One of the most critical yet misunderstood concepts in historical building restoration is "breathability." On site, this expression is often reduced to simple definitions like air permeability, wall drying, or the surface expelling moisture.
Yet, from the perspective of conservation engineering, breathability means preserving the hygrothermal equilibrium—the moisture and heat transfer mechanism—that the historical structure has maintained for centuries. This is because historical buildings are not closed, waterproof, and completely insulated systems like modern reinforced concrete structures. They are:
- Open-pore physical systems capable of absorbing and releasing water,
- Conducting vapor diffusion,
- And working in adaptation to environmental conditions.
What Does Breathability Mean?
In building physics, breathability is the capacity of a material or wall system to transmit water vapor through itself and transfer it to the external environment. This process is associated with parameters such as pore structure, capillary network system, vapor diffusion resistance, and hygroscopic behavior.
Natural stones, Khorasan mortars, lime plasters, and traditional bricks used in historical structures establish a natural moisture transfer system due to their high porosity. Through this system, the building balances moisture, reduces internal stresses, and preserves material stability.
How Does Moisture Movement Occur in Historical Walls?
Moisture movement and breathing behavior in historical walls occur through three fundamental mechanisms:
- 1. Capillary Action: Porous building materials can absorb water from the ground or atmosphere. This is observed especially in capillary-active materials such as natural stone, brick, and lime mortar.
- 2. Vapor Diffusion: Moisture inside the wall moves in the vapor phase toward lower pressure. Through this process, the building can expel indoor moisture and reduce condensation risk.
- 3. Hygroscopic Equilibrium: Traditional mineral-based materials absorb and release moisture according to the relative humidity of the environment. This behavior creates a natural climatic balancing in historical structures.
The Greatest Risk in Non-Breathable Buildings: Salt Crystallization
When the breathing capacity of a historical structure is hindered by applications such as cement-based plasters, plastic paints, acrylic coatings, and film-forming waterproofing, deterioration mechanisms accelerate. Consequently, moisture accumulation, vapor pressure, salt crystallization, and biological growth begin to increase within the structure.
One of the most destructive decay mechanisms in historical structures is salt crystallization. Dissolved salts inside the wall are transported by water movement and crystallize in evaporation zones. If the surface cannot breathe, the evaporation front shifts inward, and salt crystals grow within the stone. This can cause surface spalling, exfoliation, granular disintegration, and plaster detachment. In other words, the problem is rarely the "water" itself, but rather the water's inability to escape.
Behavior Comparison of Restoration Materials
The effects of traditional and modern intervention materials on the breathability of historical wall systems show significant differences on both microscopic and macroscopic scales:
| Material Group | Characteristic Properties & Behavior | Impact and Risks on the Historical Structure |
|---|---|---|
| Lime-Based Mortars & Khorasan (Traditional / Compatible Material) |
Possesses high vapor permeability, an open-pore structure, and a low modulus of elasticity. It exhibits flexible behavior. | Ensures controlled expulsion of moisture. It accumulates salts within the mortar matrix, preventing damage to the original stone. |
| Cement-Based Materials (Modern / Incompatible Intervention) |
Porous structure is low, vapor permeability is low, and its high modulus of elasticity creates an excessively rigid (stiff) structure. | Obstructs moisture movement, increases internal stresses, and causes damage (spalling, flaking) to the historical stone instead of the mortar. |
| Plastic & Acrylic Coatings (Impermeable Film Layers) |
Creates artificial film layers on the wall surface that completely prevent water vapor passage and clog the pores. | Shifts the evaporation front inward, leading to water accumulation under the plaster, salt crystallization, and surface flaking. |
Breathability, Energy Balance, and Contemporary Conservation
Historical structures display breathing behavior not only in terms of moisture but also temperature balance. Porous systems can store heat, buffer moisture, and mitigate sudden temperature fluctuations. Consequently, historical structures create a natural indoor climate distinct from modern buildings. On surfaces where breathability is obstructed, condensation, mold, and indoor humidity increase rapidly.
According to international conservation principles, restoration materials must be vapor-permeable, porous, minimally invasive, and compatible with the original materials. The goal is not to transform the building into a plastic shell, but to preserve the natural hygrothermal behavior it has maintained for centuries.