Vapor barrier

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Vapor barrier film on a roll
Relocated vapor barrier

The vapor barrier is a component with a defined water vapor diffusion resistance , which is a diffusion of moisture from the warm indoor air in the heat insulation hinders a building (explained extensively in Breathing wall ). A vapor barrier is intended to largely prevent condensation from forming within components of a wall (especially the insulation ) , which can lead to black mold infestation if it is insufficiently dried .

A vapor barrier can at the same time take over the tasks of the airtight layer.

Rated values ​​for moisture protection

The classification of a building material as a vapor barrier is defined by its air layer thickness equivalent to water vapor diffusion . It also takes into account the actual strength of the component and not just the material properties such as the dimensionless water vapor diffusion resistance value . She is billed to .

In Germany, component layers are defined according to DIN 4108-3 (10/2018) as being permeable, retarding, inhibiting, blocking and impermeable to diffusion. In linguistic usage and construction practice, a distinction is also made between vapor barriers and vapor barriers.

s d value Degree of tightness
s d ≤ 0.5 m open to diffusion
0.5 m <s d ≤ 10 m diffusion inhibiting
10 m <s d ≤ 100 m diffusion-inhibiting
100 m <s d ≤ 1500 m diffusion-blocking
s d > 1500 m diffusion-proof

Vapor barrier

Vapor barriers, with an s d value of over 1500 m, are practically completely vapor diffusion-tight layers. The term vapor barrier is sometimes used synonymously with vapor barrier. In the strict sense, only metals and glass are complete vapor barriers.

In the construction sector, rigid foam panels laminated with aluminum foil on one or both sides, as well as mineral or glass fiber insulation, are offered to prevent moisture penetration of the insulation material from inside the building. The problem is that water can penetrate through every small damage to the thin film, but then basically cannot evaporate inward again through the barrier layer if the evaporation to the outside is also prevented by blocking layers, as is the case, for example, in non-ventilated flat roofs is.


Simple vapor retarders usually consist of thicker plastic films , often polyethylene films. With a thickness of 0.1 mm (0.0001 m) and a water vapor diffusion resistance number (µ) of 100,000, it has a barrier effect like 10 m of air. Aluminum foil with a thickness of 0.05 mm has an sd value of 1500 m. This is a value 300 times greater than that of a polyethylene film of the same thickness.

However, other materials (e.g. PVC foils or wooden panels) can also serve as a vapor barrier, depending on the water vapor diffusion resistance, the material thickness used and the vapor barrier.

Building physics function

In winter, the heated indoor air of inhabited rooms can absorb more moisture than the cold outdoor air, which leads to a higher partial pressure of water vapor in the warm indoor air. In this situation, water vapor diffuses through the outer walls or loft extensions from the inside (high partial pressure) to the outside (low partial pressure) because concentration or pressure gradients are always trying to compensate. In building components, the vaporous water that has diffused in will condense out at this point due to the lower water vapor absorption capacity of cold air, i.e. due to the dew point within the wall and cause moisture damage or further frost cracking there .

The condensation or waterlogging takes place at that point of the wall cross-section (in the wall on a flat front) where the dew point is not reached. If there is an insulating layer, condensation takes place in the outer area of ​​the insulating material on its colder side. To avoid such condensate, the building envelope can generally be made relatively vapor-tight on the inside and become more and more permeable from the inside to the outside. Vapor retarders are therefore usually arranged on the room side, that is to say on the heated side of the thermal insulation layer. It should be noted, however, that thermal insulation composite systems on the facade quickly cool down below the dew point of the outside air at night, where dew condenses and the main waterlogging also and more likely to penetrate from the outside.

Since unplanned moisture penetration or minor damage to the vapor barrier will always lead to more water (vapor) entering the insulation than intended, it should be able to diffuse out through the outer layers of the component, which are open to water vapor diffusion. In a warm roof, for example, diffusion-open roofing or sarking membranes are used today . As a rule of thumb, the airtight layers on the cold side of the structure should be about six times more permeable to water vapor than the layers on the inside. When building in southern latitudes, it should be noted that in hot and humid climates and in rooms that are permanently cooled by air conditioning, the steam flow can also be directed from the outside to the inside.

