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What are the Insulation Types Available for Flat Roofing?

October , 2022 | 9 min. read

By Johnny Girard

Tapered insulation on a flat roof

The primary function of any roof assembly is to protect the building from the impacts of Mother Nature. Roof assemblies accomplish this goal through the use of waterproofing materials. Materials such as flat roofing membranes, flashings, and insulation as well as other components.

Roof insulation is one of a building's primary defenses against heat loss(winter) or heat gain(summer), more on that later. The use of insulation will benefit you by adding to the functionality of the roof system and the building's energy efficiency. 

So, what are the insulation types available for flat roofing?

roofer installing a roof drain on a flat roof

Many of our clients over the last 3 decades came to RoofCrafters Roofing  asking, “what type of insulation would be best for my flat roof?” It’s an important question to get answered before you start getting estimates for a flat roof replacement. 

Especially if your roof is either ponding water or the sun's heat can penetrate inside. Neither of these situations is favorable for you to have a long-lasting high-quality flat roof system. One that’ll last you for many years to come.

So in this article, we’ll cover the following:

  • Flat roof, low slope roof, and steep slope roof construction

  • What does an R-value mean for your building or flat roof

  • What are the best insulations types for flat roofing

To get a better understanding of insulation types we need to understand the 3 commercial roofing construction types. 

Flat roof, low slope roof, and steep slope roof construction

  • Flat roof construction
  • Low-slope roof construction
  • Steep slope roof construction

Flat roof construction is common in commercial buildings. It gives the engineers and architects more options for mounting rooftop equipment. Flat roof construction requires a roofing contractor to slope the roof using insulation. Eliminating ponding water on a flat roof material is both a manufacturer and building code requirement. The tapered insulation used to achieve the slope also provides the building with a higher R-value. 

However, the downside is that it does take more labor and materials to accomplish the slope. Therefore, it’ll increase the roof system's price, impacting the project's cost.

Low-slope commercial construction is when the roof already has a positive slope to drain water from the rooftop. A low-slope roof does not require tapered insulation. Low-slope roofing can be insulated from underneath(the attic) or the rooftop side. Depending on the deck type and roof system specifications. 

The downside is that a low slope is not good to install some roofing materials. For example, a low slope roof with a screw-down metal roof(PBR-panel) is not the best option. In most cases, it will result in leakage and create ongoing maintenance issues that another system would not have on a low-slope roof.

crew installing commercial flat roof

A steep slope roof on a commercial property is identical to or similar to residential construction. The most common construction types on steep slope commercial roofs are gable or hip roofs. Steep slope roofing is constructed with metal or wood trusses. It also can be custom framed with wood 2x framing materials. Insulation is typically installed in the attic or the underside of the decking.

However, in steep slope commercial construction, insulation can be installed from the rooftop side. When the roof decking doubles as the ceiling inside, there is no attic space. This is when nail-base insulation or vented nail-base insulation can be installed. Nail-base insulation is designed for steep slope construction. It’ll provide a way to achieve the desired R-Value when there’s no attic space to install conventional or spray foam insulation. 

Now that you know the 3 types of commercial roof construction, let’s discuss what R-value means and its impact on a commercial building.

What does the R-value mean for insulation types?

R-values are calculations used to determine the thermal resistance or thermal insulation. This can be a single product such as roof or attic insulation, roofing materials, doors, or windows. It also can be and is calculated by combining products. Examples would be all the materials it takes to build a wall. The wood, concrete, brick, insulation, and sheetrock. For a roof assembly, roof deck type, roof insulation, and the roofing membrane combined would provide the R-value. 

flat roof work-1

The greater the R-Value, the greater the resistance will be, which will provide a building with better thermal resistance. The objective is to keep hot air out in the summer and keep hot air in during the winter.

metal retro-fit using Iso insulation

Having a higher R-value reduces energy consumption, lowers utility bills, and helps your HVAC unit run more efficiently. There’s no downside to having a higher R-value. Unless you consider the upfront investment a downside. It will cost more, to get a higher R-value roof insulation we must use thicker insulation or higher quality insulation. 

Now that you know how the R-value impacts commercial buildings, let’s discuss the best insulation types for roofing.

What are the best insulation types for flat roofing?

