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Flat Roof Insulation

All types of flat roof insulation rely on trapped air to provide a resistance to the flow of heat.  If the insulation used is a foam it will trap the air in a cellular structure. And if it is a fibrous material it will trap the air between its fibres. The insulation efficiency of a fibrous material will come down to the orientation and the size of its fibres.

There are four main types of insulation boards which can be separated as follows:

1) Vegetable

This includes: wood fibreboard and cork.

2) Mineral

This includes: mineral wool, cellular glass and perlite.

3) Plastic Foams

This includes: polystyrene (bead), polystyrene (extruded), rigid urethane and phenolic.

4) Composite Boards

This includes: cork/urethane, perlite/urethane, perlite/phenolic, fibreboard/polystyrene.

There are a wide range of insulation boards available which have proprietary modifications to the basic material used. These modifications are made with the intention of providing superior thermal efficiency. There are also insulation boards sold with facings attached. These facings are often in metal foil, plastic paper, fibreglass tissue or bituminous roofing.

Choosing An Insulation Board For A Flat Roof

An insulation board should not be chosen solely on it's thermal efficiency. An equally important function of the insulation is to provide firm support for the waterproofing. Both the moisture and thermal movement characteristics of the insulation must be understood. The waterproofing must be appropriately bonded. This will be either fully or partially. The suitability of a particular board for the roof in question must also be considered. An insulation board which cannot be handled and fixed without damage can jeopardise the entire flat roofing system. The qualities to look for when analysing the suitability of an insulation board include: corners which are not susceptible to damage, ease of cutting, materials which no special techniques needed for application and robust working surfaces which are not abrasive to touch, dusty or easily crumbled.

Different Types Of Insulation Board

Wood Fibreboard

This was the first type of board used for the insulation of flat roofs. However, today it is very rarely used because it does not meet modern U-value requirements. additionally this type of board can suffer from decay if it gets damp. It can be used as a cost effective facing material laid on top of expanded polystyrene insulation to protect the polystyrene from the heat of hot bitumen or asphalt. Wood fibreboard can also be used as overlay board for re-roofing. One potential problem with wood fibreboard is if it absorbs moisture. This moisture absorption can cause the fibreboard to move and cause problems to the waterproofing at the joints in the fibreboard.


Cork is an insulation material which has proven itself over many years. It is made from pure granulated cork which is compressed and steam baked. The cork is held together by natural cork gum. Cork is moisture resistant and does not decay. It can be used above highly humid conditions. Cork has  low coefficient for thermal expansion meaning it will be able to tolerate a fully bonded built-up roofing specification. With a good laminar flow and compressive strength cork insulation is suitable for asphalt roofing.

Cork is quite brittle to handle so a thickness of at least 25mm is recommended. Despite being rather delicate when handled, once laid as insulation board it is firm under foot traffic. This means cork can be laid underneath a membrane for a flat roof which is going to be subject to foot traffic. If cork is to be laid over metal decking underlay may be required to provide support over the troughs of the metal decking. Cork can be seen widely used to provide heat resistance and stable facing to rigid urethane insulation. This forms a composite insulation.


Perlite is an amorphous volcanic glass. It contains a small percentage of water. This causes perlite in powder from to expand by approximately 20 times in size when it is heated. During the manufacturing process, perlite is combined with mineral fibres and binders. This results in a roof board with a thermal insulation value similar to wood fibreboard. However, perlite insulation board has a significantly lower combustible content. One side of perlite board is usually treated with a bitumen emulsion during the manufacturing process. This is to increase the resistance of the board to bitumen absorption and to bind the surface.

Generally, perlite boards have a low laminar strength but a good compressive strength. They resist decay, however any absorbed moisture can significantly weaken the board. Perlite boards are quite brittle which means it is advised to use board which is at least 25mm thick. Perlite boards have a very low coefficient of thermal expansion. These boards will also accept fully bonded waterproofing specifications. Similar to wood fibreboard, perlite provides dimensional stability and a high degree of heat shielding. This means perlite is often used to improve the performance of other insulants. This is achieved by using perlite in a composite combine with another insulant or as an overlay installed on site.

Mineral Fibre

Mineral fibre is made from rock wool. Rock wool comes from volcanic rock which is melted at extremely high temperatures. The molten rock is converted into thin fibres then added with a small amount of resin. The resin serves as a binding agent to bond the fibres together. These fibres when bound form a mat. This mat is then cured and compressed to form a rigid mineral wool insulation slab. These slabs have a high resistance to combustion, provide a good degree of thermal insulation and are dimensionally stable. For flat roofing mineral wool insulation slabs should have a glass facing tissue to provide high laminar strength.

Cellular Glass

Cellular glass is a type of insulation board which is made from pure glass. During manufacturing the glass is expanded and formed into slabs. Cellular glass will not rot or decay and is non-combustible. Slabs of cellular glass can be either a constant thickness or tapered to provide falls. A further benefit of cellular glass is it is nearly impermeable to water vapour. It can be laid without a vapour barrier as long as any joints are sealed with bitumen in line with the manufacturers specification.

Cellular glass insulation is suitable for both fully bonded built up roofing and mastic asphalt waterproofing. When mastic asphalt is applied upon cellular glass insulation two layers of building paper are required to be laid between the cellular glass insulation slab and the sheathing the mastic asphalt is to be applied on. The reason for this is to prevent the adhesion of the asphalt to the cellular glass slab.

Expanded Polystyrene

Often referred to as bead boards, expanded polystyrene boards are formed from steam heating. This steam heating process expands the beads of the polystyrene and fuses them into a installation slab. These slabs can be finished with a constant thickness of with a tapered finish to provide falls.

For roofing applications these slabs are often finished with a felted upper surface, which is overlaid by a perlite board or a fibreboard. Composite boards with either perlite or wood fibreboard laminated to the surface are also often used. The reason for expanded polystyrene often being used in unison with wood fibre or perlite is to provide heat shielding and protection from thermal movement. The reason for this is expanded polystyrene boards are heat sensitive and are susceptible to  thermal movement. Polystyrene boards are not capable of withstanding the high temperatures involved in the application of both mastic asphalt and hot bitumen.

Extruded Polystyrene

Extruded polystyrene is a foam material formed from polystyrene crystals. Similar to expanded polystyrene it is sensitive to heat and susceptible to thermal movement. However, it has superior thermal conductivity and has a high resistance to water absorption. This high level of water resistance leads to extruded polystyrene often being used to insulate inverted roofing systems.

Do You Have A Question About Flat Roof Insulation?

If you would like RJ Evans to provide any information related to flat roof insulation, please contact us or call us now on 01277 375 511. One of our friendly flat roofing specialists will be delighted to help.

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