Open and Closed Cell Spray Foam Insulation: What’s the difference and why does it matter?

by | Aug 23, 2022

Read Time: 5 minutes

Open and Closed Cell Spray Foam Insulation: what's the difference and why does it matter?


If you are currently researching different roof insulation options, you may find that identifying and installing future-proofed, cost and energy-efficient insulation solutions that meet regulatory requirements can be time-consuming and fraught. Whether it is for a new build, commercial, or retrofitted property, specifying what to install, where, and by whom can seem complicated.

In this article, we will explain:

What is Spray foam Roof Insulation?

Spray applied foam is one of your roof insulation options. Spray foam insulation, which is a polyurethane material, offers you many advantages over older types of insulation. It is made on site to suit the size, shape and technical requirements of your roof. (It can also be used as wall or floor insulation). It is the fastest growing insulation material throughout North America and Europe, with growth rates of between 7-10% per year.
Polyurethane foam is not solely used for insulation as it can be found used in many every day materials such as our bed mattresses/pillows & cushions, the dashboards of our cars, the soles of our shoes, furniture’s in our homes, fridge freezers and many more applications.

What is spray foam made of?

Spray foam is created by combining two chemicals, (polyols and isocyanates) which react to form an inert  polyurethane insulating foam.

How does it work?

It insulates by reducing heat loos by conduction while also creating an air barrier to prevent air movement – either warm air escaping or cold air entering the building.

How is the spray foam applied?

The foam is sprayed into the roof cavities on-site by a professional contractor. The foam rapidly expands to 100 times filling voids and sealing your building. (100 times for open cell, 30-40 times for closed cell)

Explanation of the two types of spray foam, open and closed cell

Cell structure, density and thermal conductivity are the three main factors that differentiate Open and Closed cell polyurethane materials.

Open cell spray foam

Open cell foam is made up of small bubble type cells which are not completely closed. These cells structures are formed with the help of water acting as the blowing agent which then evaporates during the spray process. Because they are open these cells are filled with air making it more flexible, softer to touch and more vapour open (breathable).

Closed Cell Spray Foam

The individual small bubbles within closed cell foams are completely sealed off and compressed together to create a harder, dense composition. The bubbles of a closed cell foam structure are filled with a gas type blowing agent that helps with the foam expansion. The new generation of closed cell foam has changed recently with the introduction of new chemistry allowing for low warming potential blowing agents (HFO) creating a more environmental product with a GWP (Global Warming Potential) of less than 1.
Closed cell foams are approximately four times as dense as open cell materials. Because of this density, closed cell foam will use more chemical material than open cell foam as it only expands approximately 35 times its original volume making it more expensive to install. Open cell expands up to 100 times its original volume, making it is more cost effective to fully fill between roof rafters, floor joists or timber studs.

Are both open and closed cell spray foam airtight?

Yes, both closed cell and open cell spray and injection polyurethane foams have been independently tested for airtightness and perform exceptionally well as air barriers with figures are low as 0.028m³/hr.m².

As the movement of air through the fabric of the building creates the biggest risk for moisture problems, it is essential to have an airtight envelope around the building. By creating airtightness with polyurethane spray foam, you eliminate up to 90% of potential moisture problems experienced with other insulation materials. The only way for moisture to enter the fabric is through diffusion. Water vapour diffusion is the movement of water vapour through vapour-permeable materials. Vapour diffusion happens through a solid material even when the material has no gaps or cracks (e.g., timber & plasterboard).

Polyurethane spray foam can help create a more comfortable indoor environment by eliminating draughts, while also helping to reduce energy bills.

Mechanical/ natural ventilation will be essential

Internal ventilation and moisture management of your home and building must be considered as a separate item to airtightness. Mechanical or natural ventilation is essential with a well-sealed building and must be part of the design brief when considering airtightness

Are both open and closed cell spray foam vapour permeable (breathable)?

Breathability is a material property and indicates the extent to which the material is able to transmit moisture – it’s water vapour transmission rate. Air leakage/airtightness is the uncontrolled movement of draughts through joints and gaps. Breathability is concerned with water in its gas form.

