The unvented conditioned attic code is IRC Section R806.5, “Unvented attic and unvented enclosed rafter assemblies.” It is the one section that lets you delete soffit and ridge ventilation entirely and move the insulation and air barrier up to the roof deck. The permission is not unconditional: R806.5 lists five conditions, and the fifth one carries Table R806.5, a climate-zone schedule of minimum air-impermeable insulation R-value that decides whether the assembly dries or rots.
The section text is identical in the 2021 and 2024 International Residential Code. Everything below is quoted from those editions.
What IRC R806.5 actually permits
R806.5 permits unvented attics and unvented enclosed roof framing assemblies “created by ceilings that are applied directly to the underside of the roof framing members and structural roof sheathing applied directly to the top of the roof framing members/rafters,” where all five listed conditions are met. Meeting it removes the assembly from the ventilation requirements of R806.1 and R806.2. It does not remove anything else.
The ICC commentary is explicit that the space is treated as conditioned without any requirement to duct supply air into it: the attic is “considered indirectly conditioned because of omission of the air barrier, insulation at the ceiling and leakage around the attic access opening.” The practical payoff is that ducts and air handlers in the attic sit inside the thermal envelope instead of in a 130 degree summer plenum.
Sections that still apply include R316 (foam plastic thermal barrier and ignition barrier), R302.10, R807 attic access, and the energy provisions in Chapter 11. Nothing in R806.5 modifies them.
The five conditions R806.5 requires
All five must be met, not the ones that are convenient. Conditions 1 through 4 are short. Condition 5 is where nearly every failed inspection lives.
- Inside the envelope. “The unvented attic space is completely within the building thermal envelope.”
- No Class I vapor retarder at the ceiling. “Interior Class I vapor retarders are not installed on the ceiling side (attic floor) of the unvented attic assembly or on the ceiling side of the unvented enclosed roof framing assembly.” This preserves drying to the interior.
- Wood roofing gets an air space. “Where wood shingles or shakes are used, a minimum 1/4-inch (6.4 mm) vented airspace separates the shingles or shakes and the roofing underlayment above the structural sheathing.”
- Class II vapor retarder in cold zones. “In Climate Zones 5, 6, 7 and 8, any air-impermeable insulation shall be a Class II vapor retarder, or shall have a Class II vapor retarder coating or covering in direct contact with the underside of the insulation.”
- Insulation placement and R-value. “Insulation shall comply with Item 5.3 and either Item 5.1 or 5.2.” Item 5.1 covers the conventional configurations, Item 5.2 covers the vapor diffusion port path in Climate Zones 1, 2 and 3, and Item 5.3 requires preformed insulation board to be “sealed at the perimeter of each individual sheet interior surface to form a continuous layer.”
Table R806.5: minimum air-impermeable insulation R-value by climate zone
Table R806.5 is titled “Insulation for Condensation Control.” Its single data column is headed “Minimum rigid board or air-impermeable insulation R-value.” These are the values that must sit either directly above the structural roof sheathing or in direct contact with its underside, depending on the configuration. They are reproduced verbatim from the 2021 and 2024 IRC.
| Climate zone | Minimum rigid board or air-impermeable insulation R-value |
|---|---|
| 2B and 3B tile roof only | 0 (none required) |
| 1, 2A, 2B, 3A, 3B, 3C | R-5 |
| 4C | R-10 |
| 4A, 4B | R-15 |
| 5 | R-20 |
| 6 | R-25 |
| 7 | R-30 |
| 8 | R-35 |
Footnote a: the value “contributes to but does not supersede the requirements in Section N1102.” Footnote b provides the performance alternative described below. The R-values have not changed across the 2015, 2021 and 2024 editions.
Why the required R-value rises in colder climate zones
The numbers climb from R-5 to R-35 because the air-impermeable layer has one job: hold the condensing surface above the dew point of the indoor air that reaches it. In a hybrid or air-permeable assembly, the condensing surface is the interior face of the roof deck. The impermeable layer above or against that surface is the only thing keeping it warm, and the colder the outside air, the larger a share of total assembly R-value that layer has to be.
The code states the target directly in Table R806.5 footnote b and again in Item 5.1.4. The alternative to the table is to install enough insulation above the sheathing “to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C),” assuming an interior air temperature of 68°F and an exterior temperature equal to “the monthly average outside air temperature of the three coldest months.”
