11.2.8 Protection from moisture
Closed panel systems shall be adequately protected from the effects of moisture. Details of closed panel 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 panels.
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’.
11.2.8.1 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 walls)
- extend at least 150mm below DPC
- drained and vented (for timber frame and SIP walls)
- be kept clean, free from obstructions and capable of draining freely.
Closed panel timber frame walls
Cavities to closed panel timber frame walls should be in accordance with Clause 6.2.10.
Structural insulated panels (SIPs)
Cavities to SIP walls should be in accordance with Clause 6.2.10.
Closed panel light steel frame walls
Cavities to closed panel light steel frame walls should be in accordance with Clause 6.10.16.
11.2.8.2 Protection at low level
Closed panel timber frame and structural insulated panels (SIPs)
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 walls and ground floor 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’ or ‘upstand’ to raise level of timber frame elements.
Closed panel light steel frame
The protection of light steel frame at low level should be in accordance with Clause 6.10.16.
11.2.8.3 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.
11.2.8.4 Junctions between adjoining units
At junctions between closed panel systems or between closed panel systems and surrounding construction, particularly where they incorporate factory installed external claddings, consideration should be given to:
- providing suitable access to seal laps of breather membranes and AVCLs and achieve minimum vertical and horizontal 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 closed panel systems and between closed panel systems 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