By The Roofing Brief Team | Last reviewed: October 2026
Attic foam board insulation is rigid EPS, XPS or polyiso sheet used in the few attic spots where loose fill and batts fail: the access hatch, knee walls, the shallow eave over the wall top plate, and the underside or top of the roof deck in an unvented assembly. Label R-values run about R-3.85 per inch for EPS, R-5 for XPS and R-6 to R-6.5 for polyiso, a 2 inch 4×8 sheet listed between about $31 and $64 on October 12, 2026, and every board needs a code-compliant fire covering and sealed seams to work.
This page is the material and placement guide: which board, where it goes, what the 2021 International Residential Code (IRC) asks of it, and what it costs per square foot. Sprayed foam has its own guide to open cell versus closed cell spray foam attic insulation, and the full condensation rulebook lives in our breakdown of IRC R806.5 unvented conditioned attic requirements.
Key numbers at a glance
| Figure | Value | Source |
|---|---|---|
| Polyiso label R-value | R-6.0 at 1 in, R-13.1 at 2 in (75°F mean) | Rmax Thermasheath-3 data sheet, ASTM C518 |
| Polyiso cold-climate design value | R-5.0 per inch (R-5.6 in warm climates) | NRCA guidance reported in Building Science Corporation Info-502 |
| XPS R-value by temperature | R-5.0 at 75°F, R-5.4 at 40°F, R-5.6 at 25°F per inch | Owens Corning FOAMULAR NGX 150 data sheet, Oct 2025 |
| EPS R-value | R-7.7 for a 2 in sheet (about R-3.85 per inch); R-3.8 to R-4.4 per inch by density | Henry R-Tech listing; Building America Solution Center |
| Thermal barrier over foam | 1/2 in gypsum board, unless an R316.5 exception applies | 2021 IRC R316.4 |
| 2 in 4×8 sheet price | EPS $31.21, polyiso $42.53 to $43.48, XPS $59.97 to $63.99 | Retailer listings via Google Shopping, Oct 12, 2026 |
EPS vs XPS vs polyiso: which foam board works in an attic?
For attic work, foil-faced polyiso gives the most R-value per inch and per dollar at room temperature, XPS holds its R-value best in cold air and tolerates moisture, and EPS is the cheapest sheet but needs the most thickness. In a cold attic the gap between polyiso and XPS largely closes, so the right board depends on where it sits and how cold that spot gets.
| Property | EPS (expanded polystyrene) | XPS (extruded polystyrene) | Polyiso (polyisocyanurate) | Source |
|---|---|---|---|---|
| R per inch at 75°F | About R-3.85 (R-Tech 2 in = R-7.7); R-3.8 to R-4.4 by density | R-5.0 | R-6.0 at 1 in; about R-6.5 per inch at 2 in | Henry R-Tech listing; BASC; OC FOAMULAR NGX 150; Rmax Thermasheath-3 |
| Cold-weather behavior | R-value rises as it gets colder | Rises: R-5.4 at 40°F, R-5.6 at 25°F | Can fall below label; NRCA designs at R-5.0 per inch for cold climates | OC data sheet; BSC Info-502; PIMA |
| Typical vapor class | Class III (about 2.6 perm per inch) | 1.5 perm max at 1 in (Class III), lower when thicker | Foil-faced: typically Class I; Rmax lists under 0.3 perm | BASC; OC data sheet; Rmax |
| Facer | Bare, poly film or foil, by product | Usually bare | Usually aluminum foil both sides | Maker data sheets |
| Service temperature | Check the data sheet | 165°F maximum (FOAMULAR NGX 150) | -40°F to 250°F (Thermasheath-3) | OC; Rmax |
| 2 in 4×8 sheet price | $31.21 | $59.97 to $63.99 | $42.53 to $43.48 | Listings, Oct 12, 2026 |
The service temperature row matters more in an attic than on a wall. The underside of a roof deck can get very hot in summer, and Owens Corning caps FOAMULAR NGX 150 at 165°F, so check the data sheet before pressing XPS against the deck under dark shingles in a hot climate.
