In the case of an insulated roof structure, a layer must be installed underneath the insulation to prevent too much moisture in the form of water vapour from penetrating the insulation and causing moisture damage.
This vapour-barrier or vapour-retarding layer is usually produced using suitable foils, coated cardboard or plates. A distinction is made between diffusion-dense (sd value > 1500m), diffusion-inhibiting (0.5m < sd value ≤ 1500m) and moisture-variable layers.
According to the ZVDH , both diffusion-inhibiting and diffusion-tight membranes are referred to as vapour barriers . In daily use, however, diffusion-inhibiting membranes are usually called vapour barriers. When choosing the right membrane, the roof structure and the insulation material are decisive. The vapour barrier is usually not only the diffusion-inhibiting layer, but also the airtightness level of the roof.
of underground roofing membranes as vapour barriers: Since vapour barriers are hardly ever produced anymore, diffusion-tight bitumen roofing membranes are often used as vapour barrier in construction practice. This is ideal for a roof structure with above-rafter insulation and diffusion-tight roof cladding, such as metal or green roof covering. Here, the roofing membrane is laid over the inner formwork, followed by the above-rafter insulation and then another diffusion-tight roofing membrane. However, this design must be agreed with the manufacturers.
Aluminum-laminated insulation boards: Aluminum-laminated insulation boards are always diffusion-tight. If it is not possible to bond the panels tightly, an additional vapour barrier must be installed below them to prevent leaks and the resulting moisture damage in the roof structure above.
Moisture protection: Insulation materials must be particularly protected against moisture, as this impairs the insulating effect and can lead to long-term structural damage (e.g. mould). Some insulation materials, such as cellulose, can absorb small amounts of moisture and release it back into the environment over time, but greater moisture damage also reduces the insulating effect here. Protection against moisture from the outside is guaranteed by the covering in conjunction with the underlay. However, water vapour can also cause moisture from the inside, the so-called condensation precipitation. Since most insulation materials are permeable to diffusion, water vapour flowing through them does not damage the material as long as no condensation precipitates . A vapour-permeable roof structure allows water vapour from the room air to diffuse to the outside through the insulation. In the case of a vapour-tight roof cladding without rear ventilation, a vapour barrier or a moisture-variable vapour barrier must be installed to prevent damage to the insulation level.
Computational condensation verification: In order to avoid condensation due to water vapour diffusion , the water vapour transport must function through the sequence of layers. This is achieved by decreasing the diffusion resistance of the materials from the inside to the outside. The roof cladding must therefore be more diffusion-open than the inside of the roof structure. DIN 4108-3 specifies the assignment of the Sd values for the outer and room layers. If this sequence of layers is adhered to, there is no need to provide a mathematical proof of condensation. For all other exterior components, mathematical proof of condensation water safety should be provided.
Condensation due to water vapour convection: In order to avoid internal condensation due to water vapour convection , an intact airtightness level must be guaranteed. Particularly small leaks on diffusion-tight membranes (vapour barriers) can cause severe moisture damage, as water vapour convection can transport significantly more moisture than diffusion.
Airtightness OSB boards: OSB boards of quality classes 3 and 4 are considered airtight, but care must be taken to ensure that the panel joints are also sealed tightly. A simple gluing of the tongue + groove boards is not considered permanently airtight. For this purpose, the bonding must be carried out using approved PU adhesives or adhesive tapes. A plasterboard cladding can be mounted directly on OSB boards, i.e. without an additional substructure. This provides a stable base for fixtures such as ceiling lights. It is important to ensure that the necessary electrical cables are already laid in the insulation level during the roof truss assembly and that all penetrations of the OSB boards are glued airtight.
Blower door test: Since the vapour barrier layer is part of the airtight building envelope, its tightness can be checked by means of a blower door test. It is advisable to carry out this immediately after completion of the airtight building envelope in order to be able to eliminate discovered defects without major effort.
Crown of the wall: Since moisture can escape through the chambers of the bricks into an insulating plane above, it often makes sense to place a strip of the vapour barrier with an inward overlap over the crown of the wall before the roof truss is erected. The overlap can then be glued to the vapour barrier that is then installed over the entire surface.
Adhesive substrate: In order to achieve an airtight bond of the vapour barrier to the masonry, it is advisable to apply a smooth line to the inside of the wall.
DIN EN 13984 Waterproofing membranes - Plastic and elastomer vapour barrier membranes
DIN EN 12369-1 Wood-based materials - Characteristic values for the calculation and design of timber structures - Part 1: OSB, chipboard and fibreboard
DIN EN 300 Boards made of long, flat, aligned chips (OSB) - Definitions, classification and requirements
German Roofing Trade: Regulations, published by the Central Association of the German Roofing Trade (ZVDH)
Source: bauwion
