The Science of Spray Foam Encapsulation
Spray foam encapsulation of asbestos works by creating a continuous, impermeable barrier layer that physically seals asbestos fibres within or beneath the foam. Unlike liquid encapsulants (PVA-based sealers, acrylic coatings) which penetrate and bind surface fibres, spray foam creates a thick, rigid, structural layer that covers the entire surface of the asbestos-containing material and prevents any fibre release under normal conditions.
The encapsulation mechanism of spray foam is fundamentally different from thin-film coatings. A liquid coating seals fibres at the molecular level by penetrating the surface — effective but thin and vulnerable to abrasion, impact, and weathering. Spray foam creates a 50–80mm thick barrier that is orders of magnitude more robust. The asbestos cement surface is buried beneath a continuous rigid foam layer that would require deliberate, significant effort to penetrate.
This is why spray foam encapsulation is classified as "Type 3" encapsulation (complete enclosure) rather than "Type 1" (penetrating sealant) or "Type 2" (surface coating). The foam does not merely seal the surface — it entombs it.
Why Closed-Cell Spray Foam Works
Nexseal LE is a closed-cell polyurethane spray foam with over 90% closed cells. This means the foam structure consists of millions of tiny, sealed cells that do not allow air, moisture, or fibres to pass through. The properties that make it ideal for asbestos encapsulation are:
• Impermeability: The closed-cell structure creates a vapour barrier — air and moisture cannot pass through the foam. Asbestos fibres, which are microscopic particles, are completely unable to penetrate the foam layer.
• Adhesion: The foam bonds tenaciously to the asbestos cement surface. During the expansion phase, the foam cells interlock with the surface texture of the asbestos cement, creating a mechanical bond that is stronger than the tensile strength of the asbestos cement itself. The foam cannot be pulled off without breaking the substrate.
• Rigidity: Closed-cell foam cures to a rigid solid with compressive strength of approximately 170 kPa. It cannot be crushed, dented, or displaced by normal building use. This physical robustness protects the encapsulation from accidental damage.
• Durability: Polyurethane foam is chemically stable and does not degrade under normal building conditions. It is resistant to moisture, fungi, insects, and rodents. Its service life in a protected (indoor) environment exceeds 50 years.
The Application Process Step by Step
The application of Nexseal LE to asbestos cement follows a controlled, systematic process:
Step 1: Pre-Works Assessment
Before any spray foam is applied, the asbestos cement is assessed for condition and suitability. A management survey (or existing survey data) confirms the type and condition of ACM. The surface is inspected for excessive deterioration, loose fragments, or contamination that might affect adhesion. Surface temperature and moisture levels are checked — the surface must be above 5°C and dry for optimal adhesion.
Step 2: Area Preparation
The working area is prepared: goods and equipment are moved or covered with protective sheeting, ventilation is arranged for over-spray extraction, access equipment (cherry pickers, scaffolding) is positioned, and masking is applied to areas that should not receive foam (ventilation openings, light fittings, services). Livestock or personnel are removed from the working section.
Step 3: Spray Application
Nexseal LE is a two-component system — the A-side (isocyanate) and B-side (polyol resin) are stored in separate heated drums and pumped through heated hoses to the spray gun. At the gun, the two components are mixed and sprayed onto the asbestos cement surface. The chemical reaction begins immediately — the liquid expands approximately 30:1 and cures to a rigid foam within 5–10 seconds.
The foam is applied in passes of approximately 20–25mm thickness. For a typical 60mm specification, three passes are made, allowing each pass to cure before the next is applied. This layered approach ensures uniform density and complete coverage without voids or thin spots. A skilled applicator achieves 150–300m² per day per spray rig.
Step 4: Quality Inspection
After application, the foam surface is inspected for coverage, thickness, adhesion, and surface quality. Thickness is measured with long-probe depth gauges at regular intervals across the sprayed area. Any areas of insufficient thickness receive a top-up pass. The inspection confirms that the encapsulation is continuous — no gaps, thin spots, or missed areas.