That is a solvable one-line problem. The temperature at the underside of the sheathing is the outdoor temperature plus the indoor to outdoor difference multiplied by the fraction of total R-value sitting above that surface:
T(sheathing) = T(out) + (68 minus T(out)) × R(above) / R(total)
Worked illustration using the code’s own assumptions. If the three-coldest-month average outdoor temperature at a site is 25°F, the required fraction is (45 minus 25) divided by (68 minus 25), or roughly 47 percent of the assembly’s total R-value. At a site averaging 40°F over those three months, the required fraction drops to about 18 percent. Same physics, different answer, which is exactly why the table is zoned rather than flat.
Table R806.5 is the prescriptive shortcut. Item 5.1.4 and footnote b are the calculated path for assemblies that do not fit the table, and they are the better tool when a designer is pushing an unusual total R-value or an unusual site.
Air-impermeable vs air-permeable: the definitions that decide the assembly
Which branch of Item 5.1 applies depends on a test result, not on marketing language. The IRC defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m2 at 75 Pa pressure differential as tested in accordance with ASTM E283 or E2178.” Anything above that threshold is air-permeable for code purposes.
Closed-cell spray polyurethane foam clears the bar by a wide margin. Johns Manville’s published data sheet for JM Corbond III lists air permeance of 0.00055 L/s per square meter at 75 Pa per ASTM E2178 at 3.75 inches, roughly 36 times below the code threshold. Fiberglass batts, blown fiberglass and cellulose are air-permeable and cannot serve as the layer against the deck.
Vapor retarder class is a separate test with separate thresholds. The IRC defines class by desiccant method using Procedure A of ASTM E96: Class I is 0.1 perm or less, Class II is above 0.1 up to 1.0 perm, Class III is above 1.0 up to 10 perms. Condition 2 bans Class I at the ceiling plane. Condition 4 requires Class II performance from the air-impermeable insulation in Zones 5 through 8. Both facts about the same assembly, measured different ways.
The four compliant configurations under Item 5.1
Item 5.1 names four ways to arrange insulation, and the code requires that “Item 5.1.1, 5.1.2, 5.1.3 or 5.1.4 shall be met, depending on the air permeability of the insulation directly under the structural roof sheathing.” Each is a complete compliance path on its own.
| Item | Configuration | Where the R806.5 R-value applies |
|---|---|---|
| 5.1.1 | Air-impermeable insulation only, applied in direct contact with the underside of the structural roof sheathing | No Table R806.5 minimum is invoked by 5.1.1 itself; contact with the sheathing is the requirement |
| 5.1.2 | Air-permeable insulation below the sheathing, rigid board or sheet insulation directly above the sheathing | Rigid board above the deck must meet the Table R806.5 value for the zone |
| 5.1.3 | Hybrid: air-impermeable in direct contact with the underside of the sheathing, air-permeable installed directly under it | The air-impermeable layer must meet the Table R806.5 value for the zone |
| 5.1.4 | Rigid board or sheet insulation above the deck sized by calculation | Sized to keep the monthly average sheathing underside temperature above 45°F |
Note the geometry rule that runs through all of them: the air-impermeable material goes against the sheathing, and the air-permeable material goes below it, never the reverse and never with a gap between the two. A ventilation channel or an air space left between foam and batt defeats the entire condensation strategy.
The vapor diffusion port path for Climate Zones 1, 2 and 3
Item 5.2, added after the 2015 edition and present in the 2021 and 2024 IRC, opens a route to an unvented attic built with ordinary air-permeable insulation in hot and mixed climates. It applies only in Climate Zones 1, 2 and 3, and it carries seven requirements of its own.
- An approved vapor diffusion port installed “not more than 12 inches (305 mm) from the highest point of the roof,” measured vertically to the lower edge of the port.
- Port area “greater than or equal to 1:600 of the ceiling area,” with multiple ports summed.
- Vapor-permeable membrane rated “greater than or equal to 20 perms when tested in accordance with Procedure A of ASTM E96.”
- The port serves as an air barrier between attic and exterior.
- The port protects the attic against entrance of rain and snow.
