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    April 2021
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    Proper wood moisture is the basis for success

    Proper wood moisture is the basis for success.

    Each construction material has its own individual properties, which determine its application. In addition to its positive properties, the material also has undesirable properties, which should also be taken into account, e.g.. glass is brittle, thermoplastic polystyrene, heavy steel etc.. Wood also has some undesirable properties, which primarily include the change in volume under the influence of changes in humidity. Is said to be, that the wood "works".

    Proper wood moisture is the basis for success.

    If we are processing wet wood, the result will usually be bad, and the product is worthless. It is figuratively said: “Yesterday in that tree the birds in the forest sang, and today we process them ". This, of course, is not entirely true; the drying process cannot be ignored, nor accelerate abnormally, even when using a dryer.

    Wood moisture is defined as a percentage, as the ratio of the weight of water contained in the wood to the weight of absolutely dry wood. If e.g.. the mass of the measuring sample is 12 g, and after daily drying at 105 ° C (when the weight is no longer falling) it is 10 g, it was in the sample 2 g of moisture, or 20% dry weight. The humidity of the sample is therefore 20%.

    What is the correct humidity of the treated wood?? It is impossible to answer unequivocally, because the humidity of wood intended for different purposes should be different. It is not, as many say, that the drier the wood, the better. Wood is a hygroscopic material, which reacts primarily to relative air humidity and temperature.

    Overdrying, i.e.. drying of the wood below the normal value, when drying in dryers, it has already caused more than one damage. For example. cladding a summer house with dry larch boards in the autumn period resulted in warping and finally detachment from the structure. In another case, the paneling in width was warped 5,5 m in the stairwell (Lynx.).

    Drawing. The effects of covering the wall with a lining of excessively dry boards: a) carpet after building the wall, b) warping of the floor covering after 3 months (caused by excessive drying of the material in the dryer).

    In both cases, the cause was not drying the wood in a dryer, but improperly selected drying conditions, which caused drying below the desired value. In the first case, the humidity should be 15% (and it was probably below 10%), in second 10% (and she was below 7%). Wood that is dried long enough is better for lining the outer walls of rooms, naturally (just like lumber is dried in stacks with spacers, with possible roofing, outdoor; but never in a shed or barn).

    If for a sufficiently long period of time (e.g.. several weeks or months - it depends on the thickness of the material) The dried wood is influenced by a certain relative air humidity and temperature, then the moisture content of the wood will be set at the level of. hygroscopic equilibrium.

    In living quarters, in which the relative air humidity is 50%, at temperature 20 ° C, the moisture content of the wood will settle at the values 9,6%. In flats with central heating, the air humidity drops down to 30%. This humidity, at 23 ° C, he only answers 6,1% the equivalent of wood moisture. In summer, the relative air humidity in the apartment may reach approx 60%. At this air humidity and temperature 20 ° C, the equivalent humidity of wood is 11,4%. Therefore, wood humidity varies from 6,1% to 11,4%.

    The change in the moisture content of the wood ultimately affects the change in volume; another in the direction perpendicular to the course of the fibers, another in the direction along the grain. The directions of wood drying off are shown in the figure.

    Drawing. Determination of tangent sections (A) and radial (B) according to the course of annual increases: a) tangent direction, b) radial direction, c) longitudinal direction.

    Along the grain, the wood dries very little and can be skipped (e.g.. larch 0,3%, oak 0,4%).

    The value of drying off with loss of moisture is not the same for all types of wood.

    Blackboard. Maximum drying out of various types of wood when dried to 0% humidity.

    Kind of wood Drying out,% on the cross section
    tangent radial longitudinal
    spruce 7,8 3,6 0,3
    pine 7,7 4,0 0,4
    larch 7,8 3,3 0,3
    oak 7,8 4,0 0,4
    buk 11,8 5,0 0,3
    ash 8,0 5,0 0,2
    clone 8,0 3,0 0,5
    birch 8,0 5,3 0,6
    grab 11,5 6,8 0,5

    W tablicy podano wartości zsychania się różnych gatunków drewna przy wysuszeniu do 0%. Examples of drying wood width 120 mm with moisture loss 3%, 5% i 7% are given in the table.

    Blackboard. Drying out wood width 120 mm depending on the different drop in humidity.

    Wood species and cross-section The wood is dry in width 120 mm depending on the drop in humidity, mm
    13—10% 13-8% 13-6%
    Oak - radial section 0,48 0,80 1,12
    Oak - tangent cross-section 0,94 1,56 2,18
    Beech - radial section 0,60 1,00 1,40
    Beech - tangent section 1,42 2,36 3,31

    The period of winter is critical for all wooden structures located in rooms with central heating, when the wood can dry up to 6%. As a rule, the following undesirable effects are associated with such a large drop in humidity:

    - deformation of veneered ceiling elements (drawing), door, wardrobes, cabinets etc.,

    Drawing. Deformation of the floor covering made of veneered elements of a larger format, due to a sudden loss of moisture on one side, inaccurate fixing of the plates, cracking of the varnish coat or one-sided wrapping.

    - cracks in the veneer glued on the wrong chipboard or carpentry board with an insufficient right angle gluing individual layers of the board,

    - paneling deformation (the picture on the left), steps of the stairs (the figure on the right) e.t.c. solid wood,

    Figure on the left. Deformation caused by the use of undried boards and a decrease in their humidity during the heating season: a) tangential section of the board with the right side on top, b) tangential section of the board with the left side on top, c) radial section, non-deformable with changes in humidity.

    Figure on the right. Stair deformation caused by lining with non-dried material (it is more pronounced in the heating season). Tangential section of a board (a) the step also deforms the radial section of the board (b).’ Część desek (c) cut radially does not deform

    - twisting in different directions, especially massive elements.

    These effects are not always preventable; they can only be limited by various treatments. When choosing connectors and all structural elements in general, we will try to prevent negative phenomena.

    If we convert the given changes in the humidity value to the width 120 mm of oak plank, we will find it out, that its width will periodically change from 120 mm in the later summer to 119,4 mm in late winter. This variability should be taken into account, which we will mention in the article about solid wood panels.


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