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ARMORTHANE PROTECTING YOUR WORLD

PROTECTING YOUR WORLD

ARMORTHANE PROTECTING YOUR WORLD

Roof Insulation:
R-Values, Venting
& Real Costs

Attic floor, under the deck or on top of it: where roof insulation belongs, what the 2021 code requires, how much foam keeps an unvented deck dry, and what it costs.
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  • Roof type and pitch
  • Area and rafter depth
  • Vented or unvented today
  • Any moisture or ice dam history
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Step 1 of 3
R-49 / R-60
Ceiling insulation required in climate zones 2–3 / 4–8 under the 2021 IECC
IECC 2021 Table R402.1.3 via PNNL
R-20
Minimum foam at the deck of an unvented roof in climate zone 5, for condensation control
IRC Table R806.5
1 in.
Minimum air space between insulation and roof sheathing in a vented roof
IRC R806.3
$2,500
Average cost to insulate a roof; most pay $1,700 to $3,300
Angi, 2026 data

Roof insulation means putting the thermal boundary at the roof itself, under the deck, between the rafters or on top of the sheathing, instead of on the attic floor. It is how you insulate a vaulted ceiling, a bonus room, a low-slope commercial roof or a metal building, and it is where moisture problems usually start when the details are wrong. We manufacture spray foam and roofing foam, so we spend a lot of time on roofs. This guide covers where the insulation should go, what the 2021 code asks for, how much foam an unvented roof needs to stay dry, what it costs, and two tools to size and price your roof.

Quick answer

If you have an open attic, insulate the attic floor; it is cheaper and the code target is R-49 to R-60 in most of the country. Insulate at the roof when there is no attic (vaulted ceilings), when ducts or equipment sit in the attic, or on low-slope and metal roofs. Vented roofs need a 1 inch air channel above the insulation. Unvented roofs need air-impermeable insulation, such as closed-cell spray foam or rigid board, against the deck: at least R-5 in climate zones 1 to 3 and up to R-35 in zone 8, with the rest of the code R-value made up below it.

What's in this guide
  1. 1Roof insulation vs attic insulation
  2. 2Roof insulation R-values by climate zone
  3. 3Roof insulation thickness planner
  4. 4Roof insulation types compared
  5. 5Roof insulation cost and savings calculator
  6. 6Unvented roofs: keeping the deck dry
  7. 7Low-slope and metal roofs
  8. 8FAQ
First decision

Roof insulation vs attic insulation

Every roof gets insulated in one of three places. Pick the place first, then the material.

Usually cheapest

On the attic floor

Blown or batt insulation over the ceiling joists, with a ventilated attic above. The Department of Energy's advice for unfinished attics is to "insulate between and over the floor joists," and to seal air leaks first. The roof stays cold and dry.

No attic, or equipment up there

Under the roof deck

Insulation follows the rafters, vented or unvented. This is the only option for cathedral ceilings and the right one when ducts or a furnace live in the attic. DOE notes an unvented "hot roof" lets you fit more insulation because the vent space goes away.

Reroofing or low slope

On top of the deck

Rigid board or sprayed roofing foam over the sheathing, covered by a membrane, coating or new roofing. Standard on commercial low-slope roofs and the cleanest fix for a vaulted ceiling when the roof is coming off anyway.

Do not mix themA mistake we see often is a half-and-half roof: insulation on the attic floor and foam on the deck, or batts against the deck with the soffit vents still open. Choose a vented or an unvented assembly and build it completely.
Code

Roof insulation R-values by climate zone

Two code tables matter. The energy code (IECC Table R402.1.3) sets the total R-value for the ceiling or roof. The building code (IRC Table R806.5) sets how much of that, at minimum, has to be air-impermeable insulation against the underside of the deck if the roof is unvented, so the deck stays warm enough that indoor moisture does not condense on it. The inch figures assume closed-cell spray foam at R-6 to R-7 per inch.

Climate zoneTotal ceiling R, 2021 IECCTotal ceiling R, 2018 IECCMin. foam at deck, unvented (R806.5)Closed-cell needed for that minimum
1R-30R-30R-5About 1 in.
2R-49R-38R-5About 1 in.
3R-49R-38R-5About 1 in.
4A, 4BR-60R-49R-152.1 to 2.5 in.
4C (marine)R-60R-49R-101.4 to 1.7 in.
5R-60R-49R-202.9 to 3.3 in.
6R-60R-49R-253.6 to 4.2 in.
7R-60R-49R-304.3 to 5 in.
8R-60R-49R-355 to 5.8 in.
Sources: 2018 and 2021 IECC ceiling values from the DOE/PNNL Building America Solution Center; R806.5 minimums from the 2021 IRC (zones 2B and 3B with tile roofs need none). Closed-cell R per inch from ArmorThane's spray foam R-value guide. Your state may amend either table; check the edition your county enforces.

