Spray Polyurethane Foam (SPF) Basics

Whether retro-fitting a home or choosing insulation when building a new one, spray polyurethane foam (SPF) insulation is among the best ways to increase energy efficiency and improve comfort.

What is SPF & How Does it Work?

SPF, a spray-applied cellular plastic, is made by mixing chemicals to create foam. Those chemicals react very quickly, expanding on contact to create foam that insulates, air seals and provides a moisture barrier. When properly installed, SPF forms a continuous barrier on walls, around corners and on contoured surfaces. It resists heat transfer very well and is an effective solution for reducing unwanted air infiltration through cracks, seams, and joints.

SPF insulation applied by professionals is usually described as either a high- or low-pressure foam and is available as either open- or closed-cell. Each type has advantages and disadvantages depending on the application requirements. The comparison chart below can be helpful in explaining or understanding which type of SPF insulation is best suited to a specific application.

Closed-Cell vs Open-Cell Foam: A Comparison

Closed-Cell Open-Cell
Higher R-value (greater than 6.0 per inch) R-value (approximately 3.5 per inch)
High moisture barrier (lower moisture permeability) Lower moisture barrier (higher moisture permeability)
Air barrier Air barrier at full wall thickness
Higher strength and rigidity Lower strength and rigidity
Resists water Not suggested for applications in direct contact with water
Medium density (1.75 — 2.25 lbs./ft3) Lower density (0.4 — 1.2 lbs./ft3)
Absorbs sound Absorbs sound very well

More about the Chemicals and How They React

Two liquids combine during a chemical reaction to form SPF. The two liquids come in different drums or containers, and professionals generally refer to one container as the “A” side and the other container as the “B” side.

The “A” side of a spray polyurethane system is commonly comprised of methylene diphenyl diisocyanate (MDI) and polymeric methylene diphenyl diisocyanate (pMDI). The “B” side is typically a blend of polyols, catalysts, blowing agent, flame retardant and surfactant. Note that the “A” and “B” sides may be reversed outside the U.S.

The polyols are part of the chemical reaction to make foam. The remaining ingredients in the “B” side serve different purposes to help control the creation of the foam bubbles (the “cells”) in an optimal way, and to provide the various characteristics of the finished foam product (flame retardancy, for example).

After the chemicals are mixed and reacted, the foam hardens very rapidly. The time to complete reaction depends on the type of SPF insulation and other variables.

Safety Data Sheets (SDS) are available for both “A” and “B” side chemicals. As a SPF professional, it’s important that you understand the information on safety sheets and are able to share the details with your customers.

SPF Installation: Educating the customer

Professionals will want and need to give their customers guidance about the process for installation and time when they can reenter their home after an interior, two component foam insulation application.

Part of that guidance will be explaining that interior, two component foam is applied with the professional using specific personal protective equipment (high-pressure foam is installed while using a respirator, for example). It is encouraged that professionals explain clearly to customers that this equipment, coupled with certain work and engineering practices, including ventilation, is used to minimize exposures to the chemicals used to make SPF during the job.

Further, professionals will want to share how homeowners can minimize or eliminate their exposure to the chemicals used to create spray foam by carefully following guidance about not occupying the home or space during the installation, job completion and cleanup and for an appropriate period after.