Applying standard insulation to a stone wall can potentially result in structural failure or dampness issues due to the nature of traditional stone wall construction, which relies on breathable materials. Historically, stone walls were bonded with weak, permeable mortars made of lime and sand. The external walls were coated with a lime render and limewashed, enabling the structure to breathe. When it rained, the wall would absorb moisture a few millimeters into the external surface, which would then evaporate once the rain stopped, assisted by sunlight and wind drying effects.
Internally, walls were plastered with lime and finished with simple breathable paints. Excess humidity from activities such as cooking, washing, and human presence was dispersed through open flues and natural drafts or absorbed by the breathable surfaces. Additionally, kitchen ranges or open fires were in use from autumn to spring, drawing air through the building and maintaining a steady internal surface temperature. As long as the building was properly maintained, the structure remained essentially dry.
By the late 1940s, lime began to be replaced by cement, and modern building technologies took a fundamentally different approach to moisture management. Modern methods rely on impervious outer layers, cavity walls, and barriers to moisture, which are contrary to the principles of traditional solid, breathable walls. Over recent decades, sustainability drives and efforts to reduce energy consumption in traditional homes have led to a mixing of old and new technologies. For example, retrofitting traditional houses with cement-based repairs, cement renders, plastic-based paints, waterproof sealants, damp-proof courses, or applying impervious external or internal insulation to breathable walls has caused significant issues.
This mixing of technologies traps water within permeable materials, exacerbating the very problems it aims to solve. For instance, in a stone wall where an impermeable cement render has been applied externally, a damp-proof membrane retrofitted, and inner surfaces treated with hard tanking plasterboards, moisture from the ground is forced up the wall. With no way to escape, this concentrated ground moisture accumulates, creating localized areas of dampness. Over time, this trapped water may lead to structural failure, as the moisture exerts pressure within the stone and mortar, weakening the wall fabric and compromising its integrity.
To preserve the structural and thermal performance of traditional stone walls, it is essential to use materials and methods that align with their original breathable design.
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