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How Fibres Are Sealed
The fibre sealing mechanism operates at two levels:
Surface contact sealing: When the spray foam first contacts the asbestos cement surface, the liquid components wet the surface and flow into the texture of the cement. As the foam expands and cures, it locks into the surface irregularities of the asbestos cement — the pores, ridges, and fibre ends that characterise the material. Any loose fibres on the surface are physically trapped within the expanding foam cells.
Barrier enclosure: Once cured, the foam layer creates a complete physical barrier over the entire surface. Even if fibres were to become detached from the asbestos cement (through vibration, thermal movement, or gradual deterioration), they cannot escape because the foam layer prevents any pathway to the building interior. The fibres are permanently enclosed between the asbestos cement substrate and the rigid foam barrier.
This dual mechanism — surface contact sealing plus barrier enclosure — is why spray foam encapsulation is classified as the most effective form of encapsulation for asbestos cement. Thin-film coatings provide only the surface contact mechanism; spray foam provides both.
Curing, Adhesion, and Bond Strength
Nexseal LE undergoes an exothermic chemical reaction during curing — the foam heats up as it expands and solidifies. The curing process is effectively complete within 60 seconds of application, though full dimensional stability is achieved within 24 hours. The key performance characteristics during and after curing:
• Tack-free time: 5–10 seconds
• Full rise time: 8–15 seconds
• Handling strength: Within 60 seconds
• Full cure: 24 hours for dimensional stability
• Adhesion to asbestos cement: >100 kPa (exceeds the tensile strength of the asbestos cement itself)
• Compressive strength: ~170 kPa
• Closed-cell content: >90%
• Thermal conductivity: 0.027 W/mK
The adhesion figure is critical: the bond between the spray foam and the asbestos cement is stronger than the asbestos cement itself. This means that if you tried to pull the foam away from the surface, the asbestos cement would break before the bond failed. In practical terms, the foam and the asbestos cement become a single composite structure that cannot be separated without destroying both materials.
Testing and Certification
Nexseal LE performance is evidenced by the manufacturer's declared values, independently tested to recognised European standards, covering thermal performance, durability, adhesion and fire characteristics. Building Control officers, architects and insurers can be provided with the full test evidence and installation documentation on request.
Nexseal LE has been tested to: EN 14315-1 (thermal insulating products for buildings), I.S. EN 13501-1 (reaction-to-fire classification — achieves Euroclass B-s1,d0 with DC315), and ISO 5660 (heat release rate).
For asbestos encapsulation, the survey, risk assessment and method of work matter as much as the product. Installation is carried out by operatives with recognised asbestos awareness training, with the work risk-assessed and notified to the HSA where required.
Long-Term Performance
Closed-cell polyurethane spray foam has a proven track record of long-term durability in building applications. The earliest spray foam installations date from the 1980s, and many remain in service and performing well after 40+ years. The key factors in long-term performance are:
Dimensional stability: The foam does not shrink, sag, or settle over time. The rigid cell structure maintains its shape and volume permanently, unlike open-cell foam or fibrous insulation which can settle and lose thickness.
Moisture resistance: Closed-cell foam absorbs less than 2% moisture by volume, even under prolonged immersion. In normal building conditions, moisture uptake is negligible. This means the foam does not deteriorate through moisture absorption, and its thermal and encapsulation performance is maintained.
Chemical stability: Polyurethane is chemically inert in normal building conditions. It does not react with asbestos cement, does not off-gas after the initial 24-hour cure, and is not affected by the alkaline chemistry of cement. The bond between foam and asbestos cement is permanent.
Biological resistance: Closed-cell foam does not support mould, fungal growth, or insect or rodent activity. It is not a food source for any organism and does not absorb moisture that could support biological growth. This is a significant advantage over fibrous insulation materials in buildings with asbestos.
Nexseal LE carries a manufacturer-backed product warranty when applied to the manufacturer's specification and maintained according to the maintenance schedule. The expected service life in a protected building interior significantly exceeds the warranty period.
Contact Duratite on info@duratite.ie for a technical consultation on spray foam asbestos encapsulation.