The vapor permeability of a material is expressed in its ability to pass water vapor. This property of resisting the penetration of steam or allowing it to pass through the material is determined by the level of the vapor permeability coefficient, which is denoted µ. This value, which sounds like "mu", acts as a relative measure of vapor transfer resistance compared to air resistance characteristics.
There is a table that reflects the ability of the material to vapor transfer, it can be seen in fig. 1. Thus, the mu value for mineral wool is 1, which indicates that it is able to pass water vapor as well as air itself. While this value for aerated concrete is 10, this means that it can handle steam 10 times worse than air. If the mu index is multiplied by the layer thickness expressed in meters, this will make it possible to obtain an air thickness Sd (m) equal in terms of vapor permeability.
The table shows that for each position, the vapor permeability index is indicated in a different state. If you look into the SNiP, you can see the calculated data of the mu index with the ratio of moisture in the body of the material equal to zero.
Figure 1. Table of vapor permeability of building materials
For this reason, when purchasing goods that are supposed to be used in the process of summer cottage construction, it is preferable to take into account international ISO standards, since they determine the mu index in a dry state, with a humidity level of not more than 70% and a humidity index of more than 70%.
When choosing building materials that will form the basis of a multilayer structure, the mu index of the layers located inside should be lower, otherwise, over time, the layers located inside will become wet, as a result of which they will lose their thermal insulation qualities.
When creating enclosing structures, you need to take care of their normal functioning. To do this, one should adhere to the principle that the mu level of the material that is located in the outer layer should be 5 times or more higher than the mentioned value of the material located in the inner layer.
Vapor permeability mechanism
Under conditions of low relative humidity, moisture particles that are contained in the atmosphere penetrate through the pores of building materials, ending up there in the form of vapor molecules. When the relative humidity level increases, the pores of the layers accumulate water, which causes wetting and capillary suction.
At the moment of increasing the moisture level of the layer, its mu index increases, thus the vapor permeability resistance level decreases.
The vapor permeability indicators of non-moistened materials are applicable in the conditions of internal structures of buildings that have heating. But the vapor permeability levels of moistened materials are applicable to any building structures that are not heated.
The vapor permeability levels that are part of our standards are not in all cases equivalent to those that belong to international standards. So, in domestic SNiP, the level of mu expanded clay and cinder concrete is almost the same, while according to international standards, the data differ by 5 times. The levels of vapor permeability of gypsum plasterboard and cinder concrete in domestic standards are almost the same, and in international standards the data differ by 3 times.
There are various ways to determine the level of vapor permeability, with regard to membranes, the following methods can be distinguished:
- American test with a vertical bowl.
- American Inverted Bowl Test.
- Japanese vertical bowl test.
- Japanese inverted bowl test with desiccant.
- American vertical bowl test.
The Japanese test uses a dry desiccant that is placed under the material being tested. All tests use a sealing element.
Everyone knows that a comfortable temperature regime, and, accordingly, a favorable microclimate in the house is provided largely due to high-quality thermal insulation. Recently, there has been a lot of debate about what ideal thermal insulation should be and what characteristics it should have.
There are a number of properties of thermal insulation, the importance of which is beyond doubt: these are thermal conductivity, strength and environmental friendliness. It is quite obvious that effective thermal insulation must have a low coefficient of thermal conductivity, be strong and durable, and not contain substances harmful to humans and the environment.
However, there is one property of thermal insulation that raises a lot of questions - this is vapor permeability. Should the insulation be permeable to water vapor? Low vapor permeability - is it an advantage or a disadvantage?
Points for and against"
Supporters of cotton wool insulation claim that high vapor permeability is a definite plus, vapor-permeable insulation will allow the walls of your house to "breathe", which will create a favorable microclimate in the room even in the absence of any additional ventilation system.
Adepts of penoplex and its analogues say: the insulation should work like a thermos, and not like a leaky "quilted jacket". In their defense, they make the following arguments:
1. Walls are not the "breathing organs" of the house at all. They perform a completely different function - they protect the house from environmental influences. The respiratory system for the house is the ventilation system, as well as, in part, windows and doorways.
