When working with spray foam insulation, many people are concerned about whether they can apply the insulation to certain materials. After all, spray foam is known to have certain chemicals that might be detrimental to the integrity and longevity of some materials.

When it comes to the underside of the metal roof, it’s possible to use spray foam there. It can help close up leaks in the roof, creating a barrier against pests and the elements. The addition of thermal spacers can help spray foam adhere well to metal structures.

While spray foam can extend the life of your roof, improve indoor air quality, prevent pollen infiltration, and create an air-tight seal, it’s also highly expensive. Plus, it’s difficult to work with. Let’s look at the use of spray foam on metal roof undersides and discuss some alternatives. 

Can You Spray Foam The Underside Of A Metal Roof?

The short answer is Yes. Since spray foam is made of polyurethane, it will adhere to most surfaces. It includes the metal roofing that is found on many homes and buildings.

Suppose there’s a leak in your roof, and water is dripping through the metal onto your ceiling. In this case, you can apply spray foam insulation to the underside of the roof to stop the water from coming in.

The spray foam will create a barrier preventing the water from seeping through. It’s a great way to temporarily fix a roof until you can get a more permanent solution, such as replacing the metal roof.

Another common question is whether spray foam is safe on a metal roof. The answer is Yes.

However, proper insulation is a must when applying any type of foam to the underside of a metal roof. You don’t want the foam to be in direct contact with the metal because it can cause the metal to deteriorate over time.

The best way to apply spray foam insulation to the underside of a metal roof is by using thermal spaces. These are areas where the metal is not in direct contact with the foam.

For example, you can create a thermal space by placing a layer of fiberglass insulation between the metal roof and the foam. It will protect the roof from the foam’s chemicals while providing adequate insulation.

Another option is to use closed-cell spray foam. It is denser than open-cell foam and will not deteriorate the metal roof over time.

Besides insulation, spray foam can also be used to fix other problems. For instance, it can help to seal around pipes and ductwork that run through the roof. It will prevent air and water leaks.

It’s also great for filling in gaps and cracks. Doing this will make the roof more energy-efficient and prevent pests and other critters from getting into the building.

Spray foam insulation also prevents condensation on the metal roof. Applying a light layer of spray foam will create a vapor barrier that will keep the metal roof dry. A 1-inch layer of spray foam should be sufficient for this.

Advantages Of Applying Spray Foam To Metal Roof Underside

Applying spray foam to the metal roof underside has the following benefits.

  • Reduces Energy Bills: You can expect up to a 30% reduction in your energy bills by doing this. The foam acts as an insulator, thereby reducing the amount of heat that escapes from your home. 
  • Protects Against Mold and Mildew: The foam creates a barrier between the metal roof and the elements, preventing moisture from seeping in and causing mold and mildew to grow.
  • Extends the Life of Your Roof: By protecting your roof from the elements, you can expect it to last much longer than it would without the foam.
  • Reduces Noise: The foam absorbs sound, reducing the amount of noise that comes into your home from outside.
  • Improves Indoor Air Quality: The foam prevents airborne pollutants and allergens from entering your home, improving air quality. As a result, you and your family can enjoy better health.
  • Prevents Pollen Infiltration: If you or anyone in your family has a pollen allergy, this can be a lifesaver. By keeping pollen out of your home, you can avoid the sneezing, sniffling, and watery eyes that come with it.
  • Air-Tight Seal: No other product comes close to providing the air-tight seal that spray foam does. That’s because it expands to fill any void, creating a virtually impenetrable seal.
  • Consistent R-Value: Spray foam does not settle. So, it provides a consistent R-value throughout its lifespan.

Disadvantages Of Applying Spray Foam To Metal Roof Underside

Although there are many advantages of applying spray foam to the metal roof underside, there are a few disadvantages.

The most significant disadvantage is the initial cost. Spray foam is more expensive than traditional insulation, so you must factor that into your budget.

Another potential downside is that the foam can be difficult to work with. It is essential to follow the manufacturer’s instructions carefully to avoid accidents.

Some people are concerned about the chemicals used in spray foam. However, most manufacturers use safe, non-toxic chemicals.

How to Properly Apply Spray Foam Metal Roof Underside 

Here’s a step-by-step guide on how to properly apply spray foam to the underside of a metal roof:

  • Make sure the surface is clean and dry before starting.
  • Start by applying a continuous bead of spray foam around the roof’s perimeter.
  • Then, fill in the middle area with spray foam, working in small sections until the entire underside of the roof is covered.
  • Be sure to apply the foam evenly and smoothly to avoid any gaps or voids.
  • Once the foam is applied, let it cure for 24 hours before installing the metal roofing panels.
  • It will ensure a strong and secure bond between the roofing and the spray foam.

While it may seem simple, it’s best not to DIY this project. For one, spray foam can be messy and difficult to work with. Second, it’s essential to have a professional ensure that the job is done correctly and that there aren’t any gaps or voids in the spray foam application.

You should also consider the weather when doing this project. The spray foam may not cure properly if it’s too hot or humid. Likewise, the foam may not expand properly and crack if it’s too cold. The best time to apply spray foam is in the spring or fall when the weather is moderate.

What’s The Best Insulation To Use on the Underside of The Metal Roof?

Here are some options you can use other than spray foam to insulate the underside of the metal roof.

Fiberglass Batts

Fiberglass batts are the most commonly used type of insulation. They’re made with fiberglass fibers that are spun into a mat and bonded together with resin.

The batts are available in different widths to fit standard stud spacing and are typically 16 or 24 inches wide. Since they’re cheap and easy to work with, fiberglass batts are a popular choice for DIYers.

Cellulose Loose-Fill Insulation

Cellulose is made from recycled paper products that are treated with fire retardants and other chemicals to make them resistant to pests and mold. Cellulose loose-fill insulation involves blowing small particles to the desired area using an industrial blower.

Cellulose is a good option to insulate a difficult space, such as around plumbing pipes. It’s also environmentally friendly.

Foil Bubble Insulation

Reflective foil or foil bubble is a waterproof material consisting of two layers of aluminum foil with a layer of air bubbles. It’s often used to insulate ductwork and water pipes.

Foil bubble insulation is easy to install and can be cut to fit around obstacles. It’s also good at reflecting heat, so it can help keep your home cooler in the summer.


Evan has decades of experience as a project manager for large-scale commercial renovation home-building projects throughout the US. Currently, Evan runs a successful construction management company in Virginia.

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