Vapor permeability is the ability of a material to pass or retain steam as a result of the difference in the partial pressure of water vapor at the same atmospheric pressure on both sides of the material. Vapor permeability is characterized by the value of the coefficient of vapor permeability or the value of the coefficient of permeability resistance when exposed to water vapor. The vapor permeability coefficient is measured in mg/(m·h·Pa).
The air always contains some amount of water vapor, and warm air always contains more than cold air. At an internal air temperature of 20 °C and a relative humidity of 55%, the air contains 8 g of water vapor per 1 kg of dry air, which creates a partial pressure of 1238 Pa. At a temperature of –10°C and a relative humidity of 83%, the air contains about 1 g of steam per 1 kg of dry air, creating a partial pressure of 216 Pa. Due to the difference in partial pressures between the indoor and outdoor air through the wall, there is a constant diffusion of water vapor from warm room out. As a result, in real operating conditions, the material in structures is in a somewhat moistened state. The degree of material moisture depends on the temperature and humidity conditions outside and inside the fence. The change in the thermal conductivity coefficient of the material in operating structures is taken into account by the thermal conductivity coefficients λ(A) and λ(B), which depend on the humidity zone of the local climate and the humidity conditions of the room.
As a result of the diffusion of water vapor in the thickness of the structure, moist air moves from interior spaces. Passing through the vapor-permeable fencing structures, moisture evaporates out. But if outer surface If there is a layer of material on the wall that does not or does not allow water vapor to pass through, moisture begins to accumulate at the border of the vapor-tight layer, causing the structure to become damp. As a result, the thermal protection of a wet structure decreases sharply, and it begins to freeze. V in this case there is a need to install a vapor barrier layer on the warm side of the structure.
It seems that everything is relatively simple, but vapor permeability is often remembered only in the context of the “breathability” of walls. However, this is the cornerstone in choosing insulation! You need to approach it very, very carefully! There are often cases when a homeowner insulates a house based only on the thermal resistance indicator, for example, wooden house polystyrene foam. As a result, it gets rotting walls, mold in all corners and blames the “non-ecological” insulation for this. As for polystyrene foam, due to its low vapor permeability, you need to use it wisely and think very carefully about whether it is suitable for you. It is for this reason that cotton wool or any other porous insulation materials are often better suited for insulating walls outside. In addition, it is more difficult to make a mistake with cotton insulation. However, concrete or brick houses You can safely insulate it with foam plastic - in this case, the foam “breathes” better than the wall!
The table below shows materials from the TCP list, the vapor permeability indicator is the last column μ.
How to understand what vapor permeability is and why it is needed. Many have heard, and some actively use, the term “breathable walls” - so, such walls are called “breathable” because they are able to pass air and water vapor through themselves. Some materials (for example, expanded clay, wood, all cotton insulation) allow steam to pass through well, while others transmit steam very poorly (brick, polystyrene foam, concrete). The steam exhaled by a person, released when cooking or taking a bath, if there is no hood in the house, creates increased humidity. A sign of this is the appearance of condensation on windows or on pipes with cold water. It is believed that if a wall has high vapor permeability, then it is easy to breathe in the house. In fact, this is not entirely true!
IN modern house, even if the walls are made of “breathable” material, 96% of the steam is removed from the premises through the hood and vents, and only 4% through the walls. If vinyl or non-woven wallpaper is glued to the walls, then the walls do not allow moisture to pass through. And if the walls are truly “breathable,” that is, without wallpaper or other vapor barriers, heat will blow out of the house in windy weather. The higher the vapor permeability construction material(foam concrete, aerated concrete and other warm concrete), the more moisture it can absorb, and as a result, it has lower frost resistance. Steam leaving the house through the wall turns into water at the “dew point”. The thermal conductivity of a damp gas block increases many times, that is, the house will be, to put it mildly, very cold. But the worst thing is that when the temperature drops at night, the dew point moves inside the wall, and the condensate in the wall freezes. When water freezes, it expands and partially destroys the structure of the material. Several hundred such cycles lead to complete destruction of the material. Therefore, vapor permeability building materials may serve you badly.
About the harm of increased vapor permeability on the Internet, it goes from site to site. I will not present its contents on my website due to some disagreement with the authors, but I would like to voice selected points. For example, famous manufacturer mineral insulation, Isover company, on its English site outlined the “golden rules of insulation” ( What are the golden rules of insulation?) from 4 points:
Effective insulation. Use materials with high thermal resistance(low thermal conductivity). A self-evident point that does not require special comment.