Why polyiso loses R-value in a cold attic
Polyiso’s label R-value is measured at a 75°F mean temperature, and some polyiso products lose R-value when the board’s mean temperature falls toward 40°F and below. The Polyisocyanurate Insulation Manufacturers Association (PIMA) says polyiso’s R-value may rise as temperature drops toward 40°F, then peak and fall, depending on the blowing agent blend, and tells designers to ask the maker for product-specific values.
Building Science Corporation’s Info-502 reports the National Roofing Contractors Association (NRCA) practice of designing polyiso at R-5.6 per inch in warm climates and R-5.0 per inch in cold climates. XPS moves the other way: Owens Corning publishes R-5.4 per inch at 40°F and R-5.6 at 25°F for FOAMULAR NGX 150. In a winter attic in Climate Zones 5 to 8, a 2 inch polyiso sheet designed at R-10 performs about like 2 inches of XPS, at roughly 70 percent of the XPS sheet price.
Foam board thickness and R-value by sheet
Foam board sold at home centers runs from 1/2 inch to 4 inches thick, and the R-value printed on the sheet scales with thickness. The table uses published label values where a product exists and simple per-inch math (marked “calc”) where it does not.
| Thickness | EPS | XPS | Polyiso (foil-faced) | Source |
|---|---|---|---|---|
| 1/2 in | R-1.88 | R-2.5 (calc) | R-3.2 | Insulfoam listing; Thermasheath-3 |
| 3/4 in | R-2.9 | R-3.75 (calc) | R-5.0 | Insulfoam listing; Thermasheath-3 |
| 1 in | R-3.85 (calc) | R-5 | R-6.0 | OC; Thermasheath-3 |
| 1-1/2 in | R-5.78 | R-7.5 | R-9.3 to R-9.6 | Henry listing; OC listing; JM AP Foil; Thermasheath-3 |
| 2 in | R-7.7 | R-10 | R-13 to R-13.1 | Henry listing; OC; JM AP Foil; Thermasheath-3 |
| 3 in | R-11.6 (calc) | R-15 (calc) | R-20.3 | Thermasheath-3 |
| 4 in | R-15.4 (calc) | R-20 (calc) | R-27.4 | Thermasheath-3 |
Where foam board belongs in an attic, and where it does not
Foam board earns its price in attic spots where depth is limited, where the insulation must also block air, or where the layer must stay rigid: hatches, knee walls, eaves and roof decks. On an open, vented attic floor it is usually the most expensive way to buy R-value, because blown-in fill reaches R-49 or R-60 for less and fills around framing without cutting.
| Attic location | Good fit for board? | Why | Code hook (2021 IRC) |
|---|---|---|---|
| Hatch, scuttle or pull-down stair cover | Yes | Rigid, high R per inch, glues to the panel | N1102.2.4 access insulation and weatherstripping |
| Attic side of knee walls | Yes | Air barrier plus continuous insulation over studs | Table N1102.1.3 wall R-values; R702.7 vapor rules |
| Over wall top plates at the eave | Yes, in short lengths | Adds R-value where roof pitch leaves little depth | N1102.2.1 eave credit; N1102.2.3 baffles |
| Above the roof deck at a reroof | Yes | Keeps the deck warm for condensation control | R806.5 Items 5.1.2 and 5.1.4 |
| Under the deck, cut between rafters (unvented) | Only with meticulous sealing | Code allows it; field failures come from leaky joints | R806.5 Items 5.1.1, 5.1.3 and 5.3 |
| Dropped soffits and chases | Yes, as air-seal covers | Caps large holes before fill goes on top | N1102.4 air leakage |
| Flat attic floor between joists | Rarely | Costs more per R than blown-in and leaves gaps | Table N1102.1.3 ceiling R-30 to R-60 |
Attic hatch and pull-down stair covers
The attic hatch is the most common use of foam board in an attic, because a bare plywood or drywall panel sits in an R-49 or R-60 ceiling with almost no insulation of its own. The 2021 IRC (N1102.2.4) requires hatches to be insulated to the surrounding ceiling’s R-value and weatherstripped, with a reduced standard for pull-down stairs in Climate Zones 0 to 4 (an R-10 average, R-13 over at least 75 percent of the panel, a framed opening of 13.5 square feet or less, and weatherstripped edges).
