Black mould tracking along a skirting board on a recently insulated house is a temperature symptom, not a hygiene one, and the cause is usually a break in the insulation line at the plinth. Closing that break raises the internal surface temperature at the junction, and the condensation feeding the growth stops forming. The boards that do the work sit in the XPS insulation boards range at Renders World, specified for plinths and below-damp-proof-course zones.
Plinth detailing is one stage of a wider solid-wall upgrade, and the solid-wall Victorian retrofit guide sets it inside the full build-up. What follows stays on the junction itself: what the pattern tells you, why it forms, what closes it, and what the declared figures behind that decision actually say.
What the Mould at the Skirting Line Is Telling You
Mould along a skirting line usually marks a thermal bridge at the plinth, where insulation stops short of the damp-proof course and the internal surface falls below the critical humidity threshold that ISO 13788 gives a calculation method for. That standard sets out how to find the internal surface temperature below which mould growth is likely, given the internal temperature and relative humidity — which is the useful framing, because it makes the junction a calculable quantity rather than a housekeeping failure.
The pattern reads clearly once you know what to look for, and it is worth confirming before any remedial work is priced.
- Position: a band at the wall-to-floor junction, heaviest on external walls and behind furniture, where air movement across the surface is lowest.
- Contrast with the wall above: the main elevation stays warm and clear, because it sits inside the insulation line while the base of the wall does not.
- Return after cleaning: growth reappears within weeks of being wiped down, because the surface temperature never changed.
Where all three are present, the junction is the source, and treating the surface will keep costing money until the temperature is dealt with.
Why the Plinth Becomes the Coldest Line on an Insulated Wall
Insulation was historically stopped a course or two above ground to keep boards clear of ground moisture. That habit leaves the base of the wall and the edge of the ground-floor slab outside the thermal envelope, so heat takes the shortest available path out through cold masonry, and the internal face of the junction sits several degrees below the rest of the room. Warm internal air reaching that colder surface deposits moisture on it, and sustained surface dampness is the one condition mould growth needs.
The correct target is therefore the surface temperature, not the air. Ventilation and dehumidification manage symptoms; a continuous insulation line removes the cold surface that creates them, which is why the fix is a detailing decision made once rather than a maintenance regime repeated every winter.
How to Close the Plinth Thermal Bridge
The method is to carry the insulation line down past the damp-proof course using boards specified for wet and loaded ground conditions. The GENDERKA HYDROSTYR 100 boards on this shelf declare water absorption of 3% under long-term total immersion, so a board that spends its service life in damp ground keeps the thickness the thermal calculation assumed, and 100 kPa compressive strength, so backfill pressure and paving loads at the plinth do not deform it.
The sequence that holds the detail
- Tank and level the substrate first. Nothing in the layers above corrects a wet or uneven wall below, and the boards go onto a made-good face.
- Bond with a route the sheet names. Cement-based adhesives, polyurethane foam adhesives, or water-dispersion bitumen compounds — the cement-based route sits on the EPS adhesives basecoats shelf, and solvent-based products are kept away from the boards entirely.
- Separate the boards from the soil. Below ground level the manufacturer requires a separating layer, either a reinforced layer with waterproofing over it or a dimpled membrane, so the boards never sit in direct contact with the ground.
- Work upward in staggered courses from the footing, keeping joints tight so the layer reads as one plane up to the damp-proof course.
- Restrain the exposed section mechanically. Backfill holds the buried part; above ground the plinth takes fixings to the layout the system holder specifies for that substrate.
Two points are settled at order rather than on site. The sheet describes the product in both smooth-edged and rebated forms without saying which reaches this shelf, so joints are detailed as plain butt joints until the rebate is confirmed; and the overlap between plinth board and facade board is a junction detail from the system holder, since no figure for it appears on any document read here.
Key Takeaway: Mould at the wall-floor junction is a surface-temperature problem with a calculable threshold behind it. Carrying a continuous insulation line down past the damp-proof course in boards declared for immersion and load raises that surface temperature; the overlap dimension and the fixing pattern come from the system holder, and the boards themselves come with declared figures you can hand to an assessor.