Tightness. Good sealing is a necessary condition For effective system thermal insulation! Leaking thermal insulation, regardless of its thermal insulation coefficient, can increase energy consumption for heating a building by 7 to 11%. Therefore, the airtightness of the building should be considered at the design stage. And upon completion of work, check the building for leaks.
Controlled ventilation. It is ventilation that is tasked with removing excess moisture and steam. Ventilation should not and cannot be carried out by violating the tightness of the enclosing structures!
High-quality installation. I think there is no need to talk about this point either.
It is important to note that Isover does not produce any foam insulation, they deal exclusively with mineral wool insulation, i.e. products with the highest vapor permeability! This really makes you wonder: how is it possible, it seems that vapor permeability is necessary for moisture removal, but manufacturers recommend complete sealing!
The point here is a misunderstanding of this term. The vapor permeability of materials is not intended to remove moisture from the living space - vapor permeability is needed to remove moisture from the insulation! The fact is that any porous insulation is not essentially an insulation itself; it only creates a structure that holds the true insulation - air - in a closed volume and, if possible, motionless. If suddenly such an unfavorable condition occurs that the dew point is at vapor-permeable insulation, then moisture will condense in it. This moisture in the insulation does not come from the room! The air itself always contains some amount of moisture, and it is this natural moisture that poses a threat to the insulation. To remove this moisture outside, it is necessary that after the insulation there are layers with no less vapor permeability.
On average, a family of four produces steam equal to 12 liters of water per day! This moisture from the indoor air should in no way get into the insulation! Where to put this moisture - this should not worry the insulation in any way - its task is only to insulate!
Let's look at the above with an example. Let's take two walls frame house the same thickness and the same composition (from the inside to the outer layer), they will differ only in the type of insulation:
Drywall sheet (10mm) - OSB-3 (12mm) - Insulation (150mm) - OSB-3 (12mm) - ventilation gap (30mm) - wind protection - facade.
We will choose insulation with absolutely the same thermal conductivity - 0.043 W/(m °C), the main, tenfold difference between them is only in vapor permeability:
Expanded polystyrene PSB-S-25.
Density ρ= 12 kg/m³.
Vapor permeability coefficient μ= 0.035 mg/(m h Pa)
Coef. thermal conductivity in climatic conditions B (worst indicator) λ(B) = 0.043 W/(m °C).
Density ρ= 35 kg/m³.
Vapor permeability coefficient μ= 0.3 mg/(m h Pa)
Of course, I also use exactly the same calculation conditions: inside temperature +18°C, humidity 55%, outside temperature -10°C, humidity 84%.
I carried out the calculation in thermal calculator By clicking on the photo you will go directly to the calculation page:
As can be seen from the calculation, the thermal resistance of both walls is exactly the same (R = 3.89), and even their dew point is located almost equally in the thickness of the insulation, however, due to the high vapor permeability, moisture will condense in the wall with ecowool, greatly moistening the insulation. No matter how good dry ecowool is, damp ecowool retains heat many times worse. And if we assume that the temperature outside drops to -25°C, then the condensation zone will be almost 2/3 of the insulation. Such a wall does not meet the standards for protection against waterlogging! With expanded polystyrene, the situation is fundamentally different because the air in it is in closed cells; it simply has nowhere to collect enough moisture for dew to form.
To be fair, it must be said that ecowool cannot be installed without vapor barrier films! And if you add it to the "wall pie" vapor barrier film between OSB and ecowool with inside premises, then the condensation zone will practically leave the insulation and the structure will fully satisfy the requirements for humidification (see picture on the left). However, the vaporization device practically makes no sense in thinking about the benefits of the “wall breathing” effect for the microclimate of the room. A vapor barrier membrane has a vapor permeability coefficient of about 0.1 mg/(m h Pa), and sometimes they are vapor barriered with polyethylene films or insulation with a foil side - their vapor permeability coefficient tends to zero.
But low vapor permeability is also not always good! When insulating fairly well-vapor-permeable walls made of gas-foam concrete with extruded polystyrene foam without vapor barrier from the inside, mold will certainly settle in the house, the walls will be damp, and the air will not be fresh at all. And even regular ventilation will not be able to dry such a house! Let's simulate a situation opposite to the previous one!
