Above Membrane Insulation

Above membrane insulation speaks to what is above membrane insulation and why membrane insulation for a protected membrane roof offers two advantages.

Advantage one, the membrane is protected from extremes of heat and cold and the membrane is protected from extremes of heat and cold, and the membrane is on the warm side of the insulation, where it is immune to vapor blistering problems

Because the insulation itself is exposed to water when placed above the membrane, it must be made of a material that retains its insulating value when wet and does not decay or disintegrate. Extruded polystyrene foam is the one material that has these qualities. The panels of polystyrene are either imbedded in a coat of hot asphalt to adhere them to the membrane below or are laid loose.

They are held down and protected from sunlight by a layer of ballast (protecting the polystyrene from sunlight) which may consist of crushed stone or gravel. Often is used a thin concrete layer laminated at the factory to the upper surface of the insulating board or interlocking concrete blocks.

Because the membrane in a PMR system is shielded from sunlight and temperature extremes by the insulation and ballast above, it can be expected to last approximately twice the time as in an assembly where it remains directly exposed to the elements.

However these systems do have disadvantages. When the roof insulation is exposed to precipitation, it may absorb moisture and lose some of its resistance to the flow of heat. When performing energy calculation R values for roof insulation may be slightly reduced from their standard values to account for such losses. These systems may not be appropriate for climates with extended periods of rainy, cold weather, as cool water continuously flowing around and under the insulation boards may negate much of their insulating value. The disadvantages are minimal at best.

Repairs to protected membrane roofs if required are more costly and time consuming because accessing the roof membrane requires removal of the layers of material above it.


Some forms of roof decking may also have insulation bonded to them.

The external wall insulation needs to be extended up to the underside of the roof insulation to prevent cold bridging.

All roofs should be insulated. The thickness of insulation will depend on the type of insulant used and whether you are constructing a cold or warm roof.

Insulation levels have increased dramatically.

When constructing flat roofs please consider the following, materials used must be suitable for use in a flat roof, roof needs to achieve an external fire rating, vapour barriers should be used, flat roofs should be anchored to walls to prevent uplift, flat roofs should be laid to falls and flat roofs require regular maintenance.

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