spray foam installation

Spray Foam Roofing vs Spray Foam Insulation: 4 Differences

Greg PalyaSpray Polyurethane Foam Roofing

If you’ve ever asked yourself the question, “Is spray foam that’s used for roofing the same foam that’s used for interior wall insulation?” we’ve got the perfect article for you.

Since West Roofing Systems has been installing spray foam roofs for more than 40 years, we have that side covered.

For the insulation side, we interviewed Curt Janzen, Owner of Beyond Foam Insulation.

For a quick summary, the four differences are:

  1. The actual weight of the spray foam used
  2. Closed-cell vs open-cell spray foam
  3. One materials costs double what the other does
  4. Both have several problems with application

Also, included in this article is a bonus section: What would happen if we used spray foam for roofing on interior walls, and vise-versa?

Let’s get right into it.


The 4 differences in spray foam roofing vs spray foam insulation


Difference #1 – We use different weights of spray foam

Spray foam that’s used for roofing is what is called 3lb foam. This means that a cubic foot, 1ft x 1ft x 1ft weighs 3 pounds.

Spray foam that’s used for insulation is what is called 2lb foam. This means that a cubic foot, 1ft x 1ft x 1ft weighs 2 pounds.

Same size, different weight.

This just means that the foam used in roofing is denser than the foam used in insulation.

Both are created by using an isocyanate and a resin mixed in a 1:1 ratio. The isocyanate is the same in both formulas, but the resin is what’s different that creates the different weight of foam.

Difference #2 – Closed-cell and open-cell

Any foam that is 2 lbs. or higher is considered closed-cell.

This means that both spray foam used in roofing (3 lbs.) and spray foam used in insulation (2 lbs.) are considered closed-cell.

While that’s true, there are other spray foam insulation contractors out there who utilize 0.5 lb. foam.

The biggest difference between open- and closed-cell is how they create their R-value. Closed-cell creates its R-value by trapping cell gases for insulation. Open-cell creates its R-value by trapping air for insulation. The more stagnant that air is trapped inside those open cells, the better your R-value is.

The R-value for open-cell spray foam is 3.7 per sq. in.

The R-value for closed-cell spray foam is 6.6 per sq. in.

One negative to using an open-cell foam for wall insulation is that you need to include a vapor barrier poly on it. With closed-cell, it’s a vapor barrier, insulation, and air barrier all-in-one.

Difference #3 – spray foam roofing costs more

One of the main reasons that 3lb foam costs more is because of the higher density. It takes more chemical to produce the same 1’ x 1’ square. On average, a set of 2lb foam should produce 4500-5000 BF (board feet) and a set of 3lb is around 2600 BF.

But the clearest difference between the price of the two is found in the foam’s density, thereby impacting the yield.

Spray foam used for commercial roofing costs approximately $4-$7 per sq. ft.

Spray foam used for wall insulation costs approximately $3 per sq. ft.


Why are they different?


They’re different because there’s an obstacle that spray foam insulation usually doesn’t have to hurdle, which is UV rays. Since spray foam insulation is only used indoors, they avoid the sun.

If you’re unaware, UV rays damage spray foam by an average of 1/16th of an inch per year.

On a roofing application, a roofing contractor adds two layers of coating on top of the foam to protect it from the UV rays.

A spray foam roofing contractor also adds embedded granules into the top coating to add strength and durability to withstand foot traffic, wind, and blowing debris.

The added two layers of coating, embedded granules, and the reduced yield are what make a spray foam roofing project more costly per sq. ft. than a spray foam insulation project.

Difference #4 – problems with application

There are problems that both spray foam roofing and spray foam insulation have with application, they include:

Cannot apply spray foam to a wet surface

Spraying foam to a wet surface will cause the isocyanate to react with moisture before it has a chance to react to the resin.

On a spray foam roof, the result can cause blistering.

On spray foam insulation, the foam will make a “popping” sound as it peels away from the substrate.

Both have opportunities for applicator error

On a spray foam roof, you can spray foam too thin, therefore not giving the appropriate R-value, or you can miscalculate the three energies of spray foam roofing, which can cause issues with the curing process.

On a spray foam insulation project, the applicator can spray too thick, too fast.

Too thick, too fast means the applicator didn’t wait the appropriate time in-between passes for the foam to cool before you can add another pass.

Between pass #1 and pass #2, you must wait 10 minutes.

Between pass #2 and pass #3, you must wait 1 hour.

NOTE: times can vary heavily depending on the manufacturer and the foam system in general. One pass foams are becoming popular which can handle 4” at one time.

If the applicator doesn’t allow enough time to cool, the foam can combust and possibly start a fire!

This is why having a certified and experienced spray foam installer is important.

Both have temperature restrictions

For spray foam roofing to be installed properly on a commercial building, the temperature needs to be 50 degrees Fahrenheit and rising on the surface it’s getting installed on. This is different than ambient air temperature.

For spray foam insulation to be applied correctly, most manufacturers have a cold weather formula that allows installation at 14 degrees Fahrenheit.

NOTE: one last problem that’s specific to spray foam roofing is wind and overspray potential. Here is video explaining overspray and how it’s prevented and cleaned if necessary:

Bonus Section: What would happen if we used spray foam for roofing on interior walls, and vise-versa?

Using spray foam roofing for interior insulation

Because of R-value and the cost of material, you would never want to use 3lb spray foam on the interior walls of a home.

You would need to use twice as much material to get the same amount of thickness. You would use more product to get the same R-value.

Scientists have proven that for interior wall insulation, 2 lb. foam is the point at which you can maximize R-value with the most efficient use of materials.

Spray foam insulation used for commercial roofing

Using 2 lb. foam wouldn’t be the best fit for roofing purposes because 2 lb. foam is meant to react quickly and be hard in a few seconds. This is no good for roofing because it doesn’t have as much of a self-leveling attribute that 3lb foam has.

3lb reacts slower, therefore it has time to self-level itself, which is important when you’re custom-designing a roof system to guide water towards drains and gutters.

3 lb. foam is also 50% harder than 2lb foam, which is important when handling foot traffic, weather, and debris.

The differences in spray foam types are colossal

Hopefully today, you learned that it’s never a good idea to use 3lb spray foam for interior wall insulation, and it’s equally a bad idea to use 2lb foam for roofing.

As any contractor out there will tell you, it’s beneficial to the contractor and the client when materials maximize their efficiencies.

If you’d like to learn more about spray foam insulation, view Beyond Foam Insulation’s video on how spray foam is more effective than other wall insulations:

And if you’d like to learn more about spray foam for roofing, please view our Ultimate Guide to Spray Foam Roofing, which answers the most popular questions building owners have, such as:

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

What kind of warranty can you get with a spray foam roof?

What kind of energy bill savings can I expect with a spray foam roof?


the ultimate guide to spray foam roofing

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.