The wall this time will consist of the following elements:
Aerated concrete grade D500 (200mm) - Insulation (100mm) - ventilation gap (30mm) - wind protection - facade.
We will choose exactly the same insulation, and moreover, we will make the wall with exactly the same thermal resistance (R = 3.89).
As we see, with completely equal thermal characteristics we can get radically opposite results from insulation with the same materials!!! It should be noted that in the second example, both structures meet the standards for protection against waterlogging, despite the fact that the condensation zone falls into the gas silicate. This effect is due to the fact that the plane maximum hydration gets into expanded polystyrene, and due to its low vapor permeability, moisture does not condense in it.
The issue of vapor permeability needs to be thoroughly understood even before you decide how and with what you will insulate your home!
In a modern house, the requirements for thermal insulation of walls are so high that a homogeneous wall can no longer meet them. Agree, given the requirement for thermal resistance R=3, make a homogeneous brick wall 135 cm thick is not an option! Modern walls- these are multilayer structures, where there are layers that act as thermal insulation, structural layers, a layer exterior finishing, layer interior decoration, layers of steam-hydro-wind insulation. Due to the varied characteristics of each layer, it is very important to position them correctly! The basic rule in the arrangement of layers of a wall structure is as follows:
The vapor permeability of the inner layer should be lower than the outer one, so that steam can freely escape beyond the walls of the house. With this solution, the “dew point” moves to outside load-bearing wall and does not destroy the walls of the building. To prevent condensation inside the building envelope, the resistance to heat transfer in the wall should decrease, and the resistance to vapor permeation should increase from the outside inward.
I think this needs to be illustrated for better understanding.
In order to destroy it
Calculations of units of vapor permeability and resistance to vapor permeation. Technical characteristics of membranes.
Often, instead of the Q value, the value of vapor permeation resistance is used, in our opinion it is Rp (Pa*m2*h/mg), foreign Sd (m). Resistance to vapor permeation is the inverse value of Q. Moreover, imported Sd is the same Rp, only expressed as the equivalent diffusion resistance to vapor permeation of the air layer (equivalent diffusion thickness of air).
Instead of further reasoning in words, let’s correlate Sd and Rп numerically.
What does Sd=0.01m=1cm mean?
This means that the diffusion flux density with a difference dP is:
J=(1/Rп)*dP=Dv*dRo/Sd
Here Dv=2.1e-5m2/s diffusion coefficient of water vapor in air (taken at 0 degrees C)/
Sd is our very Sd, and
(1/Rп)=Q
Let's transform legal equality using the law ideal gas(P*V=(m/M)*R*T => P*M=Ro*R*T => Ro=(M/R/T)*P) and we see.
1/Rп=(Dv/Sd)*(M/R/T)
Hence, what is not yet clear to us is Sd=Rп*(Dv*M)/(RT)
To get the correct result, you need to present everything in units of Rп,
more precisely Dv=0.076 m2/h
M=18000 mg/mol - molar mass 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= Rп*(0.076*18000)/(8.31*273) =0.6Rп or vice versa:
Rп=1.7Sd.
Here Sd is the same imported Sd [m], and Rp [Pa*m2*h/mg] is our resistance to vapor permeation.
Sd can also be associated with Q - vapor permeability.
We have that Q=0.56/Sd, here Sd [m], and Q [mg/(Pa*m2*h)].
Let's check the obtained relationships. For this I'll take specifications various membranes and substitute.
First, I’ll take the data on Tyvek from here
The data is ultimately interesting, but not very suitable for testing formulas.
In particular, for the Soft membrane we obtain Sd = 0.09 * 0.6 = 0.05 m. Those. Sd in the table is underestimated by 2.5 times or, accordingly, Rp is overestimated.
I take further data from the Internet. Over Fibrotek membrane
I will use the last pair of permeability data, 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 take 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 the following phrase on another forum:
Those. 1740 ng/Pa/s/m2=6.3 mg/Pa/h/m2 corresponds to vapor permeability ~250g/m2/day.
I'll try to get this ratio myself. It is mentioned that the value in g/m2/day is also measured at 23 degrees. We take the previously obtained value dP=1450Pa and have an acceptable convergence of results:
6.3*1450*24/100=219 g/m2/day. Cheers cheers.
So, now we know how to correlate the vapor permeability that you can find in the tables and the resistance to vapor permeation.
