How to choose insulation for an existing hollow clay block wall

Check for cracks, loose render and signs of damp. The insulation transfers loads to the substrate through its fixings and bonding layers. Covering weak render will not strengthen it. If in doubt, test adhesion and choose anchors suitable for the hollow blocks behind the paint.

Inspect layers added during earlier repairs. Uneven render thickness and old adhesive patches can affect flatness and bond strength. Allow for their assessment when preparing the façade.

Select insulation against the wall U-value. Where the Polish technical conditions referred to as WT apply to the project, the limiting Uc for an external wall enclosing a room with a design temperature of at least 16°C is 0.20 W/(m²·K). Confirm the applicable regulatory basis and scope for the particular renovation. A result in the middle of the wall does not cover the entire building.

Allow for calculation corrections and the actual fixing arrangement. A thickness selected exactly at the limit may be insufficient once fasteners or poorer masonry are taken into account. Base the choice on the buildable wall and its long-term performance.

For a worked example, take a 24 cm masonry wall with an assumed λ of 0.25 W/(m·K). Its thermal resistance is 0.96 m²K/W. Adding surface resistances totalling 0.17 m²K/W and 100 mm of insulation with λ = 0.026 W/(m·K) gives U ≈ 0.201 W/(m²·K). This is just above 0.20 before corrections. The calculation excludes plaster, render, fixings and thermal bridges.

For a 120 mm façade board with λ = 0.025 W/(m·K), the same calculation gives about 0.17 W/(m²·K). This example does not establish 12 cm of PIR as a general rule for clay block houses. Use the properties of the chosen board thickness and facing.

A rendered façade requires a board covered by the relevant external insulation system. Glass fleece facings differ from gas-tight aluminium laminates in how they bond with adhesive and the reinforced base coat. Similar thermal conductivity alone does not establish compatibility.

The system covers substrate preparation, bonding, mechanical fixing, reinforcement and render finish. Hollow blocks require suitable anchors installed correctly. Use the system documentation and loads on the building to determine the number and positions of fixings. Spacing taken from another façade may be unsuitable.

Thin insulation can help where the eaves have little overhang or windows sit deep in the wall. Measure the complete build-up, including adhesive and finish. Check sill projection, plinth flashing and downpipe offset. Include alterations to these elements in the cost comparison.

Maintain insulation continuity at reveals and leave room for the window sash to open. Detail lintels and ring beams separately. A thick insulation layer over the clay blocks can still leave an exposed strip of concrete that reduces comfort beyond what the masonry calculation suggests.

After insulation, the heating system may need different settings. Observe room temperatures and adjust the controls to avoid overheating. Maintain ventilation. New façade insulation cannot remove the moisture occupants produce or correct a faulty extract system.

Keep layer specifications and photographs of fixings and junctions for later additions such as awnings, lights or canopies. Each fixing needs an appropriate substrate and a sealed penetration through the façade. A hard-looking surface alone gives no indication of fixing capacity.