Will Foam Insulation Work If I Already Have Fiberglass in My Walls?

If you’ve been thinking about improving your home’s insulation, chances are you’ve come across information about injection foam—especially if your house has older fiberglass batts or blown-in cellulose in the walls.

It’s a popular topic on social media, and one question we see time and again is:

“How can I be sure the foam is truly filling the wall cavity if there’s already fiberglass batting in there? I want better insulation, but I don’t want to waste my money.”

That’s a valid concern—and we get it. You want to make sure that any upgrade you invest in is going to work. So let’s walk through what actually happens during our RetroFoam® injection process, how it interacts with existing insulation, and why it’s still a smart solution for older homes.

First, What Is the Injection Foam Process?

Our injection foam process is designed specifically for existing homes—typically older structures that were insulated with fiberglass or cellulose decades ago and are no longer performing well.

Here’s how the process works:

1. We remove a row of siding around the perimeter of your home—usually just a few pieces—so we can access the stud cavities behind the exterior wall.
2. We drill small holes between the studs to reach each cavity.
3. We inject a specially formulated foam into the wall cavity. This foam is lightweight, frothy, and highly flowable—kind of like a milkshake in consistency.
4. As the foam fills the cavity, it expands slightly and conforms to every nook and cranny, pushing aside and compacting any old insulation in the wall.

Once the foam has filled the cavity, it hardens into a dense material that provides excellent air sealing and thermal performance. Then we patch the holes, replace the siding, and clean up the site.

What Happens to the Old Insulation?

This is where many homeowners have concerns. If you already have fiberglass batts or cellulose in your walls, how can foam possibly get through or around it?

The answer lies in the foam’s design. Injection foam used for retrofitting is engineered to be low-viscosity and expand under light pressure. As it’s injected, it doesn’t just sit on top of the fiberglass—it pushes into the cavity, compressing and displacing the existing insulation material.

Think of it like this: If you tried to shove a thick pillow into a plastic bag that already had some stuffing in it, you’d compress what’s inside and still fill the bag. The same principle applies here. The foam compacts the old insulation and fills in all the gaps that the fiberglass or cellulose never reached.

So no, you’re not “trapping” the foam above a layer of fiberglass. You’re creating a denser, more insulated wall by maximizing the cavity’s potential.

Why Is This Worth Doing?

Here are just a few reasons homeowners choose to go this route—even with fiberglass already in their walls:

• Air sealing: Fiberglass alone doesn’t stop air movement. Foam creates a more airtight barrier, helping to keep warm air in during winter and hot air out during summer.
• Higher R-value: The added foam increases your wall’s insulation value—resulting in more comfort and better energy efficiency.
• Noise reduction: Foam insulation can also reduce outside noise more effectively than traditional batts or blown-in materials.
• Moisture control: Sealing gaps in your walls can also help prevent moisture infiltration, which reduces the risk of mold and mildew.

In short, you’re not just adding insulation—you’re transforming how your walls perform.

Will It Work in Every Case?

In most cases, yes. We’ve insulated thousands of homes across Indiana with this process—even those built in the 1960s and ‘70s that were filled with fiberglass batts. As long as the wall cavity is accessible and in reasonable condition, we can inject foam and significantly improve its performance.

If we run into a situation where something unusual is going on behind the walls—such as insulation that’s completely deteriorated, water damage, or structural issues—we’ll let you know. Our team does a thorough inspection before we start the job, and we’re always upfront about what we find.

The Bottom Line

If you’re worried that foam won’t work because your home already has fiberglass insulation, rest easy—it will. Our injection foam is designed to fill, pressurize, and insulate your wall cavities more completely than your original materials ever could.

And if you’ve been feeling drafts, noticing high energy bills, or struggling to keep your home comfortable in extreme weather, this could be the upgrade that makes all the difference.