In many European countries, supply and exhaust ventilation is installed without fail in any residential area and is perceived as the same norm as a centralized heating system in our country.
2. The penetration of water vapor through walls is a natural physical process. But at the same time, the amount of this penetrating steam in a residential area with normal operation is so small that it can be ignored (from 0.2 to 3% * depending on the presence / absence of a ventilation system and its efficiency).
* Pogozhelsky J.A., Kasperkevich K. Thermal protection of multi-panel houses and energy saving, planned topic NF-34/00, (typescript), ITB library.
Thus, we see that high vapor permeability cannot act as a cultivated advantage when choosing a thermal insulation material. Now let's try to find out if this property can be considered a disadvantage?
Why is the high vapor permeability of the insulation dangerous?
In winter, at sub-zero temperatures outside the house, the dew point (the conditions under which water vapor reaches saturation and condenses) should be in the insulation (extruded polystyrene foam is taken as an example).
Fig. 1 Dew point in XPS slabs in houses with insulation cladding
Fig. 2 Dew point in XPS slabs in frame-type houses
It turns out that if the thermal insulation has a high vapor permeability, then condensate can accumulate in it. Now let's find out why the condensate in the heater is dangerous?
First of all, when condensation forms in the insulation, it becomes wet. Accordingly, its thermal insulation characteristics decrease and, conversely, thermal conductivity increases. Thus, the insulation begins to perform the opposite function - to remove heat from the room.
A well-known expert in the field of thermal physics, Doctor of Technical Sciences, Professor, K.F. Fokin concludes: “Hygienists consider the air permeability of fences as a positive quality that provides natural ventilation of the premises. But from a thermotechnical point of view, the air permeability of fences is rather a negative quality, since in winter time infiltration (air movement from inside to outside) causes additional heat loss by fences and cooling of rooms, and exfiltration (air movement from outside to inside) can adversely affect the humidity regime of external fences. promoting moisture condensation.
In addition, in SP 23-02-2003 "Thermal protection of buildings", section No. 8, it is indicated that the air permeability of enclosing structures for residential buildings should be no more than 0.5 kg / (m²∙h).
Secondly, due to wetting, the heat insulator becomes heavier. If we are dealing with a cotton insulation, then it sags, and cold bridges form. In addition, the load on the supporting structures increases. After several cycles: frost - thaw, such a heater begins to collapse. To protect the moisture-permeable insulation from getting wet, it is covered with special films. A paradox arises: the insulation breathes, but it needs protection with polyethylene or a special membrane that negates all its “breathing”.
Neither polyethylene nor the membrane allows water molecules to pass into the insulation. It is known from a school physics course that air molecules (nitrogen, oxygen, carbon dioxide) are larger than a water molecule. Accordingly, air is also unable to pass through such protective films. As a result, we get a room with a breathable insulation, but covered with an airtight film - a kind of greenhouse made of polyethylene.
The concept of "breathing walls" is considered a positive characteristic of the materials from which they are made. But few people think about the reasons that allow this breathing. Materials capable of passing both air and steam are vapor-permeable.
A good example of building materials with high vapor permeability:
- wood;
- expanded clay slabs;
- foam concrete.
Concrete or brick walls are less permeable to steam than wood or expanded clay.
Sources of steam indoors
Human breathing, cooking, water vapor from the bathroom and many other sources of steam in the absence of an exhaust device create a high level of humidity indoors. You can often observe the formation of perspiration on window panes in winter, or on cold water pipes. These are examples of the formation of water vapor inside the house.
What is vapor permeability
The design and construction rules give the following definition of the term: the vapor permeability of materials is the ability to pass through moisture droplets contained in the air due to different partial vapor pressures from opposite sides at the same air pressure values. It is also defined as the density of the steam flow passing through a certain thickness of the material.
The table, which has a vapor permeability coefficient, compiled for building materials, is conditional, since the specified calculated values \u200b\u200bof humidity and atmospheric conditions do not always correspond to real conditions. The dew point can be calculated based on approximate data.
