How to install spray foam insulation in existing walls

Spray foam insulation is a highly effective method for improving the energy efficiency of your home. It provides superior insulation compared to traditional materials like fiberglass and cellulose, helping to reduce heating and cooling costs, improve indoor air quality, and increase overall comfort. Installing spray foam insulation in existing walls might seem challenging, but with the right approach and tools, it can be a manageable DIY project or a job for a professional. Here’s a comprehensive guide on how to install spray foam insulation in existing walls.

Understanding Spray Foam Insulation

Before diving into the installation process, it’s essential to understand what spray foam insulation is and how it works. Spray foam insulation is made from polyurethane, a material that expands upon application to fill cavities and seal gaps. There are two main types of spray foam insulation:

  1. Open-Cell Spray Foam: This type has a lower density and is more flexible, making it ideal for indoor applications. It provides excellent soundproofing and is typically cheaper than closed-cell spray foam.
  2. Closed-Cell Spray Foam: This type has a higher density and is more rigid, offering better thermal insulation and moisture resistance. It is suitable for both indoor and outdoor applications and can add structural strength to the walls.

Preparing for Installation

1. Assessing the Walls:

Start by assessing the existing walls to determine if they are suitable for spray foam insulation. Look for any signs of moisture, mold, or structural damage, as these issues need to be addressed before installation. Ensure that the walls are dry and clean.

2. Choosing the Right Spray Foam:

Select the appropriate type of spray foam for your needs. For most interior wall applications, open-cell spray foam is adequate. However, if you need extra moisture resistance or structural reinforcement, closed-cell spray foam might be the better choice.

3. Calculating the Amount of Foam Needed:

Calculate the amount of spray foam needed for your project. Measure the total wall area to be insulated and consult the product specifications to determine how much foam is required per square foot.

4. Gathering Tools and Materials:

You will need the following tools and materials for the installation:

  • Spray foam insulation kit (includes foam and applicator)
  • Protective gear (gloves, goggles, mask)
  • Utility knife
  • Measuring tape
  • Drop cloths or plastic sheeting
  • Caulking gun and caulk

Installation Process

1. Preparing the Area:

Lay down drop cloths or plastic sheeting to protect floors and furniture from overspray. Remove any wall coverings or obstacles that may interfere with the application process.

2. Drilling Access Holes:

Drill small access holes in the walls to allow the spray foam to be injected. The size and placement of these holes will depend on the specific design of your walls and the type of spray foam you are using. Generally, holes are drilled between each stud, starting from the top and working down.

3. Sealing Gaps and Cracks:

Before applying the spray foam, use a caulking gun to seal any visible gaps or cracks around windows, doors, and electrical outlets. This step helps to prevent air leaks and ensures that the spray foam insulation is as effective as possible.

4. Applying the Spray Foam:

Follow the manufacturer’s instructions for your spray foam kit. Wear protective gear to avoid skin and eye contact with the foam. Begin by inserting the applicator nozzle into the access holes and slowly spray the foam into the cavity. Start from the bottom and work your way up, filling the cavity evenly. The foam will expand to fill the space, so apply it gradually to avoid overfilling.

5. Monitoring Expansion:

Spray foam expands quickly, so monitor the application to ensure it doesn’t overflow the wall cavities. If necessary, stop and allow the foam to expand and cure before continuing. This process typically takes a few minutes, but curing times can vary depending on the product used.

6. Trimming Excess Foam:

Once the foam has fully expanded and cured, use a utility knife to trim any excess foam that protrudes from the access holes. Make sure the foam is flush with the wall surface.

7. Patching Access Holes:

Patch the access holes with appropriate materials, such as drywall or plaster, to restore the wall’s appearance. Sand the patched areas smooth, then prime and paint to match the existing wall color.

Post-Installation Considerations

1. Inspecting the Insulation:

After installation, inspect the insulated areas to ensure complete coverage and check for any missed spots or gaps. Address any deficiencies promptly.

2. Ventilation and Air Quality:

Proper ventilation is crucial after installing spray foam insulation, as the foam can release gases during the curing process. Ensure that the area is well-ventilated for a few days following installation to maintain good indoor air quality.

3. Safety Precautions:

Always follow safety guidelines when working with spray foam insulation. Wear protective gear, work in well-ventilated areas, and keep the foam away from open flames or high heat.

4. Long-Term Maintenance:

Spray foam insulation is durable and long-lasting, but periodic inspections are recommended to ensure it remains effective. Check for any signs of damage or deterioration, especially if you experience leaks or moisture issues in your home.

When to Call a Professional

While installing spray foam insulation in existing walls can be a DIY project, there are situations where hiring a professional is advisable:

  • Large-Scale Projects: For extensive insulation projects, professionals have the experience and equipment to complete the job efficiently.
  • Complex Wall Structures: If your walls have intricate designs or hard-to-reach areas, professional installers can ensure thorough coverage.
  • Health Concerns: If you have respiratory issues or sensitivities to chemicals, it’s best to leave the installation to professionals who can take proper safety measures.


Installing spray foam insulation in existing walls is a highly effective way to improve your home’s energy efficiency, comfort, and indoor air quality. By following the steps outlined in this guide, you can achieve a successful installation, whether you choose to tackle it yourself or hire a professional. Proper preparation, careful application, and attention to detail will ensure that your insulation project delivers the desired benefits for years to come.