Where does the moisture in your home come from?

This is one of the most frequently asked questions. The moisture in the house seems to be one of the biggest problems for the bigger part of the people nowadays. It is not harmful itself – the mold, which finds its ideal environment for development on damp walls, floors and ceilings, is harmful. It causes respiratory diseases, allergies and skin diseases. In order not to get there – the moisture has to stay out or as soon as possible to be taken there. For this purpose, this brief introduction to the construction physics will help you understand where moisture comes from.


In what ways the moisture enters our home?

From the roof to the basement, there are numerous possibilities that allow the moisture to penetrate into our house.

Moisture from external influences

1. clogged or leaking gutters 2. displaced or damaged shingles 3./4. defective connections with chimneys or ceiling windows 13. leaks from water mains or drainage 14./18. missing or damaged horizontal waterproofing against capillary moisture penetration 16. flooding, broken pipes, pouring rains 17. missing or damaged (perminetral) waterproofing of the underground walls 21. side rains and running water in the bioactive zone (it starts 50 cm under the ground and spreads up to 40 cm in height above the ground)


Moisture condensation

5. condensate in the steam barrier 6. moisture in the exhaled breath and perspiration of people 7. cold interior walls 8./9. thermal bridges on the balcony and window’s  transom 10./11./12. cooking, bathing, washing 15. pipelines for cold water and ventilation 19./20. plants and animals


When moisture comes from outside

Moisture penetrates from the outside in most cases when something breaks, is blocked, crashes or it is not well done. This can be a displaced or damaged shingle or leaking flat roof. Often rain penetrates through cracks and openings in the connections of chimneys or roof windows. A clogged or damaged gutter is able in a very short time to flood the facade. Cracks in the facade are vulnerable to adverse rains and gradually moisten the wall more and more, with which the thermal insulation effect is drastically reduced. This leads to cooling of the inner walls to such an extent that the moisture and mold on them are programmed. Very dangerous are the small leaks in the pipelines, which are in such place that is not visible, and remain unseen for a long time. Successfully controlled leaks and drainages can also have later unpleasant consequences, if the floor coverings and insulation are not carefully dries. Little impact has the owner when the basement is flooded. To prevent mold after that, as soon as possible measures to dry it must be taken. Very often the cause of mold in the basement is poorly made horizontal and perimeter waterproofing if the foundation.


When the moisture comes from the inside

Very often humid placed are formed as a result of condensation of moisture, which is contained in the air on cold surfaces. The reason for this is poor ventilation and /or poor thermal insulation. Along with the poor thermal insulation and the insufficient vapor barrier, there are a few trouble spots, such as convex concrete and steel structures of balconies and staircases. Also often the moisture damages the lintels of the windows due to their insufficient thermal insulation. The second reason for damp walls lies, as we already mentioned, in the improper and insufficient ventilation. It is from extreme importance especially where there is a lot of water – such as in kitchens and bathrooms. Furthermore poorly heated service premises and basements are also vulnerable due to improper ventilation. It should not also be underestimated the influence of moisture from plants, pets or sleeping people, which for one night separate from half to one liter fluids throughout sweating or breathing. All these risks can be prevented through proper ventilation.


Forming condensation

When water condenses, we talk about forming a condensation. Condensation can form on the surface of building elements, as well as in their interior. The consequences are soaked bulding materials (especially thermal insulating materials based on wool), reduced thermal insulation, forming of mold and ultimately building damage.


Condensation on the surface of building elements

The air always contains within itself a certain amount of gaseous water vapor. The percentage content of water vapor in the air is called relative air humidity and it can vary (20%, 50% and so on). When the quantity of water vapor in the air is constant and the temperature of the air increases, the relative air humidity decreases. The same applies of course to in the opposite case – when the temperature is reduced, the relative air humidity increases. At one point it reaches 100%. This is the moment when then the water vapor turns into liquid. The lower the temperature is, the quicker it gets to this state. We all know this effect when we take a bottle from the fridge and over a short period of time it is covered with water drops. This water comes from the air around the bottle, which is cooled form the bottle and cannot contain any longer the water contained in it, so it gives it to the surface. The same effect occurs when the warm air in the room gets on a cold surface – for example in the basement or in uninsulated or insufficiently insulated building elements.

