrete blockwork at least 250mm thick, or lightweight aggregate or aerated autoclaved concrete blockwork at least 215mm thick; and

b. rendering: the exposed face of the bricks or blocks should be rendered or be given no less protection. Rendering should be in two coats with a total thickness of at least 20mm and should have a scraped or textured finish. The strength of the mortar should be compatible with the strength of the bricks or blocks. The joints, if the wall is to be rendered, should be raked out to a depth of at least 10mm. Further guidance is given in BS EN 998:2003. The rendering mix should be one part of cement, one part of lime and six parts of well graded sharp sand (nominal mix 1:1:6) unless the blocks are of dense concrete aggregate, in which case the mix may be 1:0.5:4. BS 5262:1991 includes recommendations for a wider range of mixes according to the severity of exposure and the type of brick or block. Premixed and proprietary renders should be used in accordance with the manufacturer's instructions;

Diagram 10 Protection of wall head from precipitation

c. protection should be provided where the top of walls, etc. would otherwise be unprotected (see Diagram 10). Unless the protection and joints will be a complete barrier to moisture, a damp-proof course should also be provided;

d. damp-proof courses, cavity trays and closers should be provided and designed to ensure that water drains outwards:

i. where the downward flow will be interrupted by an obstruction, such as some types of lintel; and

ii. under openings, unless there is a sill and the sill and its joints will form a complete barrier; and

iii. at abutments between walls and roofs.

5.10 Insulation. A solid external wall may be insulated on the inside or on the outside. Where it is on the inside a cavity should be provided to give a break in the path for moisture and where it is on the outside it should provide some resistance to the ingress of moisture to ensure the wall remains relatively dry (see Diagram 11).

Alternative approach

5.11 The requirement can also be met by following the relevant recommendations of BS 5628-3:2001. The code describes alternative constructions to suit the severity of the exposure and the type of brick or block.

Cavity external walls

5.12 Any external cavity wall will meet the requirement if the outer leaf is separated from the inner leaf by a drained air space, or in any other way which will prevent precipitation from being carried to the inner leaf.

Technical solution

5.13 The construction of a cavity external wall could include:

a. outer leaf masonry (bricks, blocks, stone or manufactured stone); and

b. cavity at least 50mm wide. The cavity is to be bridged only by wall ties, cavity trays provided to prevent moisture being carried to the inner leaf (see paragraph 5.15 for cavity insulation), and cavity barriers, firestops and cavity closures, where appropriate; and

c. inner leaf masonry or frame with lining.

Masonry units should be laid on a full bed of mortar with the cross joints substantially and continuously filled to ensure structural robustness and weather resistance.

Where a cavity is to be partially filled, the residual cavity should not be less than 50mm wide (see Diagram 11).

Alternative approach

5.14 The requirement can also be met by following the relevant recommendations of BS 5628-3:2001108. The code describes factors affecting rain penetration of cavity walls.

Cavity insulation

5.15 A full or partial fill insulating material may be placed in the cavity between the outer leaf and an inner leaf of masonry subject to the following conditions:

a. The suitability of a wall for installing insulation into the cavity should be determined either by reference to the map in Diagram 12 and the associated Table 4 or following the calculation or assessment procedure in current British or CEN standards. When partial fill materials are to be used, the residual cavity should not be less than 50mm nominal; and

b. A rigid (board or batt) thermal insulating material built into the wall should be the subject of current certification from an appropriate body or a European Technical Approval and the work should be carried out in accordance with the requirements of that document; or

c. Other insulating materials inserted into the cavity after the wall has been constructed should have certification from an appropriate body and be installed in accordance with the appropriate installations code. The suitability of the wall for filling is to be assessed before the work is carried out and the person undertaking the work should operate under an Approved Installer Scheme that includes an assessment of capability. Alternatively the insulating material should be the subject of current certification from an appropriate body or a European Technical Approval and the work should be carried out in accordance with the requirements of that document by operatives either directly employed by the holder of the document or employed by an installer approved to operate under the document; or

d. Urea-formaldehyde foam inserted into the cavity should be in accordance with BS 5617:1985 and be installed in accordance with BS 5618:1985. The suitability of the wall for foam filling is to be assessed before the work is carried out and the person undertaking the work should operate under an Approved Installer Scheme that includes an assessment of capability.

