By The Roofing Brief Team. Last reviewed: October 2026.
Flat roof insulation works best as rigid board above the roof deck in a warm roof, and polyisocyanurate (polyiso) is the most common board at R-5.7 to R-5.9 per inch long-term (LTTR), per PIMA’s QualityMark data. On commercial low-slope roofs, 2021 IECC Table C402.1.3 sets the above-deck minimum at R-20ci in climate zones 0 and 1 (R-25ci for Group R residential occupancies), R-25ci in zones 2 and 3, R-30ci in zones 4 to 6 and R-35ci in zones 7 and 8. Two layers of polyiso reach R-30 at about 5.2 inches. The three assembly types are the warm roof (insulation above the deck, below the membrane), the cold roof (insulation between the joists, below the deck) and the inverted or protected membrane roof (insulation above the membrane).
Which flat roof insulation setup fits your roof
Start from the building type, then the climate zone, then where the insulation can physically go. The thresholds below come from the 2021 IECC, 2021 IRC and 2021 IBC sections named in each row.
| If your roof is | Use | Threshold or rule | Source |
|---|---|---|---|
| Commercial, insulation entirely above the deck | Warm roof, two staggered polyiso layers | R-20ci (zones 0 to 1) to R-35ci (zones 7 to 8) | 2021 IECC Table C402.1.3 |
| House, rigid board above the deck plus batts below (hybrid, unvented) | Board sized to the condensation table | Board at least R-5 (zones 1 to 3), R-10 (4C), R-15 (4A, 4B), R-20 (5), R-25 (6), R-30 (7), R-35 (8) | 2021 IRC Table R806.5 |
| House, insulating only from inside (cold roof) | Vented rafter space | 1 in minimum air space above the insulation; net free vent area 1/150 of the space (1/300 with conditions) | 2021 IRC R806.2, R806.3 |
| House, no room to vent, rafters 2×8 or deeper | Closed-cell spray foam against the deck (unvented) | Air-impermeable insulation directly under the sheathing; no interior Class I vapor retarder | 2021 IRC R806.5 |
| Single-ply or mod bit membrane on a dead-flat deck | Tapered insulation | Design slope of at least 1/4 in per foot (2 percent) | 2021 IBC 1507.11.1, 1507.12.1, 1507.13.1 |
| Terrace, green roof or ballasted deck | Inverted (protected membrane) roof with XPS | FOAMULAR XPS rated R-5.0 per inch at 75°F | Owens Corning data sheet |
| Cold climate (zones 6 to 8), polyiso stack | Add a mineral wool or XPS layer, or size polyiso on a lower design value | XPS rises to R-5.6 per inch at 25°F; ROCKWOOL Toprock DD to R-4.2 at 40°F | Owens Corning, ROCKWOOL data sheets |
Warm roof vs cold roof: which flat roof insulation assembly to use
A warm roof puts the insulation above the structural deck and directly under the membrane, keeping the deck, joists and interior on the warm side of the insulation. A cold roof puts the insulation between the joists, below the deck, which leaves the deck cold; on a house, 2021 IRC R806.3 then requires at least a 1 inch air space between the insulation and the roof sheathing, and R806.2 sets the net free vent area at 1/150 of the vented space. Warm roofs dominate new low-slope construction because they eliminate thermal bridging through the joists and cut condensation risk. Cold roofs survive mainly in retrofits where the deck cannot be raised. For the membrane choice and winter installation limits, see our guide to the best flat roof system for cold climates.
