- Traditional screens
- Well-defined functions
- The different types of screen
- Traditional underglazing screens with radiant barriers
- What is a radiant underroof fence?
- The classification "R"
- Regulatory recommendations
- Ventilation closures, indispensable complements
Easy and fast to implement, flexible underlays are increasingly used to protect the roof in new works. They are mandatory in certain geographic areas to enhance airtightness and wind resistance.
As the supply of manufacturers is becoming wider, it is important to implement the screen whose characteristics correspond best to the configuration of each cover so that the protection does not become a source of pathology.
Choosing a roof underlay
All flexible under-roof screens available on the French market are subject to CE marking and fall under standard NF EN 13859-1. To choose the most suitable screen for the work, you need to know the support on which it will be placed, namely:Discontinuous support It will be necessary to take into account the spacing of the rafters, in order to put the screen having the classification "R" appropriate. Ventilation on the underside of the screen will be provided by the entire volume of the attic undeveloped or by an air space of at least 20 mm. The ventilation on the surface of the screen will be ensured by a ventilated air gap provided by the installation of a counter-lathing of at least 20 mm. It is fed by the hatches or by the simultaneous installation of a sewer and a ridge ventilated. This type of installation involves an interruption of the screen a few centimeters (2 to 5 cm) from the ridge line. Types of screens recommended: bituminous screens, synthetic screens, HPV synthetic screens.
Discontinuous support: rafters or farmhouses.
Continuous support It will promote the installation of a screen with a fibrous underside that improves their adhesion during installation.
Ventilation on the underside of the support will be provided by the entire volume of the undeveloped roof or by an air space of at least 20 mm, depending on the type of roofing material (roofing shingles = 40 mm).
Continuous support: contiguous foliage, wood-based panels.The ventilation on the surface of the screen will be ensured by a ventilated air gap provided by the installation of a counter-lathing of at least 20 mm.
It is fed by the hatches or by the simultaneous installation of a sewer and a ridge ventilated.
This type of installation involves an interruption of the continuous support and the screen to a few centimeters (2 to 5 cm) from the ridge line.
Types of screens recommended: bituminous screens *, reinforced synthetic screens *, HPV synthetic screens.
* Suitable for continuous media
Direct laying on insulation
The ventilation on the surface of the screen will be ensured by a ventilated air gap provided by the installation of a counter-lathing of at least 20 mm. It is fed by the hatches or by the simultaneous installation of a sewer and a ridge ventilated.
This type of installation involves the systematic implementation of a screen whose permeability to water vapor eliminates a ventilated air gap. Indeed, when the under-roof screen is in direct contact with the insulation, the transfer of water vapor present in the wall is effected through the screen.
Types of screens recommended: HPV synthetic screens, whose Sd value, is ≤ 0.09 m.
Thermal insulator: mineral wool, vegetable wool.
Note: in renovation, the roofer generally does not have the opportunity to ensure the quality and implementation of the vapor barrier. In this case, it will promote the implementation of a two-sided ventilated screen to limit the risk of dew and condensation.
The DTU series 40 recommends the most often the installation of a screen and the regulation imposes even in some cases (mountain, seaside, roof with lowered slope...). In general, the functions provided by the under-roof screen make it a useful component of any cover in small elements:
- It helps to limit the lifting and moving of tiles under the effect of wind.
- It protects against penetration of rain, powder snow, dust... and it ensures the evacuation of the water of condensation or infiltration towards the gutter.
- More or less permeable to water vapor, it prevents the internal moisture from condensing in the insulation.
- It also reduces the risk of intrusion (birds, rodents, insects...) in the attic.
The different types of screen
They consist of a non-woven frame and synthetic fibers coated with bitumen. Exceptionally waterproof, they are less flexible than other screens and have low permeance *, which requires careful ventilation.Multilayer screens
In synthetic material (polyethylene, polypropylene...), armed and microperforated, they are much lighter.HPV screens
High-performance "Highly Permeable to Steam" screens are characterized by high permeability * facilitating the evacuation of all excess water vapor. They can be placed directly on the insulation without sparing an air gap.Reflective screens
Functioning as a mirror, the reflective screens placed under the tiles return much of the infra-red rays received, reducing the heat energy transmitted to the attic. They help to improve summer comfort.
Good to know
Under certain conditions, the use of a screen may allow a lowering of the minimum permitted slope.
* The permeance of a material characterizes its ability to let water vapor pass through it.
Traditional underglazing screens with radiant barriers
The advantages of a screen are nowadays perceived and recognized by a large audience.
Nevertheless, an evolving regulatory environment (RT2005, DPE) and a growing notion of comfort have favored the emergence of a new generation of screens offering new functions: under-roof radiant barriers.
What is a radiant underroof fence?
A radiant underroof barrier is a screen that can reflect a large part of the infrared radiation (heat) that reaches it (from the outside as well as the inside) and concentrate it in the air space that faces it..
It thus contributes, much better than a simple under-roof screen, to improving the comfort of rooms located under the eaves.
It is intended to complete a thermal insulation in place (preserved or replaced), especially in the context of the renovation of a cover.
It may in some cases provide comfort in summer and winter to the occupants of housing. This dual function can only be achieved with an HPV radiant barrier allowing the installation of an immobile air space on the underside, without risk of condensation.
As for traditional under-roof screens, there are types of radiant barriers that are suitable for roof configurations and the requirements of project owners (bituminous, synthetic reinforced, HPV synthetic).
Note: The installation of a radiant barrier of under-roofing is systematically associated with that of a counter-batten of 38 mm in order to ensure an optimal chimney effect by the organization of an "effective" air space allowing to quickly evacuate the calories on the surface of the screen.
The classification "R"
In the case of flexible screens placed on the frame supporting the cover wood, the resistance of the screen to the nail tear characterizes its classification "R" and determines the maximum allowable spacing between rafters or small supports.
The rules of the DTU series 40 require to ventilate the underside of the cover elements to avoid condensation phenomena. The installation of under-roof flexible screens, with the exception of HPV screens, involves providing an air gap of sufficient thickness as well as high and low openings in order to promote the circulation of air. DTUs 40.1 and 40.2 specify the section of ventilation openings and air gap thicknesses.
Mountain roofs, a special case
Roofs subject to a mountain climate (altitude 900 m) are governed by specific provisions. They generally make use of complementary waterproofing membranes.
Ventilation closures, indispensable complements
Ridges mechanically fixed have largely proved their superiority of resistance compared to ridge sealed during the strong climatic disturbances undergone in recent years.
Ridge closers have a dual function
- ensure watertightness of the ridge tile connection;
- allow the circulation of the existing air gap between the tiles and the sub-roof.
There are many products on the market, flexible or rigid, basic or high performance, PVC, zinc, lacquered galva, non-woven breathable...
We advise you to implement quality products with a real life and efficiency because they actively participate in the health of the roof and insulation.
To complete the installation, the implementation of ventilation comb at the foot of the slope (the sewer) helps maintain the quality of the air intake by damming insects and other pests.
(photos / visuals: BigMat, except special mention)
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