In the case of flat roofs (for example non-ventilated “ warm roofs ” with under or between rafter insulation ) the principle “outside more permeable than inside” cannot be implemented because the materials of the outer roof sealing , for example bitumen welding membrane or sheet metal, are often completely impermeable to diffusion. If water penetrates into such a non-ventilated roof structure, which is encapsulated on both sides, through imperfections in the vapor barrier or if the thermal insulation is installed in a not completely dry state, it can no longer leave the structure automatically and would have to be removed by technical drying .

As a rule, it cannot be permanently ruled out that small amounts of moisture will gradually get through the vapor barrier and accumulate in the insulation layer or on the underside of the roof cladding. The humid, warm air often finds its way through small imperfections in penetrations of the vapor barrier layer, such as cable ducts, cracked wooden stems, leaky connection points, and through the adjoining walls via "flank diffusion".

A vapor barrier on the warm side of non-ventilated flat roofs was classified in October 2008 by the “Wood Information Service” as no longer state of the art. DIN 4108-3: 2017-09 stipulates equivalent air layer thicknesses of the vapor retarders depending on the air layer thicknesses of the outer diffusion-inhibiting layers for components that do not have to be verified according to the Glaser method. It is required that a maximum area-related amount of condensation water of 1.0 kg / m² is not exceeded on roofs and external walls and that wood-based materials may only be installed up to a permissible material moisture level. DIN 68800-2 on wood protection requires a dry reserve of more than 250 g / m² to take into account the convective moisture entry. The leaflet on thermal insulation for roofs and walls from the German roofing trade recommends that if the outer roof sealing has an equivalent air layer thickness of sd, e> 2.0 m, building materials such as wood that are not durable against the effects of moisture should be avoided.

If the steam that has penetrated condenses to form water at the coldest point in the insulation layer, puddles of condensation can literally form. Non-ventilated flat roofs with mineral wool insulation between the rafters and a vapor barrier on the underside are always endangered in the long term by the constant penetration of small amounts of moisture.

Puddles of condensation on the vapor barrier of a flat roof
Moisture damage to a roof that is vapor-proof upwards and downwards; the fan shrouds installed on the top could not create sufficient ventilation from below and therefore only had a very limited range

An article on only names two applications in which a vapor barrier could be useful:

  • on the inside of insulated roofs
  • on the inside of basement walls, which are provided with internal insulation

However, the literature also describes applications of vapor barriers in the construction of external walls (when using vapor-tight external cladding or lack of ventilation) or for protecting screeds on young concrete.

Alternatives to the classic vapor barrier

An alternative that has been tried and tested since the 1990s is the use of sorptive and capillary-active insulation materials, such as cellulose flakes or calcium silicate boards, in combination with a moisture-variable vapor barrier. Sorptive (absorbent) insulation materials can temporarily store and distribute penetrated moisture, the moisture-variable vapor barrier ensures re-drying in times when the pore humidity between the insulation fibers exceeds the humidity in the heated interior.

Another alternative to the insulation between the rafters is the inverted roof , in which the roof seal is attached below the thermal insulation and can thus also act as a vapor barrier. This second sealing layer, located under the insulation, is well protected against UV light, mechanical damage and temperature fluctuations and is therefore very durable. On top of the vapor barrier / sealing layer is an insulation layer made of water-resistant, rot-proof materials, such as closed-cell XPS (extruded polystyrene rigid foam), which is either in contact with the moisture from precipitation, or whose moistening can at least be accepted is protected with a further seal in the interest of better thermal resistance. In addition, the implementation of such an additional protective layer, for example the application of a layer of gravel on the thermal insulation, can be made mandatory in building inspectorate approvals. If this outer seal leaks due to damage or aging, this only results in a slight deterioration in the thermal resistance.

With flat roofs, the construction height of the roof is often limited. Some building codes require a room height of 2.40 m, while at the same time the roof should be greened or a roof terrace should be set up, although the spacing regulations often prevent the roof from being raised. The combination of on-roof and between-rafter insulation is then often an obvious solution that is approved even for green roofs that are particularly demanding in terms of moisture .


Moisture-variable vapor barrier

Planned condensate in the wall or roof structure or unplanned moisture due to structural damage should be able to be discharged to the outside at any time to a sufficient extent to evaporate on the outer skin of the building.

