Insulating the Roof

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Insulating the Roof

Low-slope Roof Insulation.jpg

1 General

Buildings are designed to provide an interior environment (or shelter) that is not governed by the exterior environment. The roof is a fundamental component of this design and must perform several functions. As part of the roof assembly, properly designed and installed roof insulation provides the following benefits:

  • It reduces energy costs and provides a comfortable interior environment by resisting heat loss.
  • It can prevent condensation occurring on interior surfaces by raising the dew point above the roof deck.
  • It generally provides a more suitable substrate for the membrane than does the roof deck.
  • It reduces deck component temperature fluctuations and, therefore, the expansion and contraction of the deck.

The following disadvantages have been associated with “conventionally” placed roof insulation (i.e., insulation placed between the membrane and the roof deck):

  • In hot weather the insulation resists heat flow into the building, thereby increasing the surface temperature of the membrane. This accelerates the aging (or oxidization) process of the membrane. In built-up roofing this can result in either the hardening and embrittlement of the bitumen, alligatoring, or splitting.
  • By eliminating the problem of interior condensation, the insulation creates the possibility of moisture (vapour or liquid) trapped within the roof system. This can lead to numerous problems and may create the need for a vapour retarder.
  • The frequency of thermal expansion and contraction in the membrane may be increased, thereby increasing the stresses on the membrane, which can result in membrane splitting.

A protected membrane roof assembly (PMR) may eliminate some of the possible disadvantages, but this assembly has its own characteristics (see Protected and Modified Protected Roof Systems).

1.1 Theory

The function of insulation is to retard the flow of heat energy. Heat may flow in three ways:

  • Conduction: the transfer of heat energy through direct contact molecule to molecule.
  • Convection: the transfer of heat energy by the movement of liquid or gas.
  • Radiation: the transfer of heat energy by electromagnetic waves.

Through the use of solid materials such as insulation in the roof assembly, the mode of heat transfer can be considered purely conductive. The rate at which a material allows heat transfer by conduction is referred to as its thermal conductivity (K or KSI). Conductivity is the basic unit of heat flow.

Thermal conductance (C or CSI) refers to the conductivity of a specific thickness of a material. The formula for determining the thermal conductance of a material is:

C = K

Unit of Thickness

Thermal resistance (R or RSI) refers to a material's resistance to heat flow, which is the reciprocal of its thermal conductance (C or CSI). Thermal resistances are additive and are therefore useful in design calculations. The formula for determining the thermal resistance of a material is:

R= 1

OR unit of thickness

C K

The overall coefficient of heat transfer (U) is the time rate of heat energy flow for a complete assembly, including air films. To calculate the U-value, take the reciprocal of the additive total of the resistance, for example:

U = 1

R1 + R2 + R3 etc

The units for these quantities can be either metric or standard. Metric units usually include “SI” (Systeme International d'Unites) after the unit symbol.

Thin air at inside and outside surfaces provides thermal resistance. Wind reduces outside air-film thickness, thus reducing its thermal resistance (R-value) as wind speed increases. The thermal resistances of indoor air films vary with the direction of heat flow. A ceiling air film has greater thermal resistance against downward heat flow because upward heat flow is accompanied by convective currents that disturb the air film and reduce its resistance to conductive heat flow.

If the space between the structural roof deck and the suspended ceiling is used as a return-air plenum, only R-values above the plenum should be considered.

1.2 Installation

The following procedures for installing insulation in roof systems should be followed:

  1. On steel roof decks, deck flutes and felts should normally run in parallel alignment and be perpendicular to the roof slope.
  2. On steel roof decks, insulation boards should be firmly supported by steel deck flanges. When only one layer of insulation is installed, the long dimension of insulation boards should not cantilever over steel deck flutes.
  3. A staggered double-layer insulation system may provide the following benefits:
  • elimination of thermal bridges, where leakage of heating or cooling energy may occur.
  • reduced ridging, by eliminating through-joint migration of moisture vapour into the membrane, and subsequent deformation.
  • reduced ridging and splitting in the roof membrane.