ENERGY STAR’s do-it-yourself guide recommends weatherstripping the hatch and gluing a piece of rigid foam board to its back. The layer-by-layer stack math and the cover-box build are in our guide to insulating and sealing an attic hatch.
Knee walls
Foam board on the attic side of a knee wall does two jobs: it is the air barrier that stops attic air from washing through the batts, and it adds continuous insulation over the studs. Under the 2021 IRC wall table, Climate Zones 4 to 8 accept R-13 in the cavity plus R-10 continuous, which a 2×4 knee wall with a 2 inch XPS or 2 inch polyiso layer meets.
Tape every seam and seal the board to the floor and the sloped ceiling. If the board is foil-faced polyiso, do not add polyethylene on the room side; Table R702.7(2) footnote b requires an approved design when a wall has Class I layers on both sides. Our guide to knee wall and gable wall insulation covers blocking the joist bays under the wall.
Rafters and the underside of the roof deck in unvented assemblies
Foam board can serve as the air-impermeable layer in an unvented attic under IRC R806.5, either alone against the deck (Item 5.1.1) or as the top layer of a hybrid with batts below it (Item 5.1.3). Item 5.3 adds the condition that decides success: preformed board “shall be sealed at the perimeter of each individual sheet interior surface to form a continuous layer.”
In a hybrid, the board must meet the Table R806.5 R-value for the climate zone. The table below converts those minimums into board thickness, using polyiso at its NRCA cold-climate design value rather than its label, because condensation control is a cold-weather problem.
| Climate zone | Table R806.5 minimum | Polyiso at R-5.0/in | XPS at R-5.0/in | EPS at R-3.85/in |
|---|---|---|---|---|
| 1, 2A, 2B, 3A, 3B, 3C | R-5 | 1 in | 1 in | 1.3 in |
| 4C | R-10 | 2 in | 2 in | 2.6 in |
| 4A, 4B | R-15 | 3 in | 3 in | 3.9 in |
| 5 | R-20 | 4 in | 4 in | 5.2 in |
| 6 | R-25 | 5 in | 5 in | 6.5 in |
| 7 | R-30 | 6 in | 6 in | 7.8 in |
| 8 | R-35 | 7 in | 7 in | 9.1 in |
Source: 2021 IRC Table R806.5; per-inch values from BSC Info-502 (NRCA), Owens Corning and the Henry R-Tech listing. Thickness is Roofing Brief arithmetic (required R divided by R per inch). Table R806.5 also lists 0 for tile roofs in 2B and 3B.
Code permission is not the same as field reliability. Martin Holladay of Green Building Advisor describes cut-and-cobble foam in unvented roofs as risky, citing mold and rot reports, because framing swells and shrinks with humidity and can break the perimeter seals over time. Closed-cell spray foam against the deck, or rigid board above the deck at a reroof, avoids those cut joints.
Two other code paths use board without cutting it between rafters. Item 5.1.2 puts rigid board above the deck with air-permeable insulation below, and Item 5.1.4 sizes above-deck board to keep the deck’s underside above a 45°F monthly average. Both are reroof jobs, so plan them when the shingles come off. For vented rafter bays, R806.3 requires at least a 1 inch space between insulation and sheathing; see our guide to vented versus conditioned attic rafter insulation.