As a rule, an outwardly decreasing diffusion resistance of the individual component layers is provided for this. In summer, however, the high temperatures of the outside air can lead to water vapor diffusion with a pressure gradient from outside to inside (reverse diffusion). In this case, a vapor barrier located on the inside of the component would be unfavorable, since the water vapor can build up at the boundary layer with the more diffusion-open thermal insulation, which leads to an increase in the relative humidity there. If this continues to rise, this can result in the loss of condensation, which can be harmful to the component.

In this case, the use of a moisture-variable vapor barrier can be advantageous.

In the trade, these are also referred to as moisture-adaptive or intelligent vapor barriers and climate membranes . Their s d value varies with the prevailing humidity and its absorption by the plastic layer. In winter, the moisture-adaptive vapor barrier works like a commercially available vapor barrier with high diffusion resistance when the water vapor pressure is from the inside out. In summer, however, when there is a diffusion flow from the outside in, it can reduce this by absorbing the moisture. As a result, the water vapor flow is not hindered to the usual extent, but can also be diverted inwards into the room air.

The use of moisture-adaptive vapor retarders is particularly recommended for extended roofs. But they can also be used for internal insulation of external walls.

Special, moisture-variable vapor barriers have only been available for a few years. Knowledge of the operating conditions and limits is still relatively limited.

Wood as well as all traditional mineral building materials have a moisture-variable diffusion resistance, which is less pronounced than with special moisture-adaptive vapor control membranes. In contrast to these building materials, vapor control membranes are usually unable to remove moisture by capillary action, so that from a building physics point of view, constructions with traditional building materials are preferred in many cases. However, there are also capillary-active vapor retarders, such as polymer-coated fleeces, which absorb moisture and release it back into the room air at a later point in time.

The vapor diffusion resistance of plastering mortar applied on the inside can be controlled, for example, by specifically increasing the synthetic resin content.

When using wood-based panels as a vapor barrier, it should be noted that vapor diffusion resistance and capillarity are heavily dependent on the amount and type of material used. OSB boards in particular cannot always be expected to have better properties than vapor control membranes. However, the risk of accidentally perforating the plates is less.

Capillary transport

In addition to water vapor diffusion and sorption, capillary transport can occur in building materials. In principle, almost all mineral building materials (except mineral wool) as well as wood and natural materials are capable of capillary transport , so that they can dry out quickly after exposure to moisture.

Water vapor can diffuse through open-pored building materials such as mineral wool. Mineral wool can also bind liquid water by adsorption. However, if there is a large amount of liquid water in the insulation material due to an incorrect wall structure or an inadequately installed vapor barrier film, it can take a long time for it to dry out again. This is because capillary transport hardly takes place in materials such as mineral wool and polystyrene (Styrofoam) and evaporation from the insulation material is usually hindered by the surrounding component layers.

Since significantly larger amounts of moisture are moved during capillary transport than during water vapor diffusion, the use of capillary-active (i.e. absorbent) materials in combination with a moisture-variable vapor barrier is particularly important in the case of building physically critical wall and roof constructions, in order to prevent any condensation that may arise locally in the insulation layer to pull apart, thereby reducing the moisture concentration and increasing the re-drying area, and to enable re-drying both through the vapor barrier and through the diffusion-open roof membrane.

With the consistent use of mineral materials and natural building materials, the strict adherence to the rule that the diffusion resistance of the inner component layers should be significantly higher than that of the outer layers can be dispensed with. If certain conditions (e.g. high capillary activity) are met, it is possible to dispense with a vapor barrier, even with interior insulation , despite the expected condensation .

When using mineral building materials, wood and natural fibers and at the same time dispensing with capillary-breaking, water- and vapor-tight layers in ceilings and walls, the consequences of water damage can usually be reduced to a minimum. In historical buildings, moisture damage could be quickly identified by means of water stains on ceilings or walls. The moisture absorbed by the structure usually evaporated after the cause had been removed before serious damage to the building fabric occurred. Today, barrier layers such as foils or synthetic resin coatings are often used in buildings to prevent the water from draining away and evaporating. Water damage are often only discovered when already swell timber structures and forms of mold or already dry rot grows out of control.


Both vapor barriers and vapor barriers are usually installed on the room side, i.e. on the heated side of the thermal insulation layer, and must be made airtight.

Bituminous vapor barriers are usually manufactured in a bonded layer structure. This can be done over the full surface on a separating or leveling layer, for example perforated glass fleece bitumen sheeting, or in the case of a so-called "equalizing vapor barrier" in the form of loosely laid, point or strip welded or mechanically fixed bitumen welding and cold self-adhesive sheeting. Cold self-adhesive sheets are applied self-adhesive on the underside and overlapping seams are welded with a torch. For vapor barriers made of bitumen, a minimum overlap of 8 cm at seams and joints must be observed.

