Issue > Damp on exterior walls only (no cavity wall)

Most dampness in old houses are created by using the wrong material over the years for repairs, modern materials and old houses do not work well together as they are trying to achieve diffrent things, you need someone who understand how the materials used in the construction of your property all work together to prevent dampness, tanking an old wall will only cover up the damp rather than finding the source of the water ingress & solving it. Good luck Alex

Hi, From what you’ve described, the issue is most likely related to moisture being trapped within the wall structure rather than a plumbing leak. Older solid brick walls (with no cavity) rely on the ability to breathe and allow moisture to evaporate. When a tanking slurry is applied internally, it can sometimes stop that evaporation path. If moisture is entering from the outside through cracked render, defective pointing, or porous brickwork, it can become trapped behind the tanking layer. When you then install metal stud framing, insulation, and plasterboard, the space can become a cold zone where condensation forms, especially if the insulation ends up touching the wall due to moisture or sagging. That can explain why you saw green mould under the skirting. The fact that the wall started drying quickly once the insulation was removed is a strong indication that the moisture was being trapped rather than actively leaking. A few things worth checking: • External condition of the wall – cracks, failed pointing, or damaged render allowing rain penetration. • Whether the tanking system has trapped moisture in the solid wall. • Ventilation of the internal wall build-up behind the stud system. • Type of insulation used and whether a vapour control layer was installed. Often the correct approach is to fix any external defects first (cracks, pointing, render), then reconsider the internal build-up so the wall can manage moisture correctly rather than trapping it. Solid walls usually perform better with breathable systems rather than fully sealed tanking layers. A proper inspection with moisture readings and thermal imaging would confirm whether this is penetrating damp, trapped moisture, or condensation within the wall build-up. Hope that helps.

From what you’ve described, it sounds like the issue may be caused by the wall no longer being able to breathe properly. Older solid brick walls (with no cavity) rely on moisture being able to move through the wall and dry naturally. When tanking slurry is applied internally it creates an impermeable barrier, which can trap moisture in the wall. Once insulation and plasterboard are added in front, the wall behind often becomes colder and poorly ventilated, which can lead to damp forming in that void. The fact the wall dried quickly once you opened it up is usually a good sign that it just needed airflow again. Cracks or defects in the external wall could also be allowing moisture in, so repairing those externally is important too. Generally with older properties it’s best to use breathable systems internally rather than sealing the wall, so moisture can escape naturally.

Old solid brick walls with no cavity behave very differently from modern walls because they need to be able to absorb and release moisture naturally. From what you described, it sounds very likely that moisture is being trapped within the wall build-up, rather than this being simply a plumbing leak. If the wall dried quickly once the insulation was removed, that is usually a strong sign that the existing system is not allowing the wall to dry properly. In cases like this, the issue is often caused by a combination of: • moisture ingress from the outside through cracks, failed pointing or poor external condition • tanking slurry preventing the wall from breathing inward • interstitial condensation forming within the internal insulation build-up when warm internal air meets a cold solid wall • thermal bridges and poor junction detailing around floors, skirtings and openings Leaving a gap behind the insulation does not always solve the problem, because if humid air reaches a cold masonry surface it can still condense. This type of issue really needs to be looked at as a building performance and moisture behaviour problem, not just a plastering or damp-proofing issue. As a Certified Passive House Designer, this is the type of work I deal with. I carry out a full investigation to understand how the wall is behaving before recommending any repair. This can include: • thermal imaging survey to identify cold bridges and hidden moisture patterns • moisture and humidity assessment of the wall and indoor conditions • blower door / smoke testing where required to identify hidden air leakage paths • inspection of the existing wall build-up, insulation void and external wall condition • assessment of how the wall performs in relation to the five Passive House principles: continuous insulation airtightness thermal bridge reduction high-performance junction and window detailing controlled ventilation and moisture management The aim is to understand how heat, air and moisture move through the wall, because unless that is properly checked, there is a big risk of only treating the symptom rather than the actual cause. When I carry out this type of assessment, it comes with a detailed step-by-step report explaining: • what is happening • what is causing it • what should be done first • and what repair strategy is most suitable for that specific wall build-up With older solid walls, the correct solution is often to address the external defects first, then redesign the internal build-up so the wall can manage moisture safely rather than trapping it.