Rendered PIR insulation on an aerated concrete wall: what to check before installation

Check the moisture content and whether wet trades have finished. The substrate must meet the insulation system’s requirements. Covering a wet wall to shorten the build can trap moisture. If the building stood without a roof for a long time, establish the condition of the masonry before comparing insulation prices.

PIR for use beneath render forms part of a specific external thermal insulation composite system (ETICS). Some boards have a glass fleece facing designed to bond with the other layers. An aluminium-faced board with a similar-looking core does not establish the required adhesive bond or durability. Check which board the system specifies.

Confirm which face goes against the wall. On some boards the two facings serve different functions. Use the specified adhesive, reinforced base coat, fixings and render. Combining components from separate installation guides puts the assembly outside the performance demonstrated for the complete system unless that combination is covered.

Take a 24 cm masonry wall with an assumed conductivity of 0.16 W/(m·K). Its thermal resistance is 1.50 m²K/W. Add surface resistances totalling 0.17 m²K/W. With 80 mm of façade insulation at λ = 0.026 W/(m·K), the simplified U-value is about 0.21 W/(m²·K). At 100 mm with the same conductivity, it falls to about 0.18 W/(m²·K).

This comparison omits render, fixings and thermal bridges. Under the Polish technical conditions referred to as WT, the limiting Uc for this type of wall at a design indoor temperature of at least 16°C is 0.20 W/(m²·K). The designer should confirm which provisions apply, including transitional rules. Final sizing requires a full calculation with building-specific data.

A foil-faced PIR board may have a declared λ of 0.022 W/(m·K), while the façade version has a different value. In the rendered-board family used in this example, conductivity also varies with thickness. It is 0.026 W/(m·K) at 80–110 mm and 0.025 W/(m·K) from 120 mm. These figures apply to the product family described.

Use a calculator that accepts the actual product value. Entering the lowest figure advertised for a different board can underestimate the thickness required. Match each quotation to the datasheet for the exact board offered.

Prepare the aerated concrete surface and choose fixings suitable for it. Sanding dust can impair adhesion. On an existing wall, check the render too. Select the primer, adhesive and fixing method using the system instructions and the condition of the substrate.

Wind loads can be higher at corners. Determine the number of anchors for the building’s height, location, façade geometry and fixing capacity. If the existing substrate’s holding capacity is uncertain, on-site tests may be needed.

A test at one arbitrarily selected spot cannot characterise a façade with several substrates. Mark infilled openings, ring beams and repairs on the survey drawing and account for them in the fixing design.

Provide drainage and sealed junctions at the plinth, sills and downpipes. Measure the eaves overhang and size the flashings for the full insulation build-up. The reinforcement mesh and render cannot provide a windowsill flashing. Use system-compatible protection in areas exposed to impacts.

Record how the insulation runs around windows and service penetrations before finishing. Retain the system details for repairs. If a corner is damaged, reinstate its layers as part of the repair. Repainting alone can hide damage that affects the façade in later seasons.