I agree Our site saves small pieces of text information (cookies) on your device in order to deliver better content and for statistical purposes. You can disable the usage of cookies by changing the settings of your browser. By browsing our website without changing the browser settings you grant us permission to store that information on your device.
The vapour permeability of mastic asphalt is extremely low. Mastic asphalt is effectively vapour-tight in service (water-vapour diffusion resistance factor μ ≈ 20,000–100,000; equivalent air-layer thickness Sd ≈ 200–2,000 m for a 10–20 mm layer). In practice it is treated as an integral vapour control layer: WVTR is negligible compared with other envelope layers, and long-term performance depends on continuous coverage with well-sealed joints and penetrations. If controlled drying is required, use a dedicated VCL strategy rather than relying on the asphalt layer. Values are typically determined under EN ISO 12572 conditions (~23 °C/50% RH) and resistance increases further at lower temperatures.
Vapour permeability describes how easily water vapour diffuses through a material when there’s a vapour-pressure difference across it. It’s reported as permeability (e.g., ng/(Pa·s·m)) or permeance (e.g., ng/(Pa·s·m²)), and also via the dimensionless vapour diffusion resistance factor μ; the related Sd value (m) equals μ × thickness and expresses the “equivalent air layer.” Standard tests include EN ISO 12572 (dry-cup and wet-cup) and ASTM E96 (desiccant and water methods), each at stated temperature and humidity conditions. Higher permeability or permeance means vapour passes more readily; low values indicate a vapour barrier or control layer. It is not the same as air permeability—airtightness depends on sealing joints and penetrations, even if the material itself is vapour-tight. Permeability varies with temperature, relative humidity, moisture content, and microstructure; many materials become more permeable at higher RH. Designers use these values to position vapour control layers, choose “breathable” membranes, and assess interstitial condensation risk in hygrothermal calculations. Mastic asphalt is effectively vapour-tight (very high μ, large Sd), so continuity at laps, edges, and penetrations is critical.
Low vapour permeability lets mastic asphalt act as an effective vapour control and waterproofing layer in one, blocking moisture diffusion from both sides. It reduces interstitial condensation, protecting insulation and substrates (timber/steel) from damp, mould, and corrosion. By keeping moisture out, it preserves insulation performance and lowers the risk of blistering, improving durability when the substrate is dry and well primed. It can also simplify the build-up by allowing the asphalt to serve as the VCL, reducing separate layers—provided detailing is continuous at laps, upstands, and penetrations.
Mastic asphalt can act as both the waterproofing membrane and the vapour control layer (VCL), stopping liquid water and water vapour from either side. This dual role limits moisture movement by diffusion, capillary action, and minor air leakage, but only if the layer is continuous and well bonded. Use compatible primers, ensure full support (no voids), and seal laps, upstands, terminations, and penetrations with detailing that carries the VCL function through junctions. On critical works, add simple QA such as adhesion pull-off checks, visual “holiday”/pin-hole inspection, and moisture readings of the substrate before laying. Remember that trapped moisture can still create vapour pressure blisters under heat, so substrates must be dry, and early protection (boards, traffic control) should prevent point-load damage during construction.
By restricting vapour diffusion, mastic asphalt reduces interstitial condensation—where warm, moist indoor air would otherwise reach a cold layer and deposit water. Keeping moisture out preserves insulation R-value and helps prevent mould in timber and corrosion on steel or fixings. The asphalt also stabilises the dew-point position, making the build-up’s hygrothermal behaviour more predictable across seasons; this is especially valuable in warm-roof designs where the VCL should sit on the warm side of the insulation. Pair the vapour barrier with good airtightness so vapour isn’t carried by unintended air paths at edges and penetrations, and match detailing to the project’s humidity class (e.g., kitchens, pools need stricter control). Where risk is high, confirm the strategy with a simple condensation risk assessment and ensure site QA (dry substrate, sealed junctions) aligns with the design assumptions.
Keeping the build-up dry preserves insulation R-value, reduces adhesive creep, and makes the asphalt far less prone to vapour-pressure blistering under heat. It also cuts freeze–thaw and wet–hot cycling damage that can lead to ridging, scuffing, and early surface defects. Reliability hinges on laying over dry, primed, fully supported substrates; verify with moisture/RH checks, adhesion pull-off tests, and bulk-density/continuity checks (near MTD) after lay-down. Maintain good falls and clear outlets so ponding doesn’t sustain high temperatures or prolonged wetting, and adopt an O&M plan of periodic inspections and prompt hot-patch repairs to keep the barrier continuous.
