Insulated Roofing Materials

11 min read Updated 26 August 2026
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    Insulated Roofing Materials

    Insulated roofing materials are the rigid boards, composite panels and spray-applied foams that are built into a roof assembly itself, above the structural deck and below (or, in protected-membrane roofs, above) the waterproofing membrane. They are distinct from the batts and loose fill placed on an attic floor, which are covered at Roof Insulation and Insulation. In a low-slope commercial roof the insulation is typically the thickest single component of the assembly, and its selection affects Energy Efficiency, fire classification, wind-uplift and hail ratings, drainage and the service life of the membrane above it. The main product families are polyisocyanurate (polyiso), extruded polystyrene (XPS), expanded polystyrene (EPS), mineral wool (stone wool) boards, spray polyurethane foam (SPF), and the cover boards and nailbase or composite panels that combine insulation with a hard or nailable surface.

    Why insulation is placed in the roof assembly

    Placing continuous insulation above the deck avoids the thermal bridging of joists or purlins and keeps the deck on the warm side, which limits condensation. Energy codes recognize this configuration explicitly: the International Energy Conservation Code (C402.1.3) tabulates minimum R-values for roofs with "insulation entirely above roof deck," ranging from R-20 continuous insulation (ci) in the warmest climate zones to R-25ci, R-30ci and R-35ci in zones 7 and 8, the coldest.[1] Canadian requirements are set by the National Energy Code for Buildings and provincial codes, which specify effective thermal resistance of the whole assembly rather than a nominal R-value, so consultants must account for fasteners, edges and any thermal bridges. Insulation entirely above the deck is also what makes tapered systems possible: boards are manufactured with a slope to create positive drainage on flat structural decks, which minimizes ponding water and extends the life of the roof.[2]

    Product families

    Polyisocyanurate (polyiso)

    Polyiso is a closed-cell thermoset foam manufactured as boards with glass-reinforced or coated-glass facers, and it is the most widely used low-slope roof insulation in North America. Boards are produced to ASTM C1289, which classifies them by facer type (Type II for glass-faced roof boards) and compressive strength grade.[3] A representative product data sheet lists compliance with ASTM C1289 Type II, Class 1, Grade 2 (20 psi) or Grade 3 (25 psi), and long-term thermal resistance (LTTR) determined under CAN/ULC S770 of 5.7 for a 1-inch board and 11.4 for a 2-inch board, that is, about R-5.7 per inch.[4] Natural Resources Canada's design table gives polyurethane/polyisocyanurate boards a range of R-5.5 to R-6.8 per inch with a design value of R-6, and cautions that any product claiming a long-term value above R-6.5 per inch may be making an unsubstantiated claim.[5]

    Two properties matter in cold climates. First, polyiso's R-value is reported as LTTR, which accounts for the slow diffusion of blowing agent out of the cells over time; the aged design value, not the fresh value, should be used in calculations.[5] Second, the R-value of some polyiso products tends to drop at low temperatures, so NRCan advises using it as an interior insulation layer unless a cold-temperature rating is given.[5] In roofing practice this is often addressed with a hybrid assembly, polyiso below and a layer of XPS, EPS or mineral wool above, so that the polyiso stays warmer. Halogen-free polyiso formulations without brominated flame retardants are now available.[2]

    Extruded polystyrene (XPS)

    XPS is a closed-cell thermoplastic foam board made to ASTM C578, with fine, uniform cells.[6] NRCan lists it at R-5 per inch and notes its low permeability, that it does not absorb or pass moisture and that sealed joints can function as an air barrier and, at sufficient thickness, a vapour barrier.[5] Its water resistance and high compressive strength make it the standard choice for protected-membrane (inverted) roofs, where the insulation sits above the membrane under ballast or pavers, and for plaza decks and green roofs.[2] XPS has a higher embodied carbon than EPS or polyiso because of its blowing agents, although low-GWP formulations have replaced older HFC-blown products in Canada and the U.S.

    Expanded polystyrene (EPS)

    EPS ("bead board") is moulded from expanded polystyrene beads using steam, also under ASTM C578, and is available in a range of densities. NRCan gives R-3.8 per inch for Type I and R-4 to R-4.4 for Type II.[5] It has the lowest R-value per inch of the foams but the lowest cost per R, is easily cut into tapered and custom shapes, can be produced in very thick blocks, and its R-value does not drift with age because it contains no captured blowing agent. Roofing EPS may contain up to 25 percent recycled content.[2] Because it is less water-resistant than XPS, it is used below the membrane, often with a polyiso or high-density cover board above.

    Mineral wool (stone wool)

    Mineral wool roof boards are made from spun molten rock and slag bound into high-density, dual-density or bitumen-coated boards. Their defining property is fire performance: a representative low-slope roof board is classified non-combustible under ASTM E136 and CAN/ULC S114, carries a 0/0 flame-spread and smoke-developed index under ASTM E84, and is used as a component of fire-rated and Class A roof assemblies and in very severe hail (VSH) rated systems.[7] The same product lists R-7.6 at 2 inches and R-22.8 at 6 inches, roughly R-3.8 per inch, so mineral wool needs more thickness than foam for the same R.[7] Its R-value is stable over time, it is vapour-open and dries readily, and bitumen-coated versions are compatible with torched and hot-mopped modified bitumen.[7] It is often specified over steel decks in fire-rated assemblies or as the top layer in hybrid assemblies.

