Mineral wool, cellulose, polyurethane: which insulation for your attic?

When you start an attic insulation project, your operator will propose a material from three main families: mineral wool (glass wool or rock wool), blown-in cellulose or sprayed polyurethane foam. Each has a different thermal profile, fire behaviour, lifespan and cost. This article compares the three options against verifiable criteria, without commercial bias, to help you understand the recommendation you will receive.

Mineral wool: the European reference

Mineral wool covers two very close families: glass wool, made from sand and recycled glass, and rock wool, derived from melting basaltic volcanic rocks. The two variants represent close to 70% of the European thermal insulation market.

Their main asset is the very low thermal conductivity, between 0.032 and 0.040 W/m·K depending on product density. This makes it possible to reach the target thermal resistance (R ≥ 6 m²·K/W for unused attics in Spanish climate zone D or E) with a reasonable thickness.

Crucial added asset in Spain: mineral wool is rated A1 non-combustible according to EN 13501-1. It does not feed fire and withstands temperatures up to 1,000 °C without structural degradation.

Blown-in cellulose: the eco-friendly choice

Cellulose insulation is made from recycled newspaper, ground, defibred and treated with boron salts to resist mould and insects. Its thermal conductivity ranges from 0.038 to 0.042 W/m·K, slightly above mineral wool.

Its main advantage is summer thermal performance: high specific heat capacity (about 1,600 J/kg·K, versus 800 for mineral wool) slows the transmission of summer heat. This is not a minor point in Castile and León where summers are hot.

Cellulose is typically blown into unused attics, eliminating thermal bridges around joists. Its carbon footprint is five to ten times lower than that of mineral wool, making it the preferred option for ecologically oriented projects.

Sprayed polyurethane foam: maximum performance

Sprayed polyurethane foam (PUR or PIR) is applied on site by spraying. It expands on contact with air and adheres to any surface, creating a continuous envelope with no thermal bridge. Its thermal conductivity is exceptional: between 0.022 and 0.028 W/m·K.

In practice, this means that to reach the same thermal resistance as mineral wool, you need about 30% less thickness in sprayed polyurethane. This is a key argument for converted attics with low ceiling height.

Polyurethane is, however, more expensive on a per-material basis and its environmental balance is less favourable. Its fire behaviour (class E) is below that of mineral wool and requires complementary fireproofing in certain configurations.

Synthetic comparison of the three materials

Criterion	Mineral wool	Cellulose	Polyurethane
Conductivity (W/m·K)	0.032 to 0.040	0.038 to 0.042	0.022 to 0.028
Fire class	A1 non-combustible	B-s2,d0	E (needs protection)
Summer comfort	Correct	Excellent	Correct
Lifespan	50 years and more	40 to 50 years	30 to 40 years
Carbon footprint	Medium	Very low	High
CAE compatibility	Yes	Yes	Yes (variant)

Which material for which project?

In practice, CAE operators in Spain mainly propose mineral wool for standard unused attics: it offers the best performance/cost/safety ratio and requires the least logistics on site.

Blown-in cellulose is preferred for projects with a strong ecological component, old stone homes where material breathability matters, and attics where joists form complex cavities to fill uniformly.

Sprayed polyurethane foam is generally only recommended for converted attics with low ceiling height where available thickness is critical, or for sloping rafters requiring strong adhesion. It accounts for less than 10% of CAE installations in Spain.

Verifying the quality of delivered material

Whichever material is chosen, ask your operator for the technical data sheet of the installed product. It must indicate the European CE marking, the Declaration of Performance (DoP), the nominal thermal conductivity and the fire reaction class.

Manufacturers accepted in Spain's CAE programme include ISOVER, URSA, Knauf Insulation, Rockwool and Saint-Gobain for mineral wool, plus Soprema, Igluo and Cellaouate for cellulose. For polyurethane, BASF Walltite, Synthesia Spraytherm and Soprema are the most common references.

The post-intervention report should also state the actual installed thickness, the material's bulk density and the achieved thermal resistance R. Keep this document: you will need it for your EPC and for the ten-year guarantee.

Frequently asked questions

Is glass wool harmful to health?

No, provided installation is carried out by equipped professionals. Modern mineral wool fibres marketed in Europe comply with Directive 97/69/EC, which excludes carcinogenic classification. Once installation is complete and the material is covered, it is completely neutral.

Is cellulose sensitive to humidity?

Cellulose absorbs humidity faster than mineral wool, but releases it just as quickly if attic ventilation is correct. It is a hygroscopic material. Compliant installation with a vapour barrier on the warm side guarantees healthy operation over several decades.

Can two materials be combined?

Yes, it is even frequent in complex renovations: you can for example spray polyurethane foam on narrow rafters and blow-in mineral wool on bulkier unused attics. The CAE programme accepts these mixes if they are technically justified.

Which material to choose after the CAE scheme ends?

If you had to self-finance the operation, mineral wool in panels or rolls remains the most competitive solution (€20 to €35 per installed m²). Blown-in cellulose costs around €30 to €40 per m². Sprayed polyurethane is the most expensive: €40 to €60 per m².