Roof Insulation

13 min read Updated 26 August 2026
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    Roof Insulation

    Roof insulation is the layer of low-conductivity material placed in or on a roof assembly to slow heat flow between a building's interior and the outdoors. Because warm air rises and the roof is the largest surface exposed to sun, wind and clear-sky cooling, the roof and ceiling plane is usually the single most important part of the envelope for Energy Efficiency. Insulation reduces winter heat loss and summer heat gain, keeps interior surfaces closer to room temperature (which limits condensation), and, when combined with air sealing and ventilation, is the primary defence against ice dams on Sloped Roofs in cold climates.

    The term covers several quite different products and positions. In houses, insulation most often lies on the attic floor above the ceiling; in cathedral ceilings it fills or covers the rafter bays; on Flat Roofs and other Commercial Roofing systems it is normally a rigid board laid over the Roof Deck beneath the membrane. Each position has its own rules for vapour control and ventilation. This article covers materials, metrics, assembly placement, code minimums in the United States and Canada, and moisture management. Board products for low-slope roof systems are covered in more detail at Insulated Roofing Materials, and attic airflow at Attic Ventilation and Insulation Guide.

    R-value, RSI and U-value

    Thermal performance is expressed as R-value, the resistance to heat flow; the higher the R-value, the slower the heat transfer.[1] Canadian codes and product labels use the metric equivalent, RSI (m²·K/W); one RSI unit equals about R-5.68, so a design value of 0.88 RSI per 25.4 mm corresponds to R-5 per inch.[2] U-value (or U-factor) is the inverse of the total R-value of an assembly and measures heat transmission, so lower is better. Energy codes state ceiling requirements both ways: a prescriptive minimum R-value for the insulation itself, or a maximum U-factor for the whole assembly, which accounts for framing and thermal bridging.[3]

    R-value is additive: two layers of R-20 give R-40, provided the material is installed at its full thickness. Compressed batts, gaps around obstructions, and framing members that bridge the insulation all reduce the effective value of the assembly below the sum of the labels.

    Insulation materials

    The four main families used in roofs and attics are fibreglass, mineral wool, cellulose and spray foam, plus rigid boards for above-deck and cathedral applications.[1] Typical design values, from the Natural Resources Canada table used for Canadian retrofits, are summarized below.[2]

    Material Common forms Design R per inch (RSI per 25.4 mm) Typical roof use
    Fibreglass (glass fibre) batt Batts, rolls R-3.2 (0.56); range R-3.1–4.3 Between rafters or ceiling joists
    Fibreglass, blown loose-fill Loose-fill R-2.9 (0.51) Attic floor top-up
    Cellulose, blown (settled) Loose-fill, wet-spray R-3.6 (0.63); range R-3–3.8 Attic floor, dense-pack rafter bays
    Mineral wool (mineral fibre) batt Batts, boards R-3.4 (0.6); range R-3–4 Rafter bays, fire-rated assemblies
    Expanded polystyrene (EPS) Type I / Type II Rigid board R-3.8 / R-4 (0.67 / 0.7) Above-deck, tapered systems
    Extruded polystyrene (XPS) Rigid board R-5 (0.88) Above-deck, over-roof retrofits
    Polyisocyanurate / polyurethane board Rigid board, foil- or glass-faced R-6 (1.06); range R-5.5–6.8 Low-slope roofs under membranes
    Closed-cell spray polyurethane foam Sprayed in place R-6 (1.06); range R-5.2–6.5 Unvented rafter bays, air sealing
    Open-cell spray polyurethane foam Sprayed in place R-3.6 (0.63) Unvented rafter bays (with vapour control)

    Fibreglass batts and loose-fill

    Fibreglass is spun from molten glass and sold in batts sized for standard framing, or blown loose. It is inexpensive and non-combustible, but batts must be cut to fit around wiring, pipes and trusses without compression or voids; blown fibreglass settles less than cellulose but has a lower R per inch.[2]

    Cellulose

    Cellulose is made mostly from recycled paper fibre treated with borates to resist fire and pests.[1] Blown across an attic floor it fills irregular spaces well; its listed R-value applies to the settled thickness, so installers add extra depth to allow for settling.[2]

    Mineral wool

    Mineral (rock or slag) wool is denser and stiffer than fibreglass, tolerates high temperatures, and holds its shape in rafter bays. It is favoured where fire resistance or sound control matters.[1]

    Rigid boards

    EPS, XPS and polyisocyanurate boards provide the highest R-value per inch of the non-foam-in-place options and can be laid in continuous layers that cover the framing, eliminating thermal bridges. They are the standard insulation beneath Single-Ply Roofing Membranes such as TPO Roofing, EPDM Roofing and PVC Roofing, and under Modified Bitumen Roofing and Built-Up Roofing; product details, facers and tapered layouts are covered at Insulated Roofing Materials.