e. When the cavity of an existing house is being filled, special attention should be given to the condition of the external leaf of the wall, e.g. its state of repair and type of pointing. Guidance is given in BS 8208-1:1985. Some materials that are used to fill existing cavity walls may have a low risk of moisture being carried over to the internal leaf of the wall. In cases where a third party assessment of such a cavity fill material contains a method of assessing the construction of the walls and exposure risk, the procedure set out below may be replaced by that method.

Diagram 11 Insulated external walls examples

Diagram 12 Uk zones for exposure to driving rain

Table 4 Maximum recommended exposure zones for insulated masonry walls

5.16 If the map given in Diagram 12 is used, determine the national exposure and, where appropriate, apply the following modifiers:

i. where local conditions accentuate wind effects, such as open hillsides or valleys where the wind is funnelled onto the wall, add one to this exposure zone value;

ii. where walls do not face into the prevailing wind, subtract one from this exposure zone value.

(The national exposure zone value can be more accurately calculated from the larger scale maps and correction factors given in BS 8104:1992112).Determine the recommended constructions from the modified exposure zone values given in Table 4. Further guidance as to the use of this table is given in BRE Report 262113.

Framed external walls

5.17 Any framed external wall will meet the requirement if the cladding is separated from the insulation or sheathing by a vented and drained cavity with a membrane that is vapour open, but resists the passage of liquid water, on the inside of the cavity (see Diagram 11).

Cracking of external walls

5.18 Severe rain penetration may occur through cracks in masonry external walls caused by thermal movement in hot weather or subsidence after prolonged droughts. The possibility of this should be taken into account when designing a building. Detailed guidance is given in:

a. BRE Building Elements: Walls, windows and doors; and

b. BRE Report 292;

c. Guidance for choice of materials is given in BS 5628-3:2001.

Impervious cladding systems for walls

5.19 Cladding systems for walls should:

a. resist the penetration of precipitation to the inside of the building; and

b. not be damaged by precipitation and not carry precipitation to any part of the building which would be damaged by it.

5.20 Cladding can be designed to protect a building from precipitation (often driven by the wind) either by holding it at the face of the building or by stopping it from penetrating beyond the back of the cladding.

5.21 Any cladding will meet the requirement if:

a. it is jointless or has sealed joints, and is impervious to moisture (so that moisture will not enter the cladding); or

b. it has overlapping dry joints, is impervious or weather resisting, and is backed by a material which will direct precipitation which enters the cladding towards the outer face.

5.22 Some materials can deteriorate rapidly without special care and they should only be used as the weather-resisting part of a cladding system if certain conditions are met (see Approved Document supporting Regulation 7, Materials and workmanship). The weather-resisting part of a cladding system does not include paint nor does it include any coating, surfacing or rendering which will not itself provide all the weather resistance.

Technical solution

5.23 Cladding may be:

a. impervious including metal, plastic, glass and bituminous products; or

b. weather resisting including natural stone or slate, cement based products, fired clay and wood; or

c. moisture resisting including bituminous and plastic products lapped at the joints, if used as a sheet material, and permeable to water vapour unless there is a ventilated space directly behind the material; or

d. jointless materials and sealed joints, which would allow for structural and thermal movement.

5.24 Dry joints between cladding units should be designed so that precipitation will not pass through them, or the cladding should be designed so that precipitation which enters the joints will be directed towards the exposed face without it penetrating beyond the back of the cladding.

Notes: Whether dry joints are suitable will depend on the design of the joint or the design of the cladding and the severity of the exposure to wind and rain.

