What is the board's natural humidity? Natural moisture content of wood

In today's article we will explain how important the natural moisture content of wood is in the manufacture of log houses.

Log houses made of natural moisture logs

IN last years wooden architecture is becoming more and more popular and in demand. The desire of people to live in a house made of natural, environmentally friendly safe material understandable and natural. In addition, wood is a natural building material in Central Russia. The body is in perfect harmony with natural material, and wood helps get rid of negative impact hectic life of city residents.

When planning to build, you need to understand that log houses with natural humidity are durable and reliable, this is confirmed by centuries-old structures scattered throughout Russia, once made by our ancestors and easily withstood changes climatic conditions, wars, snowfalls, storms and floods. The masters of ancient architecture unanimously assert that it is necessary to build a wooden house exclusively from the natural moisture of wood.

In old times log house was built only from logs with normal natural humidity. Experts have long noticed that such a structure better withstands any disasters and sudden changes in weather conditions. The house will not shrink; the moisture content of naturally drying wood allows the walls to breathe normally without gaining excess moisture. A tree, like any living organism, requires a lot of water for normal existence. Logs used to build a house must contain an optimal amount of moisture, allowing the structure to remain strong and monolithic.

Log houses with natural humidity, advantages:

Existing types of lumber

1. Wet wood with one hundred percent humidity occurs after a tree has been in water for a long time. Such material is strictly prohibited from being used in any construction.
2. Freshly cut wood natural humidity has in its structure from fifty to eighty percent moisture. The parameters directly depend on the time of year and the location of the felling.
3. Air-dried wood has a humidity of no more than twenty percent. This material, as a rule, is stored for a long time in the open air under natural changes in weather conditions.
4. Room-dried wood it is stored for a long time in covered warehouses and the humidity does not exceed ten percent.
5. Completely dry wood obtained by forced drying in special rooms, the moisture content of the lumber does not exceed two percent.

How important is the moisture content of naturally dried wood during construction?

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Wood is a very hygroscopic material that easily changes its humidity. The moisture content of wood is the percentage of water (moisture) in it. The moisture content of wood does not depend on the type of wood. Wood moisture content is quantitative indicator moisture content in it

Wood moisture content

Moisture exchange occurs all the time between wood and air. Therefore, the moisture content of wood is a very unstable value, which changes along with the humidity of the environment. If the humidity of the wood is greater than the humidity of the surrounding air, the wood will dry out. If it's the other way around, it's hydration. And if the humidity and temperature of the environment (air) remain constant for a long time, then the humidity of the firewood will also stabilize and will correspond to the humidity of the surrounding air.

Humidity of wood at which the exchange of moisture between it and environment, called “equilibrium”

In nature, equilibrium moisture content for wood is an extremely unstable state. Because in nature it is impossible to find air with constant temperature and humidity parameters for a long enough time. However, the state of equilibrium humidity is easily achieved for wood located in an artificial microclimate, for example, in a drying chamber or simply in any other room with constant temperature and humidity.

Distinguish between absolute and relative humidity of wood

Absolute humidity of wood

Absolute humidity is the ratio of the mass of moisture that a wood sample contains to the mass of absolutely dry wood of the same sample. According to , the value of absolute humidity (W) is calculated after examining (drying) the sample, according to the formula:

W = (m - m 0) / m 0 x 100,

where (m) and (m 0) are the mass of the sample before and after drying.

The concept of the value “absolute humidity”, according to GOST 17231-78, is interpreted simply as “humidity”. Like everything “absolute”, the value of “absolute humidity” is divorced from the real world and is an extremely indigestible form when thermotechnical calculations. For example, at an absolute humidity of 25%, a kilogram of wood will contain 200 grams of water. This discrepancy in numbers confuses calculations.

The relative humidity value is more convenient and practical

Relative humidity of wood

Relative (working) humidity of wood is the ratio of the mass of moisture that a wood sample contains to its total mass. According to GOST 17231-78, the value of relative humidity (W rel.) is calculated from the value of absolute humidity (W) of the sample, according to the formula:

W rel. = 100W / (100+W)

or more simply,

W rel. = m water / m sample x 100

Relative humidity is a very simple and convenient form for taking into account evaporated water in wood-burning heat engineering calculations. The value of relative humidity directly indicates the quantitative water content in wood. For example, one kilogram of wood with a moisture content of 20% will contain 200 grams of water and 800 grams of dry wood matter.

