The Concept of ‘Breathing’ in Building Physics: Water Vapor Diffusion and Moisture Cycle

I. Introduction: The Physical Reality of “Breathing”

The expression “breathing,” frequently encountered in restoration terminology, is scientifically defined as water vapor diffusion permeability. For a structure to breathe means that water vapor passes through the building elements (stone, brick, plaster) and is evacuated due to the partial vapor pressure difference between the indoor and outdoor environments. This process maintains the hygric balance of the building and allows water within the material to transition from a liquid phase to a gas phase for dissipation.

II. Vapor Pressure and Diffusion Mechanism

Water vapor always moves from areas of high pressure to areas of low pressure. In heated indoor spaces, vapor pressure rises, and moisture tends to exit through the building envelope.

• Advantage of Mineral Structures: Traditional lime-based plasters and natural stones do not resist this vapor transition thanks to their open-pore structures.
• Synthetic Barriers: Acrylic-based paints or cement-based plasters applied to building surfaces exhibit high diffusion resistance, halting this transition. The result is the entrapment and condensation of vapor beneath the surface.

III. μ Value: Water Vapor Diffusion Resistance Factor

In building physics, the breathing capacity of a material is measured by the μ value. This coefficient indicates how much resistance the material offers to water vapor passage compared to air:

• Air: μ = 1
• Traditional Lime Plaster: μ ≈ 5-10
• Natural Stones: μ ≈ 10-40
• Acrylic Paints / Cement: μ > 100-200 (Resistance is very high)

As the resistance coefficient increases, the building envelope becomes unable to evacuate vapor, leading to “moisture entrapment.”

IV. Disruption of the Moisture Cycle and Pathological Outcomes

When the moisture cycle is interrupted, water liquefying inside the building element carries soluble salts to the surface. An impermeable layer (acrylic paint or low-quality plaster, etc.) prevents this water from evaporating. The unevaporated water crystallizes just beneath the surface (subflorescence), causing mechanical disintegration, peeling of the material, and a rapid increase in biological growth (mold, fungi).

V. Conclusion: Sustainable Conservation Approach

Intervening in the moisture balance of a historical building directly affects its lifespan. Any layer applied to the building envelope (consolidants, water repellents, or coatings) must be compatible with the material’s original diffusion capacity.

A restoration intervention that “does not breathe” initiates a silent process that decays the structure from within. Conservation ethics mandate the preservation not only of the material’s aesthetics but also its physical behavior.