The edges of insulation boards should be square, flush and have moderate contact with the edges of adjacent insulation boards. End joints between adjacent insulation boards should be staggered. For heat sensitive insulation and heat insensitive foamed insulation, RoofStar Guarantee Standards should be consulted for fibreboard overlay requirements for Five (5) or Ten (10) RoofStar Guarantee requirements.

The specifier should not simply reference the thermal performance for a roof assembly by specifying the total R-value. The generic type, thickness, C-value, and applicable standards of the insulation required for application should also be specified.

1.3 Desirable Properties

Ideally, a roof insulation would provide the following physical properties:

  1. Compatibility with Asphalt: it would not have an adverse chemical reaction upon contact with asphalt.
  2. Compatibility with Roof System Components: it would be compatible with the other components of the roof system. This is of special concern with flexible membranes (bituminous or non-bituminous) and the solvents, adhesives, application methods, etc. involved in their application.
  3. Resistance to Thermal Conductivity: the lower the thermal conductivity, the less thickness of insulation is required to obtain the desired thermal resistance.
  4. Resistance to Fire: it would not support combustion and would comply with the accepted fire rating.
  5. Resistance to the Effects of Moisture: it would not be adversely affected by moisture vapour and free water.
  6. Resistance to Cell Deterioration: it would be of a material that is durable and not subject to deterioration.
  7. Resistance to Impact: it would be resistant to impact damage - providing rigidity and strength; and of sufficient density to provide a workable surface.
  8. Dimensional Stability: it would be unaffected by varying moisture and temperature conditions, and resistant to thermal cycling.
  9. Unaffected by Asphalt Application Temperatures: it would not “burn out” or deform due to the application temperatures of asphalt.
  10. Stable K-value: the thermal conductivity would remain stable and would not drift higher with age.
  11. Attachment Capability: its surfaces would accommodate secure attachment by traditional methods (roofing nails, or screws and plates).


The remainder of this section will provide information about types of the roof insulation available in British Columbia. This includes generic information and links to other sections of this Manual, to product information for RoofStar-accepted Materials. The generic information discusses the properties of the materials relative to their use and to similar materials. Any product information that has been included is taken from referenced publications.


2 Heat-resistant Insulation

2.1 Polyisocyanurate Foam

NOTE: See also Accepted Polyisocyanurate Insulation products


Polyisocyanurate is a closed-cell rigid foam material. The insulation boards are manufactured with asphalt-saturated felt, glass fibre or acrylic facer sheets on the top or bottom of the foam core. In some cases the foam core is reinforced with glass fibre or acrylic to provide additional fire resistance and greater dimensional stability.

Polyisocyanurate foamboard roof insulation is presently manufactured to two standards:

  • CAN / ULC S704-01 (CGSB 51-GP-21M) applies to unfaced polyisocyanurate rigid insulation boards intended for applications where the continuous use surface temperature does not exceed +110°C (+230°F). This standard establishes one type (Type 4) and three classes (Class 1, 2 or 3) of polyisocyanurate insulation. The type is determined by the physical properties of the material and the class is determined by its surface burning characteristics.
  • CAN / ULC S704-01 (CAN / CGSB-51.26-M86) applies to faced polyisocyanurate rigid insulation boards intended for applications where the continuous use temperature is within -60°C to +80°C (-76°F to +176°F). This standard establishes four types (Type 1, 2, 3, or 4), four facing categories (Facing 1, 2, 3, or 4), and two surface burning characteristic classifications (surface burning characteristic “a” or “b”). The type is determined by the physical properties of the material, the facing is determined by the product the insulation is foamed between, and the surface burning characteristic is derived from the final product's flame spread classification. RoofStar Guarantee Standards require that polyisocyanurate insulation manufacturers clearly identify the manufacturing date on product labels.