- Framing and blocking must not block vapor flow, with “not less than a 2-inch (51 mm) space” between blocking and roof sheathing.
- Roof slope “greater than or equal to 3:12.”
Item 5.2 also carries a mechanical requirement that is easy to miss. Where air-permeable insulation sits directly below the roof sheathing, air must be supplied to the attic “at a flow rate greater than or equal to 50 CFM (23.6 L/s) per 1,000 square feet (93 m2) of ceiling,” from supply ductwork or a dedicated supply fan operating with the conditioning system. Two exceptions relieve it: a hybrid assembly that meets the Table R806.5 R-value, or an assembly where the only air-permeable insulation sits on the attic floor or ceiling.
The common failure: too little closed-cell foam against the deck
The failure that shows up in cold-climate forensic work is a hybrid assembly where the closed-cell foam layer was priced by the inch instead of sized by Table R806.5. Item 5.1.3 requires the air-impermeable layer to meet the table value. In Climate Zone 5 that is R-20, in Zone 6 it is R-25, and in Zone 7 it is R-30. Fall short and the condensing surface moves out of the foam and onto the cold face of the sheathing, where warm indoor air arriving through the batt below can wet it every winter night.
Put real product numbers on it. Johns Manville publishes an aged R-value of 7.0 per inch for JM Corbond III under ASTM C518, with R-14 at 2 inches and R-21 at 3 inches. Against Table R806.5:
| Climate zone | Required air-impermeable R-value | Approximate ccSPF thickness at R-7.0 per inch |
|---|---|---|
| 4A, 4B | R-15 | About 2.2 inches |
| 4C | R-10 | About 1.5 inches |
| 5 | R-20 | About 2.9 inches |
| 6 | R-25 | About 3.6 inches |
| 7 | R-30 | About 4.3 inches |
| 8 | R-35 | About 5.0 inches |
Thicknesses are calculated from Johns Manville’s published aged R-value for one specific product. Other closed-cell foams publish different values, commonly in the R-6.0 to R-7.0 per inch range, so the arithmetic has to be redone against the actual data sheet for the actual product. Do not accept a per-inch figure that is not on paper.
The practical read: a 2 inch flash of closed-cell foam under fiberglass, a very common bid in Zone 5, delivers about R-14 against a required R-20. It fails Item 5.1.3, and it fails in the direction that puts liquid water on the underside of the deck. The same pattern drives condensation on the underside of a roof deck in vented assemblies for a different reason, but the wet sheathing and the rusted fastener heads look the same from a ladder.
Vapor retarder rules people get wrong
Three separate vapor rules operate inside R806.5, and they point in different directions. Condition 2 prohibits a Class I vapor retarder at the ceiling plane in every climate zone, because most roof assemblies already have low-permeance layers on top and the assembly needs to dry inward. Condition 4 requires the air-impermeable insulation to be Class II in Zones 5 through 8. Nothing in R806.5 requires a vapor barrier at the roof deck beyond that.
Open-cell spray foam is where this bites. Half-pound open-cell foam is typically a Class III vapor retarder at usable thicknesses, so in Zones 5, 6, 7 and 8 it needs a Class II vapor retarder coating or covering “in direct contact with the underside of the insulation” to satisfy Condition 4. Closed-cell foam usually reaches Class II on its own, but thickness matters: Johns Manville lists JM Corbond III at 1.1 perm at 1 inch, which is Class III, and 0.61 perm at 1.5 inches, which is Class II. The same product changes vapor class as it gets thicker. Our breakdown of open cell versus closed cell spray foam attic insulation covers the cost and performance tradeoff between the two.
Item 5.3 adds a detail that inspectors do check on rigid-board jobs: preformed insulation board used as the air-impermeable layer “shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.” Unsealed board joints are not an air-impermeable layer, whatever the R-value says.
Unvented conditioned attic code vs the vented rules in R806.1 and R806.2
R806.5 is an alternative to ventilation, not a supplement to it. Comply with R806.5 and the net free ventilating area requirements of R806.2 no longer apply to that assembly. Fail any one of the five conditions and the assembly reverts to needing full ventilation, which is difficult to retrofit once the rafter bays are full of foam.
