Solid foam boards are probably the most recognisable form of insulation, with the rectangular boards used in residential and commercial projects for new builds and retrofitting.

Here we will explain how solid board insulations are made, as well as the different types available and the scenarios in which they can be used.

Solid board insulation typically refers to thick boards of rigid foam. These usually come in rectangular boards approximately 2,400mm x 1,200mm in size. These boards can then be cut into different shapes to fit the needs of a specific project.

Rigid insulation boards are used to create a continuous layer of insulation with a high thermal performance for its thickness. The most common materials used for these insulation foam boards are derivations of polystyrene and polyurethane.

Solid insulation boards are made from closed cell foams which have been compressed to different levels before being cut down into boards. The defining insulating factor of solid boards is that they consist of countless little cells of material, each isolated from the others around them. This creates a barrier that blocks air or moisture from passing through, trapping heat.

Depending on the chemical structure of the foam, whether it is PIR, EPS, or something else, these cells may contain air or another gas with varying levels of thermal performance.

These solid boards are often then backed with a reflective foil to further improve their thermal performance.

Rigid Insulation boards made from a closed cell foam are known for having moisture resistant properties due to the many cells in the foam matrix. However, they are not completely moisture impermeable and there will always be gaps between individual boards.

Therefore in many projects you may need to install a separate vapour control layer in projects using rigid foam insulation. This will depend on the property, and you should consult with an installer before making any decisions.

Polyurethane (PUR) insulation boards are made from a closed cell foam, the same type that is used in expanding spray foam. This synthetic material has excellent moisture and heat resistance properties that make it a good insulation material.

Polyisocyanurate (PIR) insulation boards are made from a closed-cell rigid foam created by the reaction of a polyol and an isocyanate. PIR is similar to PUR boards, being a derivative of the same technology, but it has an even better thermal performance. 

PIR also has the added benefit of being very resistant to fire, and emits little smoke when exposed to flame.

Expanded Polystyrene (EPS) boards are made from small beads of polystyrene containing a blowing agent, usually pentane, that expand massively when exposed to steam. 

This is then seasoned in special silos to allow moisture to evaporate before heated and compressed again, then shaped into boards. EPS boards are 98% air, which is what gives it its insulating properties.

Extruded Polystyrene (XPS) is made from polystyrene pellets, with additives, which are melted down together with a blowing agent in an extruding machine. This mixture then expands into a foam which is shaped into a homogenous, closed cell matrix of millions of tiny buddles. 

This gives it a better thermal performance than EPS, and a greater resistance to water. The uniform structure also gives XPS a high compressive strength.

There are other forms of insulation that commonly come in the form of solid boards. But these are different from the foam boards we are discussing here. Other solid form insulation includes corkboard, wood fibre boards, and insulated plasterboards. These have different properties and uses from synthetic foam boards.

Solid board insulation should provide an excellent boost to your home’s energy efficiency and BER rating due to its high thermal performance at a low thickness compared to many other wool materials.

Solid insulation boards can be the best choice for homes that are concerned about space for internal or external wall insulation. It is also an excellent choice for rafter insulation if you wish to avoid using spray foam directly.

See the best ways to insulate your home in our Guide to Home Insulation.