Rigid foam board is insulation in sheets. Three plastics dominate — expanded polystyrene (EPS), extruded polystyrene (XPS) and polyisocyanurate (polyiso) — and all three do something batts and blown insulation cannot: they go on as a continuous layer across the framing, so heat that would run straight through every stud, joist and rafter has to cross insulation too.
Two things decide whether board is the right call. It stops air only when every seam and edge is sealed, and it is foam plastic, so indoors the code wants a thermal barrier over it, normally 1/2-inch drywall. Get those right and board is the cheapest way to add real R-value to a foundation wall, a rim joist, a wall at re-siding or a roof at a reroof.
What is rigid board insulation?
Rigid board, foam board, insulation board, rigid panels: same product, different words for it. It comes in sheets, most commonly 4 feet by 8 feet, in thicknesses from 1/2 inch to about 4 1/2 inches, with square, scored or tongue-and-groove edges, and it cuts with a utility knife and a straight edge.
Boards come faced or unfaced, and the facing changes the product more than people expect: foil on polyiso brings permeance down to 0.05, making the board a Class I vapor retarder, while unfaced EPS at 5 perms lets a wall dry through it. One common mix-up is “rigid spray foam”: closed cell spray foam does cure rigid, but it is sprayed as a liquid and expands in place, while board is made in a plant and cut to fit on site. See spray foam insulation for the sprayed product.
EPS, XPS and polyiso: R-value per inch and how each behaves
Values are per inch at a 75°F mean temperature, the condition federal rules require for labeling. The higher the R-value, the greater the insulating power; ask your seller for the fact sheet on R-values.
EPS, expanded polystyrene
The white bead board. Building Science Corporation’s guide to insulating sheathing puts EPS between R-3.2 and R-4.4 per inch depending on density, with permeance from 5 perms down to 2. The EPS Industry Alliance lists the two grades used most in buildings, ASTM C578 Type II and Type IX, at R-4.0 and R-4.2 per inch. The Department of Energy calls EPS the least expensive foam board and notes that it absorbs some water and is not a vapor retarder. Graphite EPS, sold as GPS, has graphite blended into the beads; one maker’s chart lists 1 1/16-inch board at R-5.0 and 3-inch board at R-14, roughly R-4.7 per inch.
XPS, extruded polystyrene
The colored board, usually pink, blue or green. Owens Corning’s FOAMULAR NGX 150 data sheet gives R-5.0 per inch at 75°F, rising to R-5.4 at 40°F and R-5.6 at 25°F, with water absorption of 0.3 percent by volume or less, permeance of 1.5 perms or less at 1 inch, compressive strength of 15 pounds per square inch, and a warranty on 90 percent of the R-value for the life of the building. The Department of Energy calls XPS a good vapor retarder and the preferred material for soil contact, and says it needs a capillary break between the soil and the insulation.
Polyiso, polyisocyanurate
The highest label value of the three. Johns Manville’s foil-faced sheathing is R-6.0 at 1 inch, R-13 at 2 inches and R-26 at 4 inches; Rmax’s is R-6.0 at 1 inch and R-20.3 at 3 inches. Those foil facers make the board a Class I vapor retarder and an air barrier when the seams are treated. Polyiso is also the one board the Department of Energy says to keep out of contact with soil, and it absorbs water, so it lives above grade behind a drainage plane.
How R-value changes with age and with cold
The label value is a laboratory number at 75°F on a new sample. Three things move it.
- Age. The EPS Industry Alliance reports that EPS holds its R-value because its cells contain only air, while XPS loses about 14 percent over 50 years as its blowing agent escapes. Polyiso is sold on a Long-Term Thermal Resistance value that already builds aging in.
- Cold. EPS and XPS get better as they get colder, by about 10 percent at a 40°F mean temperature. Polyiso does the opposite. Building Science Corporation tested sixteen polyiso samples and found R-value drops once outdoor temperatures go below freezing, and reports that the National Roofing Contractors Association tells designers to assume R-5.6 per inch in warm climates and R-5.0 in cold ones. The Polyisocyanurate Insulation Manufacturers Association pushes back: its third-party testing averaged 5.78 to 5.95 per inch across 1 to 4-inch roof boards, and foil-faced wall polyiso is tested differently, typically R-6 per inch and better.
- Water. Both polystyrenes lose about 10 percent of their R-value to absorbed moisture below grade, by the EPS Industry Alliance’s figures.
