The Best Insulation for Commercial Roofs: SPF vs Rigid Foam Board

Commercial roofing is a significant expense, running from 5 figures on small roofs to 7 figures on larger ones. Understandably, it's a price you're probably reluctant to pay and may even try to put off for as long as possible. 

The good news is that one part of your commercial roof can help you save money, producing, in some cases, a net profit for your business after 5 years. 

What is this factor? The insulation.

At GIDEON, we've been counseling building owners and property managers for over 20 years on how to effectively do commercial roofing from both a quality and cost perspective. In this article, we'll compare the 4 most common kinds of commercial roofing insulation in terms of effectiveness, upfront price, and long-term savings. By the end, you'll have a clearer idea of which option will work best for you.

Table of Contents

• How to Evaluate Insulation
• Rigid Foam Board
• Expanded Polystyrene (EPS)
• Extruded Polystyrene (XPS)
• Polyisocyanurate (Poly-ISO)
• Spray Polyurethane Foam

How to Evaluate Insulation

Before we can compare insulation types and make a decision, we first need to know how to evaluate the materials we're comparing.

Fortunately, it's not too difficult. There are 4 primary factors to consider: 

#1 R-value: This measures how well a material resists conducting heat. The higher the R-value, the better the "thermal resistance" of the material. R-value is measured per inch. Thus, 3 inches of insulation material with R-4 will be labeled "R-12 Insulation".

#2 U-value: This measures the heat transfer rate through a building component as a whole, for example, an entire roof assembly. It considers that the insulation is (usually) not a single, solid component but requires screws to hold it in place and has gaps between boards. The lower the U-value, the better the insulation of the entire assembly.

#3 Upfront Price: Price, of course, is essential. While the best, most expensive insulation might be the most efficient, not everyone has the budget for it, and not everyone needs the highest level of efficiency. 

#4 Long-term effect on operating costs: The great thing about insulation is that, unlike other components of the roof assembly, which are a net cost, a better insulating system can actually create net long-term savings for your business by substantially reducing energy bills. In other words, with the right insulation, your roof could produce a net profit in 5 years (more on this later).

Rigid Foam Board

The standard insulation used on a commercial flat roof is rigid foam board. It comes in rigid, lightweight panels that can be cut and shaped to fit the roof. 

There are 3 types:

#1 Expanded Polystyrene (EPS)

Expanded polystyrene, or beadboard, is the cheapest and least effective type of foam board. 
It's created by expanding spherical beads of polystyrene in a mold using heat and pressure. This results in a network of tightly packed beads with small voids between them (imagine a styrofoam cooler).

Key Features of EPS include:

R-value: EPS has a stable long-term R-value of approximately R-4 per inch.

U-value: Like all rigid foam boards, EPS is often installed with metal fasteners and comes in multiple sheets. This creates "thermal shorting" (when the metal screws conduct heat through the board) and "thermal bridging" (when air moves through the gaps in the foam boards). Proper installation is critical to maintaining a solid U-value throughout the roof's life. 

The key to maximizing U-value on any rigid foam board system is to use two layers of insulation and overlap the seams. You'll want to discuss installation with your contractor to ensure they follow this practice.

Price: EPS is the least expensive of your insulation options.

The long-term effect on operating costs: EPS is least likely to significantly reduce your long-term heating and cooling costs because of its low R-value. 

Another aspect of EPS to consider is vapor permeability. EPS can absorb vapor and water, slightly reducing its insulating capacity, but its breathability means it will dry out quickly and perform as it originally did.

#2 Extruded Polystyrene (XPS)

Extruded polystyrene is made by mixing polystyrene and a foaming agent and then extruding the mixture through a die to form a uniform piece of board stock that is a closed-cell material. You'll often see XPS used in home siding as a pink-colored board.

Key Features of XPS:

R-value: XPS typically has an R-value of approximately R-5 per inch, but it will lose some performance over time as it ages and off-gasses. 

U-value: Being a rigid foam board, XPS will suffer from some thermal shorting and bridging, and good installation is necessary to minimize these effects.

Price: XPS is the next level up in terms of pricing.

The long-term effect on operating costs: Because of its higher R-value, XPS is more likely to reduce your energy consumption and, thus, your long-term utility costs.

Regarding vapor resistance, manufacturers claim that XPS does not absorb moisture, but contractors have sometimes discovered that it will absorb and retain vapor.

#3 Polyisocyanurate (Poly-ISO)

Polyisocyanurate, usually called "Poly-ISO," is produced through a reaction between polyol and isocyanate, creating a highly cross-linked, thermosetting plastic.

Key Features of Poly-ISO:

R-value: Poly-ISO has an R-value of around R-6 to R-7 per inch, making it the highest-performing foam board.

U-value: As with all foam boards, thermal shorting and bridging can occur, and you should discuss installation procedures with your contractor to ensure they are following best practices to minimize this.

Price: Poly-ISO is the most expensive foam board.

The long-term effect on operating costs: With the highest R-value of all foam boards, polyiso can help you save money on heating and cooling costs, provided it is installed well.

Spray Polyurethane Foam

Spray polyurethane foam (SPF) is an insulation material used in commercial roofing applications for over 50 years. It's a liquid that, when applied to the roof's surface, expands into a foam, creating a solid, extremely tough layer of insulation that (with an additional coating or single-ply membrane to protect it from the sun) can serve as an entire roofing system.

Spray foam insulation is a two-component mixture composed of isocyanate and polyol resin, making it essentially a liquid-applied version of poly-ISO board. These two components come together at the tip of a gun and form an expanding foam that is sprayed onto the roofing surface. The foam then expands and hardens, creating a solid, seamless, monolithic layer of insulation.

Key Features of SPF:

R-value: SPF has an R-value per inch of 6.5-7.0, making it comparable to a Poly-ISO board. While SPF can lose a small amount of R-value over time, this won't occur in a spray foam roofing system, as the foam is completely encapsulated by the elastomeric coating, sealing it off from the environment. 

U-value: This is where SPF outperforms every other insulator on the market. Because it is sprayed on, it forms a continuous layer of insulation with no gaps, seams, or fasteners to create thermal shorting and bridging. This solid envelope gives SPF (properly installed) the best U-value of any insulating system. 

Price: SPF is the most expensive insulating material on our list. However, because SPF is not just an insulator but forms a functional roofing system on its own, the comparison is not really apples to apples. When comparing SPF to other insulating materials, you have to take into account the cost of the roofing system as a whole.

The long-term effect on operating costs: With the highest R-value and U-value of any system, spray foam insulation will save you the most money in the long run. A Texas A&M study concluded that spray foam roofs can pay for themselves completely in just 5 years. 

Unlike foam boards, SPF is completely vapor proof, absorbing no vapor at all.

The Upshot

We've all struggled to balance upfront savings with long-term quality. Rarely are the stakes higher than with a complex asset like a commercial building, where your choices have a large, enduring financial impact. Now that you've learned from GIDEON how you can transform a seemingly costly decision - like selecting the right roofing insulation - into a significant source of savings, hop on a call with us. In a free 30-minute call, our nationally-recognized roofing experts can provide you with customized guidance.