Two allowances that help on real roofs

  • Raised-heel trusses. Where full-height, uncompressed insulation extends over the wall top plates, R-49 can satisfy an R-60 requirement (IECC R402.2.1).
  • Ceilings without attics. Where the framing simply cannot hold the full value, the code accepts R-30 for vaulted sections, limited to 500 square feet or 20% of the total insulated ceiling area, whichever is less (IECC R402.2.2). It does not cover a whole vaulted house.
Tool 1

Roof insulation thickness planner

Pick your climate zone, the way you plan to insulate, and the depth of your rafters. You get the code target, the minimum deck foam for an unvented roof, and a verdict on your framing.

Planner

Will my roof framing hold enough insulation?

Targets from 2021 IECC Table R402.1.3 and IRC Table R806.5. Sized at R-6.5 per inch for closed-cell foam, R-3.6 for open-cell, R-3.2 for fiberglass batts, R-5 for rigid board and R-2.3 to R-3.0 for blown attic insulation.

Code allowance
—
Code target
—
Foam needed at the deck
—
Best R this framing holds
—
Verdict

Materials

Roof insulation types compared

R-value per inch matters most on a roof, because rafter depth is fixed. Air sealing matters almost as much, because warm indoor air leaking into a roof assembly is what wets the deck. Click a heading to sort.

MaterialR per inchAir barrier on its own?Installed cost per sq ft (roof)Best roof use
Closed-cell spray foam6.0 to 7.0Yes, at 1 in.; vapor retarder at about 2 in.$3.50 to $7.00 (spray foam)Unvented roofs in any zone, flash-and-batt, metal roofs
Open-cell spray foam3.5 to 3.8Yes, when thick enough$3.50 to $7.00 (spray foam)Unvented roofs in zones 1 to 4; needs a vapor retarder in zones 5 to 8
Rigid board (polyiso, XPS, EPS)About 5 (3.6 to 5.0 by type)Only with taped seams$2.50 to $6.00Above the deck when reroofing; low-slope roofs
Fiberglass or mineral wool battsAbout 3.2No$1.50 to $3.50Vented rafters, or under foam in flash-and-batt
Blown fiberglass or celluloseAbout 2.3 to 3.0No$2.00 to $4.00Attic floors, not sloped roofs
Radiant barrierNot rated per inchNo$1.80 to $4.50Hot climates, with an air space, in addition to insulation
Sources: closed- and open-cell R per inch from ArmorThane's spray foam R-value guide; air and vapor thresholds from Building Science Corporation BSD-149; batt and foam rules of thumb from ENERGY STAR; rigid board range from HomeGuide; blown range implied by DOE's "R-30 (about 10 to 13 inches)"; costs from Angi's 2026 roof insulation data.

With 2x6 or 2x8 rafters, closed-cell foam is often the only material that gets close to code without furring the rafters down. With 2x12s, vented batts or flash-and-batt usually cost less.

Tool 2

Roof insulation cost and savings calculator

Enter the roof or ceiling area, what is there now and the R-value you want. The calculator prices the job with Angi's 2026 per-square-foot ranges and estimates the heating saving with the degree-day method. Energy prices default to recent EIA averages.

Cost and payback

What will roof insulation cost, and what will it save?

Heating saving = area x 24 x HDD x (1/R before minus 1/R after), divided by heating efficiency. An uninsulated roof is treated as about R-3 for drywall, decking and air films. Cooling savings are not included.

Roof or ceiling area
Base 65°F, from NOAA or degreedays.net
EIA, Feb. 2026
EIA, Mar. 2026
EIA, July 2026
—
Installed cost
—
Heating saving per year
—
Roof heat loss cut
—
Simple payback

Moisture

Unvented roofs: keeping the deck dry

In winter the underside of the roof deck is the coldest surface in the assembly. If humid indoor air reaches it, water condenses and the sheathing rots from the inside. A vented roof washes the deck with outdoor air; an unvented roof keeps the deck warm and airtight, which is the job of the air-impermeable layer in IRC Table R806.5.

  1. Air-impermeable insulation goes against the deck

    Spray foam applied directly to the underside of the sheathing, or rigid board above it. Building Science Corporation notes closed-cell foam works as an air barrier at about 1 inch and as a vapor retarder at about 2 inches.