On a wall or a roof deck here in January, the gap between polyiso and XPS is smaller than the labels suggest. The answers are to add thickness or to put the cold-tolerant board outside the polyiso.
Where rigid foam board belongs in a house here
Basement walls
Board and closed cell spray foam are the two materials that belong against concrete; fiberglass and cellulose hold water and do not. The Department of Energy’s renovation guide caps the board at what still lets the wall dry inward: up to 2 inches of XPS or unfaced polyiso, or up to 4 inches of the more vapor-permeable EPS, which it rates R-10 and R-15.
Code for a conditioned basement wall is R-10 continuous or R-13 cavity in New Jersey’s climate zone 4 counties, and R-15 continuous, R-19 cavity, or R-13 cavity plus R-5 continuous in New Jersey’s zone 5 counties, all of Connecticut under the 2021 IECC, and New York under its 2025 code. That last option solves the drying problem: 1 inch of XPS on the concrete plus R-13 batts in a stud wall in front of it complies without passing the 2-inch limit. Insulation runs from the top of the wall down 10 feet below grade or to the floor, whichever comes first (Section R402.2.9.1). The Department of Energy’s job aid sets the order: fix water intrusion, air seal penetrations, install the board in full contact from ceiling to floor with adhesive and fasteners, any facing toward the conditioned space, seams sealed, then 1/2-inch gypsum board held 1/2 inch above the slab. More in basement insulation.
Rim joists
The band of framing on top of the foundation leaks air at the sill and the subfloor, and it is the one spot where almost every homeowner feels the difference. Board is cut snug to each bay and sealed on all four edges with canned foam or caulk; the Department of Energy’s rim joist job aid says to measure each individual cavity, fit it with no gaps, and seal every edge. The Building America Solution Center recommends XPS or polyiso there, unfaced or with the film removed so the rim can still dry.
Fire rules give the rim joist an exception: foam there may stay exposed when it is no more than 3 1/4 inches thick, 0.5 to 2.0 pounds per cubic foot, with a flame spread index of 25 or less and a smoke-developed index of 450 or less (Section R303.5.11 of the 2024 IRC, R316.5.11 of the 2021 IRC). Check the data sheet first: Owens Corning lists its XPS at flame spread 10 and Johns Manville its foil-faced polyiso at 25 or less, but Rmax lists a core flame spread of 75 or less and tells installers not to leave its board exposed at all.
Crawl space walls
In a sealed crawl space the insulation goes on the walls, with the same limits: 2 inches of unfaced XPS or 4 inches of unfaced EPS, seams sealed. The Department of Energy’s job aid adds what matters down there — attach it durably, seal seams and edges, and lap the wall board under the ground vapor retarder. A crawl space entered only for repairs needs no full thermal barrier if the board is tested for the use or covered by an ignition barrier such as 1 1/2-inch mineral fiber, 1/4-inch wood structural panel or 3/8-inch gypsum board (R303.5.4 in the 2024 IRC, R316.5.4 in 2021), though storage space does not qualify. See crawl space insulation.
Continuous insulation under new siding
When the siding comes off, an existing wall gets its one good chance at exterior board. ENERGY STAR’s retrofit guidance for zones 4 through 8: blow the empty cavities full through the sheathing and add R-5 to R-10 of insulating sheathing under the new siding, or R-10 over an insulated 2x4 wall. New wood-framed walls in all three states need R-30 in the cavity, R-20 cavity plus R-5 continuous, R-13 plus R-10 continuous, or R-20 continuous alone, and continuous insulation counts on its own, with no credit for sheathing, siding or drywall (Section R402.1.4). In New York, replacing the wall covering across a full elevation triggers that continuous insulation requirement outright (Section R503.1.1.3 of the 2025 energy code).
The sequence, from the Building America Solution Center and Building Science Corporation’s sheathing guide: repair the sheathing, set the water-resistive barrier and flash the openings, run the board in one or two layers with seams offset in both directions, tape the outer layer where it is the drainage plane, then vertical furring, then cladding fastened to the tables in Section R703.15 or the siding maker’s instructions for use over foam. Windows and doors need extended jambs and new flashing, which is why this belongs with a siding job. Board choice matters outside: Building Science Corporation prefers a vapor-permeable board such as EPS or unfaced XPS in cold climates so the wall can dry outward, and a semi-impermeable board behind brick veneer and stucco, which drive moisture inward in the sun. The board also buys a concession inside — in zone 5, latex paint is accepted in place of a Class II vapor retarder where the wall carries R-5 of continuous insulation on a 2x4 or R-7.5 on a 2x6 (Table R702.7(3)). More in wall insulation.
