Spray Polyurethane Foam Application

Spray polyurethane foam (SPF) roofing is a low-slope system built in place by spraying a two-component liquid onto the roof. The liquids react and expand into rigid, closed-cell foam that bonds to the substrate, insulates, and forms a seamless layer that can be shaped to improve drainage. Because polyurethane foam degrades in sunlight, it is covered with a protective elastomeric coating or other surfacing.

Chemistry

The US Environmental Protection Agency describes SPF as a reaction between an A side, containing isocyanates such as MDI, and a B side, containing polyols with catalysts, flame retardants, blowing agents and surfactants. The two are mixed in roughly equal proportions and the reaction gives off heat as the foam forms. Roofing foam is applied with high-pressure equipment that heats and pumps both components to a spray gun.

Typical application sequence

1. Inspect and prepare. Remove loose material, wet insulation and contaminants; the substrate must be clean and dry.
2. Prime where the manufacturer requires it.
3. Mask and protect nearby surfaces, vehicles and air intakes, since overspray can travel.
4. Spray in passes. Foam is built up in layers to the specified thickness, with extra thickness to fill low spots and create slope to drains.
5. Inspect the foam surface. Texture is checked; rough or blistered areas are corrected.
6. Apply the coating. An elastomeric coating (often acrylic, silicone or polyurethane) is applied in one or more coats, sometimes with embedded granules for foot traffic.

Weather and conditions

Moisture, low temperatures and wind affect foam quality and overspray. Applicators monitor substrate and air temperatures, humidity and wind speed against the manufacturer's limits, and stop work in rain or dew conditions.

Health and safety

Isocyanates are respiratory and skin sensitisers. The EPA notes that spraying can produce aerosols and vapours, and that people already sensitised can react severely even to low concentrations. Applicators use respiratory and skin protection, and access to the area is controlled during and after spraying. The European Union requires training for professional users of products containing diisocyanates.

Other uses

In parts of Southeast Asia, foam is sprayed onto the underside of tile and metal roofs to seal and insulate them.