Over the wall top plates at the eaves
The eave is where attic insulation is thinnest, because the roof pitch squeezes the space over the exterior wall top plate. A strip of high R-per-inch board laid on the top plate, under the vent baffle, puts more R-value where blown-in fill cannot reach full depth. The 2021 IRC (N1102.2.1) lets R-38 satisfy an R-49 requirement only where the full height of uncompressed R-38 extends over the top plate at the eaves.
Keep the ventilation channel clear. N1102.2.3 requires a baffle beside soffit vents for air-permeable insulation in vented attics, and our guide to attic insulation baffles covers the sizing.
The attic floor: usually the wrong place for board
Laying foam board across a vented attic floor is legal but rarely sensible. Reaching R-49 takes about 8 inches of polyiso, or four 2 inch layers, which is about $5.44 per square foot in board alone at the October 12, 2026 Lowe’s price for Johns Manville AP Foil. HomeGuide puts installed batt and roll insulation at $0.80 to $2.60 per square foot.
Board also leaves gaps at every joist and wire. The better use of board on the attic floor is as a sealed cover over dropped soffits and open chases before loose fill goes on top, which is how ENERGY STAR’s sealing guide uses it.
Fire code: thermal barriers and ignition barriers over foam board
All three foam boards are combustible plastics, so the 2021 IRC requires them to be covered. Section R316.4 requires foam plastic to be separated from the building interior by an approved thermal barrier of at least 1/2 inch gypsum wallboard, 23/32 inch wood structural panel, or a material that passes the NFPA 275 tests, unless an R316.5 exception applies.
The attic exception, R316.5.3, drops the thermal barrier when all three of these are true: attic access is required by R807.1, the space is entered only for repairs or maintenance, and the foam is either tested under R316.6 or covered by an ignition barrier. The listed ignition barriers are:
- 1-1/2 inch thick mineral fiber insulation
- 1/4 inch thick wood structural panels
- 3/8 inch particleboard
- 1/4 inch hardboard
- 3/8 inch gypsum board
- Corrosion-resistant steel with a base metal thickness of 0.016 inch
- 1-1/2 inch thick cellulose insulation
- 1/4 inch fiber-cement panel, soffit or backer board
Some foil-faced polyiso products are tested to skip the ignition barrier. An Rmax application sheet states that Thermasheath-3 “has been tested to be left exposed in attics and crawl spaces without the code prescribed ignition barrier, provided the space is limited to servicing utilities,” up to 1 inch thick in walls and ceilings. Rely on that only with the current evaluation report (ICC-ES ESR-1864) in hand for the inspector.
An attic used for storage or living space may not meet the “repairs or maintenance” condition, and then the full thermal barrier applies. A finished bonus room with foam under the rafters needs drywall over it. Section numbers can shift between the 2021 and 2024 IRC, so confirm the edition your jurisdiction adopted.
Vapor retarder classes and foam board in the attic
The IRC sorts vapor retarders by perm rating measured with the ASTM E96 desiccant method: Class I is 0.1 perm or less, Class II is above 0.1 up to 1.0 perm, and Class III is above 1.0 up to 10 perms. Foil-faced polyiso usually lands in Class I, XPS crosses from Class III to Class II as it gets thicker, and EPS usually stays Class III.
Three attic rules turn on these classes:
- Unvented attic ceilings: R806.5 Condition 2 bans an interior Class I vapor retarder on the ceiling side (attic floor) of an unvented attic, so do not lay foil-faced board on the attic floor of a conditioned attic.
- Cold-zone unvented roofs: R806.5 Condition 4 requires air-impermeable insulation in Climate Zones 5 to 8 to be a Class II vapor retarder or to carry a Class II coating in direct contact with it. Foil-faced polyiso, rated well under 1 perm, generally satisfies it; 1 inch of XPS at 1.5 perm does not, and EPS usually needs a coating or covering.
- Knee walls and other frame walls: Table R702.7(2) does not permit Class I or Class II interior vapor retarders in Climate Zones 1 and 2, or Class I in Zones 3 and 4 (except Marine 4).
