What to check in a hygrothermal assessment before renovating

Opening the wall in one place cannot characterise an entire storey. Assess the spandrel below the window, ring beam and balcony junction separately. A conservative assumption can be revised once the construction is known. Documenting this explains the choice of insulation thickness later.

Lambda denotes a material’s thermal conductivity. For a homogeneous layer, thermal resistance is R = d/λ, where d is the thickness in metres. For example, 80 mm of material with λ = 0.022 W/(m·K) gives R of about 3.64 m²K/W. This calculation applies to that layer alone. The U-value of the finished wall and the declared properties of the chosen PIR board need separate data.

The U-value includes the full construction and the relevant surface resistances. A thin insulation board’s datasheet cannot describe the existing masonry. Include the applicable corrections when checking the requirement and investigate thermal bridges at reveals and floor junctions separately.

If an existing masonry property is uncertain, compare two plausible sets of inputs. A large difference between results shows where further investigation of the wall will improve the assessment more than refining the calculation itself.

Bedrooms, laundries and intermittently heated workshops have different humidity patterns. Use temperatures and humidity levels suited to the room. Check that ventilation still works after windows have been replaced. Collect measurements during a cold spell to supplement any single reading taken on a sunny afternoon.

Internal insulation reduces the heat reaching the masonry from the room. Consider how the wall dries and how much water reaches it from outside. Changing a humidity input in software cannot remedy a leaking gutter or rising damp.

Methods in PN-EN ISO 13788 include simplified assessments of surface humidity and interstitial condensation. They cover only certain moisture transport mechanisms. Airflow through leaks and the full effects of capillary water movement are outside the scope of these simplified calculations.

Wind-driven rain, old timber beam ends built into masonry or unusual layer sequences may call for a transient hygrothermal analysis. The designer determines its scope. A quick calculation can compare options as long as its limits remain clear.

Even with a favourable overall U-value, a cold strip can remain at a lintel. Mark reveals, external corners, floor junctions and service penetrations on the drawings. For internal insulation, include intersecting walls. Their junction with cold masonry can govern the room-side surface temperature.

Translate the results into details for continuous insulation, sealed layer junctions and the final lining. If the specified thickness will not fit behind a radiator or beside window hinges, revise the design before installation. A thinner strip chosen on site changes the conditions assessed.

Include the layer sequence, material properties, boundary conditions and limits of the method. Provide results for the existing and proposed walls so the effect of the retrofit is clear. Use product data for the facing and thickness that will actually be installed.

Photograph junctions before the lining conceals them. If opening up reveals wet masonry or a different construction, notify the person responsible for the assessment. Update the final report to describe what was actually built.