The damp guarantee that may be the wrong diagnosis

The evidence does not justify confirming rising damp. A 78% relative humidity at 18°C, black mould around a cold window reveal and a musty smell point strongly towards condensation, but the high external ground level, blocked gully and painted cement render also create a credible penetrating-damp pathway. Rising damp is possible but should not be assumed merely because a damp patch is low on a wall; chemical injection does not prove that rising damp was correctly diagnosed, and the guarantee is a contractual document, not diagnostic evidence.

I would distinguish the causes by: checking the external ground level and DPC continuity; clearing and testing the gully and rainwater goods; taking moisture profiles at several heights and depths with a calibrated meter and carbide testing; checking for hygroscopic salts, which can indicate rising damp or contamination; using thermal imaging and internal/external moisture readings; inspecting sub-floor ventilation and floor structure; and checking whether the patch correlates with weather-driven rain or with occupation and drying. A salt analysis and a monitored drying test would be more informative than another injection. I would also inspect the guarantee's scope and exclusions before advising the owner to rely on it.

The first actions should address the obvious water pathways: lower the external ground level to at least two courses below the DPC where practicable, clear and repair the blocked gully, check rainwater goods and render defects, and improve sub-floor ventilation. Internally, I would remove non-breathable coverings from the affected wall, allow the wall to dry, and treat the mould with appropriate personal protection; the cause of the moisture must be corrected before decorative repair.

Re-injection is not my first recommendation because the building already has a chemical DPC and the visible risk factors point to penetrating damp and condensation. Re-injection would not fix a bridged DPC, a blocked gully or high internal humidity, and it could waste the owner's money while masking the real problem. I would monitor moisture content and internal humidity over several months after the drainage and ventilation improvements. If rising damp is still demonstrated by a credible moisture profile and salt evidence after those works, a remedial DPC could then be considered, but the specification should be based on the confirmed cause. The report should explain the reasoning and not simply repeat the previous contractor's diagnosis.

I would report that the guarantee is a document to be read in full: it may be insurance-backed or merely a contractor's warranty, may require notification and inspection before works, may exclude condensation and penetrating damp, and may be void if the original installation was not in accordance with the specification. I would not advise the owner that the guarantee will pay for re-injection until the cause is established and the guarantee's terms are checked.

I would explain the distinction between a defect in the original installation and a new or different moisture source. If the evidence supports rising damp despite the injection, the guarantee may be relevant; if the evidence supports condensation or penetrating damp, the guarantee is unlikely to respond. I would recommend a written request to the guarantee provider, keeping photographs and readings, and obtaining their view before authorising further work. I would avoid becoming an advocate for either the owner or the contractor; my role is to report the evidence, the likely causes and the proportionate next steps.