    Spray polyurethane foam (SPF)

    Closed-cell SPF combines a high R per inch with an air barrier and, at sufficient thickness, a vapour retarder; open-cell foam is air-impermeable to air movement but vapour-open. Both are used to create unvented "cathedralized" attics by spraying the underside of the roof sheathing.[4]

    Radiant barriers and reflective insulation

    Foil radiant barriers reflect radiant heat rather than resisting conduction, so they do not carry a conventional R-value per inch; NRCan lists them separately from mass insulation.[2] They are most useful facing a ventilated air space under hot, sunny roofs, including Metal Roofing, and are a supplement to, not a substitute for, the R-values required by code.

    Structural insulated panels (SIPs) sandwich rigid foam between two sheets of sheathing, providing framing and insulation in one component; they are used in energy-focused custom construction.

    Where insulation sits in the roof assembly

    Attic floor (vented attic)

    The most common residential arrangement places insulation on the ceiling, leaving the attic cold and ventilated. It is the easiest to insulate to high levels and to inspect. The 2021 IRC requires a net free vent area of 1/150 of the vented space, reduced to 1/300 where vents are balanced between the upper and lower parts of the attic and, in Climate Zones 6–8, a Class I or II vapour retarder is installed on the warm-in-winter side of the ceiling.[5] Natural Resources Canada describes four attic conditions that dictate the retrofit approach: accessible attics, cramped truss attics, one-and-a-half-storey houses with knee walls, and cathedral, flat or mansard roofs with little or no attic space.[6]

    Cathedral ceilings and unvented roofs

    Where the ceiling follows the roof line, insulation is confined to the rafter depth. A vented cathedral ceiling keeps an air channel under the sheathing from soffit to ridge; an unvented assembly fills the bay completely. The IRC permits unvented attics and enclosed rafter assemblies only where the space is inside the thermal envelope and no interior Class I vapour retarder is installed on the ceiling side, with air-impermeable insulation against the sheathing to control condensation.[5] Building Science Corporation recommends a total of at least R-40 for unvented roofs in cold regions and R-50 in very cold regions, and notes that a vented over-roof above the insulation is needed in heavy-snow areas to prevent ice damming.[4]

    Above-deck continuous insulation

    Low-slope roofs place rigid boards on top of the structural deck, under the membrane, so the deck and framing stay on the warm side and no ventilation is required. On steep roofs the same idea, sometimes called an "over-roof," puts rigid foam above the sheathing during re-roofing; two inches of XPS over a cellulose-filled 2×8 rafter bay is the example Building Science Corporation gives for reaching an R-40 cathedral ceiling while blunting thermal bridges at the rafters.[7]

    How much insulation: code minimums and recommendations

    The 2021 International Residential Code sets prescriptive ceiling R-values by climate zone, and ENERGY STAR publishes retrofit targets for existing wood-framed homes derived from the same code.[3][8]

    IECC climate zone 2021 IRC minimum ceiling R-value (new construction) ENERGY STAR retrofit target, uninsulated attic ENERGY STAR target, attic with 3–4 in. existing
    0–1 R-30 R-30 R-25
    2 R-49 R-49 R-38
    3 R-49 R-49 R-38
    4 (except Marine) R-60 R-60 R-49
    5 and Marine 4 R-60 R-60 R-49
    6 R-60 R-60 R-49
    7–8 R-60 R-60 R-49

    A quick field check: common loose-fill and batt products deliver roughly R-3 to R-3.5 per inch, so ENERGY STAR describes R-38 as about 13–14 inches and R-49 as about 16–18 inches of attic insulation.[9] Local amendments, provincial codes and utility rebate programs may be stricter; see Roofing Building Codes and Permits.

    In Canada, ceiling requirements are set by the energy-efficiency provisions of the National Building Code (Section 9.36) as adopted and amended by each province, and are expressed in RSI. Values commonly specified for attics in the populated climate zones fall in the range of roughly RSI 8.7 to 10.4 (about R-50 to R-60), with higher values in the far north; the applicable provincial table should be checked for the exact figure. Cities such as Calgary and Edmonton combine cold winters with heavy sun and wind, which makes both the insulation depth and the air barrier especially important — see Calgary Weather and Your Roof.

