Waterproofing and Thermal Insulation of a 200-Year-Old Windmill
Dry and Warm Future for Historic Buildings
Construction and Engineering Award winner
Best Restoration Project in 2025
National Building and Construction Awards
Silver Winner in 2025
Innovative Roman lime techniques bring 200-year-old windmill into the 21st century.
High on a windswept hill in South Wales, overlooking the Bristol Channel,
stands a 200-year-old Grade II listed windmill tower.
Challenges of Retrofitting a Windmill
Like many historic towers, this windmill suffered from severe dampness and radiating cold. Exposed to relentless rain and sea spray, it had developed chronic moisture problems. After every storm, water would seep into the masonry and run down the curved interior walls, leaving green patches of damp plaster and buckets scattered across the floor to catch the drips.
Previous lime repointing and re-plastering had failed to solve the issue, and insulation could not be considered until the moisture problem was under control. Even then, the curved walls and tight internal space made it impossible to use conventional insulation systems.
Planning restrictions required the use of lime plasters only, eliminating any modern tanking or non-breathable methods.
From the outset, it was clear this was going to be a complex and delicate conservation challenge. Fortunately, our mentor and adviser, Mr. Naldo Busato, master craftsman of Venetian building restoration, provided invaluable insight. Having spent his career preserving Venice’s UNESCO-listed heritage, he reminded us:
Restoration master from Venice
“We don’t need to invent anything — the Romans already did!”
He was referring to the ingenious lime technologies described by the Roman architect Vitruvius in The Ten Books on Architecture. Guided by Mr Busato’s expertise, we developed a fully lime-based solution to tackle dampness and moisture ingress, condensation, and thermal insulation — an approach that also earned the approval of the local conservation officers in South Wales.
Ancient Knowledge, Modern Performance
Although the external pointing was recently redone and appears to be in good condition, the continued moisture ingress indicates a need for internal moisture protection as well.
Drawing from decades of research into ancient mortars, Mr Busato has studied countless Roman lime samples under the microscope, reviving several authentic formulas with extraordinary properties. For the base coat, we selected the MGN Rinzaffo Roman base, a super-microporous yet breathable lime plaster that repels liquid water while allowing vapour to pass freely. Applied to the interior walls, this original Roman lime formulation created a protective yet open layer — keeping moisture out without trapping it in.
Certain problem areas, such as behind embedded joists, required delicate intervention. By carefully removing and reinstating selected joist ends, we were able to reach and treat every last source of moisture ingress.
Achieving Insulation Without Compromise
To meet modern thermal expectations while respecting the historic fabric, we combined two highly specialised lime-based insulating materials:
Coccioterm Lime and Terracotta Insulating Plaster – used as the main insulating layer. This traditional Roman cocciopesto formulation, made with crushed terracotta, naturally regulates moisture. It was enhanced with microsilica insulating aggregates, boosting its thermal performance.
Termorasante Aerogel Lime Plaster – applied as a super-insulating top layer. This advanced lime-aerogel composite delivers exceptional insulation and thermal reflectance while remaining lightweight and vapour-permeable.
Together, these layers achieved a wall U-value close to 0.5 W/m²K, meeting modern energy-efficiency expectations with only a few centimetres of insulation. The final lime skim coat restored the traditional smooth finish, harmonising perfectly with the tower’s historic character.
Results You Can Feel
The ultimate test came with the following winter. The work was completed in October 2024, and through months of Atlantic wind and rain, the interior remained completely dry and comfortably warm.
No damp patches. No buckets. No green-stained walls — just a stable, breathable, and healthy indoor environment.
Learn More About These Unique Products:
Roman Waterproofing Base
Rinzaffo MGN keeps moisture and salts in check by turning liquid moisture into a regulated vapour flow
Cocciopesto Insulator
Coccioterm MGN is an excellent insulator and humidity regulator for cellars, bathrooms, and similarly humid environments.
Super-Insulator
TerrmoRastante Aerogel MGN is probably the best insulator on the market, a pure aerogel-lime mix. Our best selling product
Watch the Full Story
In this 12-minute video, our colleague Val presents a detailed breakdown of the windmill project during his lecture at the Building Limes Forum Conference 2025, complete with slides and technical details.
Watch now to see how Roman lime technology continues to shape the future of building conservation.
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