It remains to be convinced that the above relationship between Rп and Sd is correct. I had to rummage around and found a membrane for which both values (Q*dP and Sd) are given, 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 => Rп=0.85 =>Sd=0.6/0.85=0.51m
It doesn't add up again. But in principle, the result is not far off, considering that it is unknown at what parameters the vapor permeability is determined quite normally.
Interestingly, with Tyvek we got misalignment in one direction, with IZOROL in the other. Which means that some quantities cannot be trusted everywhere.
PS I would be grateful for searching for errors and comparisons with other data and standards.
To create favorable microclimate indoors, it is necessary to take into account the properties of building materials. Today we will look at one property - vapor permeability of materials.
Vapor permeability is the ability of a material to allow vapors contained in the air to pass through. Water vapor penetrates the material due to pressure.
Tables that cover almost all materials used for construction will help you understand the issue. Having studied this material, you will know how to build a warm and reliable home.
If we're talking about about prof. construction, it uses special equipment to determine vapor permeability. This is how the table that appears in this article appeared.
The following equipment is used today:
There is an opinion that “breathing walls” are beneficial for the house and its inhabitants. But all builders think about this concept. “Breathable” is a material that, in addition to air, also allows steam to pass through - this is the water permeability of building materials. Foam concrete and expanded clay wood have a high rate of vapor permeability. Walls made of brick or concrete also have this property, but the indicator is much less than that of expanded clay or wood materials. 
Steam is released when taking a hot shower or cooking. Because of this, increased humidity is created in the house - a hood can correct the situation. You can find out that the vapors are not escaping anywhere by looking at the condensation on the pipes and sometimes on the windows. Some builders believe that if a house is built of brick or concrete, then it is “hard” to breathe in the house.
In fact, the situation is better - in modern home about 95% of the steam escapes through the vent and hood. And if the walls are made of “breathing” building materials, then 5% of the steam escapes through them. So residents of houses made of concrete or brick do not suffer much from this parameter. Also, the walls, regardless of the material, will not allow moisture to pass through due to vinyl wallpaper. “Breathing” walls also have a significant drawback - in windy weather, heat leaves the home.
The table will help you compare materials and find out their vapor permeability indicator:
The higher the vapor permeability index, the more moisture the wall can absorb, which means that the material has low frost resistance. If you are going to build walls from foam concrete or aerated block, then you should know that manufacturers are often cunning in the description where vapor permeability is indicated. The property is indicated for dry material - in this state it really has high thermal conductivity, but if the gas block gets wet, the indicator will increase 5 times. But we are interested in another parameter: the liquid tends to expand when it freezes, and as a result, the walls collapse.
The sequence of layers and the type of insulation are what primarily affect vapor permeability. In the diagram below you can see that if the insulation material is located on the facade side, then the indicator of pressure on moisture saturation is lower. 
If the insulation is located on the inside of the house, then condensation will appear between the supporting structure and this building structure. It negatively affects the entire microclimate in the house, while the destruction of building materials occurs much faster.
The coefficient in this indicator determines the amount of vapor, measured in grams, that passes through materials 1 meter thick and a layer of 1 m² within one hour. The ability to transmit or retain moisture characterizes the resistance to vapor permeability, which is indicated in the table by the symbol “µ”.
In simple words, coefficient is the resistance of building materials, comparable to the permeability of air. Let's look at a simple example, mineral wool has the following vapor permeability coefficient: µ=1. This means that the material allows moisture to pass through as well as air. And if you take aerated concrete, then its µ will be equal to 10, that is, its vapor conductivity is ten times worse than that of air.
On the one hand, vapor permeability has a good effect on the microclimate, and on the other hand, it destroys the materials from which the house is built. For example, “cotton wool” perfectly allows moisture to pass through, but as a result, due to excess steam, condensation can form on windows and pipes with cold water, as the table shows. Because of this, the insulation loses its quality. Professionals recommend installing a vapor barrier layer with outside Houses. After this, the insulation will not allow steam to pass through. 
If the material has a low vapor permeability rate, then this is only a plus, because the owners do not have to spend money on insulating layers. And get rid of the steam generated from cooking and hot water, a hood and a window will help - this is enough to maintain a normal microclimate in the house. When a house is built from wood, it is impossible to do without additional insulation, and special varnish is required for wood materials.
The table, graph and diagram will help you understand the principle of operation of this property, after which you can already make your choice suitable material. Also, do not forget about climatic conditions outside the window, because if you live in an area with high humidity, then you should completely forget about materials with a high vapor permeability rate.
