spray foam over a metal roof

What R-Value Can Be Achieved on a Bare Metal Roof?

Greg PalyaMetal Roofing, Spray Polyurethane Foam Roofing

Quick Summary: An R-value of 26 can be achieved on a bare metal roof. Adding more than a R-26 may result in diminished returns.

So, you have a metal roof, and you’re looking at increasing its R-value.

Or, your metal roof is uninsulated and you’re looking to get started.

Spray Polyurethane Foam can be installed over the existing metal roof. Its R-value is 6.5 per inch of thickness.

  • 1 inch = R-6.5
  • 2 inches = R-13
  • 3 inches = R-19.5
  • 4 inches = R-26

In most cases, installing more than 4 inches of foam isn’t worth it. The additional cost of more foam won’t offset with increased energy savings.

Here are some other questions you might have:

  • How much does spray foam over a metal roof cost per square foot?
  • What’s the scope of work when foam is installed over an existing metal roof?
  • How long does spray foam last?
  • Can’t I just install a roof coating system instead?

How much does spray foam over a metal roof cost per square foot?

$5 – $10 is a ballpark cost for a spray foam roof over an existing metal one.

Many factors influence price, such as:

The amount of R-value desired

A 2-inch application of foam will cost more than a 4-inch application.

More R-value = More product that’s used = Higher cost.

Finished spray foam roof over an existing metal roof

The desired warranty length

I’m unsure if you’re aware, but spray foam will degrade within 72 hours when exposed to UV rays.

To counter this, an elastomeric coating is always installed over spray foam.

The thickness of that coating is what determines the warranty length. The thicker the coating application, the longer the warranty is.

  • 20 mils of coating results in a 10-year warranty
  • 30 mils of coating results in a 20-year warranty

Longer warranty = More product that’s used = Higher cost.

Size of metal roof

The larger the roof is, the less expensive it will be per square foot. It’s just simple economies of scale. Granted, all job characteristics are the same: a 10,000 sq. ft. roof will cost more per square foot (not overall) than a 100,000 sq. ft. roof.

Contractor location

The closer a contractor is, the less expensive the project will be. Never choose a contractor solely based on price. There’s probably a reason why someone is cheaper than someone else. Workers with little experience, no safety protocols, no industry or manufacturer training, etc.

The #1 most important aspect of any commercial roofing project is the trust and professionalism that the job will be done right. You don’t want to spend $100,000, then the roof leaks, the contractor doesn’t pick up the phone, the VM box is full, and now lawyers are involved.  

spray foam roofing on a metal roof going around a pipe

What’s the “typical” scope of work when foam is installed over an existing metal roof?

  • Clean the existing metal roof via pressure washing
  • Dispose of loose debris
  • If desired, remove unused rooftop equipment, such as skylights. These can be sheeted over with ¾” plywood or new metal roof panels.
  • If desired and as needed, install new perimeter edge metal, drains, downspouts, and/or gutters.
  • Install spray polyurethane foam with proper thickness for R-value and proper slope (there are minimum thickness requirements based on metal panel profiles to achieve a watertight condition)
  • Install roof coating system (in two separate coats) over the foam
  • Embed granules into the top coat of coating
  • Final touch-ups and issue warranty

How long does spray foam last?

Spray foam roofs are renewable, so they can last as long as the building is up. Here’s an example:

Let’s say you install spray foam over your existing metal roof today. You have an average thickness of 1.5” of foam and 30 mils of coating to protect the foam from UV rays.

The thickness of the coating determines the warranty length. 30 mils of coating would grant a 20-year warranty.

Over 20 years, the 30 mils of coating will naturally wear down. The amount of coating that remains in year 21 depends on foot traffic, weather, maintenance, etc.

But in year 21, there will be some coating left.

We’ve been doing spray foam and coating roofs for 40+ years, and we’ve never seen (without an act of God or improper maintenance) where the coating worn off entirely before the warranty ends.

Coating manufacturer do extensive testing and are basically 100% sure that if their coating is installed right, and there’s no foul play, it will last the length of the warranty.

In year 21, a contractor can clean the roof and install more coating.

If 30 mils of coating are installed again, a new 20-year warranty would be granted. The cycle never stops. This is how a spray foam roof is truly renewable and can last as long as the building stands.

NOTE: You may be concerned about the spray foam over this long period. Spray Polyurethane Foam doesn’t lose any R-value, waterproofing capabilities, or any other performance characteristics over time.

There are roofs sprayed in the 1980s that’ve been recoated 2-3x. They still have the original foam on them.

metal roof that's restored with spray foam roofing

Can’t I just install a roof coating system instead?

You might. If it meets a coating system’s installation requirements.

Roof coating systems on their own (no spray foam underneath) are very popular these days.

Coating can be installed right over an existing metal roof, and it’s less expensive than a spray foam and coating system.

The problem is that coating systems alone will not increase the R-value of the existing roof. Sure, they’ll make your metal roof watertight and seamless and grant you a brand new and renewable warranty, but no R-value increase.

Want to learn more?

Video – Case Study – Start to Finish – Spray Foam Over a Low-Slope Metal Roof

Video – Spray Foam Roofing Over an Existing Metal Roof

Article – 7 Problems with the Spray Foam Roofing System

The spray foam roofing cheat sheet - download now
About the Author

Greg Palya

Greg Palya is the Digital Content Manager of West Roofing Systems, Inc. He has a B.S. in Marketing from the University of Akron and an MBA in Marketing from Walsh University. When he's not trying to teach others about spray foam roofing and silicone roof coatings, you can find him on the basketball court or golf course.