Wood is hydroscopic material. It means that wood always seeks to reach balance between its own humidity and relative air humidity. When relative air humidity rises the wood absorbs its moisture and increases in volume (expands), contrastingly, when relative air humidity decreases the wood evaporates moisture and contracts. The recommended air humidity in the room with hardwood floor shouldn’t be lower than 35%.
When boards are delivered from the factory their humidity correspond to air humidity in the premises with central heating. During winter time when the central heating is on and absolute air humidity outdoors is very low, relative air humidity indoors falls that can cause narrow gaps to appear between parquet boards. During spring and summer time when relative air humidity indoors rises again, the gaps are disappeared in themselves. It indicates that parquet is made from natural materials and it doesn’t contain any synthetic components.
Taking into consideration the fact that wood is hydroscopic material, the relative air humidity indoors must be maintained whenever is possible within 40-60% at air temperature 18-24°C. It’s worth to mention that these conditions are ideal not only for your hardwood floor but for you as well.