Cathedral ceilings and roofs, at a reroof
Above the deck is where board does its best roof work, and the time for it is when the roofing is coming off anyway. In an unvented roof, fiber insulation is allowed between the rafters when enough rigid board sits directly on top of the sheathing: R-15 in New Jersey’s zone 4 counties, R-20 in zone 5 and Connecticut, R-25 in the zone 6 Hudson Valley counties (Table R806.5). For R-20 that is 4 inches of XPS or 3 inches of polyiso. Where all the insulation sits above the deck, the 2024 IECC tables in New Jersey and New York call for R-30 continuous.
Connecticut makes the trigger explicit: a roof with no insulation in the cavity, where the sheathing is exposed during reroofing, has to be insulated above or below the sheathing (Section R503.1.1 of the 2021 IECC), and New York’s 2025 code sets the same rule for roof replacements on assemblies insulated above the deck. The build-up is an air control membrane over the old sheathing, board in two or more layers with joints offset, then a nailbase for the roofing. Foam under a roof covering needs no interior thermal barrier where wood structural panel sheathing at least 15/32 inch thick separates it from the inside. More in roof insulation.
Knee walls, hatches, soffits, garages and slabs
Board earns its keep in small pieces too. The attic side of a knee wall wants a rigid air barrier, and board, plywood or drywall sealed at the plates all qualify; the joist bays under the wall get cut board sealed in with foam, and dropped soffits and open wall tops get covered the same way — air sealing rather than insulating. ENERGY STAR’s attic hatch method is rigid board glued to the back of the hatch, weatherstripping and a latch that compresses it. In a garage, board suits block walls, and the gypsum board the code requires between garage and house doubles as the thermal barrier (Section R302.6). See garage insulation and air sealing.
Slab-on-grade floors within 24 inches of grade have to be insulated: R-10 continuous to 3 feet deep under New Jersey’s 2024 energy subcode and to 4 feet in Connecticut and New York, plus full-slab insulation under a heated slab. That is 2 inches of XPS at the edge. Under a slab, compressive strength is the spec to check: XPS runs 15 to 60 pounds per square inch by grade, EPS 5 to 25.
Cut-and-cobble: board in stud and rafter bays
Cutting board into strips to fit between studs or rafters, then sealing each piece with canned foam, is called cut-and-cobble. It is honest work in some places and a gamble in others.
Where it works: rim joists, blocking under knee walls, covering soffits and open wall tops, insulating a hatch. Short runs, few seams, and the board is patching an air barrier rather than becoming one.
Where it gets risky: filling rafter bays in an unvented roof or cathedral ceiling. The code is explicit that preformed board used as the air-impermeable layer has to be sealed at the perimeter of each individual sheet to form a continuous layer (Section R806.5). Every bay is four more joints, and the sealant has to hold as framing moves for decades. When it does not, humid indoor air reaches cold sheathing behind foam that cannot dry. Building Science Corporation monitored a cut-and-cobble roof in central Massachusetts and reported sheathing moisture content peaking near 30 percent with air leaks to the roof clearly evident, though disassembly found no damage. That is a thin margin.
In walls the objection is simpler. The Department of Energy points out that framing cavities in older houses are seldom uniform, which is why batts fit them badly, and board cut for a bay that is 14 3/8 inches at the top and 15 1/8 at the bottom fits no better. Cut-and-cobble also does nothing about the framing itself, which is the reason to use board in the first place.
Our rule: board in bays where it patches or blocks, spray foam or above-deck board where an unvented roof needs a control layer that lasts.
Spray foam vs rigid foam board
Both are foam plastic, both live under the same fire rules, and they are good at different things, which is why we install both.
R-value per inch
Closed cell spray foam is R-6.9 to R-7.1 per inch, the most of anything we install. Foil-faced polyiso is close behind at R-6.0 at 1 inch, XPS is R-5.0, EPS R-3.2 to R-4.4, and open cell spray foam R-3.9 to R-4.2. Where depth is fixed, closed cell or polyiso wins on inches alone.