The inverted roof (also called a protected membrane roof, or PMR) flips the warm roof: insulation goes on top of the waterproofing membrane, protecting it from UV and temperature swings. Extruded polystyrene (XPS) is the usual board here; Owens Corning lists its FOAMULAR XPS at R-5.0 per inch at 75°F and notes that roofing jobs use the NGX THERMAPINK 25 or NGX 400, 600 and 1000 compressive grades. Ballast, pavers or a green roof holds the boards down.
| Assembly | Insulation position | Vapor/condensation control | Best for |
|---|---|---|---|
| Warm roof | Above deck, below membrane | Vapor retarder below insulation; deck stays warm | Most new low-slope roofs |
| Cold roof | Between joists, below deck | Ceiling vapor barrier plus ventilated air gap above insulation | Retrofits that cannot raise the deck |
| Inverted (PMR) | Above the membrane | Membrane stays warm and protected; XPS only | Green roofs, terraces, ballasted decks |
The failure mode that decides it is condensation. In a cold roof, warm interior air that leaks past the ceiling hits a cold deck and drops water. The 2021 IRC handles this two ways: vent the space (R806.2 and R806.3), or keep the deck warm with enough board above it, sized by climate zone in Table R806.5, from R-5 in zones 1 to 3 up to R-35 in zone 8. A warm roof keeps the deck above the dew point, so it forgives small air leaks that would rot a cold roof.
Flat roof insulation options: polyiso, XPS, EPS, mineral wool, and spray foam
The five flat roof insulation materials in common U.S. use are polyisocyanurate (polyiso, ASTM C1289), extruded polystyrene (XPS, ASTM C578), expanded polystyrene (EPS, ASTM C578), mineral wool (ASTM C726) and closed-cell spray polyurethane foam (SPF); 2021 IBC Table 1508.2 lists the board standards. Polyiso leads on R-value per inch, XPS wins where the board gets wet, EPS wins on cost, mineral wool wins on fire and sound, and SPF wins on irregular decks.
| Material | R-value per inch | Water resistance | Relative cost | Typical use | Source |
|---|---|---|---|---|---|
| Polyiso (ISO) | R-5.7 at 1 in to R-5.95 at 4.5 in (LTTR) | Absorbs water if soaked | Moderate | Primary warm-roof board | Sika Sarnatherm ISO data sheet, Aug 2024 |
| XPS | R-5.0 at 75°F; R-5.6 at 25°F | Very high (inverted roofs) | Higher | Protected membrane, wet exposure | Owens Corning FOAMULAR data sheet |
| EPS | R-3.6 to R-4.2 | Moderate | Lowest | Budget fill, tapered base layers | Rmax ASTM C578 explainer |
| Mineral wool | R-3.8 at 75°F; R-4.2 at 40°F | Water absorption 1.0 percent by volume or less | Moderate to high | Fire-rated and acoustic assemblies | ROCKWOOL Toprock DD data sheet |
| Closed-cell roofing SPF | R-6.7 at 1 in (2.8 pcf foam) | High | High | Irregular decks, monolithic recover | NCFI 10-016 data sheet |
Polyiso carries a cold-weather catch. Its LTTR is measured at a 75°F mean temperature, and PIMA’s own temperature bulletin says polyiso’s R-value can rise as the mean temperature drops toward 40°F and then falls off at colder mean temperatures. XPS and mineral wool move the other way: Owens Corning lists FOAMULAR XPS at R-5.0 per inch at 75°F, R-5.4 at 40°F and R-5.6 at 25°F, and ROCKWOOL lists Toprock DD at R-3.8 at 75°F and R-4.2 at 40°F. That is why many northern roofs pair polyiso with a mineral wool or polystyrene layer, or size the polyiso on a lower design value.
Is there R-7 per inch rigid roof insulation?
Not among the standard roof boards. Polyiso, the highest-rated mainstream above-deck board, carries a published long-term thermal resistance (LTTR) of R-5.7 per inch at 1 and 2 inches, rising to R-5.9 per inch at 4 inches. Foil-faced wall polyiso is rated higher under a different test method, but roof boards with permeable facers are not. To reach a higher R-value above the deck, add thickness.
