Wood is a material of natural origin with a complex chemical composition. Because of its fine, fibrous structure it always contains water, and it’s capable of absorbing and dispersing moisture as a result of a change in environmental humidity. Among its many components, cellulose is in the first place. According to data in technical literature, its proportion is 40-50%. Primarily this material, with fibrous structure, plays the most important role in water circulation.

Materials of natural origin, with fibrous structures, such as wood, or textile react to the effect of fluctuations in the humidity of the air. Building materials of mineral origin, concrete and brick, also behave in a similar way, but in the case of fibrous materials changes in size may occur as well, especially in the case of wood materials.

In practice, it’s crucial to know, how the moisture content of wood changes with the variation ofthe relative humidity and temperature of the air. Moisture balance has a substantial significance, because it shows what amount of moisture the wood will absorb depending on the circumstances of usage.

Relative air humidity

We can’t render ourselves independent of the above mentioned laws of physics. The utilization of this ancient, versatile and healthily applicable material is important in our living environment.

There are already a number of modern technological possibilities to eliminate the unpleasant effects.

Low humidity, which causes a problem in our case, may have multiple harmful consequences on not only wood, but primarily on the human body as well. The following diagram illustrates the humidity dependent changes of the most widely known negative factors.

The health care significance of air humidity

From the point of view of living-space and work place climate, beside temperature and air purity, humidity has a determining significance as well.

Under our geographical conditions, the risk of excessively humid environment has a relatively smaller significance. However, the negative effects of excessively dry air in the extraordinarily long, continental winter months are quite significant.

During the heating season ventilation is of no use to avoid excessively dry air. If we can’t reduce the “thirst” of the air in this period with artificial humidification, it will absorb water from the environment, from the skin, the mucous membrane, and from products manufactured from plants and wood.

The humidity of the air is measurable, which is signified by the relative humidity measurement number. The hygrometer, the measuring instrument of humidity, serves to measure this. Medically 40-60% humidityis recommended, in the case of which we perceive the air ideally humid. This value is primarily important from the point of view of our health, but it’s ideal for our plants, animals, furniture and wood floor tiles as well.

Relative humidity means, how much water the air contains at a specific temperature, compared to the maximum amount it’s capable of absorbing at that specific temperature. This is why temperature is significant, since the warmer the air is, the higher water content it can have (gr/m3) as well.

How relative humidity depends on temperature The correlation between the humidity of outside and inside air humidity example

In the winter, under the freezing point, air can only contain a maximum of 1-2 gr/m3 of water, which as it enters the living-space, would result in 12% humidity without humidification. In reality, as a consequence of supplemental moisture uptake (humans, plants, smaller humidifiers), this value generally doesn’t sink below 18-25%. However, it’s important to bring attention to the fact, that even this value is extraordinarily low.

 ·  unpleasant general sensation, reduced oxygen uptake and transfer into the circulatory system
 ·  increased risk of illnesses related to the common cold
 ·  dry skin
 ·  dust formation
 ·  damage to furniture, wood floor tiles, cracks

The characteristics of wood material related to humidity fluctuation, mentioned in the introduction, are prevalent in an even more increased manner in the case of wood floor tiles. Large surfaces are in contact with the environment, compared to the thickness of the material. In the case of prefabricated wood floor tiles, even the size of individual tiles grew.

Certain fluctuation can be observed depending on the conditions, under which the specific wood developed, but with some simplification and by rounding off the numbers its most prevalent value is 0.33%.

This means, what degree of shrinkage occurs with 1% of wood moisture reduction.

The moisture content of tongue and groove wood floor tiles is reduced from 8% to 5%, thus the probable size change will be:

0,0099 x 65 = 0,64 mm
3 x 0,0033 x 192 = 1,9 mm

This appears to be quite a frightening degree, but because of the three layered structure of prefabricated wood floor tiles, as a result of the stabilizing role of the centre layer, the shrinkage is reduced by approximately 70%. In accordance with this, even in the case of the least favourably behaving beech, in practice we can’t measure cracks wider than 0.5 – 0.6 mm per tile. In the case for instance, when the movement of individual tiles is restricted, because of a heavy piece of furniture, and in case several tiles move in unison without the creation of a crack (glued together), then these values add up.

In recent years, in predictably critical cases, fixation with glue to concrete has become more and more prevalent, instead of the floated tile laying method. In the case of floor heating we recommend this method anyway, because of the more efficient heat transfer. Perhaps, the practical realization originated from this, that this fixation method is more favourable from the point of view of the formation of cracks. It should be emphasised, that the formation of cracks can’t be avoided in this case either, under extreme climatic circumstances, but the earlier mentioned adding up of cracks is preventable.

This phenomenon is unpleasant for the owner of the apartment, but in the case of very dry climate it’s inevitable. During the summer months, with the increase of humidity the cracks disappear again. In this season the balanced moisture content may reach 11%, but this value doesn’t cause damage, because of the flexibility of wood.
Unfortunately, this concept often only means cooling in the summer heat, perhaps combined with mild re-moisturizing. We are already familiar with the significance of it during the winter, but smaller room humidifiers are most often available in stores. Their performance and controllability are questionable, as it was measurable in one of the apartments. Furthermore, the uniform distribution of the entered moisture is also significant.

There is an abundant supply of living-space humidifiers, but there are fewer which are of high quality, regarding both performance and controllability. The energy demand of humidifiers, which use heat for evaporation, is comparatively high, and warm vapour isn’t favourable from a biological point of view either. Those humidifiers which use cold, wet surfaces tend to have low performance. Ultrasonic equipment may provide the truly favourable solution. Without detailed knowledge of the professional field, according to the supplemented material, it appears that a new generation has emerged. Instead of the 40-100 kHz ultra-sound frequency, which was in accordance with the knowledge of physics till now, they operate at 1700 kHz. As a consequence of this the “l/hour” performance is considerably increased, with a mere 0.05 KWh energy consumption (supplement). However, cold humidifying equipment requires certain water pre-treatment (softening), especially in the case of harder types of water.

Considering an apartment with a 100 m2 floor area (2.70 m ceiling height), which has a 19% (meaning 3 gr/m3) relative humidity at 20°C, the total water content is:

100m2 x 2,70 x 0,003 = 0,81 kg

The achieve the ideal condition, 50% should be provided at 20 °C as well, so the total water content should be:

100 x 2,70 x 0,008 = 2,16 kg

The missing 1.36 kg doesn’t appear to be significant, but it only reflects a momentary condition. In an already dried out apartment there are a number of other moisture absorbent materials (textile, furniture, wall-paper), thus the actual “water shortage” is considerably larger than in the calculations above.

Our opinion is, that in the interest of protecting our health and property, in the professional circles of architecture and structural engineering this issue should be given much greater significance. Certain fields of modern printing, paper and textile industry couldn’t even operate without these today. And supply is abundant at speciality expos, in technical periodicals as well as on the internet.

Comment: The published data are indicatory values, originating from technical literature.