    Spray polyurethane foam (SPF)

    Closed-cell SPF is sprayed in place, expanding to form a seamless, self-flashing insulation layer that bonds to the substrate. NRCan gives closed-cell polyurethane spray foam a range of R-5.2 to R-6.5 per inch with a design value of R-6, and stresses that the LTTR design value, rather than the higher fresh value, should be used.[5] In SPF roofing, the foam itself is the roof: it is applied over a prepared deck or an existing roof and then protected with an elastomeric coating or granules. The Spray Polyurethane Foam Alliance reports that SPF roofs bond tenaciously to substrates, which enhances wind-uplift resistance, and that post-hurricane surveys by NIST and RICOWI found SPF roofs performed extremely well in wind and hail.[8] SPF must be applied by certified installers in suitable weather, protected from UV by its coating, and recoated periodically. See Roof Coatings.

    Cover boards

    A cover board is a thin, hard layer placed between the insulation and the membrane (and sometimes between the deck and the insulation) to protect foam from foot traffic, hail and fastener back-out, to improve fire ratings and wind-uplift performance, and to provide a better substrate for adhered membranes. Gypsum-core boards with fibreglass mats are the most common: a representative product is available in 1/4-, 1/2- and 5/8-inch thicknesses, is rated 0/0 flame spread and smoke development under ASTM E84, offers about 900 psi nominal compressive strength, and is marketed for resistance to fire, wind uplift, punctures and severe hail.[9] Alternatives include high-density polyiso cover boards, which offer similar protection at a fraction of the weight, as well as EPS-based, cement and wood-fibre boards.[2] Cover boards add little R-value (a 1/4-inch gypsum board is about R-0.28) but are now standard practice under single-ply and modified bitumen membranes.[9]

    Nailbase and composite panels

    For steep-slope roofs and cathedral ceilings, insulation is factory-laminated to a nailable surface. A nailbase panel bonds a closed-cell polyiso core to 7/16-inch OSB (or thicker OSB or plywood), giving a single board that insulates above the rafters and accepts shingles or metal roofing directly.[10] Ventilated versions add wood spacers between the foam and the OSB to create a standard 1-inch air space, which vents heat and moisture above the insulation, helps prevent ice dams and extends shingle life; one such panel exceeds the air-space depth in ARMA Technical Bulletin 211-RR-24.[11] Structural insulated panels (SIPs) go a step further by sandwiching foam between two structural skins and spanning between supports.

    Comparison of roof insulation products

    Product Typical R per inch (design) Standard Strengths Limitations
    Polyiso About 5.7 (LTTR); NRCan design R-6[4][5] ASTM C1289 Highest R per inch; fire performance; industry default R drops in cold for some products; must stay dry
    XPS 5[5] ASTM C578 Water-resistant; high compressive strength; inverted roofs Higher embodied carbon; UV-sensitive
    EPS 3.8 to 4.4[5] ASTM C578 Lowest cost per R; stable R; easy to taper Lower R per inch; less water-resistant
    Mineral wool About 3.8[7] ASTM C726 / CAN/ULC S702 Non-combustible; stable; hail and fire assemblies Thicker; heavier
    Closed-cell SPF 5.2 to 6.5; design R-6[5] ASTM C1029 Seamless; self-flashing; wind uplift Weather-dependent application; needs coating
    Gypsum cover board About 0.28 per 1/4 in.[9] ASTM C1177 Fire, hail, puncture protection Adds weight; no meaningful R

    Assembly design considerations

    • Layering. Two staggered layers of thinner board outperform one thick layer because joints do not line up, reducing air and heat leakage at the seams.
    • Attachment. Boards are mechanically fastened, set in low-rise foam adhesive, or hot-mopped. Fasteners through the full insulation thickness are thermal bridges; adhered upper layers and cover boards reduce this.
    • Moisture. Foam boards must be kept dry during and after installation; NRCan notes that foam plastics must be protected from prolonged exposure to sunlight and water.[5] Wet insulation loses R-value and can corrode steel decks. A vapour retarder over the deck is common in Canadian and northern assemblies where interior humidity is high.
    • Fire. Foam plastics are combustible; assemblies over steel decks commonly require a thermal barrier or a mineral wool or gypsum layer to achieve their listing. Non-combustible mineral wool or gypsum cover boards are used to reach Class A roof-covering ratings.[7][9]
    • Drainage. Tapered polyiso, EPS or mineral wool creates the minimum 1/4:12 slope most membranes require on flat decks, with crickets and saddles between drains.[2]
    • Membrane compatibility. Torch-applied membranes need a compatible, non-combustible or bitumen-coated substrate; single-ply adhesives require approved facers. See Modified Bitumen Roofing, TPO Roofing and PVC Roofing.