Choosing Board Depth and Ordering the Plinth Run
Depth follows the calculation, and the figure an assessor wants is the declared thermal resistance for the thickness chosen rather than thickness divided by conductivity — the two are not identical, and only the declared value appears on the manufacturer's declaration. The range declares λD 0.036 W/mK, so a given resistance target is reached in less depth than a poorer conductivity would need, which matters where paving levels fix how far the wrap can project.
| Board depth | Declared resistance | Where it earns its place |
|---|---|---|
| 80 mm plinth and foundation board | 2.20 m²K/W | Plinth matched to an 80 mm facade board above |
| 90 mm plinth and foundation board | 2.50 m²K/W | Below-DPC work matched to a 90 mm facade |
| 100 mm plinth and foundation board | 2.75 m²K/W | Deepest wrap and the plinth-to-wall transition |
The steps between those figures are uneven because each is declared rather than calculated, which is worth knowing when a specification is written to a resistance target. On cost, the boards are 1000 × 500 mm, so 1 m² ÷ 0.5 m² = 2 boards per m²: at £9.50 per board the 90 mm depth works out at £9.50 ÷ 0.5 = £19.00 per m², and the 100 mm at £10.50 ÷ 0.5 = £21.00 per m², both August 2026, board only and before adhesive, fixings and finish. Renders World sells them as single boards, so a plinth run is ordered to the metre rather than by the pack, and the visible zone above ground is usually closed with a hard-wearing mosaic render finish for plinths.
Designing Out Plinth Mould on the Next Project
Specifying the continuous line at design stage costs a fraction of retrofitting it, and the regulatory direction now rewards doing so. Under the government guidance on Awaab's Law, in force for the social rented sector from 27 October 2025, damp and mould hazards that present a significant risk of harm carry fixed timeframes: investigation within 10 working days, a written summary to the tenant within 3 working days of that conclusion, and safety work begun within 5 working days.
The detail that matters to a specifier sits further into the same guidance. It names a lack of thermal insulation in the structure of a home leading to excessive mould growth as a deficiency in scope, and states that where a damp and mould hazard results from poor insulation, the landlord must address the root cause — installing effective insulation, rather than managing the surface. Preventative work must be begun within five working days of the investigation concluding, or physically started within 12 weeks where that is not possible, so a junction designed correctly at the outset removes a recurring obligation rather than deferring one.
Where the plinth sits within a full external wall insulation build-up, the boards below the damp-proof course and the facade boards above it are chosen together, so the two planes finish flush and the insulation line never breaks at the transition.
Written by Mariusz Saja. Technically reviewed by Rafał Wyrzykowski. Last reviewed Aug 2026.
FAQ — Plinth Boards, Quantities and Damp Duties
How many boards does a plinth run need?
Two per square metre. At 1000 × 500 mm each board covers 0.5 m², so a plinth 24 m long at 500 mm high is 12 m², or 24 boards before an allowance for cuts at corners and returns. Ordering as single boards rather than packs keeps that allowance small.
Why not simply run the facade board down to the ground?
The two zones are specified on different properties. Below the damp-proof course the governing figures are immersion behaviour and load, which is what the plinth and foundation boards declare; above it the priorities become vapour movement and compatibility with an adhesive, basecoat and render build-up, which is where a graphite facade board belongs.
Which thickness meets a U-value target?
That is decided across the whole build-up rather than by a board on its own. Give the assessor the declared thermal resistance for the depth you are considering — 2.20, 2.50 or 2.75 m²K/W for the three depths above — and let the calculation return the thickness, since existing construction and the layers around the junction both move the answer.
Does the reaction-to-fire class matter at the plinth?
The boards declare class E, and that figure belongs to the board alone. A rendered plinth is a different assembly with its own evidence, so any performance a project needs for the finished build-up comes from the system holder rather than from a board property.
Will insulating the plinth clear mould that is already there?
A clean-down still needs doing, but it holds afterwards. Once the junction surface sits above the critical humidity threshold, the condensation sustaining the growth stops forming, so a single treatment after the thermal work replaces a cycle of repeated ones, supported by sensible internal humidity control.