Wall construction taking into account vapor permeability
Even if the walls are built from a material with high vapor permeability, this cannot be a guarantee that it will not turn into water in the thickness of the wall. To prevent this from happening, it is necessary to protect the material from the difference in partial vapor pressure from inside and outside. Protection against the formation of steam condensate is carried out using OSB boards, insulating materials such as foam and vapor-tight films or membranes that prevent steam from penetrating into the insulation.
The walls are insulated in such a way that a layer of insulation is located closer to the outer edge, incapable of forming moisture condensation, pushing the dew point (water formation) away. In parallel with the protective layers in the roofing cake, it is necessary to ensure the correct ventilation gap.
The destructive action of steam
If the wall cake has a weak ability to absorb steam, it is not in danger of destruction due to the expansion of moisture from frost. The main condition is to prevent the accumulation of moisture in the thickness of the wall, but to ensure its free passage and weathering. It is equally important to arrange a forced extraction of excess moisture and steam from the room, to connect a powerful ventilation system. By observing the above conditions, you can protect the walls from cracking, and increase the life of the whole house. The constant passage of moisture through building materials accelerates their destruction.
Use of conductive qualities
Taking into account the peculiarities of the operation of buildings, the following principle of insulation is applied: the most steam-conducting insulation materials are located outside. Due to this arrangement of layers, the likelihood of water accumulation when the temperature drops outside is reduced. To prevent the walls from getting wet from the inside, the inner layer is insulated with a material having low vapor permeability, for example, a thick layer of extruded polystyrene foam.
The opposite method of using the steam-conducting effects of building materials is successfully applied. It consists in the fact that a brick wall is covered with a vapor barrier layer of foam glass, which interrupts the moving flow of steam from the house to the street during low temperatures. The brick begins to accumulate humidity in the rooms, creating a pleasant indoor climate thanks to a reliable vapor barrier.
Compliance with the basic principle when building walls
Walls should be characterized by a minimum ability to conduct steam and heat, but at the same time be heat-retaining and heat-resistant. When using one type of material, the desired effects cannot be achieved. The outer wall part is obliged to retain cold masses and prevent their impact on internal heat-intensive materials that maintain a comfortable thermal regime inside the room.
Reinforced concrete is ideal for the inner layer, its heat capacity, density and strength have maximum performance. Concrete successfully smooths out the difference between night and day temperature changes.
When carrying out construction work, wall cakes are made taking into account the basic principle: the vapor permeability of each layer should increase in the direction from the inner layers to the outer ones.
Rules for the location of vapor barrier layers
To ensure the best performance of multilayer structures of buildings, the rule is applied: on the side with a higher temperature, materials with increased resistance to steam penetration with increased thermal conductivity are placed. The layers located outside must have a high vapor conductivity. For the normal functioning of the building envelope, it is necessary that the coefficient of the outer layer is five times higher than the indicator of the layer located inside. 
When this rule is followed, it will not be difficult for water vapor that has entered the warm layer of the wall to quickly escape through more porous materials.
If this condition is not observed, the inner layers of building materials lock up and become more heat-conducting.
Familiarity with the table of vapor permeability of materials
When designing a house, the characteristics of building materials are taken into account. The Code of Practice contains a table with information on what vapor permeability coefficient building materials have under conditions of normal atmospheric pressure and average air temperature.
Material | Vapor permeability coefficient |
extruded polystyrene foam | |
polyurethane foam | |
mineral wool | |
reinforced concrete, concrete | |
pine or spruce | |
expanded clay | |
foam concrete, aerated concrete | |
granite, marble | |
drywall | |
chipboard, OSB, fiberboard | |
foam glass | |
ruberoid | |
polyethylene | |
linoleum |
The importance of the material vapor permeability table
The vapor permeability coefficient is an important parameter that is used to calculate the thickness of the layer of insulation materials. The quality of the insulation of the entire structure depends on the correctness of the results obtained.
Sergey Novozhilov is an expert in roofing materials with 9 years of practical experience in the field of engineering solutions in construction.
