Spray Foam Chemistry Explained

A-Side and B-Side Spray Foam: How the Chemicals Work Together

Professional spray polyurethane foam begins as two separate liquid components. The A-side and B-side travel through dedicated equipment lines and do not meet until they reach the spray gun. Their controlled chemical reaction creates the expanding foam installed in attics, walls, metal buildings and other building assemblies.

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Quick answer: The A-side is the isocyanate component, commonly based on MDI or polymeric MDI. The B-side is a manufacturer-formulated polyol resin blend. When properly heated, metered and mixed inside the spray gun, they react to form polyurethane foam.

How Two-Part Spray Foam Works

The two liquid components remain separated from their drums to the spray gun. Transfer pumps feed them into a proportioner, the proportioner heats and meters them, and a dual heated hose carries them through separate internal passages. They finally meet in the gun’s mixing chamber immediately before application.

Infographic showing a red A-side isocyanate drum and white B-side polyol resin drum feeding transfer pumps, a proportioner and dual heated hose into a spray gun where the components mix and expand into polyurethane foam.

The red A-side drum and white B-side drum feed separate equipment paths. The finished foam exits through the front nozzle of the spray gun only after the components meet in the mixing chamber.

What Is in the A-Side and B-Side?

A-Side

Isocyanate Component

The A-side of most professional spray polyurethane foam systems contains methylene diphenyl diisocyanate, known as MDI, and polymeric MDI. Installers often call this side “ISO.”

The isocyanate groups react with the polyols in the B-side to create the polyurethane polymer. The A-side is stored, pumped, heated and transported through its own dedicated equipment path.

  • Common base: MDI or polymeric MDI
  • Installer shorthand: ISO
  • Primary role: Reacts with polyols to form polyurethane
  • Equipment: Dedicated drum, pump and hose passage

Uncured isocyanates require professional handling, appropriate protective equipment, work-area controls and ventilation.

B-Side

Formulated Polyol Resin Blend

Contractors commonly call the B-side “resin,” but it is not one simple ingredient. It is a manufacturer-specific formulation created to produce a particular spray foam product.

  • Polyols: React with the A-side to help form the polymer
  • Catalysts: Help control the reaction rate
  • Blowing agent: Helps create expansion and cellular structure
  • Surfactants: Help stabilize the forming foam cells
  • Additives: Support product-specific processing and performance
  • Flame-retardant ingredients: Contribute to tested fire performance

Exact B-side ingredients vary between manufacturers and products. Flame-retardant ingredients do not make spray foam fireproof.

From the Drums to the Finished Foam

  1. Separate Drums

    The liquid A-side and B-side are stored separately. They are not mixed inside the drums or transfer containers.

  2. Transfer Pumps

    A dedicated transfer pump moves each component from its drum toward the proportioning equipment.

  3. Proportioner

    The proportioner meters, heats and pressurizes the two components according to the product manufacturer’s processing requirements.

  4. Dual Heated Hose

    Both components travel through the same hose assembly while remaining separated in individual internal passages.

  5. Mixing Chamber

    The liquids meet and mix rapidly inside the spray gun’s mixing chamber. This is where the chemical reaction begins.

  6. Expansion and Cure

    The reacting material exits the gun, expands against the prepared substrate, develops its cellular structure and begins curing.

A spray rig does not pump premade foam. It precisely processes two liquid components that become polyurethane foam during application.

What Happens When the Chemicals Meet?

When the A-side and B-side mix, the isocyanate and polyol components react to form polyurethane. The reaction releases heat, which is why spray foam application is described as exothermic. At the same time, the blowing system creates gas that expands the reacting material and helps form millions of small cells.

Expansion, surface cure and full property development are related but are not exactly the same thing. Foam can appear solid soon after application while the product continues developing its final properties. Cure time, ventilation and reoccupancy guidance should come from the specific product manufacturer and installing contractor.

Polymer Formation

Isocyanates and polyols react to build the polyurethane structure that becomes the solid insulation material.

Expansion

The product’s blowing system creates gas and cellular expansion, allowing the material to fill and seal the intended area.

Heat Generation

The chemical reaction generates heat. Installers must follow the manufacturer’s pass-thickness and cooling requirements.

Why Temperature, Pressure and Ratio Matter

Spray foam quality depends on more than having the correct chemical drums. The product has to be processed within its approved application range, and the surface receiving the foam must be suitable for installation.

Proportioning

The equipment must deliver the components in the ratio specified for that system. Off-ratio material may not react or cure as intended.

Material Temperature

Temperature affects viscosity, mixing and spray pattern. Manufacturer requirements vary by product and jobsite conditions.

