11.3.8 Protection from moisture
Volumetric systems shall be adequately protected from the effects of moisture. Details of volumetric systems at low level shall fully consider the durability of materials, protection of the building from moisture ingress and condensation as a result of thermal bridging. Issues to be taken into account include:
- cavities in external walls
- protection at low level
- DPCs, DPMs and cavity trays
- junctions between adjoining units.
Guidance on temporary weather protection can be found in Clause 11.1.5. Guidance on moisture management strategies for structural timber buildings can be found in Structural Timber Association publication ‘Moisture management strategy’.
11.3.8.1 Cavities in external walls
A clear cavity in accordance with Table 1 should be provided between the cladding and insulation.
Table 1: Minimum cavity widths
| Cladding | Cavity width |
|---|---|
| Masonry | 50mm |
| Render on board background | 25mm |
| Vertical tile hanging(1) where a breather membrane is provided and fixed to sheathing | Dependant on batten support layout and spacing(1) |
| Other cladding(1) | See Chapter 6.9 Curtain walling and cladding |
Note(s)
1. See Chapter 6.9 Curtain walling and cladding.
Cavities should be:
- drained (for light steel frame backing walls)
- extend at least 150mm below DPC
- drained and vented (for timber frame backing walls)
- be kept clean, free from obstructions and capable of draining freely.
Timber frame backing walls
Cavities to volumetric units with a timber frame backing wall should be in accordance with Clause 6.2.10.
Light steel frame backing walls
Cavities to volumetric units with a light steel frame backing wall should be in accordance with Clause 6.10.16.
11.3.8.2 Protection at low level
Timber frame walls
The lowest timber should be a minimum of 150mm above finished ground level. This may be reduced to 75mm in situations where the site is not subject to a high-water table or where the cavity will not have standing water.
DPCs should be installed below the sole plates of ground floor walls and internal partitions.
Where external ground levels are raised locally to accommodate accessible entrances, consideration should be given to the following:
- appropriate drainage installed along the perimeter or ground falling away from the building
- adjacent surface finishes which do not promote splashing
- additional cavity ventilation either side of raised ground levels
- use of a ‘kerb’ to raise level of timber frame elements.
Light steel frame walls and floors
The base rail of LSF should be kept a minimum of 150mm above the external ground level (or waterproofing layer of a flat roof, balcony, or terrace) and cavity fill.
DPCs should be installed underneath the full width of the lowest section of framing, eg all ground floor walls and internal partitions, to protect the steel from corrosion due to moisture.
Locally raised ground levels (up to the internal floor finish) to less than 15% of the external perimeter (of an individual building, eg row of terraced homes, apartment blocks and detached garages, measured on plan) to accommodate level thresholds can be accepted. The cavity should be kept clear and allow drainage. Wall insulation should overlap the base rail by a minimum of 150mm.
Where the base rail or lowest steel is less than 150mm above ground level:
- light steel frame should be in accordance with Clause 6.10.16b
- hot rolled steel components should have a factory-applied protective coating system in accordance with Clause 6.5.3.
11.3.8.3 DPCs, DPMs and cavity trays
Damp proofing should:
- be installed correctly to provide a physical barrier to the passage of water, and to ensure water is directed to the outside
- use only appropriate tapes and sealant (but not solely rely on sealant) in accordance with the design and manufacturer’s recommendations
- use DPCs/DPMs where necessary, including junctions between systems and any other component or systems.
DPCs
The following materials are acceptable for use as DPCs:
| Bitumen-based materials | BS 6398, BS EN 14967 |
| Polyethylene (except as cavity trays in walls, below copings and in parapets) | BS 6515, BS EN 14909 |
| Proprietary materials | Technical Requirement R3 |
| Thermoplastics and Elastomers | BS EN 14909 |
DPMs
Where DPMs are required, they should be linked with any DPCs in the supporting structure, in order to provide continuous protection from moisture from the ground or through the supporting structure.
DPMs should be properly lapped in accordance with Chapter 5.1 Substructure and ground-bearing floors.
Suitable materials for DPMs include:
- 1,200-gauge (0.3mm) polythene sheet
- minimum 1,000-gauge (0.25) polythene sheet where assessed in accordance with Technical Requirement R3
- bitumen sheet to BS 6398
- proprietary materials assessed in accordance with Technical Requirement R3.
Cavity trays
Cavity trays should be provided at all interruptions to the cavity (eg window and door openings and air bricks) and abutments unless otherwise protected (eg by overhanging eaves) or alternative means of protection are provided (eg profiled metal flashings) such as part of a proprietary cladding system.
Clause 6.1.17 contains further guidance on the installation of cavity trays in masonry cladding.
The following materials are acceptable for use as cavity trays:
- plastic and rubber to BS EN 14909 and hold satisfactory assessment by an appropriate independent technical approval’s authority accepted by NHBC
- proprietary materials as part of a cladding system holding satisfactory assessment by an appropriate independent technical approvals authority accepted by NHBC.
11.3.8.4 Junctions between adjoining units
At junctions of volumetric units or between volumetric units and surrounding construction, particularly where they incorporate factory installed external claddings, consideration should be given to:
- providing suitable access to install breather membranes and achieve minimum laps
- providing suitable access to adequately link DPCs and DPMs in the substructure
- use of appropriate tapes and sealant (but not solely rely on sealant) in accordance with the design and the manufacturer’s recommendations
- design of suitable cladding joints in accordance with the manufacturer’s recommendations
- interaction of cavity barriers and cladding joints
- use of damp proofing materials which are compatible with adjoining components.
Connections between volumetric units and between volumetric units and surrounding construction should be designed to avoid the risk of fixing through and thus puncturing or damaging AVCLs or breather membranes.
Last updated: 2nd January 2025