The vapour barrier
Apart from choosing the right insulation material with the appropriate thickness, a correct installation highly influences the quality and insulation value as well. By covering the insulation with a vapour barrier sheet, everything will be sealed airtight. In this way, the damp indoor air won’t penetrate the isolation. Without a vapour barrier, moulds would occur and the insulation value would decrease.
Condensation: Causes and consequences
In the form of vapour, air always contains a certain amount of water. The higher the air temperature, the more water that can be taken. When the maximum amount is reached, the air is saturated.
The air will contain too much vapour when the temperature drops, which is emitted in the shape of water. This is what we call condensation. Cool surfaces like cold walls, the roof, windows, doors and domes will cool down fastest.
If the water vapour can penetrate the insulation, condensation will appear in the insulation material. Over time, this may start to cause damage such as rotting, stains and a deteriorated insulation value. In order to avoid problems like these, the installation of a vapour barrier is crucial. This barrier must be applied onto the inner side of the heated zone, in order to prevent damp air from penetrating the insulation.
When to install a vapour barrier?
When you are insulating the house on the inside, a vapour barrier is always required. The only exception to this is insulation which is already provided with a vapour resistant layer. In this case, it is important to seal the gaps and seams with a special vapour barrier tape to make sure that vapour still cannot get through.
The weakest points of a house are the ceilings, together with damp rooms like the kitchen and the bathroom. It is advised to ventilate these rooms sufficiently.
Frequently used vapour barrier materials
A vapour barrier can easily be realised with a plastic foil like Polythene (at least 0.2 mm thick). This can be attached with staples beneath the rafters or trusses on the inside. The seams must be sealed with tape and the edges of the foil should overlap a little (a few inches).
If the roof is insulated with foil-faced insulation blankets, the aluminium side can serve as a vapour barrier at the same time. However, note that one should seal the seams here as well. Next to that, you may also buy a moisture regulating vapour barrier membrane that can manage the damp even better.
Vapour barrier installation: what must be considered?
Everyone can install a vapour barrier, but attention is required when sealing the overlaps between the strips. If the finish is not carried out well enough, the vapour barrier won’t be effective at all.
Because of this, many producers provide a range of watertight strips or sealing rolls that are adapted to their product. A vapour barrier that is pierced in several places due to power lines won’t be of any use either. Unfortunately, situations like these often occur in practice.
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