Polyisocyanurate insulation provides the following properties and advantages:

  • Compatible with asphalt
  • Compatible with roof system components
  • Resistant to the effects of moisture
  • Resistant to cell deterioration
  • Resistant to impact
  • Resistant to thermal conductivity
  • Resistant to fire
  • Resistant to thermal cycling (dimensionally stable)
  • Unaffected by hot asphalt
  • Retains mechanical fasteners


The possible disadvantages or precautions involved in the use of polyisocyanurate foam insulation include:

  • Requires an insulation overlay (fibreboard or retrofit board) to prevent potential asphalt blistering
  • Aged thermal resistance Long Term Thermal Resistance (LTTR) tested to CAN / ULC -S770 should be used for design calculations.
  • Felt skin may change dimensionally if exposed to weathering (provide protection prior to installation)
  • Heavy equipment may compress insulation, causing deflections and de-bonding of the membrane
  • Will not retain nails (requires screws and plates)

2.1.1 Storage and Handling

See the Technical Update for January 12, 2012 concerning an issue of moisture in packaged polyisocyanurate insulation bundles. Also consult the PIMA Technical Bulletin #109 for storage and handling guidelines.

2.2 Mineral Fibre Roof Insulation

NOTE: See also Accepted Mineral Fibre Insulation products

Mineral fibre roof insulations are composed of rock fibres with a thermoset resin and surfaced with glass fibre scrim on the top surface. These boards can provide a suitable surface for directly mopped bituminous membranes, however, a minimum one layer fibreboard overlay is recommended.

Mineral fibre roof insulation may provide the following properties and advantages:

  • compatible with asphalt
  • compatible with most roof systems
  • resistant to thermal conductivity
  • excellent resistance to the effects of moisture
  • resistant to fire
  • resistant to cell deterioration (non-corrosive)
  • resistant to thermal cycling (dimensionally stable)
  • stable K value (thermal conductivity does not change with age)
  • conforms to substrate irregularities
  • can be hot mopped or mechanically fastened

The possible disadvantages or precautions involved in the use of mineral fibre roof insulation:

  • heavy equipment may compress the insulation and cause delamination of the membrane
  • not intended for use under high traffic deck surfaces

Mineral fibre roof insulation should meet or exceed CAN / ULC S126.M86.

2.3 Composite Board Insulation and Overlay Boards

NOTE: See also Accepted Composite Insulation products

Composite board roof insulation products consist of an insulation bonded with another insulation and / or a variety of other products (typically fibreboard, perlite, membranes, etc.) to form a unified, multi-layered insulation board. The top and bottom surfaces of the board may be impregnated and / or coated with asphalt (or other binders), and covered with facing materials such as roofing felts, foils, kraft paper modified bituminous membranes, etc.

The properties and performance of composite board roof insulation varies with the components of the board. Generally, the boards are manufactured to provide some of the following properties and advantages:

  • Compatible with asphalt,
  • Resistant to thermal conductivity,
  • Resistant to fire,
  • Resistant to the effects of moisture,
  • Resistant to cell deterioration (durable),
  • Resistant to impact, and / or
  • Unaffected by asphalt application temperatures (no overlay may be required).

The possible disadvantages and precautions involved in the use of composite board insulations include:

  • Complexity of manufacture and dissimilar materials,
  • Sometimes expensive, and
  • Limited availability

2.4 Perlitic Insulation

NOTE: See also Accepted Perlitic Insulation products

Perlite ore is a volcanic glass with a natural water content characteristic. When this ore is heated to approximately +930oC (+1700oF) the water vaporizes and the ore expands into glass spheroids. The expanded perlite ore may be mixed into a water slurry formulation containing cellulose fibre, a small amount of asphalt, and sometimes starch. The slurry is then dried into boards and cut to size. The top surface is usually treated to minimize bitumen absorption.

Perlitic roof insulation provides the following properties and advantages:

  • Compatible with asphalt
  • Compatible with most roof system components
  • Resistant to thermal conductivity
  • Resistant to fire
  • Resistant to impact
  • Resistant to thermal cycling (dimensionally stable)
  • Unaffected by asphalt application temperatures
  • Stable K-value
  • Retains roofing nails

The possible disadvantages or precautions involved in the use of perlitic roof insulation include:

  • Absorbs moisture (protect from the effects of weathering)
  • Heavy equipment may compress boards causing possible debonding of the insulation from the deck
  • Extremely friable (crushable); proper compounding and correct grading is important
  • Relatively low thermal values.