While both open and closed cell foams are excellent air barriers, they both have different levels of vapour permeability (breathability).

Open cell is more vapour open than closed cell with a μ factor of 3.2. Open cell will absorb and release moisture vapour freely, similar to timber which is completely airtight but will absorb and release moisture through diffusion.

Closed cell foam blocks most of the moisture but is not a complete barrier, so has a small degree of vapour permeability. Unlike open cell foams, closed cell is watertight and impervious to water penetration. Liquid water has much larger chemical structure and is unable to pass into or through the closed cell foam barrier.

Why does the difference between the open and closed cell matter?

Because open cell polyurethane foam has the ability to absorb moisture from the surface and diffuse it through the thickness of the material, it is important that this moisture has somewhere to be released on the exterior side of the construction.

Ventilation Paths / Breathable Membranes Must be Present with Open Cell Spray Foam

It is essential that a ventilation path or breathable membrane is present in your roof structure if you are considering using Open Cell Spray Foam. This is because all moisture that diffuses through the open cell should have a free path to continue to diffuse through other materials such as breathable membranes or should have suitable ventilation paths to allow the moisture to be eliminated.
Watch a video of how moisture diffuses using our ENERTITE open cell spray foam.

What is the risk if there is insufficient roof ventilation?

If the membrane is not breathable or there is no ventilation provision, there is a high risk of condensation forming on the cold surface of the exterior impermeable material. Water vapour itself may not be harmful but over time the moisture build up can lead to significant deterioration of building fabric.

The average family of two adults and two children produce approximately 24 pints of water a day through their modern daily living requirements and this moisture can be extremely dangerous if ignored.

4 Key Takeaways about Open and Closed Spray Foam Insulation

Have a Moisture/Vapour Management Plan before you Insulate

Before you choose to insulate, you will need to plan for appropriate ventilation, depending on the type of spray foam you wish to use. While polyurethane spray foam eliminates air leakage and helps prevent excessive moisture entering the building fabric, it should not be treated any differently to other insulations when designing moisture management in any roof, wall or floor. Additional consideration must also be giving to the requirements for a vapour control layer on the inside of the building, with design requirements reflecting building regulation requirements and relevant associated documentation.

Understand the Roof Pre-conditions that must Exist before Installing Spray Foam

Not all roofs are suitable for spray foam insulation. Your installer will evaluate the condition of your roof prior any agreement to install or an engage an independent quantity surveyor to give you an assessment of the suitability of your roof.

Understand the relevant building regulations

You may find it helpful to read our blog: Spray Foam Roof Insulation – which Building Regulations apply in the UK?

Use only certified materials and reputable installers

A roof will only comply with the Building Regulations if it is designed and constructed in accordance with Clause 12.4 of British Standard BS5250. Ensure that your materials and installers are certified and reputable.
Information about ECON Polyurethanes’ Open Cell Spray Foam product – ENERTITE

Quality of the Raw Material: ENERTITE is manufactured by BASF and is one of a range of BASF’s construction polyurethanes. (BASF is the largest chemical producer in the world operating in more than 90 countries with six integrated production sites and 241 other production sites in Europe, Asia, Australia, the Americas and Africa. BASF has around 90,000 customers in nearly all sectors in almost every country in the world.) ENERTITE is installed to be BS 5250 Moisture Management compliant and is KIWA and NSAI certified. View the ENERTITE Certificates and Technical information here.

Quality of Installation: We train all our contractors (i.e. installers) in our BASF-approved contractors scheme, known as FOAM MASTERS. This ensures that contractors who install our products are fully trained and supported by our technical team. Contractors must attend courses at our training centre in BASF Alfreton, Derbyshire and on completion are included in our database of accredited installers and a photo identity card is issued. Once a trained FOAM MASTER, our technical team will help our contractors to ensure compliance with installation methodology and to offer advice on correct application and intricate interfacing with other construction elements. You can read more here.

Need More information?

Contact us for more information about any aspect of spray foam insulation on

Useful links

BS 5250: 2021 Management of moisture in buildings – Code of practice | BSI (

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