    Vapour retarders, air sealing and moisture

    Insulation slows heat, but water vapour and air leaks move moisture. Building Science Corporation groups vapour retarders into Class I (0.1 perm or less, e.g. polyethylene and foil), Class II (0.1–1 perm, e.g. kraft-faced batts) and Class III (1–10 perm, e.g. latex-painted drywall), and warns against "double vapour barriers" that prevent an assembly from drying in either direction.[10]

    Placement depends on climate:

    • Cold and very cold climates (Canada, IECC Zones 6–8): a Class I or II retarder on the warm-in-winter side of the ceiling is standard practice. NRCan specifies that the vapour barrier be located on the warm side of the insulation, or no deeper than one-third of the way into the insulating value (the one-third/two-thirds rule); for example, if a 3½-inch joist space holds RSI 2.1 (R-12) below the polyethylene, at least RSI 4.2 (R-24) should be installed above it.[2][6]
    • Mixed and hot-humid climates: interior polyethylene is discouraged because summer air conditioning reverses the vapour drive and can trap moisture against the cold ceiling; a Class III retarder (painted drywall) is usually sufficient.[10]
    • Unvented roofs in any climate: no interior Class I retarder, so that the assembly can dry inward; condensation control comes from air-impermeable insulation at the sheathing.[5][4]

    In every case the air barrier matters more than the vapour retarder. NRCan places air sealing of the ceiling plane — around plumbing stacks, chimneys, recessed lights, top plates and the attic hatch — ahead of ventilation as the first line of defence against attic moisture.[6] Ducts and air handlers in attics should be sealed and buried in insulation or brought inside the envelope.

    Ice dams: insulation, air sealing and ventilation together

    Ice dams form when the upper roof is warm enough to melt snow while the eaves stay below freezing. Building Science Corporation identifies the common causes as insufficient insulation or thermal bridging, warm air leaking into the roof space, heat sources such as poorly insulated ducts or hot-water piping in the attic, and uneven snow depth combined with solar radiation.[7] The remedy is layered: seal ceiling air leaks first, bring insulation up to the full climate-zone R-value with baffles that keep it from blocking soffit vents, and then provide balanced ventilation so the attic tracks outdoor temperature. NRCan describes a cold, well-vented attic as less prone to ice dams at the eaves and treats ventilation as the second line of defence after air sealing.[6] An ice-barrier membrane at the eaves protects the Roof Deck from the water that any residual dam holds back. Removal and emergency measures are covered at How to Prevent and Remove Ice Dams, and vent sizing at Roof Ventilation.

    Installation issues that reduce performance

    • Compression — batts stuffed into shallow cavities or squeezed under ducts lose R-value; the code counts only the installed value where a cavity is shallower than the label thickness.[3]
    • Voids and gaps — even small uninsulated areas around framing, hatches and lights allow disproportionate heat loss.
    • Blocked eave vents — insulation pushed into the soffit stops intake air and promotes ice dams and condensation.
    • Recessed lights and chimneys — need airtight, rated fixtures or non-combustible clearance boxes before insulation is added.
    • Thermal bridging — wood framing at 16 or 24 inches on centre conducts heat past cavity insulation; a continuous layer above or below the framing addresses it.[7]
    • Roof leaks — wet insulation loses most of its value and supports mould; NRCan advises finding and fixing any staining or rot before adding insulation.[6]

    Retrofits also affect the Roof Structure: deep loose-fill adds little weight, but rigid over-roofs, SIPs and ballasted assemblies must be checked against the framing's capacity, and improved insulation changes attic temperatures in ways that can raise the risk of condensation if ventilation is not adjusted.

    Environmental impact

    Roof insulation is among the lowest-cost carbon reductions in a building because it cuts heating and cooling energy for the life of the structure. Cellulose is largely recycled paper, fibreglass and mineral wool contain recycled glass and slag, and modern spray and board foams have moved away from high-global-warming-potential blowing agents. Insulation contributes to energy credits under LEED Certification and complements Cool Roofs, Green Roofs and other Energy-Efficient Roofing Systems; it also protects Roof Integrity by reducing thermal cycling of the membrane and deck.

    Frequently Asked Questions

    How much roof insulation do I need?