Using mastic asphalt as the VCL + waterproofing can remove a separate vapour layer, reducing interfaces, incompatibility risks, and programme steps. Fewer layers mean fewer potential failure points at laps, terminations, and penetrations, but continuity still must be carried through upstands, outlets, and collar details. Coordinate with the insulation system (adhesives/foils), fire classification, slip/skid requirements (where trafficked), and the intended dew-point position so the vapour strategy remains correct. Lock this into clear drawings and QA hold points—substrate dryness, primer type/coverage, detail mock-ups—and consider electronic leak detection after cure to verify the simplified build-up performs as intended.
Applications with strong vapour drives or moisture sensitivity gain the most: warm roofs and terraces over occupied space, especially above high-humidity interiors like pools, commercial kitchens, and showers. Below-grade tanking and retaining walls benefit by blocking ground moisture and inward vapour diffusion. Podium decks, balconies over habitable rooms, and wet-room floors use the asphalt as both waterproofing and an effective VCL to prevent interstitial condensation. It’s also valuable over timber or steel substrates (avoiding rot/corrosion) and in cold-store builds where a vapour-tight layer controls reverse vapour drive.
For warm roofs and terraces over occupied spaces mastic asphalt can act as both waterproofing and VCL, stopping moisture from high-humidity interiors (kitchens, showers, pools) migrating into cooler roof zones. To control the dew point, place the VCL on the warm side and carry its continuity through upstands, parapets, penetrations, and movement joints; align joints with the substrate so the asphalt isn’t forced to bridge movement. Pay special attention to thermal bridges at thresholds and balustrade fixings—use thermal breaks and upstand insulation so cold spots don’t undermine the vapour strategy. Substrates must be dry, sound, and fully supported: verify with moisture/RH checks, correct primers, and pull-off tests where required. Finish with clear QA hold points (substrate dryness, primer coverage, detail mock-ups) and consider electronic leak detection before over-burden or finishes are applied.
For below-grade tanking and retaining walls, low vapour permeability of mastic blocks ground moisture and inward vapour diffusion. This complements the asphalt’s waterproofing under hydrostatic head (Type A protection per BS 8102). Protect the membrane with drainage/protection boards of adequate compressive strength and creep resistance, and provide perimeter drains/cavity systems to reduce water pressure and avoid point-load damage during backfill. Ensure continuous detailing at laps, corners, kicker joints, and service penetrations; use compatible collars/sleeves and mirror structural joints through the asphalt to prevent random cracking. Substrates must be clean, dry, and well primed—cast concrete laitance should be removed and moisture verified—then backfill carefully with rounded material to avoid puncture. Plan inspections at each stage (primer, membrane, protection, drainage) and document as-built photos and test results so continuity is proven before concealment.
Both podium decks and mastic asphalt balconies over rooms cover habitable space. Using a vapour-tight mastic asphalt layer prevents moisture migrating into cooler deck zones, cutting interstitial condensation and protecting ceilings/finishes below. In wet rooms, the same layer blocks downward water/vapour drive into the structure, so substrates (timber, screeds, MEP penetrations) stay dry and durable. Carry VCL continuity through thresholds, door frames, upstands, outlets, and balcony edge details; mirror movement joints from the structure and avoid forcing the asphalt to bridge gaps. Provide falls to drains, compatible primers, and high-density protection boards under pedestals or heavy finishes to stop point-load damage. Build in QA: substrate moisture/RH checks, primer coverage records, detail mock-ups, and a water test or electronic leak detection before covering.
Timber and steel are moisture-sensitive: a vapour-tight asphalt layer limits vapour ingress that can trigger timber decay or steel corrosion at interfaces and fixings. In cold stores, vapour drive runs from warm ambient to the cold interior, so a continuous VCL on the warm side is critical to prevent frost/ice formation and loss of insulation performance. Maintain continuity across upstands, fixings, hangers, and service penetrations; use sleeves/collars and thermal breaks to avoid cold spots that defeat the vapour strategy. Pair with continuous insulation to control surface temperatures and check junctions (parapets, floor-wall) with 2D/3D modelling where needed. Verify substrate dryness (wood MC, steel prep/priming), ensure airtightness so moisture isn’t carried by unintended air paths, and schedule inspections to keep the barrier intact over time.