Inverted flat roof thermal insulation

Inverted Flat Roof Thermal Insulation


Unlike conventional warm and cold roof constructions, HEXATHERM inverted flat roof insulation protects the waterproof membrane from a number of stresses; freeze-thaw action, solar radiation, damage caused by interstitial condensation, ultra-violet degredation and mechanical damage caused by foot traffic during construction maintainenance. Consequently, the life expectancy of the membrane is greatly increased.

Inverted Flat Roof Insulation Boards Benefits

HEXATHERM Insulation Benefits

  • Domestic, commercial and industrial applications
  • 250-750 kPa high compressive strength
  • Able to withstand heavy point loads
  • Closed cell structure
  • Suitable for areas in close proximity to water
  • Blown with reclaimed CO2
  • 100% Recyclable
  • Excellent life long thermal performance
  • Range of thickness’ available 20-160mm
  • Super low U-values achievable

HEXATHERM Accreditations

Inverted Flat Roof Construction details

Inverted Roof Thermal Insulation Construction details

Suitable application products




Suitable applications products



More Information

Inverted Roof thermal insulation information

Thermal Shock Protection

The graph below illustrates two temperature variations: the red area shows a typical unprotected warm roof membrane; the yellow shows a membrane protected with Cellecta inverted insulation boards.


Loading Required

The roof structure must be strong enough to withstand the maximum expected loads with a suitable factor of safety. Inverted roofs are subject to three main loads: dead, wind and imposed – refer to BS 6399: Parts 1, 2 & 3 for further guidence.

The insulation must be covered with either ballast or paving slabs to protect it from ultra-violet degredation and to prevent wind up-lift, or floating, if ponding occurs. For severe exposure zones or tall buildings, specialist advice should be sough. BRE Digest 311 Wind scour of gravel ballast on roofs should be used when a calculation is required for a specific building project.

Filter Layer

Damage to the membrane may occur if fines are washed between the insulation boards. To prevent this, the insulation should be covered with either a suitable water permeable non-woven polyester filter layer or HEXATEX thermal improvement membrane, which will also improve the thermal efficiency of the insulation boards.

Inverted Roof Thermal Insulation Requirements

The thickness of insulation used on an inverted roof must be increased by 20% to allow for the intermittent cooling effect of rainwater on the waterproof membrane. On roofs with fall of 1:60 to 1:80 – the cooling effect can be reduced to only 2% by replacing the non-woven filter layer between the insulation and the ballast with HEXATEX vapour permeable, thermal improvement membrane.


HEXATEX Physical Specifications

  • Thickness: 0.5mm Roll size: 1.5m x 50m Weight: 11kg per roll
  • Water vapour resistance (BS 3177): 0.18 MNs/g
  • Water vapour transmission (BS3177): 1149 g/m2/day Water column:> 2m
  • Tensile strength longitudinal (BS 2782-3): 264 N/5cm
  • Tensile strength transverse (BS 2782-3): 210 N/5cm
  • Fire rating (CE): E, Temperature range: -40c to + 80c, UV Stability: 4 months

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