For comparison, let’s put a “live” example into a table. This is a table for the same our sample. Let us determine and compare the values ​​of its absolute and relative humidity:

Wood moisture level

According to humidity, all wood is divided into three groups: wet (humidity more than 35%), semi-dry (humidity from 25 to 35%) and dry (humidity less than 25%). Initially, the humidity of freshly cut trees is 50-60%. Then, during natural drying under a canopy in the air, the wood loses up to 20-30% of its moisture over the course of one and a half to two years and reaches a state of relative humidity. After this, the moisture content of the wood no longer changes significantly, and its value is ≈25%. Such wood is called air-dry. To reduce the moisture content of wood to a room-dry state (7...18%), it must be dried forcibly in drying chambers, or moved to long time into an artificial microclimate with specified conditions (for example, move it to a room or other premises).

There are the following degrees of wood moisture content:

• Splavnaya(humidity 60% or more)
  This could be a tree that has been in water for a long time. For example, driftwood, or wood after sorting in a water basin, or simply a well-wetted (damp) log.
• Freshly cut(humidity 45...50%)
  This is wood that has retained the moisture of a growing tree.
• Air dry(humidity 20...30%)
  This is wood that has been kept outdoors for a long time, with good ventilation.
• Room dry(humidity 7...18%)
  This is wood that has been in a living room or in another heated and ventilated room for a long time.
• Absolutely dry(humidity 0%)
  This is wood dried at a temperature of t=103±2°C to constant weight.

Calorific value of wet wood

The calorific value of wood is directly dependent on its moisture content. The moisture content of firewood is a determining indicator of its quality. That dry wood burns better than wet wood is known to many, if not everyone. And everyone knows that wet firewood can always be dried, and dry firewood, on the contrary, can be wetted. Accordingly, the quality of the fuel will change - improve or deteriorate. But is this really important for modern heating equipment? For example, wood-burning pyrolysis boilers allow you to burn wood with a humidity of up to 50%, and even up to 70%!

The table shows generalized indicators of the calorific value of wood for each degree of its moisture content.

The table shows that the lower the moisture content of the wood, the higher its calorific value. For example, air-dried wood has a working calorific value almost twice as high as freshly cut wood, not to mention wet wood.

Wood with a humidity of 70% or higher practically does not burn.
Perfect option For wood heating- this is to use firewood in a state of room-dry humidity. Such firewood provides the maximum amount of heat. But, since drying firewood to such a state is associated with additional energy costs, the most the best option For heating, air-dried wood will be used. Bringing firewood to an air-dry state is relatively easy. To do this, it is enough to prepare them for future use and store them in a dry, ventilated area.
Finally, I would like to note that the moisture contained in firewood not only worsens its calorific value. Increased moisture content in fuel negatively affects the combustion process itself. Excess water vapor serves as the basis for creating an aggressive environment, which causes premature wear of the heating unit and chimneys.
Manufacturers of modern heating equipment recommend using air-dry wood as fuel, with a humidity of no more than 30-35%

From logs of what moisture content can reliable and warm log houses be created? Experts believe that for construction it is necessary to choose only material with natural moisture. Why?

Secrets of the ancient wooden house construction, They say that a log house should be built from logs of natural moisture. And this is not surprising, because such material has many advantages that can be justified from the point of view of modern data. Although it must be said that durability log houses built in antiquity, which have been standing for several centuries, is in itself proof that the ancient masters were right. But still, why is the natural moisture content of wood so important?

From properties building material, in particular, now we are talking about wood, they strongly depend strength characteristics Houses. When the air humidity is unbalanced, the tree takes or gives away moisture, and its diameter changes. So, with high humidity, logs absorb water and increase in volume, and with dry air, the tree dries out. If there is a sharp change in air humidity, stresses arise inside the log, which cause cracking and deformation. To avoid this, it is necessary to control the moisture content of lumber during production (at all stages).

There are the following types of lumber:

• Wet wood. Its humidity is almost one hundred percent. This happens when lumber for a long time stayed in the water. Such material is never used in construction.
• Wood of natural moisture, recently felled. The moisture content of such lumber depends on the type of tree, the time of felling and ranges from fifty to eighty percent.
• Air-dried lumber. This tree has been stored outdoors for a long time, its humidity ranges from fifteen to twenty percent.
• Room-dry lumber. This is a tree that has been stored in a warehouse for a long time, its moisture content is from eight to ten percent.
• Completely dry lumber. This wood, after forced drying in special chambers, has a moisture content of no more than two percent.

