Choosing PIR insulation for hollow clay block walls

“A Porotherm house” can mean different products and masonry methods. One block’s value cannot be assigned to the whole family. Obtain the specific model’s documentation and mortar details. In an older building, this may require opening up the construction.

Record plaster and any existing insulation, and check ring beams, lintels and reinforced-concrete columns. These differ from the clay blocks. A masonry resistance may be used for the flat wall calculation, but state how it was obtained. An assumed example value must not appear to be a verified property of the existing wall.

Aluminium-faced PIR and façade boards intended for rendering are not interchangeable at will. Appropriately approved glass-fleece-faced boards are one option within rendered systems. Their lambda can depend on thickness: for this example use 0.026 W/(m·K) at 100 mm and 0.025 at 120 mm. These are two assumed variants, not universal values.

Do not enter lambda = 0.022 merely because another PIR board has that value. The difference can affect a thickness close to the target. Adhesive, mesh and render must also belong to a compatible complete system. Combining arbitrary materials does not automatically reproduce its properties.

For comparison only, assume masonry plus plaster resistance of 0.80 m²·K/W and surface resistances totalling 0.17 m²·K/W. The example target is U no higher than 0.20 W/(m²·K). These assumptions are not assigned to any particular clay block.

At 100 mm and lambda = 0.026, insulation resistance is about 3.85 and whole-wall U about 0.208. At 120 mm and lambda = 0.025, U falls to about 0.173. This simplified calculation omits corrections and thermal bridges, but shows why comparing neighbouring thicknesses is preferable to ordering “what the neighbour used”.

A good U-value in the middle of the wall does not describe the whole building. Heat flow at ring beams, balconies and windows is more complex. Insulation ending before a reveal leaves a weak point, which thicker boards elsewhere do not fix.

Detail these junctions in drawings and use more detailed heat-flow calculations where necessary. Measure space at sills and beneath eaves before purchase; flashings may need replacement. Allowing for that in the estimate is easier than altering it after the boards are bonded.

New masonry must be ready to receive further layers. Old walls need the plaster’s adhesion checked. Do not bond insulation onto peeling paint or dust. ETICS instructions require a dry, sound and load-bearing substrate.

Choose fixings for the substrate; hollow blocks need anchors suited to their construction. Anchor length alone does not prove suitability. Wind suction and edge zones also matter. A generic fastener count from a guide cannot replace selection for the actual house.

Lambda describes thermal conductivity in a material. Resistance increases with layer thickness. U applies to the complete assembly and falls as total resistance rises. These values are not interchangeable. Tell the supplier whether you need a board property or a whole-wall result.

Keep the masonry model unchanged when comparing options and vary one factor at a time: insulation type or thickness. That reveals the source of the difference. Comparing new blocks with thin PIR against old masonry with thicker insulation does not demonstrate the insulation material’s advantage alone.

Supply the wall section, area and detail photographs, plus the board type, expected finish and installation method. Request all system components. Compare complete façades rather than PIR packs alone.

The designer should confirm the applicable thermal requirement, including current rules and any transitional provisions. The 0.20 target above illustrates the calculation only. Base the order on the project and product documents. A little more information at the start prevents buying good thermal performance with the wrong facing.

No. Selection depends on the masonry properties and complete wall assembly.

Only within a solution intended for that board. A PIR core alone does not establish that its facing is suitable for render.