Closed-cell foam

Closed-cell foam is rigid plastic-foam insulation, EPS, XPS, PIR, or phenolic, made of sealed, gas-filled cells. The sealed cells give it high R-value per millimetre, compressive strength, and low vapour permeability.

The defining feature is the cell structure. In a closed-cell foam each cell is a discrete sealed bubble, so the material does not soak up water, resists compression, and slows vapour diffusion. (Open-cell foams, by contrast, are softer, more vapour-open, and absorb water.) The common closed-cell boards are:

• EPS (expanded polystyrene): the most economical, available in densities from light to high-compression grades.
• XPS (extruded polystyrene): denser, higher compressive strength, very low water uptake.
• PIR (polyisocyanurate): the highest R per millimetre and better fire performance, usually foil-faced.
• Phenolic: very high R per millimetre.

Three properties matter on site:

• High R per millimetre, useful where insulation depth is limited.
• Compressive strength, so the right grades work under a slab or under trafficked finishes.
• Low vapour permeability, which is a feature in cool climates (it can act as a vapour-impermeable layer) but a condensation trap in warm, humid ones if placed where the assembly needs to dry.

For a builder the practical points are to pick the foam type and grade for the job, EPS for general/economy, XPS or high-density EPS where compression matters, PIR or phenolic where you need maximum R in minimum depth, and to think about the vapour behaviour, because closed-cell foam in the wrong climate position can trap moisture. Detail it with the NCC condensation provisions in mind, not just the R-value.

Also known as: Rigid foam (closed-cell), foam board.

Category: Insulation / Materials.

Related

• Rigid foam
• EPS

See also

• PIR insulation
• Vapour impermeable

References

• Rigid foam (Chalkline) (verified 2026-06-01)

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Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.