Relative Humidity

Wood is a natural, heterogeneous material. Due to its fine, fibrous structure, it always contains water and can absorb or release moisture in response to changes in environmental humidity. Among its many components, cellulose stands out, making up 40-50% according to literature. This fibrous structure plays a key role in water regulation.

Natural, fibrous materials like wood and textiles respond to changes in air humidity. Similarly, mineral-based building materials such as concrete and brick exhibit comparable behavior, but in fibrous materials, this can also lead to dimensional changes, particularly in wood.

In practice, it is important to understand how wood moisture changes with variations in air relative humidity and temperature. Moisture equilibrium is crucial as it indicates the moisture content wood will maintain under different usage conditions, based on air relative humidity and temperature.

These physical laws cannot be ignored. The use of this traditional, versatile, and health-supportive material in our living environment is essential. Today, there are numerous modern technical solutions to mitigate undesirable effects.

Low humidity, which can be problematic in our case, not only affects wood but can have multiple harmful consequences on the human body. The following diagram illustrates the changes in well-known negative factors dependent on humidity.

For an ideal indoor and workplace climate, humidity plays a decisive role alongside temperature and air cleanliness. In our geographical conditions, excessively humid environments are relatively less frequent. When they do occur, humidity should be reduced. We can induce an excessively humid environment through closed, well-sealed windows, non-ventilating walls, and frequent moisture introduction such as cooking and drying clothes.

In contrast, the excessively dry air during the long, continental winter months has significant negative impacts. Ventilation during the heating season does not alleviate excessively dry air. Without artificial humidification during this period, the air will draw moisture from its surroundings, including the skin, mucous membranes, plants, and wooden products.

Air moisture content is measurable and expressed as relative humidity. A hygrometer, or humidity measuring instrument, is used for this purpose. Medically, a relative humidity of 40-60% is recommended, as this range is considered ideal for human health, as well as for plants, animals, furniture, and parquet flooring.

Relative humidity indicates the amount of water air contains at a given temperature compared to the maximum it can hold at that temperature. Hence, temperature is significant, as warmer air can hold more water (g/m³).

Relative humidity can decrease to about half due to heating, which is what happens when we ventilate. In winter, air at freezing temperatures may contain only 1-2 g/m³ of water, leading to a relative humidity of 12% indoors without humidification. In practice, additional moisture from people, plants, and small humidifiers typically keeps this value from falling below 18-25%, although this is still very low.

The consequences of excessively dry air include:

• Discomfort, reduced oxygen intake and transport in the bloodstream
• Increased risk of colds
• Dry skin
• Dust formation
• Damage and cracking of furniture and parquet

The moisture-related properties of wood mentioned earlier are even more pronounced in parquet. Large surfaces interact with the environment relative to thickness, and in finished parquet, the dimensions of individual elements have increased.

There is some variability depending on the conditions under which the wood grew, but a typical shrinkage value of 0.33% is observed for a 1% decrease in wood moisture content.

Example 3: If the moisture content of tongue-and-groove parquet decreases from 8% to 5%, the expected dimensional change is: 3 x 0.33% = 0.99% If the element's width is 65 mm, the expected gap size is: 0.0099 x 65 = 0.64 mm

Example 4: In finished parquet: 3 x 0.0033 x 192 = 1.9 mm Although this seems significant, the shrinkage is reduced by about 70% due to the stabilizing middle layer in the three-layer construction of finished parquet. In the most unfavorable case, even with beech wood, gaps larger than 0.5-0.6 mm per element are unlikely. If the movement of elements is restricted, such as by heavy furniture, the movement is transferred to multiple elements without gap formation, combining these values.

To mitigate critical cases, there is a growing trend to use adhesive installation on concrete instead of floating installation. This is recommended for floor heating to improve heat transfer. Practical experience shows that this installation method is more favorable in preventing gap formation, although gaps cannot be completely avoided under extreme climatic conditions. However, the accumulation of gaps can be prevented.

Adhesive installation requires a significantly better, more even concrete surface to avoid excessive adhesive use. In multi-story buildings, attention should also be drawn to the need for sound insulation, as underlayment foam cannot be used.

This phenomenon is inconvenient for homeowners but unavoidable in extremely dry climates. In the summer, with increased humidity, the gaps disappear. During this season, equilibrium moisture content can reach 11%, which does not cause damage due to the elasticity of wood.

Air Conditioning Unfortunately, this term often only refers to cooling during the summer, sometimes combined with moderate rehumidification. We already know its importance in winter, but smaller room humidifiers can be found on the market as a solution. Their performance and regulation are questionable, as demonstrated in one residence. Even distribution of introduced moisture is also crucial.

There is a wide range of residential humidifiers available, but fewer options for high performance and regulated systems. Heating vaporizing humidifiers have relatively high energy requirements, and warm steam is biologically less favorable. Cold evaporators with moist surfaces generally have low performance. Ultrasonic devices can offer a favorable solution. Recent developments suggest a new generation has emerged. Operating at 1700 kHz instead of the previously known 40-100 kHz ultrasound frequency, performance in "l/hour" significantly increases with only 0.05 kWh energy consumption. However, cold humidifiers require some water preparation (softening), especially with harder water.

Taking a 100 m² apartment with a ceiling height of 2.70 m, where relative humidity at 20°C is 19% (i.e., 3 g/m³), the total water content is: 100m² x 2.70 x 0.003 = 0.81 kg To reach the ideal state of 50% at 20°C, the total water content should be: 100 x 2.70 x 0.008 = 2.16 kg The missing 1.36 kg might seem insignificant, but it reflects a momentary state. In a dried-out apartment, many other hygroscopic materials (textiles, furniture, wallpaper, curtains) are present, making the actual "water deficit" much larger than the calculated amount.

In our opinion, to protect our health and values, much greater importance should be given to this issue within architectural and building engineering circles. Modern printing, paper, and textile industries could not function without addressing this. A wide range of options is already available at trade fairs, in professional journals, and on the internet.

Note: The data provided are indicative values taken from literature.