Flat Roof Insulation: why insulate the roof at all?

Insulation plays the role of an additional barrier inside a ‘roofing pie’. Firstly, it prevents heat loss in winter. Secondly, it keeps the building cool in summer. These are the main functions of any insulation. But that is not all. According to experienced roofers, flat roof insulation protects the building from street noises and even mechanical damage.

Three ways to insulate any flat roof properly

Each roofing project has its own insulation, appropriate materials, thickness and density requirements and installation method. There is no ideal insulation approach that fits all commercial and residential buildings.

Before choosing the right method of roof insulation, let’s answer two questions:

  • Is your roof vented or unvented? In the first case, there should be a ventilated space between the roof and the ceiling – so called air gap. There is no such space in unvented roofs.
  • What R-value is good for your roof? R-value (per inch) is an indicator of thermal insulation. The higher the number, the better the insulation prevents heat loss.

Depending on the answers, isolation can be:

  • Exterior (the insulation materials are placed above the roof sheathing)
  • Interior (the insulation materials are placed below the roof sheathing)
  • Combined (the insulation materials are placed both above and below the roof sheathing)

If your roof is vented, it is crucially important that the ceiling is as airtight as possible. If air tightness is not achieved, condensed moisture will appear on the surface of the sheathing several weeks/months after the project is completed. This is a direct path to mold and fungi.

Don’t forget to air seal, because it prevents the leaking of hot or cold air. Air sealing is as important as insulating your roof. But that’s a topic for another guide.

Unvented flat roof insulation

Insulating a flat roof with non-ventilated structure is much easier than insulating ventilated ones. A common difficulty with insulating ventilated roofs is the inaccessibility of the places where the insulating material needs to be installed.

There is no such problem with unvented structures. This is one of the reasons why a lot of large commercial building roofs are unvented roofs.

Several ways to insulate an unvented roof

Here are some common methods for insulating a non-ventilated roof of small area:

  1. Exterior flat roof insulation with foam
  2. Exterior insulation with ‘closed cell’ foam
  3. Interior insulation with ‘closed cell’ foam
  4. Exterior insulation with foam + interior insulation using fiberglass or cellulose

Commercial flat roof insulation

Proper insulation of commercial roofs is just as important as the insulation of the houses where we live with our families.

Proper commercial roof insulation not only saves money, but increases the comfort of people working inside the building as well as reduces environmental pollution.

Materials and method of installation of insulating materials for flat roofing materials depend on:

  1. Local building codes
  2. Area of the roof and building specialization
  3. Thermal and energy efficiency you want to endure
  4. The R-value

High-Density Extruded Polystyrene (XPS)

XPS (R-value is 5) is a rigid foam made of polystyrene polymer. Common release form is foam board (different thickness). Nine times out of ten this material is pink, but there may be other options depending on the added dye and brand. Insulation thickness differs according to the roofing project.

  • Medium price material
  • Optimal ratio of cost and R-value
  • Suitable for IRMA and PMR roof systems
  • XPS is also used to insulate walls and other vertical surfaces
High-Density Extruded Polystyrene - Flat Roof Insulation
High-Density Extruded Polystyrene


Polyurethane is almost as high an R-value as Phenolic Foam (we will talk about it later in the article). According to the manufacturers, the R-value of this insulating material is 7.0-7.2. It is usually supplied as 1-component or 2-component spray foam.

  • One of the best R-values in the US market
  • Actively used for roof insulation of commercial buildings with refrigeration equipment
  • Resistant to moisture
  • This material can be used in different climatic conditions
  • Suitable for insulating a flat roof with irregularly shaped surfaces or roof assemblies
  • High fire-resistant
Polyurethane - flat roof insulation


Polyisocyanurate (polyiso) – closed-cell foam. The R-value declared by the manufacturers is almost 7.0. Boards made of this material have a high density and can be used on exploited roofs (with some restrictions).

  • Good choice for insulating a flat roof concerning advantages and R-value
  • Foam can be adhered to various surfaces including glass and metal
  • Resilient
  • Lightweight
  • Modern polyiso materials are more stable. However, the R-value decreased over time for foams produced according to the older formula.
  • Modern materials based on polyiso are more environmentally friendly, but in the past this material did not meet high environmental standards.

High-Density Expanded Polystyrene (EPS)

EPS (R-value is 4.4-4.6) – this material is made from the same synthetic raw materials as XPS. The release form can be almost any shape you need.

  • Medium price material
  •  Retains thermal insulation properties longer over time (compared to XPS)
  • Sheets can be cut for easy installation
  • Lightweight material
  • Absorbs water
  • Not compatible with some adhesive and coatings
Flat roof insulation - EPS
Flat roof insulation – EPS

Perlite Insulation Board

Perlite board is a material with a relatively low R-value (2.5-2.7), which is produced from environmentally friendly recycled materials. It is produced as an independent insulating material, and as an element of composite construction boards.

  • The material has been used in construction for almost 80 years
  • Relatively inexpensive
  • Material used with old-school roofing systems
  • Perlite board requires additional protection as it absorbs moisture
  • This material is susceptible to rotting
Perlite Insulation Board
Perlite Insulation Board

Phenolic Foam

Phenolic Foam is the material with the highest R-value. According to the manufacturers, the R-value of phenolic foam reaches 7.4-7.5. However, this foam can lead to corrosion and have a leaching acidic impact on metal materials and fasteners.

  • One of the best R-values for flat roof foam insulation
  • Due to the potential risks of roof damage, it can only be seen on older buildings
  • Due to some concerns it is quite difficult to find the manufacture of this foam in the US today
Flat roof insulation - Phenolic Foam
Flat roof insulation – Phenolic Foam

Glass Fiber and Mineral Fiber

This material is considered to be technically obsolete. Insulation is usually produced as panels or batts. As single-ply roofing membrane technology becomes more popular, glass fiber and mineral fiber materials are being phased out. However, they can still be ordered, including as formaldehyde-free mineral wool.

  • Not suitable for EPDM, PVC or TPO
  • Safety precautions must be observed, as the material may be hazardous to the respiratory tract during installation and operation.
  • High fire resistance
  • Does not mold
  • Chemically inert material
  • Suitable for insulating metal roofs
Glass Fiber and Mineral Fiber
Glass Fiber and Mineral Fiber

Please note that flat roof insulation material, especially for commercial building roofs, is selected according not only to R-value, but UL or FM flat roof insulation requirements, compliance with ASHRAE Standard and any other building code.


Roof installation is a task for professionals. Just a small mistake during installation or in the calculations can lead to large financial costs and long repairs.

It does not mean that you cannot learn more about the installation and repair of roofs. Our materials are helpful guides in which we talk about solving complex roofing tasks and projects in simple terms.

Do you want to know more about how to insulate a flat roof, about roof repair, installation, replacement or renovation approaches?

Subscribe to our emails. No spamming – just useful information, roofing tutorials and DIY guides.

Get tips and advice on flat roof repair and maintenance

* indicates required
5/5 - (1 vote)