Home bathroom and sanitarySteam brake vs. Vapor barrier - differences simply explained

Steam brake vs. Vapor barrier - differences simply explained

  • The difference
  • Steam brake or vapor barrier "> Advantages and disadvantages
  • Check airtightness
  • Why vapor breaks and vapor barriers

In new construction as well as in the renovation of old buildings, builders inevitably do not pass vapor barriers to protect the insulation of the house from moisture. Quickly the question arises: what is better? Steam brake or rather vapor barrier? Although the name difference is quite small, the differences between vapor barrier and vapor barrier are quite large. So that the insulation and the building fabric are not damaged, the client should know the differences.

In addition to the normal development of steam in the room by, for example, cooking, showering, bathing or even by plants, a 3-4 person household produces about 10 to 15 liters of water vapor per day, which it releases into the air by breathing. The air takes with it the water vapor that it carries along with every movement of air through the house. But not only here, air also diffuses through the masonry and gives off some of its moisture to the building fabric of the house. In order to prevent too much moisture from being released to the building fabric and especially the insulation, resulting in moisture damage, mold or reduced insulating properties, vapor barriers or vapor barriers must be used.

The difference

A vapor barrier is used when the Sd value of a material is between 0.5 meters and 1, 500 meters. It is said that the material is vapor diffusion-inhibiting .

A vapor barrier is used when the Sd value of a material is greater than or equal to 1, 500 meters. It is said that the material is vapor-tight .

Vapor barrier and vapor barrier - the difference

The real difference between vapor barrier and vapor barrier is included in the name. While the vapor barrier allows the moisture to diffuse in small quantities, the vapor barrier does not allow any moisture to pass through at all, which is referred to as absolute vapor-tightness.

It should be noted at this point that caution is required when naming. Colloquially, the word vapor barrier has been established for both vapor barrier and vapor barrier.

In the context of vapor barrier and vapor barrier, this is also referred to as the Sd value (diffusion resistance number). The value is a virtual value. It indicates how long water vapor is needed to penetrate an airtight component. The higher the value, the longer the water vapor needs through the barrier. The thickness of the barrier depends on the insulation and the surrounding building materials. The unit of value is meters.

Example: A 10 cm thick polystyrene plate has a Sd value of approx. 50 x 0.10 m = 5 m.

According to this, water vapor needs as much time to penetrate a 10 cm thick polystyrene plate as it does through the penetration of 5 m air.

Steam brake or vapor barrier ">
Principle and problem of the vapor barrier

But not only these mistakes can lead to moisture damage. The components of the house itself already contain water. Bricks can absorb and store water. Roof battens may have been dried before installation, but a downpour is enough and they already have some water in them again. Here and there some water in the components adds up and moisture damage is the result.

For this reason, in the construction practice almost completely waived vapor barriers and preferred the vapor barrier . Vapor barriers are only to be used where static moisture can occur. Static humidity means that moisture only ever wants to penetrate from one side into a component. But this does not occur in the home, but is only the case with steam baths and cold stores .

Advantages and disadvantages


  • Protection of the insulation from wetness
  • heating costs


  • with insufficient ventilation mold in the house
  • Incorrect installation causes accumulation of water that damages the building structure and the insulation

Self-regulating vapor breaks

If normal vapor barriers and vapor barriers have only one Sd value, which is always valid, intelligent vapor barriers, which are able to influence the diffusion of water vapor and have both the properties of vapor barriers and vapor barriers, have been available for some years now. Depending on the season and the prevailing humidity, the barrier changes its Sd value. Thus, the self-regulating vapor barrier acts as a vapor barrier in winter, allowing only small amounts of moisture to pass through and being permeable to vapor in summer. This allows moisture that has accumulated in winter to escape again. However, this only works if suitable insulation is used, which is permeable to vapor.

Check airtightness

Even the smallest holes in the barrier, as already explained, can have devastating effects on the building fabric. For this reason, in homes that have a vapor barrier or vapor barrier and an airtight building envelope have performed an airtightness test, the so-called blower-door measurement method .

This measuring procedure closes all windows and front doors. Interior doors are all open. Hoods, keyholes and other openings where air can enter or exit are sealed. In a door or a window of the house is then installed a blower-door fan.

With the help of the fan, air is continuously drawn from the building until a vacuum of 50 Pascal is reached. Subsequently, an overpressure of 50 Pascal is built up in the house. The values ​​of the measuring procedure are recorded by the blower door computer. If there were leaks, it would not be possible to maintain a negative pressure of 50 Pascal on one side as air flows into the house. On the other hand, no overpressure of 50 Pascal could be built up because the air would escape from the house. The mean value of the negative pressure and overpressure measurement is determined by the blower door computer and indicates whether the building envelope is sealed in accordance with the EnEV (Energy Saving Ordinance) and there is no leakage.

Why vapor breaks and vapor barriers

Vapor barriers and vapor barriers are primarily installed in houses, as the Energy Saving Ordinance prescribes. It requires an airtight building envelope in order to achieve the lowest possible heat transfer and to protect the insulation from moisture.

save energy

Through walls and roofs, which were provided with a vapor barrier or vapor barrier, no more or less water vapor can pass through. The result is often that the moisture that gets through the roofs and walls gets stuck in wall and ceiling constructions. If this happens, a mold damage is not far, in addition a damp insulation loses its insulating effect.

For this reason, proper ventilation in homes with vapor barriers and vapor barriers is essential. Ventilation 2 to 3 times a day is advisable, with new buildings 3 to 5 times. Several windows in the house are fully opened for 10 - 15 minutes (not tipped off). The more people live in the household, the longer and more often it has to be ventilated.

Incidentally, a misconception is that you can not ventilate in the winter in fog, snow or rain, because the air from the outside would be too wet. The air in the open air is humid, but dries very quickly due to the warmth of the living space and takes moisture on its way through the apartment again outside.

If a regular ventilation can not be guaranteed, a controlled ventilation in the house should be considered.

Tips for quick readers

  • Vapor barriers or vapor barriers protect the insulation from moisture
  • Moisture in the insulation minimizes the insulation properties
  • Vapor barriers prevent water vapor from diffusing through the barrier
  • Steam brakes slowly allow water vapor through the barrier in small quantities
  • The value is a virtual value that indicates how long water vapor takes to penetrate an airtight component
  • Vapor barriers have a Sd value greater than or equal to 1, 500 meters
  • Steam brakes have a Sd value between 0.5 and 1, 500 meters
  • The thickness of the barrier or the Sd value depends on the surrounding materials of the wall
  • For air-tight building envelopes, perform a blower-door measurement procedure
  • Vapor barriers are hardly used anymore
  • Incorrectly applied vapor barriers can cause severe structural and moisture damage
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