Vapour barrier or vapour retarder?

Insulation that works perfectly needs protection. In everyday construction, two different terms are often used: the vapour retarder and the vapour barrier. Although the two systems differ in their degree of permeability, they share the same fundamental aim: the targeted regulation of moisture within the building structure. Both systems control the movement of water vapour through the building components to prevent harmful moisture build-up in the insulation layer. However, using the correct system in each case is crucial for ensuring a building remains free from damage in the long term, for preserving its structural integrity and for promoting a healthy living environment.

Find out everything here about the physical differences, the benefits for airtightness and the appropriate system components from ISOCELL. 

The key difference lies in the so-called sd value (equivalent air layer thickness), which describes a material’s resistance to water vapour transmission:

• Vapour barrier (from diffusion-retardant to diffusion-impermeable): It regulates the flow of moisture. It allows a controlled amount of water vapour to pass through the building component. Modern, humidity-responsive vapour barriers (such as the ISOCELL Airstop Diva vapour barrier) adjust their sd-value to suit the climatic conditions: In winter, they act as a barrier to protect the building element from moisture; in summer, they open up to allow any moisture present to dry out towards the interior.
• Vapour barrier (diffusion-tight): It completely blocks water vapour (sd value > 1500 m). Nowadays, it is only used in very specific, exceptional cases (e.g. in extremely damp rooms such as indoor swimming pools or saunas) as it also prevents the building element from drying out.

The vapour barrier acts as an airtight layer, thereby protecting the structure from convection, i.e. uncontrolled air flow.

• Preventing structural damage: Warm indoor air can hold a lot of moisture. If this air enters the cooler insulation layer through leaks (convection = uncontrolled air flow), it condenses there. This can lead to mould growth and long-term damage to the timber structure.
• Energy efficiency & comfort: Only a completely airtight building envelope prevents uncontrolled draughts and ensures that warm air in winter stays where it belongs – in the living space.
• Quality assurance through measurement: Air-tightness is tested using a differential pressure measurement (blower door test). According to current guidelines (e.g. ÖNORM B 8110-1 / DIN 4108-7) must not exceed the following values: 3.0 1/h for buildings with window ventilation, 1.5 1/h for buildings with domestic ventilation, and 0.6 1/h for passive houses.

Even the best materials are of no use if mistakes are made during installation. Clear rules apply, particularly when used in conjunction with blow-in insulation (such as ISOCELL cellulose):

New builds vs. refurbishments

• In new builds: Installation usually takes place on the room side, directly onto the rafters or studs. Services (e.g. electrical cables and pipes) should always be routed on the warm side of the insulation layer or laid in a separate service layer to minimise penetrations of the vapour barrier.
• In refurbishment (sub-and-top installation): If the roof is being refurbished from the outside, the vapour barrier is laid in a loop over the rafters. In this case, strict care must be taken to ensure that the membrane is connected airtight and without gaps to the masonry and existing structural elements. 

Important installation guidelines:

• Tension relief for blow-in insulation: When compartments are retrofitted with cellulose insulation, blow-in pressure is generated. The vapour barrier must therefore be mechanically secured (e.g. using cross-battens spaced at a maximum of 30–40 cm) against this pressure to ensure that the bonded joints remain permanently airtight.  
• Clean penetrations: If pipes or cables need to pass through the layer, they must be sealed airtight using special sleeves.
• Substrate preparation: Adhesive tapes will only adhere to substrates that are free from dust, grease and ice. On rough or porous surfaces (e.g. softwood fibreboards, concrete or plaster), prior treatment with an adhesive primer is required. In the case of very uneven masonry, sealants (such as AIRSTOP SPRINT sealant) can be used to connect the membrane.

Every construction project and every refurbishment presents its own challenges. Our experienced ISOCELL Bautechnik team will be happy to assist you with planning the correct airtightness layer, calculating U-values and condensation calculations, or answering any questions you may have regarding detailed design.

Contact the Construction Technology Team now