Often in construction articles there is an expression - the vapor permeability of concrete walls. It means the ability of the material to pass water vapor, in a popular way - "breathe". This parameter is of great importance, since waste products are constantly formed in the living room, which must be constantly brought out.
General information
If you do not create normal ventilation in the room, dampness will be created in it, which will lead to the appearance of fungus and mold. Their secretions can be harmful to our health.
On the other hand, vapor permeability affects the ability of the material to accumulate moisture in itself. This is also a bad indicator, since the more it can hold in itself, the higher the likelihood of fungus, putrefactive manifestations, and destruction during freezing.
Vapor permeability is denoted by the Latin letter μ and is measured in mg / (m * h * Pa). The value shows the amount of water vapor that can pass through the wall material in an area of 1 m 2 and with a thickness of 1 m in 1 hour, as well as a difference in external and internal pressure of 1 Pa.
High capacity for conducting water vapor in:
- foam concrete;
- aerated concrete;
- perlite concrete;
- expanded clay concrete.
Closes the table - heavy concrete.
Tip: if you need to make a technological channel in the foundation, diamond drilling in concrete will help you.
aerated concrete
- The use of the material as a building envelope makes it possible to avoid the accumulation of unnecessary moisture inside the walls and preserve its heat-saving properties, which will prevent possible destruction.
- Any aerated concrete and foam concrete block contains ≈ 60% of air, due to which the vapor permeability of aerated concrete is recognized as good, the walls in this case can "breathe".
- Water vapor freely seeps through the material, but does not condense in it.
The vapor permeability of aerated concrete, as well as foam concrete, significantly exceeds heavy concrete - for the first 0.18-0.23, for the second - (0.11-0.26), for the third - 0.03 mg / m * h * Pa.
I would especially like to emphasize that the structure of the material provides it with effective removal of moisture into the environment, so that even when the material freezes, it does not collapse - it is forced out through open pores. Therefore, when preparing, this feature should be taken into account and appropriate plasters, putties and paints should be selected.
The instruction strictly regulates that their vapor permeability parameters are not lower than aerated concrete blocks used for construction.
Tip: do not forget that the vapor permeability parameters depend on the density of aerated concrete and may differ by half.
For example, if you use D400, they have a coefficient of 0.23 mg / m h Pa, and for D500 it is already lower - 0.20 mg / m h Pa. In the first case, the numbers indicate that the walls will have a higher "breathing" ability. So when choosing finishing materials for D400 aerated concrete walls, make sure that their vapor permeability coefficient is the same or higher.
Otherwise, this will lead to a deterioration in the removal of moisture from the walls, which will affect the decrease in the comfort level of living in the house. It should also be noted that if you used vapor-permeable paint for aerated concrete for the exterior, and non-vapor-permeable materials for the interior, the steam will simply accumulate inside the room, making it wet.
Expanded clay concrete
The vapor permeability of expanded clay concrete blocks depends on the amount of filler in its composition, namely expanded clay - foamed baked clay. In Europe, such products are called eco- or bioblocks.
Tip: if you can’t cut the expanded clay block with a regular circle and a grinder, use a diamond one.
For example, cutting reinforced concrete with diamond wheels makes it possible to quickly solve the problem.
Polystyrene concrete
The material is another representative of cellular concrete. The vapor permeability of polystyrene concrete is usually equal to that of wood. You can make it with your own hands.
Today, more attention is being paid not only to the thermal properties of wall structures, but also to the comfort of living in the building. In terms of thermal inertness and vapor permeability, polystyrene concrete resembles wooden materials, and heat transfer resistance can be achieved by changing its thickness. Therefore, poured monolithic polystyrene concrete is usually used, which is cheaper than finished slabs.
Conclusion
From the article you learned that building materials have such a parameter as vapor permeability. It makes it possible to remove moisture outside the walls of the building, improving their strength and characteristics. The vapor permeability of foam concrete and aerated concrete, as well as heavy concrete, differs in its performance, which must be taken into account when choosing finishing materials. The video in this article will help you find more information on this topic.