Pressure Balance

Proper equipment pressure helps the two components reach and mix correctly inside the spray gun.

Substrate Condition

The installation surface should be clean, compatible and within the manufacturer’s acceptable moisture and temperature range.

Pass Thickness

Foam should be installed within the product’s approved pass thickness. Excessive thickness can trap heat and affect quality.

Ambient Conditions

Houston heat and humidity can change material handling, substrate conditions and the installation plan.

How the Chemistry Produces Open-Cell or Closed-Cell Foam

Open-cell and closed-cell spray foam both use the two-component A-side/B-side concept, but they are not the same product. Their manufacturer-designed formulations create different densities, expansion rates, cellular structures and performance characteristics. An installer cannot convert one into the other by changing a single machine setting.

Open-Cell Spray Foam

Open-cell systems are formulated to create a softer, lower-density cellular structure and typically expand more during application. Product selection must still match the building assembly and local code requirements.

Learn about open-cell spray foam insulation

Closed-Cell Spray Foam

Closed-cell systems are formulated to create a denser and more rigid cellular structure. They are often considered for metal buildings and other applications requiring their particular moisture and structural characteristics.

Learn about closed-cell spray foam insulation

What Properly Installed Spray Foam Should Show

Installers evaluate more than the visible surface. They monitor the equipment during application and inspect whether the finished foam is consistent with the selected product and approved installation instructions.

Consistent Texture

The foam should display the texture and cellular appearance expected for that specific product.

Proper Adhesion

The material should adhere to a properly prepared and compatible substrate without abnormal pull-away.

Specified Thickness

Depth should be checked against the project scope rather than estimated only from surface appearance.

Expected Expansion

Expansion should remain consistent with the product, processing settings and environmental conditions.

No Persistent Tackiness

Persistent softness, tackiness or abnormal odor may justify further evaluation by the installer or manufacturer.

Installation Records

Homeowners may ask which product was installed and request the relevant product and safety documentation.

Uncured Chemicals and Cured Foam Are Different

The raw A-side and B-side are reactive chemical components requiring trained handling, appropriate personal protective equipment, work-area isolation and ventilation. Once properly reacted and cured, the material has different handling characteristics than the liquid components.

This does not support one universal reoccupancy time for every installation. Customers should follow the requirements of the product manufacturer and the contractor responsible for the job. Safety Data Sheets explain chemical hazards, Technical Data Sheets describe product properties, and application instructions explain how the particular foam system should be processed.

Ask your contractor for the product name, manufacturer, Safety Data Sheets, Technical Data Sheets, application instructions and written reoccupancy guidance relevant to your installation.

Spray Foam Chemistry Across Greater Houston

The core A-side/B-side reaction does not change by city, but the building, substrate and environmental conditions do. That is why local pages should connect the application back to correct product selection, surface preparation and installation control.

Angleton spray foam projects Brenham spray foam projects Hockley spray foam projects Kemah spray foam projects

For broader application information, explore spray foam insulation for Houston metal buildings or review Phantom Foam’s spray foam insulation FAQ .

A-Side and B-Side Questions

What is the A-side of spray foam?

The A-side is the isocyanate component of a two-part spray polyurethane foam system. Most professional systems use MDI or polymeric MDI, commonly called ISO by installers.

What is the B-side of spray foam?

The B-side is a manufacturer-formulated polyol resin blend. It can contain polyols, catalysts, a blowing agent, surfactants, flame-retardant ingredients and other product-specific additives.

Where do the A-side and B-side mix?

They meet inside the spray gun’s mixing chamber immediately before the reacting material exits through the front nozzle.

Why does spray foam become warm?

The chemical reaction is exothermic, meaning it releases heat. Installers must follow the manufacturer’s pass-thickness and cooling instructions.

Are open-cell and closed-cell chemicals identical?

No. Both use the A-side/B-side concept, but their complete product systems and B-side formulations are designed to produce different cellular structures and performance characteristics.

What can happen when spray foam is processed incorrectly?

Incorrect ratio, temperature, pressure, substrate conditions or pass thickness can affect expansion, adhesion, texture and cure. Abnormal tackiness, brittleness, cracking or pull-away should be evaluated.

What documents should a homeowner ask about?

Ask for the product and manufacturer name, Safety Data Sheets, Technical Data Sheets, application guidance, warranty information and product-specific reoccupancy instructions.

Technical Spray Foam Information From Phantom Foam

Written for Phantom Foam by Cole Moore, Owner and Founder

Technical review: Josh Evans, Spray Foam Insulation Expert

Information should be confirmed against the documentation for the specific spray foam product used on each project.

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