Does Vapour-Permeable Underlay Need Ventilation?
Not necessarily—but “vapour permeable” alone is not an answer. Check the named underlay, its classification, and the roof construction. One manufacturer describes two vapour-permeable products with different ventilation guidance.
Start with the roof geometry, then stop
Use the roof pitch calculator to record the roof's rise and horizontal run as a ratio or degrees. That result can help you read a product sheet, but it does not establish an underlay's ventilation arrangement. The next checks are product and construction specific.
Read the specification in this order
Use the exact current product document; do not assume two vapour-permeable membranes work the same way.
Cold and warm roof assemblies can be addressed differently in a manufacturer's guidance.
Check ventilation, vapour-control-layer notes, fixing details, and roof covering conditions as one system.
A limited manufacturer example
Klober groups membranes as LR (low resistance) and HR (high resistance), and describes Permo Air as both air open and vapour permeable. On the same support page, Klober states that Permo Air eliminates the need for high-level ventilation products in a cold roof.
The same page describes Permo Extreme as vapour permeable but classed as an HR underlay under BS 5250, and says it requires ventilation at low and high level in line with the roof construction. The contrast is the point: neither product statement is a universal rule for every membrane or roof.
Klober's page also says that a vapour-control layer does not remove the ventilation requirement it states for Permo Extreme. If your named product, construction, or current instructions differ, the correct result here is not confirmed.
Can a roof pitch figure decide the ventilation layout?
No. Pitch is a geometric measurement. Ventilation guidance depends on the named membrane and roof system. Use pitch to locate the relevant product instructions, then follow the complete documentation for the actual project.
For the wider distinction between a secondary layer and a complete roof system, read what roof underlay can do—and what it cannot decide.
Sources and scope
- Klober Technical Support — Membranes — product-specific LR/HR descriptions and the Permo Air / Permo Extreme ventilation statements used on this page.
This page is a reading aid for named manufacturer information. It is not a roof design, code interpretation, or project approval.