Relative Humidity, Condensate and Building Materials

relative humidity

Air has the ability to either hold onto or release water. This process happens immediately based on the air temperature and changes in temperature. When air is warmed up, its ability to hold water increases. Warm air can hold much more water than cold air. Conversely, if air is cooled down, it must release water in the form of condensation or dew. In construction, this is important to consider for cold components, especially thermal bridges or poorly insulated parts, as they can cool down the air around them.

For construction planning or evaluating building damage the relative humidity needs to be considered in relation to the potential saturation humidity. When air is saturated, the humidity is 100%. For instance, at an air temperature of 20°C, air can hold a maximum of 17.3 g/m³ of water in its gaseous state. If the water content exceeds this, free water or mist forms. This excess moisture becomes visible as frost in winter or condensation on colder surfaces.

Relative humidity (Φ) is always connected to the air temperature. For example, if the relative humidity is 50% at 20°C, the absolute humidity is 8.65 g/m³. If the air temperature rises while the absolute water content remains the same, the relative humidity drops. However, if the air cools down, the relative humidity rises while maintaining the same absolute humidity.

Relative humidity (Φ) is the ratio of the moisture concentration (c) to the saturation moisture (cs) at a specific temperature.

Φ = c / cs

With

c = water vapor concentration in [g/m³] and

cs = saturation moisture in [g/m³].

Condensation and Building Materials

Heat and moisture cannot be separated in building physics and both aspects have to be considered. Changing the temperature level automatically changes the relative humidity. Surface temperatures of enclosing building components are especially important. The larger the temperature difference (∆T) between warm room air and cold surface temperatures, the greater the risk of condensation forming on surfaces. This becomes especially evident in winter and can lead to mold growth on thermal bridges or poorly insulated components.

Besides surface temperature, the sorptive properties of building materials are also significant, especially for mineral and natural materials. These properties distinguish materials like plaster, concrete, wood, or masonry from materials like glass, metal, or plastic where we see condensation occur. Mineral and natural materials maintain a constant relationship with room humidity due to their moisture content and equilibrium moisture. This means these materials are never completely dry. Processes of adsorption, absorption, and desorption are in a constant dynamic.

When the relative humidity in a room exceeds 60%, a sorbate film forms in the capillaries of the building material. This leads to moisture migration into the material alongside water vapor diffusion. This moisture movement is also supported by the size of the capillaries. Here, the driving force is relative humidity, not vapor pressure. Water transport into pores and capillaries occurs in liquid form. This free water in capillaries becomes a basis for surface mold growth.

Managing condensation - Constructional Aspects

• One key strategy is enhancing the insulation of building components, such as roofs, walls, and floors. Adequate insulation reduces temperature differentials between indoor and outdoor surfaces, minimizing the potential for condensation to form.

• Thermal bridges are areas of a building that allow heat to bypass insulation, creating colder surfaces prone to condensation. Proper design and materials can help reduce or eliminate these bridges, preventing localized condensation points.

Managing condensation - Environmental Aspects

• Proper ventilation is crucial in managing indoor humidity levels. Mechanical ventilation systems, such as exhaust fans and balanced ventilation systems, ensure a continuous exchange of air, expelling moisture-laden air and replacing it with fresh, drier air.

• Maintaining indoor humidity levels within recommended ranges helps prevent condensation. Using dehumidifiers and incorporating vapor barriers can assist in keeping moisture in check.

• Consistent indoor temperature plays a vital role in condensation prevention. Managing temperature variations between surfaces and the surrounding air reduces the likelihood of dew point temperature being reached.

To manage condensation means to manage temperature and moisture conditions in tandem. By improving insulation, addressing thermal bridging, ensuring effective ventilation, and controlling indoor humidity, builders and architects can create structures that are less susceptible to condensation-related issues. These strategies collectively contribute to healthier indoor environments, reduced maintenance costs, and increased overall building durability.