Humidity in Wooden Carcass Houses

One of the effects of moisture is mildew. Photo:


At the same time to the existing tendency of taking care about the energy efficiency (about this read more in an article – Energy Efficient house in Latvia) and house hermetic, insulating and providing houses with new and new heating isolation materials, increases problems, which are connected with humidity. Why does it happen? Does it make sense to refuse of building wooden houses, because wood obeys to humidity? But now, so many people care for the house to be CO2 efficient, built from a wooden frame! Should we refuse the insulation of houses to avoid moisture-induced mold? No, in fact, the root of the problem is to be found elsewhere. Where? Read about this in this article.

Formation of humidity in wooden houses

The reason for the inner humidity formation are activities of the house livers (dishwashing, cooking, bath, and shower etc. activities with the water). External humidity comes from precipitation, groundwater and irrigation systems. The humidity also is in all of the building constructions, concrete, wood etc. materials during the construction, but it has to be fired off when the building process is finishing. The outer construction of the building, roof, and walls, are especially influenced by the rainwater. The humidity, which circulates in inner rooms, has to be regulated for not forming condensation accumulation and construction damages. Humidity is the problem not only for wooden constructions it also forms rust in metal and cracks in concrete. So, it is an actual question in all of the types of private houses. This occurs without observing the water vapor permeability indicators of building materials, creating solutions that can form water vapor condensation and limited humidity drying capacity. An unwanted cause in the process of the development of the condensation could be also constructive resolutions and material connections, which are formed to meet certain architectural requirements.

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There are four types of humidity prevalence: liquid type, capillary, air flow, and diffusion.

  • Liquid type moisture spread: in rain, watercourses.
  • Capillarity is the movement of liquid water in porous materials caused by surface-pressure forces. Capillarity or capillary suction may also occur in a small room formed between two materials.
  • Airflow refers to the movement of water vapor caused by air flow through rooms and materials
  • Diffusion is the movement of water vapor caused by the difference in vapor pressure.

The prevalence of the humidity in liquid and capillary way influences the building the most, an explanation is that builders pay the most attention to protect homes against precipitation and groundwater. Airflow and diffusion are considered less important and responsible for the occurrence of moisture problems

Humidity in dependence on the environment:

  • Hydro-thermal regions
  • Very cold
  • Cold
  • Partly humid
  • Hot humid
  • Hot dry or partly dry
  • Rainwater zones
  • Extreme (up to 1524 mm a year)
  • High (from 1016 to 1524 mm a year)
  • Low (lower than 508 mm a year)
  • Interior climate classes
  • Uncontrollable (garages, stores, storehouses)
  • Adjustable (houses, flats, offices, schools, commercial and retail rooms)
  • Controllable (hospitals, museums, swimming pool corpora, and the computer equipment).

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How to get off the humidity?

There are four ways, how to get off the humidity: drainage, drying, diversion and the use of durable materials. Diversion of the rainwater (by using gutters and rinse) applies to the water diversion from the surface of the building, but the other methods are used, when projecting the house for avoiding the humidity, when it somehow got inside.


The method of the diversion is a general part of the projecting, which historically has proved an effective way, how to reduce the rainwater influence on the outer walls. It includes:

  • to place the building in the way to prevent the dominant winds,
  • providing a large roof cover and water collection device near the walls of the exterior walls,
  • providing architectural details that collect rainwater.

The only thing, which can provide a long lasting durability for the wooden carcass building, is a roof with enough wide overhang. In the method of the diversion are also included window sills. Cladding and hermetic are also protection mechanisms.


Drainage is the next rain dismissal method after the diversion method. Building construction models, which include the principles of drainage, include horizontal elements of an inclined roof and inclined surface usage. Then drainage is used, when collecting incidental moisture savings in the wall structure and using the gravity flow, returning it to the outside surface of the cladding or even outside it. The easiest way to get this result through the mounting process by adding a drainage plane between the cladding and the sheath. In the wooden frame construction drainage plane usually consists from the humidity barrier (building paper, felt and home casing), and, what’s the most important, it works together with a window and door decoration. Drainage usually is the main wall construction release tool.