Owens Corning notes that XPS permeance “decreases as thickness increases.” As a Roofing Brief estimate, 2 inches of a board rated 1.5 perm at 1 inch lands near 0.75 perm, inside Class II. Confirm the actual figure on the data sheet before relying on it for Condition 4.
How to install foam board in an attic
A foam board job in an attic follows the same order every time: confirm the code path, air seal, cut and fit, seal every joint, then cover it. The steps below apply to knee walls, eaves and rafter bays; hatch builds have their own sequence.
- Pick the code path first. Decide whether the attic stays vented or becomes unvented under R806.5, find your climate zone, and note the Table R806.5 and Table N1102.1.3 R-values. Ask the building department whether the space counts as “entered only for repairs or maintenance” under R316.5.3.
- Inspect the framing and deck. Stained or soft sheathing gets repaired before it is covered. Foam hides leaks.
- Air seal penetrations. Caulk or foam around wires, pipes, top plates and light boxes before the board goes on, because the board will cover them.
- Measure each bay and cut the board 1/4 inch undersize. Score with a utility knife and snap, or use a fine-tooth saw for boards over 2 inches. Undersizing leaves room for a continuous foam bead.
- Fit and fasten. Press board against the deck (unvented) or against a baffle (vented), and hold it with construction adhesive rated for foam, cap nails or screws with washers.
- Seal every perimeter and joint. Fill the gap at each edge with low-expansion foam sealant and tape face seams with the board maker’s tape. This is the Item 5.3 requirement for unvented work.
- Stagger a second layer. Where more R-value is needed, offset the second layer’s seams from the first so no joint runs straight through.
- Cover it. Install 1/2 inch drywall as the thermal barrier, or a listed ignition barrier where R316.5.3 applies, unless the product’s evaluation report allows exposure.
Air sealing foam board joints so they stay sealed
A foam board layer is only as airtight as its edges, and edges are where attic installs fail. Air that leaks around a board carries indoor moisture to the cold side of the assembly, which is why R806.5 Item 5.3 makes sealing the perimeter of each sheet a code requirement rather than a suggestion.
- Edges against framing: low-expansion canned foam in a 1/4 inch gap, not caulk alone, because framing moves with humidity.
- Face seams: foil tape on foil-faced polyiso, and the maker’s seam tape on XPS or EPS.
- Two layers: stagger seams and seal both layers, so a gap in one layer is covered by solid board in the other.
- Check it: on a cold day, look for frost or daylight at the joints from the attic side. A whole-house blower door test also measures the result.
The broader sealing sequence for the attic floor is in our guide to air sealing an attic.
What attic foam board costs per sheet and per square foot
A 2 inch, 4×8 foam board covers 32 square feet and listed at $31.21 for EPS, $42.53 to $43.48 for foil-faced polyiso and $59.97 to $63.99 for XPS on October 12, 2026. Per square foot that is about $0.98, $1.33 to $1.36 and $1.87 to $2.00, before adhesive, foam sealant, tape and the drywall cover.
| Board (2 in x 4 ft x 8 ft) | Label R | Price per sheet | Per sq ft | Per R per sq ft | Listing |
|---|---|---|---|---|---|
| Henry R-Tech EPS | R-7.7 | $31.21 | $0.98 | $0.127 | Supply Ontime |
| Johns Manville AP Foil polyiso | R-13 | $43.48 | $1.36 | $0.105 (label), $0.136 at R-5.0/in | Lowe’s |
| Rmax Pro Select R-Matte Plus-3 polyiso | R-13.1 | $42.53 | $1.33 | $0.101 (label) | Supply Ontime |
| Owens Corning FOAMULAR NGX F-250 XPS | R-10 | $59.97 | $1.87 | $0.187 | Home Depot |
| Owens Corning FOAMULAR NGX F250 XPS | R-10 | $63.99 | $2.00 | $0.200 | White Cap |
Prices are single-sheet listings seen through Google Shopping on October 12, 2026; they vary by store, region and quantity. Per-square-foot figures are Roofing Brief arithmetic: sheet price divided by 32.
