Insulation and ventilation
Choosing a roof underlay membrane: which specifications matter?
A roof membrane is selected primarily according to the layer build-up and the type of covering, not by grammage alone. It is worth checking the product’s intended use, the tightness of joints and the permitted exposure time. The underlay membrane on the outer side drains water from beneath the covering, while the inner layer limits the passage of moist air and vapour in line with the design. These are different tasks and working conditions, so casually calling both of them foil leads to mistakes. The selection decision is left to the designer, who assesses the entire build-up.
How does a membrane differ from a vapour barrier?
The underlay membrane sits on the outer side of the roof layers and helps drain water from beneath the covering. The layer on the inside limits the passage of moist air and vapour in accordance with the design. These are different tasks and different operating conditions. Colloquially calling both ‘foil’ leads to mistakes when buying and installing.
Swapping their positions can disrupt the drying of the building component. Equally problematic is omitting the airtight sealing of the interior joints. Moisture can enter the insulation through small leaks around installations. A good membrane on the outside will not neutralise any amount of such moisture. The selection should cover the entire cross-section and the way the rooms are used.
What does the Sd value mean?
Sd describes the diffusion resistance expressed as an equivalent air layer. A lower value means lower resistance to vapour diffusion. However, it is not an indicator of watertightness against precipitation or tear resistance. A product may have favourable diffusion resistance yet still not suit the support method or installation conditions on a given roof.
Do not automatically choose the lowest Sd without looking at the other layers. The covering, insulation and interior layer all matter. In a system with PIR in aluminium cladding, diffusion may behave differently than with mineral wool. A coherent concept for the building component is needed. A single number in a catalogue will not replace a moisture assessment of the roof.
Why is grammage alone not enough?
Grammage specifies the mass per unit area of the material. It can help compare products, but it does not describe all properties. Resistance to tensile stress, tearing at the nail and behaviour after ageing are important. Water resistance must also be checked. Two membranes of similar mass may have a different construction and a different purpose.
On site, how the membrane is walked on and where tools are set down matters. Even a robust material can be cut by a sharp edge. Damage must be repaired with material compatible with the system. Do not assume that a small hole under the metal sheet will always stay dry. Water can reach it during strong wind, thaw or condensation on the underside of the covering.
How to assess resistance to sunlight and temperature?
The manufacturer specifies the permitted exposure time before the covering is laid. This does not mean the membrane may be left as a temporary roof indefinitely. Actual conditions may include strong wind, water and intense solar radiation. If construction is delayed, the condition of the material must be assessed before it is covered. Damage is not always visible as a large hole.
Under dark metal sheeting, elevated temperatures occur. The membrane’s scope of application should correspond to the conditions beneath the chosen covering. Open gaps and local exposure to light are also important. Not every product is suitable for all types of covering. It is worth checking the documentation rather than assuming that the term roof means unrestricted application.
How should laps be laid and penetrations sealed?
The size of the overlap depends on the instructions, the pitch and the roof conditions. The upper strip should direct water to the outer side of the lower strip. Joints must be made on a clean surface and with suitable tapes. Adequate pressure and installation temperature affect the durability of the bond. A random tape may lose adhesion after the roof is closed.
At the chimney, window and duct, the continuity of the layer must be restored, not just a hole cut. Counter-battens and fasteners require a solution intended for the specific system. At the eaves, water must drain to the outside. An edge terminated above the insulation will not work correctly, even if all other overlaps have been executed very carefully.
What data should be collected before purchase?
Prepare the type of covering, the pitch, the method of support and information about the insulation. Also specify the planned time between laying the membrane and covering. The schedule should include tapes, seals and repairs to details. The roll area must be reduced by the overlaps and the cutting layout taken into account. The price per nominal metre is not yet the cost of the completed layer.
It is worth choosing a solution that the installer knows and can lay in accordance with the instructions. After installation, an inspection is needed before covering. A well-chosen membrane does not draw attention to itself but protects the structure in situations where the covering alone is not sufficient. Its role is particularly important on a roof with many penetrations and hard-to-reach details.
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