Why You Shouldn’t Use Cement on an Earthen Wall
When homeowners of French stone and character properties notice dampness emerging at the base of an old wall, whether it’s made of earth, stone, or adobe, the natural reflex is often to apply a cement render or build a cement plinth. Although this may seem like a solid solution, it often causes more harm than it prevents.
In this article, we’ll explore why cement is incompatible with historic masonry, the damage it causes, and smarter, more respectful alternatives.
Old Walls Need to Breathe
Walls made from earth, adobe, ‘pisé‘ or soft stone are breathable and alive:
- They absorb moisture and release it seasonally.
- They regulate humidity.
- They expand and contract naturally with weather changes.
Applying an impermeable material, such as cement, interrupts these vital processes.
Cement Blocks Moisture Escape
A cement base coat traps moisture:
- Vapour becomes imprisoned within the masonry.
- It then rises higher, moving past the cement layer.
- This leads to blistering, mould growth, deterioration and cracking.
What was intended as a fix ultimately has worsened the situation.
The Misleading “Bastard Mortar”
“Bastard mortar” (a mix of cement and lime) is often promoted as a compromise. In reality, the cement component dominates, making it nearly as impermeable as pure cement and equally harmful. The result? Moisture damage simply transfers higher up the wall.
See the Problem Visually
Real consequences: Black mould and cracks appear on treated walls, while interior coatings bubble, peel and fracture.
Don’t Forget Ventilation
Modern renovations often focus on materials, yet proper ventilation is essential:
- A household releases up to 12 litres of water vapour a day (from cooking, showers, breathing).
- Without effective ventilation, this builds up and condenses on cool surfaces, leading to:
- Mould and mildew
- Damaged paint and plaster
- Health issues, such as allergies or respiratory irritation.
Recommended Solutions
- Regular airing: A few minutes after showers or cooking.
- Mechanical Ventilation (MVHR):
- Single-flow vents out moisture from kitchens and bathrooms.
- Double-flow systems filter and pre-warm incoming fresh air.
- Hygro-adjusting ventilation: Adapts airflow to real humidity levels inside.
Compatible and Respectful Alternatives
No matter your wall type, choose solutions that work with its nature, not against it:
- Natural lime renders (aerial or hydraulic): Flexible, breathable, gentle on historic fabric.
- Clay or earth-lime renders: Ideal for adobe, pisé or cob structures.
- Drainage-friendly plinths: Stone or brick bases laid in lime mortar.
- Perimeter drains: Divert external moisture away from the foundation.
Three Core Principles
- Avoid cement plinths, they obstruct moisture discharge.
- Combine with proper ventilation: either MVHR or simple, regular airing.
- Use breathable materials: lime and earth over cement.
This trio of tips ensures a renovation that respects the heritage and longevity of your building.
In Conclusion
Renovating old walls means understanding their behaviour, not just covering visible issues. Cement is often a quick fix with long-term consequences.
Choose reversible, vapour-permeable, and compatible materials to preserve the quality and character of your home. The result? A dry, healthy, and durable wall that ages gracefully.
Looking for Professional Help?
For more property tips, please contact Christophe at Dievertis, who wrote this article, or visit his website. Merci Christophe for your kind contribution. Finally, please visit our sister website for your insurance needs in France.
