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Vapor barrier, vapor barrier, diffusion – exterior wall in the prefabricated house

The exterior walls of prefabricated houses are usually made of prefabricated cladding of a load-bearing wooden frame. Inside the compartments are installed insulation materials. Construction and insulation materials, vapor retarders and diffusion openness differ. What diffusion, diffusion-open walls, the vapor retarder and vapor barrier are all about, you can find out here.

the essentials in brief

  • So-called vapor barriers do not allow water vapor to pass through. On the other hand, differently constructed vapor retarders reduce diffusion to different degrees – they are therefore open to diffusion to different degrees.
  • The building envelope of diffusion-open new buildings must also be sealed airtight.
  • It causes great damage when water vapor condenses into water in well-insulated exterior walls. Vapor retarders can prevent this – but lower diffusion openness also affects the indoor climate.
  • Different insulation materials tolerate diffusion differently (buffering) – especially with ecological insulation materials (e.g. wood fiber, cellulose), construction can be particularly open to diffusion.

Exterior walls in timber frame prefabricated house

The wooden frame of a prefabricated house consists of solid wooden posts spaced 62.5 or 81.5 cm. The so-called foot and head ledgers close the framework at the bottom and top. The so-called compartments of the supporting wooden frame are boarded with prefabricated wall elements. The wall elements are made of gypsum boards or wood-based panels (e.g. OSB boards or particle boards) or multilayer building boards. It is also said that the wooden frame “stiffens” as a result. Insulation materials are installed in the cavity within the interlocking compartments. (We have compiled more information for you on timber frame construction, also known as wood stud construction ).

From the outside, a prefabricated house can look different: Facades of wood or masonry are possible. However, timber frame prefabricated houses are considered timber houses even if the prefabricated facade is made of masonry due to their wooden framework – the wooden framework is what matters for the definition of timber house.

Thermal insulation for prefabricated house walls

In prefabricated houses, the thermal insulation is incorporated into the compartments interconnected by wall elements. It is about 20 to 40 cm thick. In most cases, components of the thermal insulation layer are also surfaced or plastered on the outside.

The most common insulation materials are mineral fiber, polystyrene or rigid foam boards. In contrast, wood fiber insulation boards, cellulose, cork or sheep’s wool are natural and ecologically sustainable insulation materials. Ecological insulation materials usually cost more.

Learn more about insulation here.

Vapor barrier, vapor retarder and diffusion openness

All insulation materials must be specially protected against moisture. Otherwise, massive damage can occur, especially in the case of prefabricated houses, which can only be repaired by large-scale renovation. But moisture does not only come from the outside. This is because the occupants are also made up of about 70% water and sweat occasionally; in addition, water vapor is generated by cooking and showering.

After all, a two-person household produces about 5 liters of water vapor a day – if more people are visiting, the volume increases quickly. This results in the so-called water vapor partial pressure, from which the relative humidity is also derived. If this vapor pressure is higher inside than outside, water vapor diffuses through the walls. However, this can only happen in so-called diffusion-open houses. In contrast, so-called vapor barriers are supposed to ensure absolute tightness, and so-called vapor retarders are supposed to ensure diffusion that is retarded to varying degrees.

In the case of older houses, the water vapor produced in the living space can additionally escape through the walls if ventilation is less severe. However, the heating costs are then also higher. In new buildings with high insulation values, this natural process is hindered or even completely prevented. It is then particularly important to have a well thought-out design. This is because insulation materials and effects, air tightness, diffusion openness and the installation of vapor retarders and ventilation systems must be coordinated.

Vapor retarders impede the diffusion of water vapor through the outer walls

Vapor retarders are usually made of plastic film (often polyethylene films) attached to the inside. However, you can also do without such films, for example, with OSB wood panels installed on the inside and fiberboard on the outside. Also used are rigid foam panels covered with aluminum foil, PVC film and fiberglass and mineral insulation acting as a vapor barrier.

The vapor barrier is designed to protect the insulating materials from the moisture generated in the living space. But this entails another set of problems. This is because self-regulating diffusion – i.e. the permeability of water vapor – is now reduced. This is not always necessary, as wall cladding and also the insulating materials can buffer a certain amount (depending on the properties of the material used) of water vapor. The natural diffusion then leaves no damage.