    The 2021 IRC requires ceiling insulation of R-30 in Climate Zones 0–1, R-49 in Zones 2–3 and R-60 in Zones 4–8, and ENERGY STAR uses the same targets for uninsulated attics. Canadian provincial codes express attic minimums in RSI, commonly in the range of about RSI 8.7–10.4 (R-50–R-60). Existing attics with 3–4 inches can usually be topped up to reach these levels.[3][8]

    What is the best type of roof insulation?

    No single material is best; installation quality and air sealing matter more than the product. Blown cellulose or fibreglass suits open attic floors, batts or mineral wool fit rafter bays, closed-cell spray foam is used for unvented cathedral ceilings, and rigid polyiso, XPS or EPS boards are standard above the deck on low-slope roofs where continuous coverage eliminates thermal bridging.[1]

    Does roof insulation need a vapour barrier?

    In cold climates a Class I or II vapour retarder on the warm side of the ceiling is normal practice, placed no deeper than one-third of the way into the insulation's R-value. In hot-humid climates interior polyethylene is discouraged, and unvented roof assemblies must not have an interior Class I retarder so they can dry inward. Air sealing is required in every climate.[10][2]

    Can insulation alone stop ice dams?

    Insulation is necessary but not sufficient. Ice dams are driven by heat reaching the roof deck through thin insulation, thermal bridges, air leaks and warm ducts. Sealing ceiling air leaks, insulating to the full climate-zone R-value with baffles at the eaves, and then ventilating the attic to keep it cold together prevent most ice dams; an eave ice-barrier membrane limits damage from any that remain.[7]

    Should a cathedral ceiling be vented or unvented?

    Both work when built correctly. A vented cathedral ceiling needs a clear air channel from soffit to ridge above the insulation. An unvented one needs air-impermeable insulation against the sheathing, no interior Class I vapour retarder, and, in cold and very cold regions, a total of about R-40 to R-50; in heavy-snow areas a vented over-roof above the insulation is recommended.[4][5]

    Sources

    1. ↑ 1.0 1.1 1.2 1.3 1.4 NAIMA Insulation Institute — Comparing Insulation Types (definition of R-value; the four main residential insulation materials; installation and air sealing outweigh material choice).
    2. ↑ 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Natural Resources Canada — Keeping The Heat In – Section 3: Materials (Table 3-1 insulation values in RSI per 25.4 mm and R per inch; vapour barrier on the warm side; one-third/two-thirds rule).
    3. ↑ 3.0 3.1 3.2 3.3 International Code Council — 2021 International Residential Code, Chapter 11 [RE] Energy Efficiency (Table N1102.1.3 (R402.1.3) minimum ceiling R-values by climate zone; U-factor alternative in Table N1102.1.2).
    4. ↑ 4.0 4.1 4.2 4.3 Building Science Corporation — RR-0108: Unvented Roof Systems (minimum total R-40 for unvented roofs in cold regions and R-50 in very cold regions; interior vapour barriers not recommended in unvented assemblies).
    5. ↑ 5.0 5.1 5.2 5.3 International Code Council — 2021 International Residential Code, Chapter 8 Roof-Ceiling Construction (R806.2 minimum vent area 1/150 with 1/300 exception; R806.5 unvented attic and enclosed rafter assemblies).
    6. ↑ 6.0 6.1 6.2 6.3 6.4 Natural Resources Canada — Keeping The Heat In – Section 5: Roofs and attics (attic types; air sealing before ventilation; ice dam causes and remedies; one-third/two-thirds example for insulation over polyethylene).
    7. ↑ 7.0 7.1 7.2 7.3 Building Science Corporation — BSD-135: Ice Dams (causes: insufficient insulation or thermal bridging, air leakage, heat sources in the roof, uneven snow; exterior rigid insulation example reaching the R-40 recommendation).
    8. ↑ 8.0 8.1 ENERGY STAR — Recommended Home Insulation R-Values (retrofit table for attic floors by climate zone, based on the 2021 IECC).
    9. ↑ ENERGY STAR — How to Check Your Home's Attic Insulation Level (R-3 to 3.5 per inch estimate; R-38 for the southern and R-49 for the northern United States as minimum retrofit levels).
    10. ↑ 10.0 10.1 10.2 Building Science Corporation — BSD-106: Understanding Vapor Barriers (Class I/II/III definitions; polyethylene as Class I, kraft facing as Class II, latex paint as Class III; avoid double vapour barriers and interior polyethylene in air-conditioned assemblies).

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