Insulation Joints and Overlays

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Insulation Joints and Overlays

Division E - General Information

Insulation Joints and Overlays

NOTICE TO READER: This is an information page only. To read the standards applicable to a particular Waterproofing or Water-shedding System, refer to the actual Standard located in Division B.


Insulation overlays are used to separate the roof membrane from the primary insulation, usually for one of two reasons:

  • to prevent the insulation from affecting the performance of the roof membrane
  • to prevent the roof membrane and / or the method of application from affecting the insulation

The insulation can affect the performance of the roof membrane in the following ways:

  • the insulation may be incompatible with the membrane, resulting in a loss of physical properties
  • different coefficients of thermal expansion may result in membrane splits / cracks.

The roof membrane and / or the method of application can affect the insulation in the following ways:

  • the insulation may not be capable of withstanding asphalt application temperatures and may “burn-out”
  • some insulation materials will not accommodate direct torch-applied membranes
  • insulation materials that are too “soft” may result in delamination or damage to the membrane or insulation when exposed to heavy loads or traffic
  • the solvents and adhesives used for flexible membranes may adversely affect the insulation.

NOTE: All hot asphalt-adhered asphaltic membranes (built-up roofing, modified bitumen, etc.) require an insulation overlay over heat sensitive insulations and heat resistant foam insulation. RoofStar Guarantee Standards for heat sensitive insulation materials require a two-layer overlay board for a RoofStar Roofing Practices Manual for a RoofStar 5-Year Roofing Guarantee, RoofStar 10-Year Roofing Guarantee, or a RoofStar 5-Year Waterproofing Guarantee. The RoofStar Guarantee Program requires the use of a two-layer overlay board over heat sensitive insulation in all cases, and also requires a single-layer overlay board application over heat insensitive insulation for all hot asphalt-adhered membranes. For single-ply, non-asphaltic membrane roofing systems, the membrane manufacturer should be consulted for proprietary options.

Insulation Joint Tape

Where the RoofStar Guarantee Standards or the roof system specifications call for taping fibreboard or insulation joints, consult the list of Accepted Materials: Insulation Joint Tape section of this Manual.

Asphalt and Paraffin-Impregnated / Coated Fibreboard Roof Insulation and Insulation Overlay Board

Under construction

Proprietary Overlay Boards

Under construction


To view materials accepted for use in the RoofStar Guarantee Program, see the tables under Insulation Overlay Board.

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