2.5 Lightweight Insulating Concrete

NOTE: See also Accepted Insulating Concrete products

2.6 Glass Fibreboard (Fibreglass)

NOTE: See also Accepted Glass Fibre Insulation products

Glass fibreboard roof insulations are composed of fine glass fibres compressed into rigid insulation boards. These boards are most commonly available top-surfaced with a glass fibre-reinforced asphalt and kraft paper. This provides a suitable surface for directly mopped bituminous membranes and for some flexible membrane roof systems. In addition, boards made of glass fibres bound in a resinous binder are available either top-surfaced for bituminous roofing, or plain for use under ballasted membrane systems.

Glass fibreboard roof insulation may provide the following properties and advantages:

  • Compatible with asphalt,
  • Compatible with most roof system components,
  • Resistant to thermal conductivity,
  • Resistant to fire,
  • Resistant to the effects of moisture,
  • Resistant to cell deterioration (durable),
  • Resistant to petroleum solvents,
  • Resistant to impact (if top-surfaced),
  • Resistant to thermal cycling (dimensionally stable),
  • Unaffected by asphalt application temperatures (no fibreboard overlay required),
  • Stable K-value (the thermal conductivity does not change with aging),
  • Conforms to minor deck irregularities, and / or
  • Retains roofing nails (first layer in a two-layer application only).


The possible disadvantages or precautions involved in the use of glass fibreboard insulation include:

  • The kraft paper facing may change dimensionally if allowed to absorb moisture (protect from weather prior to application), and / or
  • Heavy equipment may compress the insulation and cause delamination of the membrane.

Glass fibreboard roof insulation should meet or exceed CAN / CGSB-51.31-M84.

3 Heat-sensitive Insulation

3.1 XPS (Extruded Polystyrene Foam)

NOTE: See also Accepted XPS Insulation products
(See also Expanded Polystyrene Foam below, and Protected and Modified Protected Roof Systems)

Molten polystyrene and a blowing agent HCFC (142B) are mixed, under pressure, in an extruder. As this solution is extruded through an orifice into ambient temperature and controlled conditions, the blowing agent vapourizes causing the polystyrene to expand approximately 30 times its original size. The continuous extrusion process produces boards with a surface “skin” and closed cell structure and, for this reason, has been used extensively for protected membrane roof assemblies. The rigid insulation boards are expanded to a specific thickness during manufacture and have an approximate density of 32 kg / cu. m (2 lb / cu.ft).

Extruded expanded polystyrene foam roof insulation is combustible and, when used on steel decks or over a roof that is subject to fire exposure from below, a fire-rated underlayment or thermal barrier (such as gypsum board) may be required between the roof deck and the insulation (consult Building Code and insurance requirements).

Extruded expanded polystyrene roof insulation is manufactured to CAN / ULC-S701 standard and is currently available in three types (depending on physical properties) and in four forms, as follows:

  • Type 4 for use on conventional or protected membrane roofing systems
  • Type 4 with a factory-applied latex-modified concrete topping for protected membrane roofing systems
  • Type 3 for use on conventional roof systems
  • Type 2 for use on conventional roofing systems [density of 24 kg / cu.m (1.5 lb / cu.ft)]

Extruded polystyrene roof insulation provides the following properties and advantages:

  • Compatible with asphalt
  • Resistant to the effects of moisture
  • Resistant to cell deterioration (durable)
  • Resistant to impact
  • Resistant to thermal conductivity
  • Stable K-value

The possible disadvantages or precautions involved in the use of extruded polystyrene roof insulation include:

  • Flammable (combustible)
  • Affected by solvents (i.e. adhesives and cleaners used for single ply membranes)
  • Heat sensitive [requires an insulation overlay, such as fibreboard, to prevent “melting” (burnouts) from hot asphalt]
  • Will not retain nails (requires screws and plates)
  • Dimensional instability may result from improper curing at the factory

3.2 EPS (Expanded Polystyrene Foam)

NOTE: See also Accepted EPS Insulation products
(See also Extruded Expanded Polystyrene Foam)

Third Party Certification: The RoofStar Guarantee Program welcomes and accepts the industry recommendation presented by all EPS manufacturers currently active in the B.C. market to require third party certification of the EPS products meeting CAN / ULC-S701-97 / (CAN / CGSB-51.20-M87) requirements. It is understood that such a certification program requires annual inspections / re-certification by an independent testing lab.

Molten polystyrene and a blowing agent are mixed and formed into uniform closed-cell structures (“beads”). These are later expanded up to 40 times by steam in a pre-expander. (The amount of expansion determines the density and, therefore, the thermal conductivity of the final product.) The expanded beads are stabilized in curing bins, fused into a billet in a block mold, and cut into the desired size, shape and thickness. This process produces rigid boards of interconnecting closed cells of densities varying from 16 to 48 kg / cu.m (1 to 3 lb / cu.ft). Expanded polystyrene insulation is commonly referred to as “beadboard” or “popcorn”.

Expanded polystyrene roof insulation is presently manufactured to the CAN / ULC-S701-97 / (CAN / CGSB-51.20-M87) standard and is available in four types, depending on the physical properties of the material. Basically, the strength (tensile, compressive, flexural, and shear) and thermal resistance properties increase, and the water vapour permeance and water absorption properties decrease from Type 1 to Type 4. (For Type 2 and Type 4 see also section 6 above, Extruded Expanded Polystyrene). Note: Type 4 is usually, if not always, extruded.

Expanded polystyrene foam roof insulation is combustible and, when used on steel decks or over a roof that is subject to fire exposure from below, a fire-rated underlayment or thermal barrier (such as gypsum board) may be required between the roof deck and the insulation (consult local building bylaws and the latest edition of the BC Building Code, together with insurance requirements).

Expanded polystyrene foam roof insulation provides the following properties and advantages:

  • Compatible with asphalt
  • Resistant to the effects of moisture
  • Resistant to cell deterioration (durable)
  • Resistant to impact
  • Resistant to thermal conductivity
  • Stable K-value (the thermal conductivity does not change with aging)

The possible disadvantages or precautions involved in the use of expanded polystyrene roof insulation include:

  • Flammable (combustible)
  • Affected by solvents (i.e. adhesives and cleaners used for single-ply membranes)
  • Heat sensitive [requires an insulation overlay (such as fibreboard) to prevent “burnouts” from hot asphalt]
  • Will not retain nails (requires screws and plates)
  • Dimensional instability may result from improper curing at the factory.

3.3 Polyurethane Foam

NOTE: See also Accepted Polyurethane Foam Insulation products


Polyurethane foam is the result of a chemical reaction between two liquids, isocyanate and polyols, in combination with additives and catalytic agents. The mixture begins to foam instantly and quickly expands to approximately 30 times its original volume. The foam hardens into an airtight mass, becoming tack-free in minutes.

The insulating properties of the foam are derived from fluorocarbon vapour trapped in the foam's cells. Polyurethane roof insulation boards are generally available in either:

  • flat sheets of varying size and thickness that have been cut from large buns (or billets)
  • surfaced boards where the polyurethane is foamed between two skins (felt, aluminum foil, etc.) which become an integral part of the product.

Polyurethane foam roof insulation is combustible and, when used on steel decks or over a roof that is subject to fire exposure from below, a fire-rated underlayment or thermal barrier (such as gypsum board) should be installed between the roof deck and the insulation.

Polyurethane foam roof insulation is presently manufactured to two standards:

CGSB 51-GP-21M applies to unfaced polyurethane rigid insulation boards intended for applications where the continuous use surface temperature does not exceed +80°C (+176°F). This standard establishes three types (Type 1, 2 and 3) and three classes (Class 1, 2 or 3) of polyurethane insulation. The type is determined by the physical properties of the material and the class is determined by its surface burning characteristics.