Air sealing
This is the real difference. Spray foam arrives as a liquid and bonds to whatever it touches, so it is an air barrier the moment it cures. Board is an air barrier only if it is taped, caulked or foamed at every seam, edge and penetration, and the Department of Energy is blunt about the consequence: even small gaps make the insulation less effective and give moisture a cold spot to condense on. On a flat, square foundation wall a careful crew seals board well; on stone, rubble and uneven block it is close to impossible, which is why Building America researchers found spray foam both the most successful and the most practical way to insulate the inside of masonry walls. The same goes for joist ends crowded with pipes and for old, out-of-square framing.
Moisture and vapor
Closed cell spray foam becomes a vapor retarder at about 1 1/2 inches; open cell stays vapor-open. Among boards, foil-faced polyiso is a Class I vapor retarder at 0.05 perms, XPS runs 1.1 to 1.5 perms at 1 inch and tightens as it thickens, and EPS at 2 to 5 perms lets an assembly dry through it. Which one is right depends on the direction the assembly has to dry.
Labor, cost and DIY
The Department of Energy’s relative cost ranking puts rigid board above batts and blown insulation and below spray foam. Board is priced on sheets and on labor: cutting, fitting, fastening, taping, sealing and detailing every opening. Spray foam is priced on board feet, area times inches, and carries a rig, a crew and time out of the house while it cures.
Board is also the one a homeowner can sometimes handle: cutting it into rim joist bays and sealing the edges is within reach for a careful DIYer, while exterior board under siding is not, because the flashing and fastening decide whether the wall stays dry. Do-it-yourself spray foam kits exist, and the EPA warns that the people using them are often unaware of the inhalation and skin hazards and may lack the protective equipment.
When each one wins
- Spray foam for rim joists, stone and block foundation walls, crawl space walls, shallow rafters and unvented roof decks, and anywhere the surface is irregular.
- Rigid board for flat poured foundation walls, continuous insulation at re-siding, above a roof deck at a reroof, slab edges, garage block walls, and knee wall and soffit air barriers.
- Both together more often than either alone: closed cell at the rim and board on the walls in the same basement; board above the deck with fiber below it in an unvented roof, which is how the code writes it; or foam against the sheathing with batts under it, the flash-and-batt approach, when the cavity cannot hold the whole R-value.
For the two spray foams, see open cell and closed cell spray foam; for pricing, spray foam insulation cost.
Fire code: board needs a thermal barrier indoors
Foam plastic must be separated from the interior by an approved thermal barrier: 1/2-inch gypsum wallboard, 23/32-inch wood structural panel, or a material that passes both NFPA 275 fire tests (Section R303.4 of the 2024 IRC in New Jersey and New York, R316.4 of the 2021 IRC in Connecticut). The makers say it too: Owens Corning states its XPS cannot be left exposed on basement walls, and Rmax tells installers in capitals not to leave its polyiso exposed. The exceptions change what a finished job looks like:
- Attics and crawl spaces entered only for repairs: an ignition barrier, or foam tested for that use (R303.5.3 and R303.5.4; R316.5.3 and R316.5.4).
- Rim joists: exposed within the thickness, density and flame spread limits above.
- Under a roof covering, or behind masonry: wood structural panel sheathing at least 15/32 inch thick, or 1 inch of masonry or concrete, counts as the separation.
- Specifically approved products: foam approved on a large-scale fire test of the actual assembly (R303.6, R316.6). Some foil-faced polyiso boards carry evaluation reports that let them stay exposed on basement and crawl space walls. Settle that with the building department first.
Two product limits: polystyrene degrades in sunlight, so board is not left uncovered for long, and XPS tops out at a 165°F service temperature, which keeps it away from flues.
What rigid foam board insulation costs
Most of our projects run between $3,500 and $12,500. On board work the number moves with the area, the thickness and layers, the board type (the Department of Energy calls EPS the least expensive foam board), how many seams and penetrations there are to seal, whether the foam needs drywall over it, and what happens around it: furring, flashing, window extensions, a nailbase. Rim joists alone are a small job; a re-sided house is not.
State and utility programs offer rebates; see New York and New Jersey insulation rebates and Connecticut insulation rebates. The federal 25C credit ended for anything placed in service after 31 December 2025. If a quote specifies a depth of insulation and never mentions air sealing, it is half a job.
Common questions about rigid foam board insulation
What is rigid board insulation?
Foam insulation in sheets, most often 4 feet by 8 feet and 1/2 inch to about 4 1/2 inches thick, in three plastics: expanded polystyrene (EPS), extruded polystyrene (XPS) and polyisocyanurate (polyiso). Its advantage over batts and blown insulation is that it goes on as a continuous layer across framing, foundations and roof decks, so heat cannot bypass it through the studs and joists.
