Spray Polyurethane Foam (SPF) Basics

Spray Polyurethane Foam (SPF) is a versatile and highly effective insulation and sealing material widely used in construction, automotive, marine, and industrial applications. It’s unique because it’s applied as a liquid and then expands to form a seamless, monolithic foam layer.

SPF is created on-site by mixing two liquid components, typically referred to as the “A” side and the “B” side, at the spray nozzle:

• “A” Side: Primarily composed of isocyanates, most commonly methylene diphenyl diisocyanate (MDI) and polymeric MDI (pMDI).
• “B” Side: A blend of polyols and various additives, including catalysts (to control reaction speed), blowing agents (to create the foam structure), surfactants (to stabilize cells), and flame retardants.

When these two liquids are combined and sprayed, a rapid chemical reaction occurs. This reaction causes the mixture to expand significantly and then harden into a rigid or semi-rigid foam. This expansion allows SPF to fill irregular shapes, gaps, cracks, and crevices, creating a continuous barrier.

The two primary types of SPF are categorized by their cell structure and density:

1. Open-Cell Spray Foam (ocSPF):
   • Density: Lower density (0.4-1.2 lbs/ft³).
   • Structure: Consists of tiny cells that are not completely enclosed, meaning air can permeate the foam.
   • R-value: Generally lower (around R-3.5 to R-4.0 per inch).
   • Properties: Softer, more flexible, and allows some moisture vapor to pass through. It also excels at sound dampening.
   • Applications: Often used in interior wall cavities, attics, and ceilings where sound absorption and cost-effectiveness are priorities. It functions primarily as an air barrier, but not a vapor barrier.

1. Closed-Cell Spray Foam (ccSPF):
   • Density: Higher density (1.75-2.25 lbs/ft³ for standard, up to 3 lbs/ft³ for roofing).
   • Structure: Composed of tightly packed, fully encapsulated cells, which trap a gas (often a blowing agent) within them.
   • R-value: Much higher (typically R-6.0 to R-7.0 per inch), providing superior thermal resistance.
   • Properties: More rigid, stronger, and acts as a vapor barrier in addition to an air barrier. It also resists water absorption.
   • Applications: Ideal for exterior applications, roofs, crawl spaces, basements, and areas where moisture control, structural integrity, and maximum insulation are crucial. It can add significant racking strength to wall systems.c

Building Insulation: Walls, roofs, attics, crawl spaces, basements in residential, commercial, and industrial buildings.

  Roofing: As a primary roofing material (often with a protective coating) for flat or low-slope roofs, offering insulation and waterproofing.

  Air Sealing: Sealing around windows, doors, pipes, electrical conduits, and other penetrations to prevent air leaks.

  Soundproofing: Especially open-cell foam for reducing noise transmission between rooms or from outside.

  Cold Storage: Insulating refrigerated warehouses, freezers, and processing plants.

General Properties of Polyurethane Foam:

  Low Density: Compared to many other materials, especially metals.

  Thermal and Sound Insulation: Both flexible (open-cell) and rigid (closed-cell) foams offer significant insulation properties.

  Mechanical Properties: Can vary greatly depending on the formulation, including compressive strength, tensile strength, tear resistance, and elasticity.

  Durability: Generally a resilient material that does not deteriorate significantly over time.

  Versatility: Can be formulated into a wide range of hardnesses and flexibilities, and used in various forms (foams, elastomers, coatings, adhesives, sealants).

  Chemical Resistance: Can be formulated to be resistant to various chemicals, oils, and solvents.

  Water Resistance: Closed-cell rigid foams are particularly good at resisting water absorption.

Fire safety is a critical consideration for polyurethane foam, especially in building and automotive applications. Polyurethane is an organic material and is combustible. However, foams are formulated with fire retardants to meet specific fire safety standards and building codes.

Flammability: Unmodified polyurethane foam will burn when exposed to a sufficient heat source.

Fire Retardants: Additives are incorporated during manufacturing to improve the foam’s resistance to ignition and reduce the rate of flame spread.

Building Codes: Building codes specify the required fire performance for insulation and other foam applications, often mandating the use of fire barriers (like drywall) in conjunction with foam plastics to achieve the necessary fire rating.

Important Note: Fire-retardant foams are more resistant to ignition and burning than unmodified foams, but they are not fireproof. Proper system design and installation, including the use of prescribed fire barriers, are essential for fire safety.

Polyurethane foam’s remarkable versatility is a direct result of its ability to be engineered into different forms with a wide range of properties. Classifying PU foam by its fundamental structure (open vs. closed cell), flexibility (flexible vs. rigid), manufacturing method, and key properties like density and fire resistance provides a clear framework for understanding this complex material. By grasping these classifications, professionals and consumers alike can navigate the vast landscape of polyurethane foams and select the ideal type for their specific project or product.