CAN / CGSB-51.26-M86 applies to faced polyurethane rigid insulation boards intended for applications where the continuous use temperature is within -60°C to +80°C (-76°F to +176°F). This standard establishes four types (Type 1, 2, 3, or 4), four facing categories (Facing 1, 2, 3, or 4), and two surface burning characteristic classifications (surface burning characteristic “a” or “b”). The type is determined by the physical properties of the foam, the “facing” is determined by the product the polyurethane is foamed between, and the surface burning characteristic is derived from the final product's flame spread classification.

Polyurethane insulation provides the following properties and advantages:

  • Compatible with asphalt
  • Compatible with roof system components
  • Resistant to the effects of moisture
  • Resistant to cell deterioration
  • Resistant to impact
  • Resistant to thermal conductivity
  • Unaffected by asphalt application temperatures.


The possible disadvantages or precautions involved in the use of polyurethane foam insulation include:

  • Flammable (combustible)
  • Requires an insulation overlay (fibreboard, vented base sheet, etc.) to prevent potential asphalt blistering
  • Aged thermal resistance should be used for design calculations
  • Felt skin may change dimensionally if exposed to weathering (provide protection prior to installation)
  • Heavy equipment may compress insulation, causing deflections in the deck and debonding of the insulation
  • Will not retain nails (requires screws and plates)

4 Tapered Insulation

NOTE: See also Accepted Tapered Insulation products

Tapered roof insulation systems are generally available in three forms:

  • Field-sloped lightweight insulating concrete roof fill
  • Field-tapered perlitic insulation boards
  • Factory-tapered insulation boards

For a discussion of field-sloped lightweight insulating concrete roof fill systems see the Section entitled Roof Decks.

Field-tapered perlitic insulation board systems consist of multiple layers of square-edged perlite boards that are tapered by cutting or grinding in the field.

Factory-tapered insulation board systems are the ones most commonly used because they are inexpensive and easy to use. The insulation boards can be tapered to provide slopes of 1:200 (1/16" in 12") or greater. The following types of insulation are factory-tapered:

  • Polyisocyanurate
  • Mineral wool
  • XPS
  • EPS
  • Fibreboard

5 BUR and Hot-mopped Applications only

5.1 Fibreboard Roof Insulation

NOTE: See also Accepted Fibreboard Insulation products

Fibreboard insulation is composed of wood, cane, or other vegetable fibres and waterproofing binders. A water slurry containing the wood fibres and waterproofing binders is deposited onto a moving screen. The water drains through the screen and the remaining mass is heated in an oven or dryer to produce the finished product. Fibreboard roof insulation is produced in several forms:

  • Plain
  • Impregnated with asphalt or petroleum based paraffin
  • Coated with asphalt

Fibreboard Roof Insulation that is adhered in a roof assembly with hot asphalt or asphaltic-based adhesives must have an asphalt coating on the top and bottom surfaces (minimum coated two sides) to meet RoofStar Guarantee Standards. Fibreboard for use as roofing insulation is manufactured to meet or exceed the requirements of CAN / ULC-S706 (Insulating Fibreboard), Type I (Roof Board).

Fibreboard roof insulation provides the following properties and advantages:

  • Compatible with asphalt
  • Compatible with most roof system components
  • Resistant to thermal conductivity
  • Resistant to cell deterioration (durable)
  • Resistant to impact
  • Resistant to thermal cycling (dimensionally stable)
  • Unaffected by asphalt application temperatures (no overlay required)
  • Stable K-value (the thermal conductivity does not change with aging)
  • Retains roofing nails


The possible disadvantages or precautions involved in the use of fibreboard insulation include:

  • Flammable (Combustible)
  • Absorbs moisture (protect from the effects of weathering)
  • Low thermal values
  • Organic in nature and will rot on exposure to moisture.


Insulation Joints and Overlay

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