5.25 Each sheet, tile and section of cladding should be securely fixed. Guidance as to appropriate fixing methods is given in BS 8000- 6:1990117. Particular care should be taken with detailing and workmanship at the junctions between cladding and window and door openings as they are vulnerable to moisture ingress.

5.26 Insulation can be incorporated into the construction provided it is either protected from moisture or unaffected by it.

5.27 Where cladding is supported by timber components or is on the facade of a timber framed building, the space between the cladding and the building should be ventilated to ensure rapid drying of any water that penetrates the cladding.

Alternative approach

5.28 The requirement can also be met by following the relevant recommendations of:

a. BS CP 143118 for sheet roof and wall coverings made from the following materials:

Part 1:1958 Corrugated and troughed aluminium

Part 5:1964 Zinc

Part 10:1973 Galvanised corrugated steel

Part 12:1970 (1988) Copper

Part 15:1973 (1986) Aluminium

Part 16:1974 Semi-rigid asbestos bitumen sheets

Recommendations for lead are included in BS 6915:2001.

b. S 8219:2001;

c. BS 8200:1985;

d. BS 8297:2000;

e. BS 8298:1994;

f. MCRMA Technical Paper 6;

g. MCRMA Technical Paper 9.

These documents describe the materials and contain design considerations including recommendations for fixing.

Joint between doors and windows

5.29 The joint between walls and door and window frames should:

a. resist the penetration of precipitation to the inside of the building; and

b. not be damaged by precipitation and not permit precipitation to reach any part of the building which would be damaged by it.

5.30 Damp-proof courses should be provided to direct moisture towards the outside:

a. where the downward flow of moisture would be interrupted at an obstruction, e.g. at a lintel;

b. where sill elements, including joints, do not form a complete barrier to the transfer of precipitation, e.g. under openings, windows and doors;

c. where reveal elements, including joints, do not form a complete barrier to the transfer of rain and snow, e.g. at openings, windows and doors.

5.31 In some cases the width of the cavity due to thermal insulation and the 50mm clearance for drainage may be such that the window frame is not wide enough to completely cover the cavity closer. The reveal may need to be lined with plasterboard, dry lining, a support system or a thermal backing board. Direct plastering of the internal reveal should only be used with a backing of expanded metal lathing or similar.

5.32 In areas of the country in driving rain exposure zone 4 checked rebates should be used in all window and door reveals. The frame should be set back behind the outer leaf of masonry, which should overlap it as shown in Diagram 13. Alternatively an insulated finned cavity closer may be used.

Diagram 13 Window reveals for use in areas of severe or very severe exposure to driving rain (see paragraph 5.32)

Door thresholds

5.33 Where an accessible threshold is provided to allow unimpeded access, as specified in Part M, Access to and use of buildings, it will meet the requirement if:

Diagram 14 Accessible threshold for use in exposed areas128 (see paragraph 5.33)

a. the external landing (Diagram 14) is laid to a fall between 1 in 40 and 1 in 60 in a single direction away from the doorway;

b. the sill leading up to the door threshold has a maximum slope of 15degree.

Further advice for the development of accessible thresholds is given in BRE GBG 47 and the TSO document.

External walls (resistance to damage from interstitial condensation)

5.34 An external wall will meet the requirement if it is designed and constructed in accordance with Clause 8.3 of BS 5250:2002, and BS EN ISO 13788:2001.

5.35 Because of the high internal temperatures and humidities, there is a particular risk of interstitial condensation in the walls of swimming pools and other buildings in which high levels of moisture are generated; specialist advice should be sought when these are being designed.

EXTERNAL WALLS (RESISTANCE TO SURFACE CONDENSATION AND MOULD GROWTH)

5.36 An external wall will meet the requirement if:

a. it is designed and constructed so that the thermal transmittance (U-value) does not exceed 0.7W/m2K at any point; and

b. the junctions between elements and details of openings, such as doors and windows, are designed to Accredited Construction Details99, or follow the guidance of BRE IP17/01.