Two Approaches to Asbestos Cement Roof Encapsulation
Asbestos cement roofing can be encapsulated from the inside, the outside, or both. Each approach uses a different product and delivers different benefits alongside the core encapsulation function. Understanding the differences is essential for specifying the right system for your building.
Duratite offers both systems: Nexseal LE closed-cell spray foam for interior encapsulation (tested to BS EN 14315-1) and GacoFlex S42/SF42 silicone coating for exterior encapsulation (UL Listed, CRRC rated). Both are installed by approved installers with recognised asbestos awareness training, and both carry a manufacturer-backed product warranty (20 years for GacoFlex S42/SF42). This dual capability means we can recommend the genuinely best solution for each building — not just the one product we happen to sell.
Interior Spray Foam: Nexseal LE
Nexseal LE is a two-component closed-cell polyurethane spray foam applied to the underside of asbestos cement roof sheets from inside the building. The foam is sprayed as a liquid which expands and cures within seconds, forming a rigid, continuous layer that bonds directly to the asbestos cement surface.
The encapsulation mechanism is physical: the spray foam creates a continuous, impermeable barrier over the entire interior surface of the asbestos cement. Every fibre on the surface is sealed beneath the foam layer. The adhesion is permanent — the foam cannot delaminate or separate from the asbestos cement because the cell structure interlocks with the surface texture during expansion.
Key properties of Nexseal LE:
• Thermal conductivity: 0.027 W/mK (among the best of any spray foam)
• Closed-cell content: >90%
• Vapour barrier: Integral (no separate VCL needed)
• Structural contribution: Adds rigidity and windload resistance
• Fire performance: Euroclass B-s1,d0 with DC315 coating
• Independently tested: manufacturer’s declared values to BS EN 14315-1
• Typical thickness: 50–80mm for encapsulation + thermal upgrade
Exterior Silicone Coating: GacoFlex S42/SF42
GacoFlex S42/SF42 is a single-component silicone roof coating (SF42 is the fibre-reinforced grade; S42 is unreinforced) applied to the exterior surface of asbestos cement roof sheets by airless spray, brush, or roller. The coating cures to form a seamless, flexible membrane that bonds to the asbestos cement surface and encapsulates the exposed exterior fibres.
Silicone coatings are uniquely suited to asbestos cement roofs because they remain permanently flexible — unlike acrylic or polyurethane coatings which can become brittle over time and crack, especially on the expansion joints of corrugated profiles. Silicone's permanent flexibility accommodates the thermal movement of asbestos cement sheets without cracking or loss of adhesion.
Key properties of GacoFlex S42/SF42:
• Chemistry: 100% silicone (not silicone-modified)
• UV stability: Exceptional — does not degrade under UV exposure
• Flexibility: Permanently flexible, accommodates thermal movement
• Waterproofing: Creates seamless membrane over laps, fixings, and penetrations
• Weather resistance: Designed for exposed, wet and windy conditions — 20-year manufacturer-backed product warranty
• Colour retention: Available in grey, white, green — colour stable
• Typical coverage: 2 coats at 0.5–0.8 litres/m² per coat
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Head-to-Head: 12-Factor Comparison
| Factor | Nexseal LE (Interior Foam) | GacoFlex S42/SF42 (Exterior Coating) |
|---|---|---|
| Encapsulation | Interior surface fully sealed | Exterior surface fully sealed |
| Thermal insulation | Excellent — U-value 0.35–0.5 | Minimal — reflective only |
| Condensation control | Eliminates condensation | No interior condensation benefit |
| Waterproofing | Not applicable (interior) | Full seamless waterproofing |
| UV protection | Not applicable | Exceptional UV barrier |
| Structural reinforcement | Adds rigidity and windload | Minimal |
| Sound reduction | Significant rain noise reduction | Minor rain noise reduction |
| Interior disruption | Moderate — phased working | None — all work is external |
| Weather dependency | None — indoor work | Dry conditions required |
| Application speed | 150–300m²/day per rig | 200–400m²/day per crew |
| Relative cost per m² | Higher | Lower |
| Warranty | Manufacturer-backed | 20-year manufacturer-backed |
When to Use Interior Spray Foam
Interior spray foam (Nexseal LE) is the right choice when:
• The building is heated — the thermal insulation provides 30–50% energy savings
• Condensation is a problem — spray foam eliminates condensation completely
• The roof exterior is in good condition — no leaks or waterproofing issues that need addressing from outside
• The building needs a thermal upgrade — for Building Regulations compliance, BER improvement, or energy cost reduction
• Exterior access is difficult — fragile asbestos cement roofs are dangerous to work on without edge protection and crawl boards; interior spray from a cherry picker is often safer
• All-weather working is needed — interior spray is unaffected by rain or cold
When to Use Exterior Coating
Exterior silicone coating (GacoFlex S42/SF42) is the right choice when:
• The building is unheated — no thermal benefit is needed (open storage, unheated warehouses, agricultural stores)
• The roof is leaking — exterior coating seals leaks at laps, fixings, and deteriorated areas
• Interior access is impossible — high-bay racking to the roof, mezzanine floors, or stored goods preventing cherry picker access
• The primary objective is weatherproofing — the owner wants to stop leaks and extend roof life as much as manage asbestos
• The building exterior needs improvement — exterior coating transforms the appearance of an old asbestos cement roof
• Budget is constrained — exterior coating is the lighter-touch option compared with interior spray foam
The Combined Approach
For buildings where both thermal performance and weatherproofing are important — and the budget allows — the combined approach delivers maximum protection. Interior Nexseal LE provides thermal insulation, condensation control, and structural reinforcement, while exterior GacoFlex S42/SF42 provides waterproofing, UV protection, and weather resistance.
The combined system encapsulates the asbestos cement from both sides, creating a "sandwich" with the ACM permanently sealed between two certified encapsulation layers. This provides the highest level of fibre containment, the best building performance, and the longest service life.
The combined approach is particularly recommended for warehouses and distribution centres where the asbestos cement is deteriorating from both sides, and for buildings undergoing a comprehensive refurbishment where the owner wants to address all building performance issues in a single project.
Contact Duratite on info@duratite.ie for a free assessment and recommendation.