If the exterior walls allow no or very little diffusion, moisture can remain on furniture, for example, and cause clamminess if there is not enough ventilation. However, additional components are often installed to buffer moisture, which largely solve the problem of clamminess.

In houses with highly effective vapor barriers, dehumidification of the living space can hardly be mastered with manual ventilation alone. Automatic ventilation with ventilation systems (preferably with heat recovery) then ensures sufficient dehumidification.

Learn more about ventilation systems here.

Vapor retarder or vapor barrier?

The term vapor barrier (absolute impermeability) is often mistakenly used synonymously with the term vapor retarder (reduced permeability). The property of absolute impermeability to water vapor is actually possessed only by glass and metal. However, with stronger aluminum foil or foam glass, this can actually be implemented structurally. Vapor retarders, on the other hand, have different values depending on the constructional implementation.

Risk of mold due to faulty vapor barrier or retarder

The risk of mold growth in the insulation materials of the walls is high, especially in the case of faulty vapor barriers. For example, the internally applied film can easily have defects due to a screw, subsequently installed sockets or improper adhesion of the film. An absolute vapor barrier is therefore also not easy to implement structurally. Because even through small perforations water vapor enters the interior of the walls. However, it can then no longer escape, since the outer surfaces are also absolutely or highly diffusion-tight (e.g. due to installed polystyrene). Condensation and mold growth in the exterior walls is the result. This problem also affects vapor retarders that have a high sd value.

The sd value measures the degree of diffusion tightness

The degree of diffusion tightness of a vapor retarder is measured with the so-called sd-value. What does the sd value result from? Each building material has a specific vapor diffusion resistance number (µ-value), which indicates the diffusion properties of the building material. This value is multiplied by the installed thickness of the material(s-value). So the sd-value results from the following simple formula:

s d -value = µ value × s -value

The smaller the sd value, the better the diffusion of water vapor. If the sd value is above 1500m, water vapor blocking properties are present – practically no more water vapor can get through here.

sd ≤ 0.5 m diffusible
0.5 m < sd < 10 mdiffusion-inhibiting
10 m < sd < 100 mdiffusion-inhibiting
100 m < sd < 1500 mdiffusion blocking
sd > 1500 mdiffusion-tight
according to DIN 4108-3 (10/2018)

Houses open to diffusion contribute to better indoor climate

So-called diffusion-open houses deliberately rely on the fact that the walls can “breathe” – as you often hear in this context. However, this is a bit misleading, because water vapor is a dry gas that has nothing to do with oxygen. In addition, the building envelope of diffusion-open new buildings is also sealed airtight. Because this is the requirement of the legislator for new buildings.

In houses that are open to diffusion – where vapor retarders with a rather low sd value are installed – water vapor generated in the living space can also diffuse through the exterior walls naturally and without damage. Such vapor retarders can be constructed, for example, without polyethylene films and with OSB wood panels inside and wood fiber boards outside.

Normal ventilation without ventilation systems should then be sufficient. However, these designs must ensure that water vapor diffuses through the walls as a dry gas (even in winter when the interior surfaces are very warm and the exterior surfaces are cold). The danger here is that the water vapor condenses into water inside the walls instead (dew point). This can lead to major damage. (Ventilation is especially important for good housing health – ventilate properly and avoid mistakes).

The dew point must be outside the walls

Especially in heavily insulated houses, during the cold seasons, the interior surfaces are very warm compared to the exterior surfaces of the outer wall. Warm air absorbs more water vapor than cold air – as a result, the diffusion pressure increases. Particularly in the case of exterior walls with a low sd value, it must be ensured that the construction keeps the so-called dew point, at which the dry gaseous water vapor condenses into water, outside the walls. Otherwise, damp insulation materials and mold growth on the walls will occur, resulting in the need for significant renovation. Normally, the design of the prefabricated house ensures that the dew point is not located inside the walls. Otherwise, construction defects are to be assumed.

In principle, walls open to diffusion become increasingly permeable to water vapor from the inside to the outside. Denser materials (e.g. OSB boards) are installed on the inside and more diffusion-open materials (e.g. wood fiber boards) on the outside. Water vapor that passes through the denser interior surface then quickly diffuses throughout the wall construction without condensing into water and leaving moisture damage.