Worked example, Climate Zone 5 hybrid roof. Table R806.5 calls for R-20 of air-impermeable insulation. Two layers of 2 inch polyiso (R-26.2 label, R-20 at the cold design value) cost about $2.72 per square foot in board. Two layers of 2 inch XPS (R-20) cost about $3.75. A 1,000 square foot roof deck takes 31.25 sheets per layer, so plan about 35 sheets per layer with a 10 percent cutting allowance.
Foam board vs spray foam vs batts in an attic
Foam board is the cheapest way to buy a sealed, high R-per-inch layer if you supply the labor, closed-cell spray foam is the most reliable air seal against a roof deck, and fiberglass batts are the cheapest R-value but do not stop air. Each wins in a different attic location.
| Factor | Foam board (polyiso) | Closed-cell spray foam | Fiberglass batts |
|---|---|---|---|
| R per inch | About R-6 label; R-5.0 cold design | About R-6 (BASC, high-density) | R-2.5 to R-4 (DOE) |
| Air barrier | Yes, only with every joint sealed | Yes, as sprayed | No |
| Vapor class | Foil-faced: typically Class I | Typically Class II (BASC) | Depends on facing |
| Cost basis | $1.33 to $1.36 per sq ft per 2 in layer, material only (Oct 2026 listings) | $1.15 to $2.00 per board foot installed (ProSeal Spray Foam, Apr 2026) | $0.80 to $2.60 per sq ft installed (HomeGuide) |
| DIY friendly | Yes | No for full jobs | Yes |
| Fire covering | Thermal or ignition barrier (R316) | Thermal or ignition barrier (R316) | None for unfaced batts |
| Best attic use | Hatches, knee walls, eaves, above-deck reroofs | Unvented roof decks, irregular framing | Vented attic floors and vented rafter bays |
At the ProSeal range, R-20 of closed-cell foam at the BASC figure of about R-6 per inch is about 3.3 board feet per square foot, or roughly $3.80 to $6.70 per square foot installed. The board version of the same R-20 is cheaper in material, but the sealing labor is where board jobs succeed or fail.
Which one should you pick?
Match the board to the job rather than buying one product for the whole attic. The table maps common attic situations to the usual choice; confirm the code path with the local building department before buying.
| Your situation | Usual pick | Why |
|---|---|---|
| Cold drafty attic hatch or pull-down stairs | Foil-faced polyiso stack or cover box, weatherstripped | Most R in the least thickness |
| Knee walls in Climate Zones 4 to 8 | 2 in XPS or polyiso over R-13 batts, seams taped | Meets the R-13 plus R-10 continuous wall option |
| Knee walls in a cold attic where winter R-value matters most | XPS | R-value rises in the cold; polyiso can fall |
| Unvented roof deck, roof not being replaced | Closed-cell spray foam, or board with meticulous Item 5.3 sealing | Leaky board joints are the known failure |
| Roof being replaced anyway, cold climate | Polyiso above the deck sized to Table R806.5 | Warm deck, no cutting between rafters |
| Shallow eaves over the top plate | Short strips of polyiso under the baffle | Adds R where fill cannot reach full depth |
| Open, vented attic floor | Air seal, then blown-in or batts; board only for soffit covers | Board costs more per R and leaves gaps |
| Hot climate, board against the deck | Foil-faced polyiso; check any XPS service temperature first | Thermasheath-3 is rated to 250°F; FOAMULAR NGX 150 to 165°F |
Frequently asked questions
Is it a good idea to use foam board insulation in my attic?
Yes, in the right spots. Foam board works well on attic hatches, the attic side of knee walls, shallow eaves over the top plate, and above the roof deck at a reroof. On an open, vented attic floor it usually costs more per R-value than blown-in fill and leaves gaps at every joist, so air seal and blow insulation there instead.
What are the disadvantages of foam board insulation in an attic?
