Breathable films are usually called membranes, i.e. films that provide protection against the penetration of atmospheric moisture, while remaining practically transparent for water vapor to escape from the inside. High vapor permeability (Sd < 0.05 m) is achieved due to the special microstructure of the membranes, which are non-woven materials made from synthetic fibers.
Fig. 1 Roof waterproofing device using a breathable membrane
Membranes owe their appearance to a sharp tightening of norms for the heat conservation of building structures in Western countries. Today, in connection with the adoption of similar standards for heat conservation in our country (SNiP II-3-79 *, 96, Construction heat engineering), breathable membranes have become widely used in our country..
The indisputable advantage of breathable membranes is that only they allow the most efficient use of all the space between the rafters for thermal insulation. Breathable membranes, unlike all other types of films, are laid directly on the heat-insulating material, so their use allows you to abandon the ventilation gap, which eats up to 50% of the space intended for roof insulation.
Fig. 2 The use of breathable membranes allows the entire space between the rafters to be used for thermal insulation. Roof structure using:
A – traditional waterproofing film; B – breathable membrane.
1 – roofing; 2 – lathing; 3 – counter lattice; 4 – waterproofing layer; 5 – rafters; 6 – thermal insulation; 7 – vapor barrier layer; 8 – breathable membrane.
For example, if the height of the rafters in the cross section is 150 mm, then when using non-breathable films, the thickness of the insulation that can be laid between the rafters is about 80 mm. According to modern requirements, this is almost two times less than the norm (150 mm – 80 mm = 70 mm is the minimum ventilation gap (including at least 20 mm for the film slack), which must be left to ventilate the insulation). Application <breathable> the membrane creates additional space for thermal insulation, allowing you to lay insulation with a thickness equal to the height of the rafters (in our example, it is 150 mm), which, as a rule, meets modern standards for heat conservation.
Breathable membranes are especially widely used in attic construction. Their use is optimal when converting a cold attic into an attic room, without replacing the existing rafter structure.
There are several types of under-roof breathing membranes on our market. Among them:
These membranes have only one drawback – a higher price. True, the difference in price between a breathable membrane and other types of films is a drop in the ocean compared to the cost of the entire roof and even more so compared to the total cost of the entire building.