Open-cell or closed-cell foam?

Given its many advantages, spray PU foam insulation has been enjoying growing popularity in recent years. Being an excellent thermal insulation material, the potential of polyurethanes is employed by owners of single homes, residential developers and businesses who look for an optimal thermal retrofitting material for their premises. When looking for PUR-based thermal insulation for a building, you have two basic types to choose from: open-cell PUR foam and closed-cell PUR foam. What are the differences, properties, and which should you choose for a project? This article provides the answers to the most important questions about open-cell and closed-cell PUR foam!

Published: 28-01-2021

Key advantages of spray foam insulation

Short installation time, penetration of even the smallest crevices and a low heat transfer coefficient are just a few of the advantages PUR foam has over conventional thermal insulation materials, such as EPS, mineral wool, or PIR panels. As a thermal insulation material, PUR foam works well not only in thermal retrofitting of residential buildings, but also in farm buildings, warehouses, shops, industrial facilities and many other structures. A big advantage of retrofitting with PUR foam is the uniform structure of the foam layer, which is free of visible joints or points of contact. These two features cause cold bridges in thermal insulating layers of EPS or mineral wool panels, resulting in loss of heat from the interior of a house.

What is polyurethane foam?

Polyurethanes (PUR) are commonly used thermal insulation materials in the group of ester-amide derivatives of carbonic acid. PUR foam is produced from two liquid components mixed at the application site. The components are ready to use and mixed with dedicated equipment directly at the work site. PUR foam is applied by spraying and this thermal retrofitting method is viable for all types of buildings. The key features of PUR foam include:

  • Low water absorption;
  • Low heat transfer coefficient (low lambda);
  • Insulating layer integrity;
  • Easy and quick installation;
  • Low insulating layer weight;
  • Resistance to attack by mould, fungi and biological corrosion.

Applications of PUR foam

Thanks to its many advantages, PUR foam’s applications are common in industrial sectors. It is not only used in thermal retrofitting systems, but is also a material used in furniture and upholstery solutions, installation and sealing of doors and windows, installation of bathroom tapware, bonding and joining of materials and sound insulation of structures.

PUR foam types

In terms of chemical properties, there are many classifications and criteria for defining PUR foam types. For commercially available thermal insulation systems, the differentiation into open-cell and closed-cell PUR foam is key. The properties of each type directly affect the thermal insulation parameters, the applicability of cell structure to specific types and elements of buildings and the performance of the complete thermal insulation solution.

Open-cell PUR foam

As the name of this type suggests, open-cell PUR foam predominantly features open cells, by which is it open to diffusion. The structure is porous and sponge-like. The material has a low weight, which is a big advantage of thermal insulation installed in attics (due to a low load applied to the roof structure). Open-cell PUR foam provides an optimal level of ‘breathability’ of thermal insulation and when applied, its volume is multiplied as the foam ‘swells’, filling out all corners and crevices.

Application method

Open-cell PUR foam is applied by spraying with dedicated dispensing units (which preheat the foam components). Technically advanced processing units provide a uniform structure of the foam which ensures identical thermal insulation performance throughout the spray-applied layer. The foam is sprayed very efficiently and an experienced contractor team can apply thermal insulation on up to 200 square metres during a single day of work.

Applications of open-cell PUR foam

Open-cell PUR foam finds many uses in thermal retrofitting of single houses, industrial buildings and structures and residential development projects. Given its structure and being open to diffusion, it is a perfect thermal insulation material for indoor applications. The advantages of open-cell PUR foam thermal insulation include:

  • No forming of cold bridges;
  • Excellent adhesion to the substrate;
  • Air permeability;
  • Stability of thermal insulation layer with no risk of displacement;
  • Protection of indoor spaces against overheating;
  • Elimination of indoor heat losses;
  • Low water absorption;
  • Lower building heating costs with resistance to mould, fungi and vermin;
  • Non-toxic composition which is safe to allergy sufferers and pets.

Closed-cell PUR foam

The second category of spray foam insulation is closed-cell PUR foam. Unlike its open-cell counterpart, the structure of this PUR foam features closed cells. The classification of a foam material as being closed cell is carried out through dedicated testing based on a relevant test standard. It is easy to understand that a closed-cell structure endows the PUR foam with lower air permeability and a much higher weight than open-cell foam. The basic advantages of closed-cell PUR foam include high resistance to crushing and considerable rigidity.

Application method

Closed-cell PUR foam is also applied by spraying. The material is applied directly on the substrate to be thermally insulated, which promotes its thorough sealing and elimination of cold bridges. An undisputed advantage of spray foam insulation is thermal retrofitting of even the most unusual structures which cannot be effectively covered with mineral wool or EPS.


Given its low water vapour breathability and a high mechanical resistance, closed-cell PUR foam is used as thermal insulation of foundation structures, flooring, various types of tanks and basins and as an anti-condensation barrier of sheet metal roofing. It can be applied outdoors as thermal insulation of farm and industrial buildings. If the substrate’s structure is strong enough, closed-cell PUR foam can be used for thermal retrofitting of roofs, although the considerable deadweight of the material should never be discounted.

Insulation advantages

The key advantages of closed-cell PUR foam insulation include:

  • Superior hardness, which is critical to the durability of thermal insulation;
  • Stability of performance over time;
  • Removal of all cold bridges by a smooth, jointless surface;
  • Lower building heating costs;
  • Forms an additional protective layer of the building;
  • Low water absorption.

Conclusion: Open-cell or closed-cell foam?

It is evident that each of the two suggested spray foam insulation systems has many advantages. Not the price, but the performance of the thermal insulation material should drive the choice between open-cell or closed-cell PUR as the most efficient thermal insulation material for a specific project. Open-cell PUR foam is light and flexible, reasons for which it is more often recommended for indoor applications. Closed-cell PUR foam is much more rigid and less water-absorbing, which makes it perfect for outdoor applications. Ultimately, the choice between the two PUR foam types must be guided by a thorough analysis of performance needed and optimal for the project in question. Read also: where to buy ingredients for polyurethane foam.

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