The figures come from a Polyisocyanurate Insulation Manufacturers Association (PIMA) performance bulletin reporting the 2015 QualityMark verification, in which an independent third party pulled boards from the plants and FM Global administered the testing. Tested boards averaged slightly above their published values at every thickness.
| Polyiso roof board thickness | Published LTTR per inch | Average tested LTTR per inch (2015) | Boards tested |
|---|---|---|---|
| 1 inch | R-5.7 | R-5.78 | 33 |
| 2 inches | R-5.7 | R-5.74 | 33 |
| 3 inches | R-5.8 | R-5.85 | 33 |
| 4 inches | R-5.9 | R-5.95 | 33 |
Some designers use a lower number on purpose. The same bulletin notes that the 2016 NRCA Roofing Manual recommends a design value of R-5.0 per inch for all polyiso thicknesses, a figure PIMA disputes. If a spec sheet or sales quote promises R-7 per inch for a roof board, ask which test method and facer the number is based on before you size the layers. Pair this with the cold-weather derating above: in a northern climate, plan the stack on a conservative per-inch value.
What R-value do you need for a flat roof?
Flat roof insulation requirements are set by climate zone under the International Energy Conservation Code (IECC), not by preference. For commercial low-slope roofs with insulation entirely above the deck, 2021 IECC Table C402.1.3 prescribes continuous insulation from R-20ci in zones 0 and 1 to R-35ci in zones 7 and 8. Residential codes push ceiling and roof R-values higher: the 2021 IECC residential table asks R-30 in zone 1, R-49 in zones 2 and 3 and R-60 in zones 4 to 8, and R402.2.2 allows R-30 on a ceiling without an attic only over 500 sq ft or 20 percent of the ceiling, whichever is less. Confirm the edition your jurisdiction has adopted, because states amend the code.
| 2021 IECC climate zone | Example regions | Above-deck ci, all other occupancies | Above-deck ci, Group R | Two-layer polyiso stack (Sika LTTR) |
|---|---|---|---|---|
| 0 and 1 | South Florida, Hawaii | R-20ci | R-25ci | 1.8 in + 1.8 in = R-20.6 at 3.6 in |
| 2 and 3 | Gulf Coast, Georgia, Arizona, coastal CA | R-25ci | R-25ci | 2.2 in + 2.2 in = R-25.2 at 4.4 in |
| 4 to 6 | Mid-Atlantic, Midwest, PNW, New England | R-30ci | R-30ci | 2.6 in + 2.6 in = R-30.0 at 5.2 in |
| 7 and 8 | Northern Minnesota, Alaska | R-35ci | R-35ci | 3.0 in + 3.1 in = R-35.4 at 6.1 in |
Sources: 2021 IECC Table C402.1.3 (R-value method); stack values from the Sika Sarnatherm ISO LTTR table (August 2024). Example regions are approximate; check the county map in IECC Chapter 3.
The thickness of rigid insulation used on a flat roof follows from the target and the material R-value per inch. To hit R-30 with polyiso at R-5.7 per inch you need about 5.3 inches; with Sika’s published LTTR for 2.6 inch boards (R-15.0 each), two layers reach R-30.0 at 5.2 inches. At R-25 that drops to roughly 4.4 inches. Two offset layers matter: a single thick layer leaves aligned board joints that leak heat and let water track to the deck.
Is tapered insulation worth it on a flat roof?
Tapered insulation is rigid board factory-cut on a slope, usually 1/4 inch or 1/2 inch of fall per foot, to move water toward drains on a deck that is structurally flat. On commercial work it is often a code item, not an option: 2021 IBC sections 1507.11.1, 1507.12.1 and 1507.13.1 require a design slope of at least 1/4 unit in 12 (2 percent) for modified bitumen, EPDM and TPO or PVC membranes, and the IBC defines positive roof drainage as slope that drains the roof within 48 hours of precipitation.
