How does Spray Polyurethane Foam (SPF) Insulation Work?

How spray foam insulation works? Spray polyurethane foam (SPF) is made by mixing and reacting chemicals to create a foam. The mixing and reacting materials react very quickly, expanding on contact to create foam that insulates, air seals and provides a moisture barrier. SPF insulation is known to resist heat transfer extremely well, and it offers a highly effective solution in reducing unwanted air infiltration through cracks, seams, and joints. There are different types of SPF. Here, we will discuss the types typically installed by professionals, which are either a high pressure foam and/or a low pressure foam.

Whether retro-fitting a home or choosing insulation when building a new one, homeowners are learning that spray polyurethane foam (SPF) insulation is a great way to save on energy costs and improve comfort. It is a spray-applied cellular plastic that forms a continuous barrier on walls, around corners and on all contoured surfaces. SPF insulation applied by professionals is generally described as a high pressure foam or a low pressure foam and is available as “open-cell” or “closed-cell” foam. There are several major differences between the two types, leading to advantages and disadvantages for both, depending on the desired application requirements. It is important to discuss with your contractor which type of SPF insulation may be best suited to your application. A side by side comparison highlights some of the typical differences between closed-cell and open-cell foam.

How is it made?

Two liquids combine during a chemical reaction to form spray polyurethane foam. 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. 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).

Safety Data Sheets (SDS) are available for both “A” and “B” side chemicals. Your contractor can provide these to you or discuss these with you.

During the SPF installation:

After the chemicals are mixed and reacted, the foam hardens very rapidly. When the reaction is fully complete depends on the type of SPF insulation and other variables. Your contractor can give you guidance about when you can reenter your home after an interior, two-component foam insulation application.

Your contractor will apply interior, two-component foam using specific personal protective equipment (high pressure foam is installed with a respirator, for example). This equipment, coupled with certain work practices and engineering practices including ventilation, are used to minimize exposures to the chemicals used to make SPF during the job. As a homeowner, you can minimize or eliminate exposure to the chemicals used to create spray foam by carefully following your contractor’s guidance about how long to leave the home during the installation, job completion, and cleanup.


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