What is the R-value of foam board per inch?
At the 75°F test temperature: EPS R-3.2 to R-4.4 per inch by density, graphite EPS about R-4.7, XPS R-5.0, and foil-faced polyiso R-6.0 at 1 inch. In the cold that order shifts. EPS and XPS gain about 10 percent at a 40°F mean temperature, while polyiso loses R-value below freezing, and roofing designers are told to assume R-5.0 per inch for it in cold climates.
Which is better, EPS, XPS or polyiso?
It depends on the location. XPS below grade and against soil, because it resists water and is the Department of Energy’s preferred board for soil contact. EPS where the assembly has to dry through the insulation and where budget matters. Foil-faced polyiso above grade where inches are tight or where you want the board to act as the vapor retarder and air barrier. Polyiso does not go in contact with soil.
Is spray foam better than rigid foam board?
For air sealing and for irregular surfaces, yes: it bonds and seals in one pass, and Building America researchers found it the most successful and practical way to insulate the inside of masonry. Board wins on flat surfaces where a continuous layer over the framing is the goal, outside the wall at re-siding and above a roof deck at a reroof, and the Department of Energy’s cost ranking puts board below spray foam. Many houses get both.
What is the best foam board for a crawl space?
XPS or EPS on the walls of a sealed crawl space, with the amount capped so the wall can still dry: the Department of Energy’s limits are up to 2 inches of unfaced XPS or 4 inches of unfaced EPS, all seams sealed. Lap the wall board under the ground liner. In a crawl space entered only for repairs, foam needs an ignition barrier or a product tested for the use.
What is the best rigid insulation for a rim joist?
XPS or polyiso, cut snug to each bay and sealed on all four edges with canned foam or caulk, unfaced or with the film removed so the rim can dry. Air seal the sill and any penetrations first. Foam can stay exposed there only within the code’s limits: no more than 3 1/4 inches thick, 0.5 to 2.0 pounds per cubic foot, flame spread 25 or less, smoke-developed 450 or less. Some polyiso boards list a core flame spread of 75 and cannot be left bare.
Can rigid foam board be left exposed?
Usually not. The residential code requires a thermal barrier over foam plastic inside a building, typically 1/2-inch drywall. The exceptions are rim joists within the limits above, attics and crawl spaces entered only for repairs with an ignition barrier, foam under a roof covering behind 15/32-inch sheathing, foam behind an inch of masonry or concrete, and products with a specific approval from a large-scale fire test. A few foil-faced polyiso boards are sold with that approval for exposed basement walls.
Do the seams on foam board have to be taped?
If you want the board to stop air, yes. Board only becomes an air barrier when every seam, edge and penetration is sealed with tape, caulk or canned foam, and the Department of Energy warns that even small gaps reduce performance and give moisture a cold surface to condense on. In an unvented roof the code goes further and requires each individual sheet to be sealed at its perimeter.
How thick does foam board need to be on basement walls?
The code asks for R-10 continuous in New Jersey’s zone 4 counties and R-15 continuous in its zone 5 counties, Connecticut and New York — 2 and 3 inches of XPS. Because the Department of Energy caps XPS at 2 inches for drying, the usual answer in the R-15 states is either 4 inches of EPS or 1 inch of XPS against the concrete plus R-13 batts in a stud wall in front of it, which the code allows as an alternative.
How much does rigid foam board insulation cost?
Most of our projects run between $3,500 and $12,500. Where board work lands depends on area, thickness and layers, the board type, the number of seams and penetrations to seal, and the work around it: furring, flashing, window extensions or drywall. State and utility rebates can offset part of it.
Rigid foam board across New Jersey, Connecticut, the Hudson Valley and Long Island
We walk the house before we quote board. The surface decides the material — flat poured concrete takes board well, rubble stone does not — and what is coming off the house decides the timing, because siding and roofing are the two moments when exterior insulation is affordable. Then you get a written scope: board type, thickness and R-value for each area, the seam detail, and how the foam gets covered. The energy audit is free.
Call 914-554-2289 or use the contact form, 8am–8pm, seven days. The Home Insulators is family-owned, based in New Rochelle, NY since 2015, with a 100-plus-person in-house crew that never subcontracts. We install rigid foam board and spray foam across all of New Jersey, all of Connecticut, the Hudson Valley including Westchester, and Long Island; see our service area.