Experts say that if logs for a house kit are subjected to forced drying or stored unassembled for a long time, then they can become deformed.

The tree loses its moisture until its humidity reaches a certain level. The level depends on atmospheric conditions. The same process occurs when moisture is absorbed (sorption).

If completely dry lumber is taken outside, brought to the construction site, it will begin to absorb moisture from the air, which will cause it to swell or bend. Practice has shown that a house built from dry wood does not shrink evenly.

Unlike natural moisture log construction, A log house made from forced-dried logs does not decrease in size (dries out), but increases (swells). In this case, the walls bend outward, and the crowns and roof may diverge. To completely remove the consequences of this process is not easy, expensive and often unrealistic. Even pine, which is considered a strong wood, loses its strength by up to six percent, while its toughness increases by an average of ten percent.

If the house set is made from logs of natural moisture, then the decrease in humidity in the tree occurs gradually. At the same time, the volume of the house decreases; under the influence of its own weight, the crowns fit tightly into place. As a result of the tight fit of the logs, the walls are blown out less, which means you will need to spend less effort and money on insulation.

When constructing structures made of wood with natural humidity You can’t do a rigid fixation. For about two years, the main shrinkage of the house structure takes place, which must not be interfered with. Shrinkage can range from three to seven percent - the amount depends on the type of wood and environmental conditions.

It is important to remember that shrinkage and swelling of lumber along and across the fibers occurs at different rates, therefore, all vertical elements of the house must be equipped with compensators.

Using compensatory devices, you can adjust the height of the house and geometric proportions. For reliability, the crowns are attached to each other using wooden dowels, which are mounted in a checkerboard pattern one and a half meters apart.

When craftsmen create log houses, they remember that shrinkage is an uneven process. It can go differently outside and inside the house.

Therefore, during the production of a house kit, technological gaps are made that prevent an initial tight fit along the groove, because this subsequently causes large gaps to appear.

There are many secrets to building log houses and only experienced craftsmen can build a warm and reliable home, which will last for several centuries.

As a material, wood for building a house is distinguished by its strength, durability and longevity. IN suitable conditions it persists for many centuries. They are dying wooden buildings not because of old age - the excessive moisture content in the wood has a detrimental effect on them.

There is water in a living tree, just like in any natural material, is required. Once cut down, it begins to dry out, losing moisture and losing weight. But in natural conditions, in the rain or snow, this process takes more than one year. Therefore, wood for building houses must be dried forcibly.

Bound and free moisture

The relative humidity of wood for building houses is measured as the ratio between the value of the original weight of the tree and the weight of the completely dry tree. In the structure of the material, moisture permeates the cell membranes (this is bound, or hygroscopic moisture) and fills the cell cavities and intercellular spaces (free, or capillary moisture).

When drying, free moisture first evaporates from the material, then the bound moisture gradually leaves. IN industrial production This process is accelerated by using artificial drying of wood for building a house in special chambers.

3 dangers of using wet wood to build a house

1. Damage to wooden structures. Wood has an uneven structure, so the results of drying it may vary. In addition, when purchasing, there is a chance that you will end up with the product of a careless manufacturer who does not comply with technological requirements.

   The use of under-dried wood for building houses is fraught with deterioration in the quality of wooden structures and even their complete destruction. Excessive moisture content leads to changes in the shape and size of structures and products. The wood expands, swells, warps or bends.

2. Rotting of wood and finishing materials. In addition, wood that is not completely dried will dry out naturally, releasing moisture to the surrounding air. If the tree is “hidden” behind finishing and insulation materials(for example, laid as a floor beam), then the water will have nowhere to go, and it will begin to settle on the surface of the wood (condense). Eventually, the moisture will seep into the ceiling or walls, creating dark and unsightly stains.

   The accumulation of condensation over time will lead to the formation of fungi and mold, and the appearance of rot. Over time, they destroy wood cells and cause cellulose to decompose and split. If the process is not prevented or stopped in a timely manner, then in a few months the rot will “eat through” the tree. As a result, the beam or truss structure will collapse, pulling other construction and Decoration Materials.

