Looking to add insulation to existing walls? You might be wondering what type of insulation material you should use.

While there are several insulation materials that you can add to existing walls, spray foam is the easiest and most pliable material to install. It is budget-friendly and does not only help to resist heat from penetrating inside, but it also serves as a moisture barrier and prevents condensation. In fact, builders recommend it more compared to other types of insulation solutions

Thus, spray foam insulation in existing walls is worth it.

Not yet convinced? This article discusses why spray foam is an excellent insulation material for existing walls. 

What Is Spray Foam Insulation?

Made by combining two synthetic materials, spray polyurethane foam is an insulation solution, as well as an air barrier, that comes in two forms: closed-cell and open-cell spray foam. Essentially, it is used to fill and seal gaps and cavities within different assemblies in both residential and commercial buildings.

Spray foam insulation is commonly sprayed into walls, floors, and ceilings. It can also be used to insulate attics, crawl spaces, and rim joists in newly constructed and existing homes. Aside from that, it is also an excellent insulation material to use in hard-to-reach and hard-to-fill areas too, such as spaces around light fixtures, and electrical outlets, as well as the space where doors and windows meet.

To sum it up, it can be used in gaps, cracks, crevices, and open cavities as long as it is accessible.

Can You Use Spray Foam In Existing Walls?

Spray foam can be used in both newly constructed and existing homes. But unlike other insulation materials, it is the most convenient and pliable to install within existing assemblies such as walls. The material is installed by simply spraying or blowing it into the walls. Builders, and even homeowners, can do it by drilling holes in the walls. These holes will serve as channels to blow the spray foam into the walls.

So, yes. Spray foam insulation can be used to insulate existing walls. Professional builders do highly recommend it too when insulating existing assemblies.

On the other hand, keep in mind that you cannot add spray foam to existing walls with fiberglass insulation installed within it. But if you are determined to replace it, then you will have to remove it first. The same goes for moldy and damp insulation—no matter the type of material it is.

The Pros and Cons of Spray Foam In Existing Walls

Aside from it being the most convenient material to install on existing walls, there are other perks spray polyurethane foam provides when used on walls. At the same time, there are also a few drawbacks that homeowners need to be wary about. 

The Benefits:

  • Spray foam offers high-quality thermal performance
  • Spray foam prevents condensation within assemblies
  • Spray foam helps reduce energy consumption better than other materials
  • Spray foam does not lose its shape, sag, compress, or settle over time
  • Spray foam blocks unwanted sound and reduces noise
  • Spray foam adds to the walls’ sturdiness
  • Spray foam lasts longer 

The Disadvantages:

  • The material is expensive
  • Installation cost is significantly high
  • It usually requires a professional to install it
  • DIY is not recommended
  • High exposure to it may prompt health risks

How to Add Spray Foam Insulation in Existing Walls

While spray foam is the easiest insulation material to install in existing walls, blowing it to the assembly is not a piece of cake. In fact, it requires an experienced person to do the job as there are different ways to add spray foam to walls, depending on the scenario. It is especially true if you do not want to remove your drywall.

Still, the best way to add spray foam to existing walls is to entirely remove the drywall. Professional builders explained that spray foam, when blown into walls, expands rapidly. And if installed incorrectly, it may cause the drywall to break due to the pressure the insulation material is putting inside the plasterboard.

Such a procedure is messy, laborious, and time-consuming. Not to mention that the removal of drywall alone requires an expert to complete the job appropriately. Hence, it is better to hire a team of professional installers.

On the flip side, there is also an alternative way to fill existing walls with spray foam without removing the drywall. And it is through drilling strategic holes in the walls. These holes will then serve as channels to blow the insulation material to the walls. Some use blown-in fiberglass too, instead of polyurethane foam.

But do take note that such a method is not highly recommended by professional builders.

Injection Foam Insulation

Another way to install foam insulation to existing walls is through injection foam insulation. Injection foam insulation is another type of insulation material that is virtually the same as spray foam. However, unlike polyurethane foam, it expands slowly when blown to enclosed cavities. Hence, it can be installed without removing drywall. 

Injection foam insulation can be installed in both interior and exterior walls. However, the process for adding insulation to existing exterior walls may differ depending on the type of siding. 

Here’s how: 

Exterior Wall Insulation

  • For brick exterior. Adding foam insulation for houses with bricks as wall exterior is a bit laborious and time-consuming. The process involves drilling tiny holes up into the mortar between the bricks. Then into each stud cavity from the top, in the middle, and at the bottom. 

    These holes serve as channels to blow the insulation material. Once done, simply fill the holes using standard gray mortar. 
  • For vinyl and aluminum siding. For vinyl and aluminum, the process is a bit easier. First, you remove a row of the siding around the house’s perimeter. This gives you access to the sheathing.

    Once done, simply drill small holes into each stud cavity then fill it with injection foam.
  • For wood siding. For wooden sidings, the process is almost like how foam insulation is injected into vinyl and aluminum siding. Simply remove a row of wood siding around the perimeter of the house and start drilling holes.

    However, if the siding cannot be removed, you will need to drill holes through the wood siding and then patch them after using wooden plugs. 

Interior Wall Insulation

For interior wall insulation, the process is the same as adding spray foam. Simply drill strategic holes in the walls. These holes will then serve as channels to blow the insulation material into the walls.

Hiring a Professional Installer vs. DIY

Do-it-yourself projects are a trend these days. And perhaps, you are wondering if you can install foam insulation to your existing walls without professional help. Of course, the answer is always yes.

Rather, the real question here is whether you should do it yourself or not.

As explained earlier, adding spray foam insulation to existing walls is a meticulous, time-consuming, and laborious process. Not to mention that the removal of drywall alone requires a skilled person to execute and complete the job appropriately.

Apart from that, hiring a professional installer can benefit you greatly in the long run. It will save you from spending more money on fixing issues that may arise due to poor installation.

Thus, it is indeed wise to hire a team of professional installers.

Why Should You Not Add Spray Foam Over Existing Insulation

As mentioned earlier, spray foam should not be added on top of any type of insulation material—specifically fiberglass. The reason is that doing so degrades the effectiveness of both insulation solutions.

But not only that! Here’s why you should not add spray foam over existing insulation:

  • Spray foam does not stick well to fiberglass insulation
  • It reduces its ability to resist moisture
  • Because of decreased moist resistance, mold and mildew will likely appear
  • It degrades the material’s thermal performance

All in all, you should not add spray foam over existing insulation materials because it takes away its primary role in reducing energy consumption.


Justin's been in construction for over 20 years in both new build and renovation. With experience in both commercial and residential construction, he specializes in healthier and more energy-efficient homes.

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