There are a number of factors that are acting on the roof, both external (wind, noise, rain, UV exposure) and internal (water vapor, temperature variations). For these reasons, the tightness of the roof and façades is decisive for the establishment of indoor comfort. An airtight roof limits the risk of condensation, improves the insulation efficiency and makes significant energy savings, improves the functioning of the ventilation system.
The most dangerous is uncontrolled vapor circulation, especially in the cold season when large quantities of vapor tend to escape outward through the non-airtight zones. According to estimates, the average amount that accumulates in a house in 24 hours is approx. 12 l which affects the inside space and risks producing undesirable effects over time.
It is therefore necessary to protect the roof as effectively as possible, especially thermal insulation. To this end there are two major categories of membranes that ensure the long efficiency of insulation: An outdoor vapor barrier is used and an indoor anti-condensation sheet.
The vapor barrier is intended to prevent the flow of indoor air to the outside and limits the amount of vapor being transported out.
The anti-condensation sheet allows the vapor that has reached the thermal insulating layer to escape as quickly as possible to the outside and prevents the formation of condensation that would affect the thermal insulation. At the same time, condensation may be formed under the cover into the thermal insulation layer.
To choose the appropriate type of membranes to protect your roof, you will need to consider a number of criteria:
- vapour permeability (Sd);
roof type (hard boarding / partly-boarded roofs, ventilated / non-vented, uninhabited attic / inhabited mansard);
- type of cover (ceramic tiles, concrete, metallic, other types).
One of the determinative factors in the choice of roof membranes is the vapor permissivity (given By The Sd value). Depending on this value you can make a first split by category:
Sd < 0.5 m – anti-condensation membrane (outward). For the anti-condensation sheet The Sd value must be as low as possible. The most efficient anti-condensation sheet have Sd values between 0,15 and 0,02 m.
Sd > 2 m - vapor barriers (inward). In this case, The Sd must be as high as possible. The most efficient vapor barriers have Sd values between 100 and 150 m.
Roof membranes with Sd values between 0,5 and 2 m can replace anti-condensation membrane in certain types of coatings providing thermal ventilation or in the case of uninhabited attics, ventilated bridges.