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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 have perlite board which is at least 25mm thick.

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.