PIR, EPS, XPS or mineral wool: which insulation suits your roof?

PIR, EPS and XPS are rigid insulations with different cellular structures. Mineral wool has a fibrous structure and is made in different grades. Each family includes products for different uses. A façade board, for example, cannot automatically carry loads below a roof waterproofing layer. First identify what the insulation will have to do.

Read the declaration of performance and the product data sheet. Thermal conductivity, reaction to fire, water absorption and mechanical performance describe separate properties. Even within PIR or XPS they vary. The finished roof also includes the structure, control layers, covering and internal lining.

Calculate layer resistance R in m²·K/W by dividing thickness in metres by thermal conductivity λ in W/(m·K). At equal thickness, lower conductivity gives higher resistance. The table illustrates this relationship for selected products.

Using these particular values, 10 cm of PIR gives roughly the same resistance as 14.1 cm of the selected EPS or 15.9 cm of the selected wool. The calculation covers the insulation layer only. Calculating the U-value of the roof also requires the structure, other layers and applicable corrections.

A thinner build-up can help beside a threshold, parapet or existing junction. On a pitched roof a continuous layer over the rafters can reduce thermal bridging through the timber. Its effectiveness depends on board joints and edge details. Windows, wall plates and fixings still require attention.

Rigid boards need accurate cutting. Flexible wool may be easier to fit around irregular framing. An over-rafter system also needs a designed path for loads through the insulation to the supporting structure. A board’s thermal resistance tells you nothing about its ability to act as a structural member.

The example PIR board is declared as CS(10/Y)120, with water absorption of up to 2% by volume under full immersion. These are results for specified test conditions, not a claim of watertightness. The selected XPS is rated CS(10)300 and is also intended for demanding moisture exposure. Check the grade against the actual load and moisture conditions.

Compressive stress at 10% deformation is not an allowable permanent roof or floor load. Use the appropriate design values for sustained pressure. EPS and mineral wool also come in grades with different mechanical properties. Compare products intended for the same application.

Some mineral wool products achieve reaction-to-fire class A1. The foil-faced PIR product considered here is rated F at 20–49 mm and E at 50–250 mm. The example EPS and XPS products are class E. Check the declaration for the exact product and thickness supplied.

Reaction to fire of a board and fire resistance of a complete roof are different assessments. Acoustic performance also depends on the assembly. Mineral wool can improve sound attenuation in a suitable build-up. Matching its thermal resistance with a thinner PIR board does not necessarily reproduce its acoustic performance.

A facing affects vapour diffusion, but joints, perimeters and penetrations determine continuity across the roof. Select the vapour control layer for the design. Two boards with the same core and different facings may be intended for quite different roof systems.

Check compatibility with adhesives, waterproofing and fixings for every material. Assess drying potential before enclosing moisture. Gaps, discontinuities and leaky wall junctions can compromise a roof despite good declared insulation values.

Consider PIR where thickness and continuity are priorities. EPS can be economical in an approved build-up. XPS grades are available for moisture exposure and compression. Consider mineral wool where fire performance, acoustics or fitting around framing are important.

Price complete roof build-ups meeting the same thermal target. Include fixings, membranes, facings and labour at junctions. PIR’s depth advantage can be quantified using conductivity, but the remaining properties and costs depend on the specific product and building.