Where should you use EPS, XPS and PIR insulation in a building?

Set a thermal target too. R = d/λ is useful for an initial comparison, but equal thermal resistance does not make products interchangeable: mechanical behaviour, fire performance and moisture properties may differ. Mapping each location helps exclude unsuitable offers early.

Distinguish insulation from waterproofing. Low water absorption does not make a board a substitute for a waterproofing layer with properly formed joints and penetrations. Specify the two functions separately.

On a rendered wall, insulation must be compatible with the adhesive, fixings, reinforcement and render. Façade systems use specific EPS grades. PIR needs a facing intended for rendering. A foil-faced board is not a substitute simply because it has lower thermal conductivity.

XPS may suit particular zones or systems; that does not extend its use to the whole façade. Check the board surface and system documentation. Around the plinth, assess water exposure and mechanical damage. Detail transitions between materials so insulation and finish remain continuous.

Insulation beneath a screed carries loads over long periods. Check the mechanical data and the floor assembly. Compressive stress at 10% deformation is not an allowable service load; assess creep and concentrated loads as well.

PIR can save depth where a renovation leaves little room. Damp locations need boards intended for that exposure, including suitable XPS grades. EPS floor grades also vary. Allow for waterproofing, services and the load-distributing layer when choosing.

In a warm roof, insulation sits beneath the waterproofing. Properly designed PIR or EPS systems can work if fixings and roofing are compatible. Do not assume that every adhesive or high-temperature application is suitable; check the documentation for the particular system.

In an inverted roof, insulation lies above the membrane and is exposed to water. Use products approved for that arrangement, often specific XPS grades. Check ballast, drainage and structural loads. A price per pack does not reflect these differences.

Thermal conductivity does not tell you how a board behaves under sustained load. Water absorption refers to a particular test, not roof watertightness. The material’s reaction-to-fire class does not establish the fire performance of the entire assembly. Each value answers a different question.

Record declared values for the chosen variant and thickness range. Check edge profiles, tolerances and the intended use of facings. A trade nickname may cover several products; specify the designation and properties to prevent substitutions.

Price each suitable option in whole packs, allowing for cuts. Add fixings, seals, protective layers and labour. If thinner insulation avoids rebuilding a threshold or sill, show that saving separately so the reason for the cost difference is clear.

Include total depth, installation conditions and inspections in the comparison. A low-priced board may require expensive detailing; equally, a lower conductivity may offer little practical benefit where space is plentiful. Decide for each part of the building.