    Longevity and maintenance

    Properly installed and kept dry, roof insulation is expected to last the life of the membrane above it, and dry boards are often reused during re-roofing. The main threats are leaks that saturate boards, foot traffic that crushes low-density foam (hence cover boards at walkways and around equipment), and wind uplift that pulls fasteners. Infrared or nuclear moisture surveys during Roof Inspection and Maintenance locate wet insulation so that only affected areas need replacement. When a roof is replaced, adding insulation to current code levels is often the most cost-effective energy upgrade available to a building owner, and it can contribute to LEED Certification and other green-building programs.

    Frequently Asked Questions

    Which roof insulation has the highest R-value per inch?

    Polyisocyanurate. A representative polyiso roof board is rated at LTTR 5.7 per inch (11.4 for 2 inches), and Natural Resources Canada lists polyurethane/polyisocyanurate boards at R-5.5 to R-6.8 per inch with a design value of R-6, compared with R-5 for XPS, R-3.8 to R-4.4 for EPS and about R-3.8 for mineral wool.[4][5][7]

    Why is polyiso paired with another insulation in cold climates?

    Because the R-value of some polyiso products falls at low temperatures; NRCan advises using it as an interior layer unless it carries a cold-temperature rating. Hybrid assemblies place polyiso below and XPS, EPS or mineral wool above so the polyiso stays warmer, while the top layer also adds hail and fire protection.[5][7]

    What is a cover board and is it required?

    A cover board is a thin, hard layer, usually gypsum-core or high-density polyiso, placed between foam insulation and the membrane. It protects against foot traffic, hail and punctures, improves fire and wind-uplift ratings, and gives adhesives a better substrate. Codes rarely mandate it, but most manufacturers require it for hail and fire-rated warranties.[9][2]

    How much above-deck insulation does code require?

    The IECC's Table C402.1.3 sets minimums for insulation entirely above the roof deck from R-20ci in the warmest climate zones to R-25ci, R-30ci and R-35ci in zones 7 and 8. Canadian projects follow the National Energy Code for Buildings or provincial codes, which specify effective assembly thermal resistance and account for thermal bridging by fasteners.[1]

    What is LTTR?

    Long-term thermal resistance is the aged R-value of foam insulation whose cells slowly lose blowing agent. It is measured by an accelerated test (CAN/ULC S770 in Canada) that predicts the resistance averaged over years of service, and NRCan recommends using LTTR rather than the higher fresh value when calculating how much insulation a roof needs.[5][4]

    Sources

    1. ↑ 1.0 1.1 UpCodes — IECC C402.1.3 Insulation Component R-Value-Based Method (Table C402.1.3) (minimum R-values for insulation entirely above roof deck by climate zone, R-20ci to R-35ci).
    2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Carlisle SynTec — Insulation (polyiso, EPS and XPS product families, tapered insulation for drainage, polyiso and EPS cover boards, halogen-free polyiso).
    3. ↑ ASTM International — ASTM C1289 Standard Specification for Faced Rigid Cellular Polyisocyanurate Thermal Insulation Board (the material standard for polyiso roof boards).
    4. ↑ 4.0 4.1 4.2 4.3 Johns Manville — ENRGY 3 Polyisocyanurate Roof Insulation data sheet (ASTM C1289 Type II Class 1 Grade 2/3, LTTR values by thickness per CAN/ULC S770, glass-reinforced facers).
    5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 5.12 5.13 Natural Resources Canada — Keeping the Heat In, Section 3: Materials (Table 3-1 R-value per inch for foam boards, mineral fibre boards and spray foams; LTTR explanation; cold-temperature R-value drop of some polyiso; protection of foam from sunlight and water).
    6. ↑ ASTM International — ASTM C578 Standard Specification for Rigid, Cellular Polystyrene Thermal Insulation (the material standard covering XPS and EPS boards).
    7. ↑ 7.0 7.1 7.2 7.3 7.4 7.5 7.6 ROCKWOOL — Toprock DD low-slope roof insulation (non-combustible stone wool board, R-value by thickness, dual-density for point loads, VSH hail use; DD Plus bitumen-coated variant for torched, hot-mopped and self-adhered membranes).
    8. ↑ Spray Polyurethane Foam Alliance — Benefits of SPF (SPF roofing wind-uplift and hail performance, NIST and RICOWI post-storm findings, combined insulation and air sealing, industry EPDs).
    9. ↑ 9.0 9.1 9.2 9.3 9.4 Georgia-Pacific — DensDeck Prime Roof Board (gypsum-core fibreglass-mat cover board, thicknesses, ASTM E84 0/0, compressive strength, fire, wind, puncture and hail resistance).
    10. ↑ Hunter Panels — H-Shield NB (polyiso core laminated to 7/16-inch OSB or plywood for steep-slope new construction and re-roofing).
    11. ↑ Hunter Panels — Cool-Vent (ventilated nailbase: polyiso, wood spacers forming a 1-inch air space, and OSB or plywood top layer for steep-slope ventilation).

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