Vapor retarders made of plastics such as polyethylene foils are mostly laid loosely on rough surfaces and an additional leveling layer. In addition, they can later be mechanically fixed by ballast or with the thermal insulation or roof cladding. Seams are connected by self-adhesive seam tapes, hot air or source welding.

In order to avoid a leak, the film should be carefully glued and if staples are used, an additional sealing tape should be applied in the affected area.

An improperly installed vapor barrier will cause condensation in the insulation layer. Even a few leaks (e.g. cable openings, sockets) render a vapor barrier ineffective. Warm and humid indoor air gets into the insulation, cools down there, and the moisture contained in the air condenses in the form of condensation . The thermal resistance of damp insulation material is reduced, which can further increase condensation. Water damage and mold growth on the building fabric are possible consequences.

The proper tightness of the building envelope is verified with a differential pressure measurement method .

Since a completely airtight installation of the vapor barrier is only possible in very few cases, additional protection can be achieved by installing negative pressure ventilation in particularly endangered wall or ceiling structures.

Indoor climate

If further (space-enclosing) components with a thickness of at least 1.5 cm that are capable of storing (buffering) moisture are installed on the inside of the vapor barrier, the moisture-barrier effect of the vapor barrier only has a minimal effect on the room climate . This is because the moisture balance through the buffer effect takes place predominantly in the first one to two centimeters of the wall structure, so that the vapor barrier behind it has hardly any influence on the moisture content of the indoor air. The amount of moisture that is released into the open through a so-called breathing outer wall that is open to diffusion is also very low compared to the air moisture that is usually transported out through simple ventilation of the interior.


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Web links

Individual evidence

  1. a b Susanne Rexroth, Friedrich May, Ulrich Zink: Thermal insulation of buildings: contemporary and versatile. VDE-Verlag, Berlin 2014, ISBN 978-3-8007-3570-9 , pages 175 ff
  2. ^ Eberhard Schunck , Thomas Finke, Richard Jenisch, Hans J. Oster: Dach Atlas: Inclined roofs. Birkhäuser, Berlin 1996, ISBN 3-7643-6479-3 , page 204
  3. German Institute for Standardization: DIN EN ISO 10456: 2010. Building materials and building products - Thermal and moisture-related properties - Tabulated rated values ​​and methods for determining the thermal insulation nominal and rated values ​​(ISO 10456: 2007 + Cor. 1: 2009); German version EN ISO 10456: 2007 + AC: 2009. Beuth Verlag, Berlin 2010.
  4. ARGE: Dampfsperre - The vapor barrier protects the building structure and helps save heating costs , In:
  5. Michael Bonk (Ed.): Lufsky building waterproofing. 7th edition. Vieweg + Teubner, Wiesbaden, ISBN 978-3-8351-0226-2 .
  6. Consensus of the speakers at the “Wood Preservation and Building Physics” congress on February 10/11, 2011 in Leipzig on the subject of “Unventilated wooden roofs”. Seven golden rules for a proof-free flat roof.
  7. Holzbau Quadriga Nt. 5/2011, Bernd Nusser, Martin Teibinger, Holzforschung Austria, Vienna: Green roof versus foil roof , pages 13–23.
  8. see point 4.7 of the study Study ( Memento of the original dated February 3, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. of the vapor control membrane manufacturer "pro clima" (PDF; 2.1 MB) @1@ 2Template: Webachiv / IABot /
  9. Matthias G. Bumann: Sorption ( Memento of the original from December 19, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. - A consideration on the subject of “moisture in the external wall component” (PDF; 965 kB). This treatise is to be read critically. Not all statements seem to be conclusively proven. @1@ 2Template: Webachiv / IABot /
  10. Peter Cheret, Kurt Schwaner: Holzbausysteme - an overview. In: , accessed in December 2016
  11. Application brochure for internal insulation of external walls. Gutex Thermoroom, as of August 2015; accessed in November 2016
  12. EU Köhnke, ö.buv expert for timber house construction: The fault is always the other person - How does moisture get into a floor separating ceiling? , Magazine "Die neue Quadriga", pages 44ff, 4/2012