Sealing refers to the measures and products used to prevent the uncontrolled passage of moisture, water vapour, air or gas through building elements such as foundations, walls, roofs, joints and penetrations. The term covers a range of distinct functions—damp proofing, waterproofing, air sealing and gas resistance—that are often confused but require different materials and detailing strategies.
Functions and Types
Damp proofing prevents the capillary rise of moisture from the ground into masonry, typically achieved with a damp proof course (DPC) built into the wall structure or a damp proof membrane (DPM) laid beneath a ground floor slab. Waterproofing addresses hydrostatic water pressure, such as in basements or retaining structures, and generally demands tanking systems or structural waterproofing to BS 8102. Air sealing is a separate discipline concerned with controlling uncontrolled air leakage through the building envelope, measured in air changes per hour (ACH) at a reference pressure of 50 Pa, or in m³/(h·m²) of envelope area. Gas-resistant sealing is required where ground gases such as methane or radon are present, using membranes that meet BRE 211 specifications rather than standard polythene DPMs.
Materials and Products
Common sealing materials include polythene DPMs with thickness typically ranging from 0.2 to 1.0 mm depending on exposure conditions, bituminous and self-adhesive membranes, liquid-applied coatings, and sealing tapes used at junctions in timber and mass-timber construction. For ground-floor applications in the UK, DPMs are commonly specified to comply with BS 6515. For airtightness in timber frame or cross-laminated timber (CLT) buildings, sealing membranes and tapes are applied at panel joints, service penetrations and wall-floor junctions to achieve the air-change-rate targets required for Passivhaus certification, typically ≤0.6 ACH at 50 Pa.
Design and Installation Considerations
Continuity of the sealing layer is the primary design requirement. DPMs must be installed with overlaps at joints and adequate upstands at perimeters, abutments and service penetrations to prevent bypass paths for moisture. In masonry facades, sealing details require particular attention at movement joints, DPC transitions and service penetrations, since these locations are the most common sources of water ingress and air leakage. For airtight envelopes, sealing tapes and membranes must be compatible with the substrate and remain durable over the building's service life, as poor adhesion or UV degradation at exposed laps is a frequent cause of performance loss.
Testing and Verification
Airtightness performance is verified through blower-door testing, expressed as ACH at 50 Pa or as air permeability in m³/(h·m²) of envelope area at a reference pressure difference, commonly below 3 m³/(h·m²) for near-passive designs. These air permeability figures describe air leakage only and are not equivalent to water-tightness criteria, which are assessed separately under hydrostatic or driving-rain test methods. Waterproofing and damp proofing performance is not typically expressed as a single pass/fail airtightness figure but through compliance with product standards such as BS 6515 for DPMs or BS 8102 for structural waterproofing.
Sealing in Our Coverage
Our reporting on damp proof membrane installation sets out practical requirements for lap continuity, upstand heights and compliance with BS 6515, noting that gas-resistant membranes to BRE 211 are required instead of standard DPMs where ground gases are present. Coverage of the largest Passivhaus schools in England describes how CLT's airtightness potential, combined with sealing membranes and tapes, supports the ≤0.6 ACH at 50 Pa target required for certification. A review of Wienerberger UK's facade reference projects notes that the published material includes no test data or construction details on sealing and weatherproofing detailing, only visual documentation of the completed facades.