The drainage hole is a more complex feature, which introduces the air space between the cladding and the plane of drainage. The airspace serves for forming of capillaries by preventing an excessive accumulation of the humidity in the drainage plane. The air space provides also diversion method by removing the humidity through the cladding drainage holes.


This method releases walls from the humidity with ventilation and steam disperse. It has to be taken into an account all the characteristics of the cladding and carcass materials to promote airflow circulation. The coating permeability, moisture barrier, vapor barrier, and interior finishes will have a significant effect on the overall drying potential of the wall. This is an area where researchers are currently engaged.

Long lasting materials

They are used everywhere, where the level of the humidity is higher, where above-mentioned methods – diversion, drainage, and drying cannot be effective to reduce the level of the humidity lower than 28%, there is a necessity to upper the durability of the timber. In the variation of the wooden carcass house frame, it has been achieved by processing the frame under pressure with wood preservatives. Usually, the wide variation of this method is processed logs usage in connection with foundations.

The house has to be considered from the point of long-lasting in case of usage of any of these methods. Intermediate usage of different materials has to be considered carefully because the good microclimate of the house is not in the walls and a roof, an important thing is the usage of the materials and their intermediate connection. Decisive is not the only quality ratio of each material, but also the quality of the work done and an ability to take a responsibility for it. To avoid a temporary result problem, adequate and coordinated work quality control measures are required in all processes of the wooden frame building – design, planning, construction, management of the structure, etc. until the building is put into operation. It is not a secret, that with a limited budget during the project processing and building there are less qualitative and cheaper labor force, less qualitative and untested materials. That’s why you That’s why you have to keep up with a quality ratio, quality control during the process, including water vapor permeability observations in building elements.


How important is the evaluation of the building humidity?

Construction standard LBN 002-15 “Thermal engineering of building envelope constructions” requires the assessment of water vapor permeability in building elements. The building elements of external building blocks are exterior walls, roofs, loft covers, ceilings that come in contact with the outside air (also above the passageways), floors over unheated paving stones, cold pits and floors to the bottom, basement exterior walls facing the outdoors air or soil, outside windows, doors and gates, as well as internal walls and other surfaces, if they are confined to rooms with an air temperature difference of 5 ° C or more.

In many cases, both the client and the author of the work believes, that an in-depth study of moisture transfer processes is unnecessary and only prolongs the project’s time-to-cover. Unfortunately, it means, that the risk of the condensation is not being redeemed, which due to time identified, could be avoided. Damage to the damage caused by invisible steam generates several tens of thousands of people, not to mention the public health problems caused by mold fungus. So the opinion, that steam can not cause visible damages, is a fool. This has to be considered during the projecting and investigation.

Humidity and wooden buildings

Timber and water are compatible definitions. Wood is a hygroscopic material, which means, that it can give or absorb humidity to achieve to the level of humidity being in the air. As a part of this natural process, wood can absorb a big amount of water, before it achieves a level of humidity, which is favorable for fungal growth. To provide durable wooden carcass houses, the design construction and also envelope construction (envelope structure – outer walls and roof constructions) should be based on the understanding of the factors, which influence humidity content and changing, which form because of the changing of the content of the humidity.

For the control of the humidity, you have to check all the building process. Despite the fact, that at the object is delivered an ideal humidity-dry timber, during the building process it can take up too much humidity. That’s why, to avoid it, there are some events, which are recommended to do:

  • all the timber materials to maintain dry, until they are at the storehouse;
  • minimize installed material humidity;
  • at the building site to provide ventilation and drainage of the materials.

When installing wooden carcass or wooden panels at the building site, where they have exposed the humidity, they should be covered with a moisture barrier as soon as possible. This will protect the overall structure from the effects of weather conditions, while at the same time, adequate ventilation, heat carriers, etc. should be provided at the facility.

A comprehension about the humidity content in the timber has a significant meaning, because:

  • different level of the humidity content causes shrinkage and swelling;
  • high level of the humidity can cause the formation of the mold and fungus.

