Insulation types compared: R-value, moisture, fire, and barrier behavior

One-line orientation

Choose insulation by more than R-value. Also consider moisture tolerance, fire behavior, and whether the insulation can help control air or vapor. For example, closed-cell foam can qualify as a vapor retarder, while open-cell foam generally does not.

Key points

• R-value per inch measures heat-flow resistance per inch of thickness; the foams generally lead, the fibers trail — but R/inch alone never decides the choice.
• Foam plastics are combustible and generally need a code-compliant thermal or ignition barrier.
• Barrier behavior splits the field: rigid foams and spray foams can contribute to the air barrier only when joints, penetrations, and continuity are sealed. Vapor-control behavior depends on product, facer, and thickness; closed-cell spray foam and faced rigid foams can qualify as vapor retarders, while open-cell spray foam, fiberglass, and mineral wool do not act as vapor barriers by themselves.
• Moisture tolerance: XPS, closed-cell foam, and mineral wool tolerate water best; EPS, open-cell foam, and fiberglass are weaker when wet (wet fiberglass loses R-value).
• Polyiso is the highest-R rigid foam but loses R-value in cold and is unsuitable below grade.
• Mineral wool is the fire/sound specialist — non-combustible, retains performance when wet, used in fire-rated and acoustic assemblies; fiberglass is the low-cost general batt.

R-value per inch ranks them — but it does not decide the choice

Sorted by R/inch, highest first — yet moisture, fire and barrier status scatter independently down the columns.

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Key

• yes
• conditional
• no
• no — and the tested trap: open-cell foam is an air barrier but not a vapor barrier

Trap — open-cell foam is an air barrier, but unlike closed-cell it is not a vapor barrier (amber ring).

conditional = if continuous / sealed, or thickness- / facer-dependent. * Polyiso: rated R falls in the cold — don’t assume it below grade or in a cold-climate exterior. Fiberglass and mineral wool are both non-combustible, but mineral wool is the standout: it withstands very high heat and is the choice for fire-rated assemblies.

Confusions / comparison

Insulation	Approx R / inch	Moisture tolerance	Fire behavior	Air barrier?	Vapor barrier?
EPS (expanded polystyrene)	3.6 – 4.2	Moderate; absorbs water over time	Combustible; barrier/assembly required by code	If continuous/sealed	Product/thickness dependent
XPS (extruded polystyrene)	4.5 – 5.0	High; resists water	Combustible; barrier/assembly required by code	If continuous/sealed	Product/thickness dependent
Polyiso (polyisocyanurate)	5.6 – 6.5	Low; degrades with prolonged moisture	Combustible; chars, somewhat better than EPS/XPS	If continuous/sealed	Facer/thickness dependent
Closed-cell spray foam	6.0 – 7.0	Excellent; watertight seal	Combustible; needs fire/ignition barrier	Yes, if continuous	Thickness dependent
Open-cell spray foam	3.5 – 3.8	Poor; absorbs moisture	Combustible; needs fire/ignition barrier	Yes	No
Fiberglass batt	2.9 – 3.8	Moderate; loses R-value wet	Non-combustible	No	No
Mineral wool (rockwool)	about 3.2–4.3	Excellent; performs when wet	Non-combustible; high-heat resistant	No	No

Related

→ The four enclosure control layers (this topic): where thermal control sits in the stack · Vapor retarders: perm classes and placement (this topic): when an insulation’s vapor-barrier behavior helps or hurts · Window and door performance (this topic): U-factor as the glazed-opening analog of R-value.