In order to destroy it
Calculations of units of vapor permeability and resistance to vapor permeability. Technical characteristics of membranes.
Often, instead of the Q value, the vapor permeability resistance value is used, in our opinion it is Rp (Pa * m2 * h / mg), foreign Sd (m). Vapor permeability is the reciprocal of Q. Moreover, imported Sd is the same Rp, only expressed as an equivalent diffusion resistance to vapor permeability of an air layer (equivalent diffusion thickness of air).
Instead of further reasoning in words, we correlate Sd and Rn numerically.
What does Sd=0.01m=1cm mean?
This means that the diffusion flux density with a difference dP is:
J=(1/Rp)*dP=Dv*dRo/Sd
Here Dv=2.1e-5m2/s diffusion coefficient of water vapor in air (taken at 0°C)/
Sd is our very Sd, and
(1/Rp)=Q
Let's transform the right equality using the ideal gas law (P*V=(m/M)*R*T => P*M=Ro*R*T => Ro=(M/R/T)*P) and see.
1/Rp=(Dv/Sd)*(M/R/T)
Hence Sd=Rp*(Dv*M)/(RT) which is not clear to us yet
To get the correct result, you need to represent everything in units of Rp,
more precisely Dv=0.076 m2/h
M=18000 mg/mol - molar mass of water
R=8.31 J/mol/K - universal gas constant
T = 273K - temperature on the Kelvin scale, corresponding to 0 degrees C, where we will carry out calculations.
So, substituting everything, we have:
sd= Rp*(0.076*18000)/(8.31*273) \u003d 0.6 Rp or vice versa:
Rp=1.7Sd.
Here Sd is the same imported Sd [m], and Rp [Pa * m2 * h / mg] is our resistance to vapor permeation.
Also Sd can be associated with Q - vapor permeability.
We have that Q=0.56/Sd, here Sd [m] and Q [mg/(Pa*m2*h)].
Let us check the obtained relations. To do this, take the technical characteristics of various membranes and substitute.
To begin with, I will take the data on Tyvek from here
As a result, the data is interesting, but not very suitable for formula testing.
In particular, for the Soft membrane we obtain Sd=0.09*0.6=0.05m. Those. Sd in the table is underestimated by 2.5 times or, accordingly, Rp is overestimated.
I take further data from the Internet. By Fibrotek membrane
I will use the last pair of data permeability, in this case Q*dP=1200 g/m2/day, Rp=0.029 m2*h*Pa/mg
1/Rp=34.5 mg/m2/h/Pa=0.83 g/m2/day/Pa
From here we will extract the difference in absolute humidity dP=1200/0.83=1450Pa. This humidity corresponds to a dew point of 12.5 degrees or a humidity of 50% at 23 degrees.
On the Internet, I also found on another forum the phrase:
Those. 1740 ng/Pa/s/m2=6.3 mg/Pa/h/m2 corresponds to vapor permeability ~250 g/m2/day.
I'll try to get that ratio myself. It is mentioned that the value in g/m2/day is also measured at 23 deg. We take the previously obtained value dP=1450Pa and we have an acceptable convergence of the results:
6.3*1450*24/100=219 g/m2/day Hurrah Hurrah.
So, now we are able to correlate the vapor permeability that you can find in the tables and the resistance to vapor permeability.
It remains to make sure that the relation between Rp and Sd obtained above is correct. I had to dig and found a membrane for which both values are given (Q * dP and Sd), while Sd is a specific value, and not "no more". Perforated membrane based on PE film
And here is the data:
40.98 g/m2/day => Rp=0.85 =>Sd=0.6/0.85=0.51m
Again it doesn't fit. But in principle, the result is not far off, which, given the fact that it is not known at what parameters, the vapor permeability is determined is quite normal.
Interestingly, according to Tyvek they got misalignment in one direction, according to IZOROL in the other. Which suggests that you can’t trust some values everywhere.
PS I would be grateful for the search for errors and comparisons with other data and standards.