  2. Enough of it for your zone

    The deck layer has to meet the R806.5 minimum on its own: R-5 in zones 1 to 3, R-15 in 4A and 4B, R-20 in zone 5, up to R-35 in zone 8. Batts below it make up the rest of the code R-value.

  3. A vapor retarder in cold zones

    In climate zones 5 through 8 the IRC requires the air-impermeable insulation to be a Class II vapor retarder or to be covered by one. Closed-cell foam at full depth qualifies; open-cell foam needs a vapor retarder coating or membrane.

  4. No vents, no gaps

    Seal soffit and ridge vents, and run the foam continuously over the top plates and into the gable ends. A vent left open behind an unvented assembly defeats it.

Shingle warrantiesSome shingle manufacturers have historically required a vented deck for full warranty coverage. Before foaming the underside of an existing shingle roof, read the warranty on your shingles or ask the maker in writing.

For vaulted rooms specifically, our cathedral ceiling insulation guide goes rafter by rafter.

Commercial and ag

Insulating low-slope and metal roofs

On a flat or low-slope roof the insulation almost always goes above the deck, because there is no attic and the deck is often steel or concrete. The two main systems are rigid board under a membrane and sprayed polyurethane foam (SPF) roofing, where the foam is both the insulation and the roof substrate and a coating protects it from UV.

Board and membrane

Rigid insulation above the deck

Polyiso or other board, fastened or adhered, under a single-ply or built-up membrane. Seams between boards are thermal bridges unless the layers are staggered. Commercial buildings follow the commercial energy code; use the DOE's COMcheck tool to confirm the R-value for your project.

Seamless

Spray foam roofing

Roofing-grade foam sprayed over the existing roof or deck, sloped to drain, then coated. Our ArmorFoam Roof 200 comes in 2.5, 2.7 and 3.0 pcf densities for low-slope, pitched and barrel roofs. No seams means no thermal bridges at board joints, and it can often go over an existing roof.

Metal roofs on shops, barns and barndominiums bring a second problem: condensation dripping from the underside of the panels. Insulation applied directly to the steel, such as closed-cell foam, stops warm air from reaching it. See spray foam roofing, the ArmorFoam Roof 200 product page and our metal building insulation guide. For a living space under a metal roof, see barndominium insulation.

Where radiant barriers fit

A radiant barrier is not insulation in the R-value sense; it blocks radiant heat from a hot roof. The Department of Energy says studies show cooling cost cuts of 5% to 10% in a warm, sunny climate, that the foil must face an air space of at least 1 inch, and that in cool climates more insulation is usually the better buy. It makes sense under the deck in a vented attic in the South, not as a substitute for R-value.

Avoid these

Roof insulation mistakes to avoid

Moisture

Batts against the deck, no vent channel

Air-permeable insulation touching the sheathing in a roof that is neither vented nor sealed. The deck gets wet every winter.

Moisture

Too little foam in flash-and-batt

An inch of foam under R-30 batts in zone 5. The foam surface runs cold enough to collect condensation. Size the foam to R806.5.

Energy

Compressed batts

An R-30 batt stuffed into a 2x8 is not R-30. Use foam, deeper framing or rigid board instead.

Code

Ignoring the thermal barrier

Spray foam in a habitable attic generally needs a thermal barrier such as drywall or a listed coating; attics used only for service may qualify for an ignition barrier instead. Check with your inspector before you spray.

If the attic has an open floor and nothing up there, you may not need roof insulation at all; see attic insulation cost for the attic-floor route. Related guides from our insulation series: barndominium insulation, floor insulation, basement ceiling insulation and garage door insulation.

Roof insulation quotes

Vaulted, low-slope or metal? Get it sized right.

A certified ArmorThane applicator can spray closed-cell foam under the deck or ArmorFoam Roof 200 on top of it, sized to your climate zone and finished to code.

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FAQ

Roof insulation questions

What is the best insulation for a roof?

Under a sloped deck with shallow rafters, closed-cell spray foam, because it gives R-6 to R-7 per inch and air-seals the deck in one step. With deep rafters, flash-and-batt or a vented batt assembly costs less. On a low-slope roof, rigid board under a membrane or spray foam roofing on top of the deck.

How much does roof insulation cost?

Angi's 2026 data puts the average at $2,500, with most homeowners paying $1,700 to $3,300 and a range of $1.50 to $7.00 per square foot. Batts run $1.50 to $3.50, rigid board $2.50 to $6.00 and spray foam $3.50 to $7.00 per square foot.