There are many types of insulation used in flat roofing. Oftentimes choosing the right insulation comes down to 3 factors.

The deck type, the flat roofing materials being installed, and the desired R-value.

Here are the 7 most frequently used insulation types installed under flat roofs:

metal retro-fit insulation on a flat roof

  1. Polyisocyanurate (Polyiso or Iso)

  2. Expanded polystyrene (EPS)

  3. Extruded polystyrene (XPS)

  4. Gypsum board

  5. Cement Board

  6. Perlite

  7. Wood fibered board


Polyisocyanurate insulation (Polyiso or Iso)

Polyiso is closed-cell insulation. It comes with varying face sheets but the most common in roof assemblies are fiberglass face sheets. It comes in 4-foot by 4-foot sheets or 4-foot by 8-foot sheets in thickness ranging from ½ inch up to 4 inches. 

tapered insulation over steel deck

Polyiso is also made in tapered sheets to provide positive water drainage on flat roof assemblies. Tapered sheets come in ⅛ inch per foot, ¼ inch per foot, and ½ inch per foot slopes. Different roof slopes allow contractors flexibility in achieving maximum performance. This is important for water drainage and R-Value.

Expanded polystyrene (EPS)

tapered insulation for flat roofing

Expanded polystyrene is one of the more cost-effective insulation. Its rigid insulation that’s commonly used under white single-ply roofing membranes. It has a lower thermal resistance. Therefore to get a higher R-value you may need to combine it with other insulations. If you are not concerned with increasing the R-Value of the roof assembly EPS insulation can be a good option. It's available in various thicknesses of flat 4 ft by 8 ft sheets. EPS also comes in tapered panels to create a slope on flat roof construction. 

Extruded polystyrene (XPS)

XPS is a foam plastic insulation that has low water permeability, therefore it's highly water resistant. It has a higher thermal resistance than EPS insulation. Its commonly used on roof assemblies that have greater exposure to water, such as green roofs. Also under ballast roof systems. It only comes in flat 4 ft by 8 ft sheets. Tapered is not an option.

Gypsum board


flat roof assembly with gypsum insulation

Commonly used in roof assemblies, it’s a gypsum core faced with fiberglass mats. It’s often used to provide a fire-resistance rating. It’s a strong insulation board that also provides wind, hail, and puncture resistance ratings. Typical glass-faced gypsum boards are used with other insulation boards. It's commonly used as a cover board to achieve one of the mentioned ratings.

Cement Board


roofer installing insulation on a flat roof

Cement boards are made up of cement and aggregates with fiberglass facings. Because XPS and EPS don’t allow the roof to have a fully adhered roofing membrane. A cement board is commonly used as a cover board with EPS and XPS when the roofing is a fully adhered system. It is mold, fire, and water resistant. It's used to provide fire resistance over combustible materials. It withstands prolonged moisture. It’ll also provide a high impact rating.



Perlite insulation boards are commonly used in conjunction with hot tar and modified bitumen roofing. Perlite is a volcanic glass and is made with wood fibers. It is a rigid insulation board. It comes in flat or tapered boards 4 ft by 4ft. Like the Gypsum and cement board, it is typically used as a cover board to achieve the desired outcome in a roof assembly.

Wood fibered board

Woof-fibered boards come in a variety of density strengths. The wood fibered board installed in roof assemblies are high density which allows for walkability. Classified as environmentally sustainable. These are completely natural, recyclable, eco-sustainable insulation boards. Similar to gypsum, cement, and perlite it can be used by itself or as a cover board with another insulation. 

Ready to Hire a Flat Roof Contractor?

Now that you’ve learned the difference between flat roofs and low-slope roofing. You understand how having a higher R-value will impact your overall energy efficiency. You’ve also learned about the 7 most popular insulation types and their benefits. You’re probably ready to hire a flat roof contractor.

Our team at RoofCrafters Roofing has been helping clients in South Carolina, Georgia, and Florida for nearly 3 decades. We’ve done so by helping them understand which insulation types to use to get the most out of their flat roof investment.

If you're in one of our service areas and you'd like to speak with one of our flat roof experts. Schedule your inspection today and we'd love to help guide you through your flat roof replacement.

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Johnny Girard

At RoofCrafters, our mission is to provide job opportunities for others to thrive and grow while making a meaningful impact within our communities.