Caring For Cob Built Cornish Cottages

Best Practices, Pitfalls, and the Importance of True Vapour Permeability

Cob-walled cottages are a cherished feature of Cornwall’s architectural heritage, known for their distinctive, thick earthen walls and historical significance. Proper maintenance of these buildings is essential to preserve their integrity, especially considering their unique construction. This article delves into the best practices for caring for cob-walled cottages, including the risks of using non-vapour permeable materials, and provides guidance on avoiding common pitfalls related to so-called “breathable” products.

Understanding Cob and Its Maintenance Needs

What is Cob?
Cob is a traditional building material made from a mixture of clay, sand, straw, and water. It is applied in thick layers to form robust walls that offer excellent thermal insulation and breathability. Cob walls have been used for centuries in Cornwall and are valued for their:

Thermal Mass: The ability to store and release heat slowly, maintaining comfortable indoor temperatures.
Breathability: The capacity to allow moisture to evaporate, preventing internal dampness and preserving structural integrity.

Best Practices for Maintaining Cob-Walled Cottages

1. Use Vapour Permeable Materials

Why Vapour Permeability Matters: Cob walls need to “breathe” to maintain their health and prevent issues associated with trapped moisture. Vapour permeable materials allow moisture to escape, thus preserving the wall’s integrity and preventing damp problems.

Recommended Materials: Lime-Based Plaster: Lime plaster is breathable and flexible, accommodating the natural movement of cob walls and allowing moisture to escape.

Natural Paints: Use breathable paints such as lime wash, clay or silicate-based paints that do not form a barrier on the wall surface, ensuring moisture can evaporate properly.

2. Avoid Non-Vapour Permeable Materials

Risks of Non-Vapour Permeable Materials

Cement-Based Products: Cement renders and mortars are impermeable, trapping moisture within the walls and leading to damp issues and potential structural damage.

Gypsum Plaster: Gypsum is non-permeable and can absorb moisture, leading to the deterioration of both the plaster and the cob behind it.

Plastic Paints: Modern plastic-based paints create a seal that prevents moisture from escaping, increasing the risk of trapped dampness and subsequent damage.

Consequences of Using Inappropriate Materials

Damp Issues: Trapped moisture can lead to internal dampness, causing plaster to deteriorate and mould to develop.

Structural Damage: Prolonged exposure to moisture can erode the cob material, potentially leading to significant structural issues or collapse.

Beware of Misleading Claims – SD Values and the Reality of “Breathable” Products

Understanding SD Values: The SD (sorption diffusion) value measures a material’s resistance to water vapour diffusion. A lower SD value indicates higher vapour permeability, which is essential for maintaining the health of cob walls. However, many products marketed as “breathable” may have misleading claims.

Common Pitfalls: Misleading Claims: Some products advertise themselves as “breathable” but have high SD values of 0.5 and above, meaning they do not effectively allow moisture to pass through. These products can trap moisture against the walls, leading to damp issues.

Inadequate Testing: Products may lack proper testing or certification for their breathability, making it difficult to assess their suitability for cob buildings.

SD Value Examples
– Standard acrylic masonry paint – 0.7 SD – 50g/m²/day
– Microporous acrylic masonry paint – 0.5 SD – 180g/m²/day
– Silicone masonry paint – 0.5 SD – 190g/m²/day

– Earthgborn Ecopro Silicate paint – 0.06 SD – 1254g/m²/day
– Beekosil Silicate paint – 0.01 SD – 1920g/m²/day
– Keim Mineral paint – 0.01 SD – 1968g/m²/day
– Limewash – 0.01 SD – 1968g/m²/day

How to Avoid Pitfalls:

Check SD Values: Always check the SD value of any material you plan to use. Look for products with an SD value below 0.10 to ensure true vapour permeability.

Consult Experts: Seek advice from conservation specialists or architects experienced with cob buildings to select appropriate materials and avoid potential issues.

Resources and Further Reading

For detailed guidance on maintaining historic cob buildings and understanding material suitability, refer to the following resources:

Historic England – Provides comprehensive advice on maintaining cob buildings, including the use of appropriate materials.

Cornwall Heritage Trust – Offers resources and support for preserving Cornwall’s historic structures, including cob-walled cottages.

The Society for Protection of Ancient Buildings (SPAB) – Offers guidance on preserving historic buildings and using materials that support their longevity.

Caring for cob-walled cottages in Cornwall involves understanding the unique properties of cob and using materials that support its health and longevity. By opting for true vapour permeable materials and avoiding those with misleading “breathable” claims, you can help preserve the structural integrity and historical value of these treasured buildings. For further assistance and expert advice, the resources listed above provide valuable support and guidance.