Good indoor climate even without ventilation system

Walls open to diffusion contribute to a balanced indoor climate and better residential health. Ventilation systems do not necessarily have to be installed in many cases. However, for correct estimation it should be added that the amount of moisture diffusing through walls is still very small compared to the amount of moisture transported out by ventilation.

Ultimately, the insulation material determines the degree of diffusion openness

Vapor retarders are open to diffusion to varying degrees. They are also installed in houses open to diffusion. You can make a conscious decision to use vapor retarders that are particularly open to diffusion. In fact, many architects and builders prefer the highest possible diffusion openness. But then you need to use appropriate insulation materials. Ultimately, it is the insulation material that determines how permeable the vapor retarder can be. Certain insulation materials tolerate (“buffer”) diffusion better (e.g. natural insulation materials such as wood fiber and cellulose) and others less well (e.g. mineral wool).

Wood fiber and cellulose – natural insulation materials for high diffusion coefficient

Wood fiber and cellulose are the best material for insulation with simultaneous high diffusion openness. Therefore, these insulation materials also ensure the comparatively best indoor climate, which contributes to the health of the home.

Wood fiber and cellulose are natural insulating materials that are ecologically sustainable in terms of production, installation and disposal. Another advantage of wood fiberboard and cellulose as an insulating material is fire protection: chemically produced insulation materials melt in fires, favor the fire and develop toxic gases. However, natural insulation materials are usually somewhat more expensive.

Here you can learn more about ecological insulation materials.

Building materials have different degrees of diffusion

It should be noted that the label “diffusion openness” still says nothing about the amount of water vapor that can actually diffuse through the exterior walls. This is because different wall constructions and building materials ensure different degrees of diffusion.

The so-called vapor diffusion resistance number (µ-value) of a building material provides information about the properties of the building material with regard to diffusion. Multiplied by the actual thickness(s-value) of the material used, the sd-value comes out (sd-value = µ-value × s-value). The sd value measures the degree of diffusion. The smaller this value, the better the diffusion of water vapor.

sd ≤ 0.5 m diffusible
0.5 m < sd < 10 mdiffusion-inhibiting
10 m < sd < 100 m diffusion-inhibiting
according to DIN 4108-3 (10/2018)

If suppliers suggest the installation of a ventilation system in a diffusion-open house, it can be assumed that the vapor diffusion resistance number is relatively high (about 1.5 upwards).


Vapor barriers (absolute diffusion tightness) are now unanimously discouraged. They are difficult to implement in terms of construction, as they are very susceptible to defects. And defects in vapor barriers lead to major damage. In addition, they have a negative impact on the indoor climate.

That is why most exterior walls of prefabricated houses are open to diffusion to varying degrees. This means that vapor retarders are installed that have a different sd value depending on the type of construction and thus ensure a different degree of diffusion. If the sd value is very low, manual ventilation without automatic ventilation with ventilation systems is sufficient to sufficiently dehumidify the living space. When houses are equipped with ventilation systems, it can usually be assumed that they have a comparatively low degree of diffusion openness.

Ultimately, the insulation material determines how much vapor retarder is needed. This is because different insulation materials have different buffering properties – they allow water vapor to diffuse to different degrees. For example, non-natural insulation materials such as mineral wool require a vapor retarder with a high sd value. In contrast, particularly natural insulating materials such as wood fiber or cellulose only require a low sd value. Ecological insulation materials and diffusion openness therefore go together perfectly.

Many suppliers offer good solutions with OSB boards installed on the inside and wood fiber boards installed on the outside – often no plastic film needs to be installed here. The OSB boards function as a vapor barrier, since the water vapor passes through them only relatively strongly retarded. The walls then become more permeable to the outside. Water vapor can diffuse easily through the exterior wood fiber boards without condensing in the walls.

Autorin Sarah Völkl

Sarah Völkl hat Architektur studiert und ist seit Jahren das Gesicht von a better place. Mit ihren Videos ist sie bei YouTube vielen Personen schon länger bekannt. Sarah teilt Ihr Wissen jetzt auch bei den Bautipps von Almondia.

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