Tapered systems are usually specified as polyiso or EPS, often with EPS as the thick sloped base and a flat polyiso layer on top for R-value. The taper adds height fast: at 1/4 inch per foot, a drain 40 feet from the high edge needs 10 inches more board at that edge than at the drain, so check curb heights, door thresholds and edge metal before ordering. The layout is engineered from the drains outward using crickets and valleys.
- Locate the drains or scuppers, the low points the water must reach.
- Set the slope, commonly 1/4 inch per foot, and calculate board thickness at each point.
- Build crickets (small ridges) between drains to split water and prevent flat valleys.
- Top with a flat board layer so the finished R-value meets code at the thinnest point.
Ponding is water still standing 48 hours after rain, the same window the IBC uses for positive drainage. For drainage design, see our guides on ponding water on a flat roof and flat roof drainage design.
Flat roof insulation cost
Flat roof insulation with rigid foam board (EPS, XPS or polyiso) runs about $2.50 to $6.00 per square foot installed, per HomeGuide’s roof insulation cost guide (November 2025), within a broader $2 to $7 or more per square foot for all roof insulation types. HomeGuide puts insulation labor at $40 to $80 or more per hour. Thickness drives the price: an R-30 stack is about 45 percent more board than an R-20 stack (5.2 vs 3.6 inches of polyiso).
| Cost item | Range | What moves it | Source |
|---|---|---|---|
| Rigid foam board (EPS, XPS, ISO), installed | $2.50 to $6.00 / sq ft | Board type and total thickness | HomeGuide, Nov 2025 |
| Roof insulation, all types, installed | $2 to $7+ / sq ft | Material, access, removal of old insulation | HomeGuide, Nov 2025 |
| Insulation labor | $40 to $80+ per hour | Region and roof access | HomeGuide, Nov 2025 |
| Board thickness for code (polyiso) | 3.6 in (R-20ci) to 6.1 in (R-35ci) | Climate zone | 2021 IECC C402.1.3; Sika LTTR table |
Insulation is usually bundled into a full membrane job rather than priced alone, so ask each bidder to list board type, layer thicknesses and the stated R-value on the quote, for example two 2.6 inch polyiso layers at R-15.0 each for R-30. Our breakdowns of flat roof replacement cost and TPO roof installation cost show how the insulation line sits within a complete assembly.
Vapor control and condensation on insulated flat roofs
Condensation, not rain, destroys most poorly insulated flat roofs. When warm interior air reaches a surface below its dew point inside the assembly, water forms and the deck rots from above the ceiling where no one sees it. A warm roof solves this by keeping the deck warm. A vented cold roof needs the 1 inch air space and the 1/150 vent ratio from 2021 IRC R806.2 and R806.3. An unvented residential roof must not carry an interior Class I vapor retarder on the ceiling side, per R806.5, because the assembly has to dry inward.
On commercial warm roofs in cold and mixed climates, designers often add a vapor retarder between the deck and the insulation to stop interior moisture from reaching the cold outer boards. Air sealing at penetrations, curbs and the roof-to-wall junction matters as much as the vapor layer, because most moisture rides in on air leaks, not diffusion. For hybrid residential roofs, the board above the deck has to meet the R806.5 minimum for your zone, for example R-15 in zones 4A and 4B, before any batts go below. Our insulation R-value chart converts material and thickness to R-value across the same climate zones.
Do you run EIFS wall foam up beyond the roof deck on a warm roof?
Yes, in most cases. On a low slope warm roof with exterior wall foam such as EIFS, the wall insulation and the above-deck roof boards should form one unbroken layer. Where there is a parapet, the thermal bridging proposal drafted for the 2024 IECC has the wall’s continuous insulation run up both sides of the parapet to the top. A parapet left bare on the inside face bypasses the roof insulation.