3. Deterioration of the microclimate. Increased relative humidity levels in wooden house largely levels out positive traits such a structure. Instead of a healthy atmosphere, the homeowner receives a microclimate in which pathogenic bacteria develop faster. And mold and fungi can cause allergies, especially in children.

The danger of low wood humidity in house construction

At the same time, it must be understood that absolutely dry wood can only be used in furniture production(for indoor use). All wood for building houses must have a certain (equilibrium) level of moisture. With this indicator, the material does not release or absorb moisture and reliably serves for many years.

Solution to the problem. How to dry wood in a built house

To avoid problems in the future, you should not immediately make repairs in a newly built wooden house (made of logs or laminated veneer lumber) and do not bring furniture into it. The building must “stand” for at least six months. During this period, the structural wood will finally dry out and gain the necessary strength.

However, if over time in a finished and lived-in house in wooden structures Problems with moisture content are detected (stains on the ceiling, the smell of mold or rot), then the wood will need to create conditions for drying, that is, provide access to air.

To do this, you will have to dismantle all the finishing and insulating materials that cover the problematic structure. In such conditions, the tree will dry out quite quickly, within three to four weeks. Under no circumstances should you speed up the process using heaters or hair dryers. Mold or fungi found on wood must be removed, the surface cleaned and coated with a protective antiseptic composition.

Tree - living plant, which naturally absorbs water to support life. Recently cut, it contains a significant percentage of moisture, different breeds it is individual. However, this value must be taken into account when purchasing material in order to understand whether the wood is suitable for further processing and use or still needs drying.

Types of humidity

Absolute - the ratio of the mass of liquid in a volume of wood to the mass of a dry sample of the same size.

Relative shows the amount of moisture relative to the mass of the same sample of wood in the same condition.

In natural materials, bound and free moisture coexist:

• The free one is located in the intercellular space and makes up the arteries of the tree. This is easily removed during drying and does not affect the quality of the workpiece;
• Bound is contained in the cells of the tree.

Norms

The moisture content of wood species is regulated by regulatory documents:

• SNiP II-25-80;
• SP 64.133330.2011;
• GOST 16483.7-71*;
• GOST 17231-78.

There are moisture standards for wood according to the class of material used. For each individual species, the value is difficult to determine: in what conditions the plant grew, what season it was, in what area the felling was done - all factors are reflected in the amount of moisture in one individual. Average values:

Wood moisture content: the norm according to GOST is determined for ready-made structures V different conditions operation.

Humidity determination

To determine what percentage is the natural moisture content of wood, you should use one of the methods:

1. The simplest and most common is the use of a moisture meter - electrical appliance, whose work is based on measuring the electrical conductivity of the measured raw materials. 3 needle electrodes are inserted into the wood, the device sends a discharge through them and displays the desired value on the screen.
2. Four-needle chip probes are based on the same operating principle.

How to determine wood moisture content without a moisture meter

An accurate calculation with a measurement error of no more than 1% is described by GOST 17231-78. Simplified algorithm:

1. Take a piece of wood and weigh it.
2. The sample is placed in drying chamber with air temperature +103°C until constant mass is obtained.
3. The dried section is cooled to room temperature and weighed again.
4. The moisture content of the source is determined using the formula:
   • W=(m-m0)/m0*100%, where m and m0 are the mass of wood before and after drying.

Determination by instruments can give an error of up to 10%:

• The electrodes penetrate to a shallow depth, and the water in the barrel or product is unevenly distributed. As the top layers dry, they become drier.
• Each breed deals with moisture differently.
• Specific electrical resistance different types trees individually; universal devices do not take this into account.

The methods described by GOST are cumbersome and time-consuming; determining the value can take several days and requires special equipment.

Experienced craftsmen determine the moisture content of wood by its appearance:

• According to the color of the breed;
• According to availability ;
• For warping, etc.

You can determine the readiness of wood for processing yourself:

• According to the quality of the bark: in a wet tree it is rich and soft, during the drying process the bark becomes coarser;
• At the place of cutting: the fresh plant is damp to the touch;
• Regarding the chips: when compressing the bundle, they should not stick together - such chips are not ready for processing, the product will shrink significantly.
• The presence of evenly distributed shallow cracks indicates the maturity of the material, deep cracks indicate drying out and heterogeneity.