Foam board is combustible, so the 2021 IRC requires a thermal barrier such as 1/2 inch drywall or, in maintenance-only attics, an ignition barrier. Every joint must be sealed or air leaks around it. Polyiso can lose R-value in cold air, XPS has a 165 degree F limit on FOAMULAR NGX 150, and board costs more per R than blown-in fill on attic floors.
Do you need an air gap with foam board insulation?
It depends on the assembly. In a vented rafter bay, IRC R806.3 requires at least a 1 inch space between insulation and the roof sheathing for airflow. In an unvented assembly, the air-impermeable board goes in direct contact with the underside of the deck with no gap. A 3/4 inch air space against a foil facer adds reflective R-value in walls.
Can foam board be left exposed in an attic?
Usually not. Under 2021 IRC R316.5.3, foam in an attic entered only for repairs or maintenance can skip the thermal barrier but still needs an ignition barrier, such as 3/8 inch gypsum or 1-1/2 inch mineral fiber, unless the product passed R316.6 testing. Rmax says Thermasheath-3 up to 1 inch was tested for exposed use in service-only attics.
Which is better for an attic, XPS or polyiso?
Polyiso gives more R-value per inch and per dollar at room temperature, about R-6 to R-6.5 per inch versus R-5 for XPS. In a cold attic, XPS gains R-value, reaching R-5.6 per inch at 25 degrees F per Owens Corning, while NRCA designs polyiso at R-5.0 in cold climates. Pick polyiso for hatches and warm spots, XPS for cold spaces.
What is the cheapest way to insulate my attic?
For a vented attic, air sealing the attic floor and then adding blown-in fiberglass or cellulose is usually the lowest cost per R-value, because loose fill flows around framing without cutting. HomeGuide puts installed batts at $0.80 to $2.60 per square foot. Foam board is cheaper only for small jobs where its rigidity matters, such as hatches and eaves.
Can I put rigid foam against the underside of the roof deck?
Yes, if the assembly follows IRC R806.5. The board must be in direct contact with the deck, every sheet must be sealed at its perimeter under Item 5.3, and in a hybrid it must meet the Table R806.5 R-value, such as R-20 in Climate Zone 5. Green Building Advisor warns that cut-and-cobble joints in unvented roofs can fail over time.
Sources
- International Code Council, 2021 International Residential Code: R202 definitions (air-impermeable insulation, vapor retarder class), R316.3 to R316.6, R702.7 and Tables R702.7(1) to (3), R806.3, R806.5 and Table R806.5, R807.1, N1102.1.3, N1102.2.1 to N1102.2.4.
- Building Science Corporation, Info-502: Temperature Dependence of R-values in Polyisocyanurate Roof Insulation (April 2013), reporting NRCA design values.
- Polyisocyanurate Insulation Manufacturers Association (PIMA), Temperature: How It Impacts R-Value.
- Owens Corning, FOAMULAR NGX 150 XPS product data sheet (October 2025).
- Rmax, Thermasheath-3 data sheet and attic and crawl space application sheet; ICC-ES ESR-1864.
- Building America Solution Center (PNNL), Typical R-values and vapor retarder classifications of common insulation materials.
- U.S. Department of Energy, Energy Saver: insulation materials.
- ENERGY STAR, A Do-It-Yourself Guide to Sealing and Insulating with ENERGY STAR.
- Martin Holladay, Cut-and-Cobble Insulation, Green Building Advisor (November 2013).
- Retail listings for Johns Manville AP Foil, Rmax Pro Select R-Matte Plus-3, Owens Corning FOAMULAR NGX and Henry R-Tech EPS (Lowe’s, Home Depot, White Cap, Supply Ontime), checked October 12, 2026; HomeGuide batt and roll insulation cost (November 2025); ProSeal Spray Foam closed-cell cost (April 2026).
Code citations are for reference and do not replace review by the authority having jurisdiction. Adopted code editions and local amendments vary. Prices are dated listings and change often.