Parapet framing is exposed to outdoor air on both faces, so insulation tucked inside the framing does little there. A GAF and Siplast continuing education course on parapets makes the same point and ties insulation continuity across the parapet to both energy performance and condensation on cold surfaces. The ICC thermal bridging proposal (draft section C402.6.5, Parapets) sets out four cases:
| Wall insulation | Roof insulation | What the draft parapet provision calls for |
|---|---|---|
| Continuous insulation outside the wall (EIFS, exterior foam) | Entirely above deck (warm roof) | Wall continuous insulation extends up both sides of the parapet to the top |
| Continuous insulation outside the wall | Below the deck | Continuous insulation extends up the exterior side of the parapet to at least the height of the top surface of the roof assembly |
| Cavity or integral insulation only | Entirely above deck | Wall insulation extends into the parapet up to the exterior face of the roof insulation, or at least 2 feet horizontally inward on the underside of the roof deck |
| Cavity or integral insulation only | Below the deck | Wall and roof insulation abut at the roof, ceiling, and wall intersection |
The proposal also allows an approved design whose wall U-factor already accounts for the parapet thermal bridge. It was drafted as commercial section C402.6.5 for the 2024 IECC cycle, so ask the building official which edition and amendments apply before you detail the parapet; under the 2021 IECC the roof itself still has to meet the R-20ci to R-35ci above-deck minimums. Where the roof has no parapet, the same logic applies at the roof edge: carry the wall foam up until it meets the edge of the above-deck boards so no strip of deck or framing is left outside the insulation. For board selection, attachment, and pricing on larger buildings, see our guide to commercial flat roof insulation types, R-value, and cost.
Frequently asked questions
What is the best insulation for a flat roof?
Polyisocyanurate (polyiso) is the most common pick for flat roofs because it carries a long-term R-value of about R-5.7 to R-5.9 per inch, the highest of the mainstream rigid boards. In cold climates, pairing polyiso with mineral wool or XPS offsets polyiso’s drop in R-value at low temperatures; Owens Corning lists XPS at R-5.6 per inch at 25°F. XPS is the usual board on inverted roofs where it sits above the membrane.
Should I use a warm roof or a cold roof?
Use a warm roof for almost all new low-slope construction, because insulation above the deck eliminates thermal bridging through the joists and keeps the deck above the dew point. Choose a cold roof only in retrofits where the deck cannot be raised; on a house, 2021 IRC R806.3 then requires a 1 inch air space above the insulation and R806.2 a net free vent area of 1/150 of the space.
How thick does flat roof insulation need to be?
Thickness depends on the R-value target and material. To reach a common code target of R-30 with polyiso at about R-5.7 per inch, you need roughly 5.3 inches, usually installed as two staggered layers. Under 2021 IECC Table C402.1.3, zones 7 and 8 require R-35ci (about 6.1 inches of polyiso in two layers), while zones 0 and 1 allow R-20ci (about 3.6 inches) for non-residential buildings. Confirm the edition your local code has adopted.
Is tapered insulation necessary?
Tapered insulation is necessary on any structurally flat deck that lacks positive drainage. The 2021 IBC requires a design slope of at least 1/4 inch per foot for modified bitumen and single-ply membranes and defines positive drainage as slope that drains the roof within 48 hours of precipitation. Roofs framed with that slope built in may skip it.
How much does flat roof insulation cost?
Rigid foam board insulation (EPS, XPS or polyiso) costs about $2.50 to $6.00 per square foot installed, per HomeGuide (November 2025), and labor runs about $40 to $80 or more per hour. The R-value target sets the thickness: from 3.6 inches of polyiso for R-20ci to 6.1 inches for R-35ci. Insulation is usually priced inside a full membrane replacement rather than on its own.
Can you insulate a flat roof from the inside?
Yes, two ways under the 2021 IRC. A vented cold roof keeps a 1 inch air space between the insulation and the deck with vents equal to 1/150 of the space (R806.2, R806.3). An unvented roof puts air-impermeable insulation, such as closed-cell spray foam, directly against the underside of the deck, or adds rigid board above the deck sized to Table R806.5 (for example R-15 in zones 4A and 4B) before batts go below.