The moisture condensates very often on surfaces in the house to which the warm heated air do not reach: behind cabinets, chests and wardrobes which are placed close to the wall. If this wall happens to be also an external wall, then the situations gets worse. The simplest way to prevent the formation of condensation is the ventilation. The “exhaled” air is replaced with fresh one. This does not mean anything else besides reducing the relative air humidity. The room air absorbs water during doing laundry, showering, washing, cooking, breathing and sweating. Through ventilation the moist air is transported outside and it is replaced with dry air from. Normally we live at a temperature of 21°С and 55% relative air humidity.                                     


Special case: the basement during summer

The formation of mold in the basement, however, does not come from the improper ventilation. During the summer, at high air humidity and heats above 35°С, the warm and moist air penetrates through the open (for ventilation) windows in the relatively cold basement and condensates on the cold surfaces, creating the ideal environment for forming mold.

Consequently this means that in the summer in our climate conditions we should not ventilate. In this case only the machine water vapor removing or the good old known mixture of sand and salt, which absorbs the moisture from the air helps.


Condensation on the inside of the building components

For condensation on the inside of building elements, the key term is diffusion of water vapor. The present in the air water vapor and the air temperature form a certain form of water vapor pressure. As the pressure of water vapors in the buildings and out in the open is different, there is a natural desire for this difference between inside and outside to be equalized. The pressure of the water vapor passes through the building elements in most cases from the inside out. This process is called diffusion of water vapor. When the temperature of the inner and outer air differs greatly from one another, the temperature in the different layers of the building construction reduces in proportionally to their thermal resistance (see the chart).

Let’s go back to the problem of condensation of water vapor. As we already learned – by reducing the temperature of the air, its relative air humidity is increased. When it reaches 100% in the building element is formed condensation. If this process goes on in time, the building element gets soaked with water. In order to prevent the formation of condensation in the inner walls, the penetrating in the building element water vapors from the warm side to not be more than the outgoing in the outside air on the other side.

The following must be taken into consideration when arranging the individual layers of building materials in the sections of an outer wall:

-       on the inner side of the wall advantage bring the materials with higher number of diffusion resistance to water vapor (vapor dense), because they prevent the penetration of water vapor and their passage in bigger volume to the outer layers of the wall

-       on the outer side of the wall must be selected materials with low number of  diffusion resistance to water vapor (open for diffusion). This way in winter, water vapor will not be detained in the construction and will be able to pass through the outside air.

-       the layers in the sections of the wall must be arranged so that their number of diffusion resistance to water vapor decreases from the inside out, and the resistance to thermal conductivity grows.


Number of diffusion resistance to water vapor (µ)

In the previous lines, we often mentioned the term „number of diffusion resistance to water vapor“. This number describes to what extent the construction material prevents the passage (diffusion) of water vapor through it and it is necessary to assess the individual layers in building structures. Its value shows how many times greater is the resistance to the passage of water vapor in a layer of building material to equally thick layer of air at a rest. The greater is the number – the more vapor dense is the building material.

Each different layer can be open, preventing or dense regarding the diffusion. What exactly it is can be determined when multiplying the thickness of the layer (d) with the number of diffusion resistance to water vapor (µ) of the material it is made of. This way is obtained the so-called diffusion-equivalent thickness of the concerned layer (its Sd value). Layers with

-       Sd ≤ 0,5 m                                 are open for diffusion layers

-       0,5 m < Sd < 1500 m              are preventing the diffusion layers

-       Sd ≥ 1500 m                             are dense for diffusion layers

The diffusion equivalent thickness is a comparative value that indicates what the thickness of the air layer in rest, which corresponds to the concerned building element. One brick layer (with thickness 20 cm and µ=5) has a diffusion equivalent thickness of 5*0,2m=1m, which means that through 20 cm thick brick layer passes such amount of water vapor that would have passed through a 1 m thick air layer at rest.

Graphically the vapor permeability of the different construction materials relative to that of the air can be demonstrated in the following manner:

In the following table are indicated the values of the number of diffusion resistance to water vapor (µ) of some typical building materials. Very often a range of values is indicated (for example a lightweight brick with holes: µ = 5/10). According to the case of application you always take the worse value (for the cold exterior side of the wall, and for the warmer interior side of theс wall the lower values).



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