Do I need roof insulation if my attic floor is insulated?

No. With a vented attic and a well-insulated, air-sealed attic floor, the roof deck should stay cold and ventilated. Adding insulation to the deck as well creates a half-vented assembly that can trap moisture. Insulate one plane or the other.

How thick should roof insulation be?

Thick enough to reach your zone's code value: R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 to 8 under the 2021 IECC. That is roughly 7 to 10 inches of closed-cell foam for R-49 to R-60, which is why most roofs combine foam with batts or rigid board, or use the R-30 allowance on small vaulted areas.

Can you insulate a roof from the outside?

Yes, and it is often the best option when the roof is being replaced. Rigid board goes over the sheathing with new sheathing or nail base on top, or spray foam roofing goes directly over the old roof and is coated. Either way the R806.5 minimums still apply if the rafters below hold any air-permeable insulation.

Is spray foam under a roof deck a good idea?

When it is designed as a complete unvented roof, yes: enough foam for your zone, vents sealed, and a vapor retarder in zones 5 to 8 where required. Problems come from thin foam, open vents behind it, or open-cell foam in cold climates without a vapor retarder.

Does roof insulation need a vapor barrier?

For vented roofs, the IRC's interior vapor retarder rule (R702.7) is written for walls, not ceilings; a well air-sealed ceiling matters more. Unvented roofs in zones 5 to 8 need the air-impermeable insulation to be, or be covered by, a Class II vapor retarder. Closed-cell foam at full depth meets that; open-cell needs a coating or membrane.

What R-value do I need in a cathedral ceiling?

The same as any ceiling in your zone, but the code allows R-30 where the framing cannot hold more, limited to 500 square feet or 20% of the total ceiling area, whichever is less. DOE notes 2x12 rafters fit a 10 inch R-30 batt plus a vent space, and high-density R-30 batts fit 2x10 framing.

AT
Written by the ArmorThane technical team. We have manufactured spray foam insulation, roofing foam, polyurea and protective coatings in Springfield, Missouri since 1989. Code values are summaries of the 2021 IECC and IRC; your state or county may amend them, so confirm with your building department. Updated October 2026.

Sources

  1. DOE/PNNL Building America Solution Center, 2009–2021 IECC minimum insulation requirements: basc.pnnl.gov/…/2009-2021-iecc-and-irc-minimum-insulation-requirements-new-homes
  2. ICC, 2021 International Residential Code, Chapter 8 (R806 roof ventilation, R806.5 unvented attics): codes.iccsafe.org/content/IRC2021P2/chapter-8-roof-ceiling-construction
  3. ICC, 2021 IECC Residential, Chapter 4 (R402.2.1, R402.2.2): codes.iccsafe.org/content/IECC2021P1/chapter-4-re-residential-energy-efficiency
  4. Green Building Advisor, how to build an insulated cathedral ceiling: greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling
  5. Building Science Corporation, BSD-149 unvented roof assemblies for all climates: buildingscience.com/documents/digests/bsd-149-unvented-roof-assemblies-for-all-climates
  6. U.S. Department of Energy, where to insulate in a home: energy.gov/energysaver/where-insulate-home
  7. U.S. Department of Energy, radiant barriers: energy.gov/energysaver/radiant-barriers
  8. U.S. Department of Energy, Building America guide to climate regions: energy.gov/…/ba_climate_region_guide_7.3.pdf
  9. DOE Building Energy Codes Program, COMcheck: energycodes.gov/comcheck
  10. Angi, roof insulation cost (2026): angi.com/articles/roof-insulation-cost.htm
  11. ENERGY STAR, insulation fact sheet (R-value rules of thumb): energystar.gov/…/Insulation Fact Sheet.pdf
  12. HomeGuide, insulation R-value per inch by material: homeguide.com/costs/basement-insulation-cost
  13. U.S. EIA, residential natural gas prices: eia.gov/dnav/ng/ng_pri_sum_dcu_nus_m.htm
  14. U.S. EIA, Electricity Monthly Update: eia.gov/electricity/monthly/update/end-use.php
  15. U.S. EIA, heating oil and propane update: eia.gov/petroleum/heatingoilpropane
  16. U.S. EIA, fuel heat content: eia.gov/energyexplained/units-and-calculators/british-thermal-units.php
  17. ArmorThane, spray foam R-value guide: armorthane.com/knowledge-center/spray-foam-r-value-guide

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