The vented path is governed by a different arithmetic entirely: a minimum of 1 square foot of net free ventilating area for every 150 square feet of vented space, reduced to 1 in 300 when the balance and vapor retarder conditions are met. We work that calculation line by line in our guide to the 1/300 attic ventilation rule and its 1/150 fallback, which is the direct counterpart to this page. For the underlying reasons ventilation exists at all, see our overview of how attic ventilation works.
Choosing between them is mostly a question of where the ducts are and what the roof geometry allows. A complex hip roof with valleys, dormers and mechanicals in the attic is a poor candidate for balanced intake and exhaust, and a strong candidate for R806.5. A simple gable roof with all ductwork inside the conditioned floor area rarely needs the expense. Our comparison of insulating at the rafter line versus the attic floor walks through that decision.
Frequently asked questions
Does an unvented conditioned attic need any ventilation under the IRC?
No. An assembly that meets all five conditions of IRC R806.5 is exempt from the attic ventilation requirements of Sections R806.1 and R806.2. No soffit vents, ridge vents or net free area calculation is required. The exemption is conditional: if any of the five conditions is not met, the ventilation requirements apply again in full.
How much closed-cell spray foam does R806.5 require in Climate Zone 5?
In a hybrid assembly under Item 5.1.3, Table R806.5 requires R-20 of air-impermeable insulation in direct contact with the underside of the roof sheathing in Climate Zone 5. Using Johns Manville’s published aged value of R-7.0 per inch for JM Corbond III, that is roughly 2.9 inches. Verify the per-inch value on the specific product’s data sheet.
Can fiberglass or cellulose be used in an unvented conditioned attic?
Yes, in two ways. Under Item 5.1.2 air-permeable insulation may sit below the sheathing if rigid board meeting the Table R806.5 R-value is installed above the deck. Under Item 5.1.3 it may sit directly beneath a compliant air-impermeable layer. In Climate Zones 1, 2 and 3, Item 5.2 also allows air-permeable insulation with a vapor diffusion port.
Is a vapor barrier required in an unvented conditioned attic?
R806.5 does the opposite at the ceiling: Condition 2 prohibits an interior Class I vapor retarder on the ceiling side of the assembly in every climate zone, so the attic can dry inward. In Climate Zones 5 through 8, Condition 4 does require the air-impermeable insulation to be a Class II vapor retarder or to carry a Class II coating in direct contact with its underside.
What is the 45°F alternative in Table R806.5 footnote b?
Footnote b and Item 5.1.4 allow a calculated path instead of the table. Install enough continuous insulation directly above the structural roof sheathing to keep the monthly average temperature of the sheathing underside above 45°F, assuming 68°F indoors and an outdoor temperature equal to the monthly average of the three coldest months. It suits assemblies the prescriptive table does not fit.
Does open-cell spray foam comply with R806.5 in a cold climate?
It can, but not on its own. Open-cell foam at typical densities is a Class III vapor retarder, so in Climate Zones 5, 6, 7 and 8 Condition 4 requires a Class II vapor retarder coating or covering in direct contact with the underside of the insulation. The assembly also has to satisfy whichever branch of Item 5.1 applies to its configuration.
Which IRC edition contains the vapor diffusion port option?
Item 5.2, the vapor diffusion port path for Climate Zones 1, 2 and 3, appears in the 2021 and 2024 International Residential Code and is not present in the 2015 edition. Because jurisdictions adopt editions on their own schedule and often amend them, confirm the adopted edition and any local amendments with the authority having jurisdiction before designing to it.
Sources
- 2021 and 2024 International Residential Code, Section R806.5 and Table R806.5, Unvented attic and unvented enclosed rafter assemblies.
- 2024 International Residential Code, Chapter 2 definitions: Air-Impermeable Insulation; Vapor Retarder Class.
- ICC IRC Code and Commentary, Section R806.5 commentary on condensing surfaces and indirectly conditioned attic space.
- Johns Manville, JM Corbond III closed-cell spray polyurethane foam product data sheet (ASTM C518, E96, E283, E2178 values).
- ASTM E96 Procedure A, ASTM E283, ASTM E2178, ASTM C518.
By The Roofing Brief Team. Last reviewed: August 2026. Code citations are provided for reference and do not replace review by the authority having jurisdiction. Adopted code editions and local amendments vary by state and municipality.