Often in construction articles there is an expression - vapor permeability concrete walls. It means the ability of a material to allow water vapor to pass through, or, in popular parlance, to “breathe.” This parameter has great importance, since waste products are constantly formed in the living room, which must be constantly removed outside.
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 a material to accumulate moisture. This is also a bad indicator, since the more it can retain it, the higher the likelihood of fungus, putrefactive manifestations, and damage due to freezing.
Vapor permeability is denoted by the Latin letter μ and measured in mg/(m*h*Pa). The value indicates the amount of water vapor that can pass through wall material on an area of 1 m2 and with a thickness of 1 m in 1 hour, as well as a difference in external and internal pressure of 1 Pa.
High ability to conduct water vapor in:
Rounding out the table is heavy concrete.
Advice: if you need to make a technological channel in the foundation, diamond drilling of holes in concrete will help you.
The vapor permeability of aerated concrete, as well as foam concrete, significantly exceeds heavy concrete - for the first it is 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 moisture in environment, so that even when the material freezes, it does not collapse - it is forced out through open pores. Therefore, when preparing, you should consider this feature and select appropriate plasters, putties and paints.
The instructions strictly regulate that their vapor permeability parameters are not lower than aerated concrete blocks used for construction.
Tip: do not forget that vapor permeability parameters depend on the density of aerated concrete and may differ by half.
For example, if you use D400, their coefficient is 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 selecting finishing materials for walls made of aerated concrete D400, make sure that their vapor permeability coefficient is the same or higher.
Otherwise, this will lead to poor drainage of moisture from the walls, which will affect the level of living comfort in the house. You should also take into account 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 damp.
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.
Advice: if you can’t cut the expanded clay block with a regular circle and grinder, use a diamond one.
For example, cutting reinforced concrete with diamond wheels makes it possible to quickly solve the problem.
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 yourself.
Today, more attention is beginning to be paid not only to the thermal properties of wall structures, but also to the comfort of living in the structure. 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 ready-made slabs.
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. 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 Additional information on this topic.
During the construction process, any material must first of all be assessed according to its operational and technical characteristics. When solving the problem of building a “breathing” house, which is most typical of buildings made of brick or wood, or vice versa, achieving maximum resistance to vapor permeation, you need to know and be able to operate tabular constants to obtain calculated indicators of vapor permeability of building materials.
Vapor permeability of materials- the ability to transmit or retain water vapor as a result of the difference in the partial pressure of water vapor on both sides of the material at the same atmospheric pressure. Vapor permeability is characterized by a vapor permeability coefficient or vapor permeability resistance and is standardized by SNiP II-3-79 (1998) “Building Heat Engineering”, namely Chapter 6 “Vapor Permeability Resistance of Enclosing Structures”
The vapor permeability table is presented in SNiP II-3-79 (1998) “Building Heat Engineering”, Appendix 3 “Thermal Indicators of Construction Materials”. The vapor permeability and thermal conductivity indicators of the most common materials used for construction and insulation of buildings are presented in the table below.
Material | Density, kg/m3 | Thermal conductivity, W/(m*S) | Vapor permeability, Mg/(m*h*Pa) |
Aluminum | |||
Asphalt concrete | |||
Drywall | |||
Chipboard, OSB | |||
Oak along the grain | |||
Oak across the grain | |||
Reinforced concrete | |||
Cardboard facing | |||
Expanded clay | |||
Expanded clay | |||
Expanded clay concrete | |||
Expanded clay concrete | |||
Ceramic hollow brick (gross 1000) | |||
Ceramic hollow brick (gross 1400) | |||
Red clay brick | |||
Brick, silicate | |||
Linoleum | |||
Minvata | |||
Minvata | |||
Foam concrete | |||
Foam concrete | |||
PVC foam | |||
Expanded polystyrene | |||
Expanded polystyrene | |||
Expanded polystyrene | |||
EXTRUDED POLYSTYRENE FOAM | |||
POLYURETHANE FOAM | |||
POLYURETHANE FOAM | |||
POLYURETHANE FOAM | |||
POLYURETHANE FOAM | |||
Foam glass | |||
Foam glass | |||
Sand | |||
POLYUREA | |||
POLYURETHANE MASTIC | |||
Polyethylene | |||
Ruberoid, glassine | |||
Pine, spruce along the grain | |||
Pine, spruce across the grain | |||
Plywood |
Table of vapor permeability of building materials