Humidity content

  • 19%. We can call the timber “dry” if its humidity content is 19% or less. This type of the timber is recognized S-DRY (surfaced-dry), which is obtained in a dry way or became during the production process.
  • 28%. It is a middle fiber saturation point for wood, in which all the wood fibers are completely saturated with water. The collapse of the wood starts, when the humidity level during the long period exceeds fiber saturation. Fiber saturation point is wood swelling.

Wood thickens or swell, when the humidity content changes, but only when the water is drawn out or separated from cell walls and when the wood changes the moisture content below the fiber saturation point. Indoor wood stabilizes 8-14% moisture content; outdoors 12-18%.

The less is humidity in a wooden material, the more stable is material with less opportunity to shrink by saving more or less the same size, as in the mounting process. To avoid the Shinkle risk, sometimes there are composite materials, for example, OSB plates, wedge-jointed timber, I-beams, structural composite materials, which are produced by interlacing smaller pieces of wood. When gluing pieces of wood, they are directed in different directions to compensate for the different directions of expansion or contraction in one plate.

Wood and damages

As far as wood is the natural material, which biologically decomposes, it is a perfect environment for different fungus, mold, crops. Mold does not make structural damages for the timber, but crops and fungus do.

How to get off the humidity?

There are various air dryers and moisture collectors that can help convert moist air into water, which can be drained from a special reservoir. There are no need to spend a lot of money installing humidity collector. Small dishes are also available with an immersion salt tablet that over time absorbs moisture. Larger electrical appliances also automatically control the required mitral level in the room by draining air and storing the condensate in the tank.

To provide timber against humidity exposure – mold, fungus, rotting and insects (crops etc.), there are three preventive actions: double proceeding of the timber with special impregnation aids, spray a wooden panel house environment by preventing appearance of the mold and microbiological organism, and proceeding of the timber under the pressure by preventing mold and termite formation.

Double woo impregnation

This method is the most common producing type, which protects timber both against mold and insects, rotting. The first layer of the timber impregnation provides first of all the protection layer for all the timber edges (lateral edges and logs ends) by providing an ability to reject water, but also at the same time by letting humidity to evaporate from the timber, by letting the wood to breathe and maintain normal humidity level. So the diversion of fungus also happens. Impregnated timber has a durability to extreme weather conditions and the construction stability. The next layer is covered with DOT  (Disodium Octaborate Tetrahydrate) fungicides and pesticides (registered by the EPA), which is mainly for insect repellency. The timber is processed in a factory and delivered for the building ready. So, it saves time for builders, because there is no necessity to proceed and later to check the timber. Proceeded this way, the timber can stay under an open sky until six months. As far as each timber material element and its edges are individually processed, this way wooden carcass is fully protected from the humidity and its problems. In a wooden carcass structure, it is possible to process all the structure elements – carcass frame timber, plywood, OSB plates, panel intermediates, fastenings, laths, and trim boards. Consequently, both the roof and wall and floor frame systems can be protected.


Fogging, spraying of the environment

There is a wide range of the ability to process an environment against the mold and to destruct it. Spraying uses chemical treatments, so after the spraying process, construction can be restarted only after several days. There are many variations in terms of both chemical processing and application techniques. The effect is estimated at about 25 years, but it affects only the mold, it does not provide protection against insects and rot. Similarly, this method does not destroy mold, it only stops the propagation of molds. Moreover, if the method is applied after the completion of the home, it does not access many structures that require processing.

Processing under the pressure

This method has 70 year old history. Its advantages are in the result of the pressure to get inside the layers of the timber by protecting the detail from the rotting, termits and crops. Timber material is placed in a large-sized tank in which the pressure is reduced, with the aim of ejecting all air from the wood. At the moment, when in the tank is high pressure, there is stuffed a pretection aid, so it goes inside a timber. This processing makes timber unpardonable for insects and very durable to mold. Such processing is included in the building codes, certain building blocks of the timber frame, in particular the lower boards – the frame of the lower curtain guard (sill plate). As far as the construction is on the lower crochet, it carries a huge load, so it has to be especially durable. In regions, where the concentration of termits is especially high, builders use the processing under the pressure not only for the lower crochet, but then a construction becomes heavier, so it is harder to work with it. The processing under the pressure plans to material be durable about 20 years in very extreme conditions.