Insulation materials compared: wool, EPS, XPS, PIR

1. Lambda, R-value and U-value

The key property of an insulation material is its thermal conductivity, λ (lambda), in W/m·K: the lower, the better it insulates. A layer’s thermal resistance is its thickness divided by lambda: R = d / λ (with d in meters). For example, 10 cm of a material with λ = 0.035 gives R ≈ 2.9 m²·K/W.

Building rules usually set a maximum U-value for the whole wall, roof or floor, which includes all its layers. The Insulation calculator works out the thickness needed for a target R-value with the lambda of your product.

2. The main materials compared

The values are typical ranges; always use the declared value of the product you buy.

3. Mineral wool (glass and stone wool)

Mineral wool is made from melted glass or rock spun into fibers. It is non-combustible, absorbs sound very well and fits snugly between studs and rafters. It must be kept dry, because wet wool loses much of its insulating value. Wear gloves, long sleeves and a dust mask when cutting it, because the fibers irritate skin and airways.

4. Polystyrene: EPS and XPS

EPS (expanded polystyrene, white or grey beads) is light, cheap and the most common material for external wall insulation systems. Grey EPS contains graphite and insulates better. XPS (extruded polystyrene, a smooth closed-cell board) absorbs very little water and resists compression, so it is used below ground, under floors and on inverted flat roofs.

Both are combustible and must be protected by render, boards or another approved covering; follow local fire rules, especially on tall buildings.

5. PIR and PUR boards

Polyisocyanurate (PIR) and polyurethane (PUR) boards have the lowest lambda of common materials, so they reach the same R-value with the thinnest layer. That makes them useful where space is tight, such as floors or roofs. They are more expensive per square meter and, like other foams, absorb little sound.

6. Wood fibre and other natural materials

Wood fibre boards and batts insulate slightly less per centimeter but store heat well, which helps keep attic rooms cool in summer. They are vapor-open, which suits timber constructions. Other natural options include cellulose, hemp and sheep’s wool; check fire treatment and installation rules for each.

7. How to choose

• Between studs or rafters: mineral wool or wood fibre batts.
• External walls: EPS or stone wool in an approved system; stone wool where fire rules require it.
• Below ground and in contact with moisture: XPS.
• Little space available: PIR.
• Between rooms for sound: mineral wool.

Whatever the material, gaps and thermal bridges reduce its effect, so fit it tightly and continuously.

Common mistakes

• Comparing materials by thickness instead of by R-value, which depends on lambda.
• Letting mineral wool get wet or compressed, which reduces its insulating value.
• Using foam insulation where non-combustible material is required.
• Leaving gaps between boards, which create cold spots and condensation.
• Forgetting the vapor control layer on the warm side in timber constructions.

Frequently asked questions

Sources

• U.S. Department of Energy — Insulation Materials
• Wikipedia — Building insulation material