Understanding HPV roofing underlay: definition, role and Sd value
HPV: what it actually means and what the HPV underlay is for
HPV stands for High Vapour Permeability. An HPV roofing underlay is a flexible membrane installed beneath the roof covering, on the outside side, to complement watertightness and airtightness. Its role is straightforward: protect the insulation and frame from accidental ingress (wind-driven snow, rain, dust) and limit air flushing in sloped roofs, provided the overlaps, taping and continuity specified in the installation instructions are respected.
Sd value: how to read it and why it changes everything for vapour transfer
Sd is the equivalent air layer thickness (m). The lower it is, the more readily the membrane allows vapour to diffuse outward. This is the criterion that governs hygrothermal performance and, depending on the system requirements, whether the underlay can be installed in direct contact with the insulation or whether a ventilated air gap must be kept.
| Product | Typical Sd value (m) |
|---|---|
| HPV underlay | ≤ 0.09 |
| Non-HPV underlay | > 0.09 |
| Vapour barrier | ≥ 18 |
Don’t confuse roofing underlay with a roof vapour barrier: two products, two opposite sides
Roofing underlay goes on the cold side, beneath the battens. The vapour barrier goes on the warm side, behind the internal lining, with a high Sd. Swapping them, or treating the underlay like a vapour barrier, blocks moisture transfer in the wrong place. The result: condensation risk within the build-up and difficult justification in the event of a claim or inspection.
Knowing when direct contact is allowed… and when an air gap remains mandatory
Direct-to-insulation installation: conditions to meet with an HPV roofing underlay
Direct contact is acceptable only if the underlay is genuinely HPV, with vapour permeability consistent with the system, and installed without discontinuities (laps, tapes, junctions at penetrations). In practice, check the Sd value on the ETA or technical data sheet before fitting the insulation.
| Underlay type | Sd (m) | Contact with insulation |
|---|---|---|
| HPV | ≤ 0.09 | Permitted |
| Non-HPV | > 0.09 | Not permitted |
Air gap: cases where it is essential despite an HPV underlay
Even with an HPV roofing underlay, the air gap under the covering must be preserved, because it provides ventilation and drainage. Keep an air gap as well if the underlay is old, unidentified, or has been rendered non-HPV by unsuitable tapes, sealants or bonded layers.
Roof underlay and insulation: key checks for attics, sloped roofs and sarking
In attic spaces, do not block air inlets at the eaves or the continuity of ventilation. In sloped roofs, pay close attention to junctions at purlins, valleys and verges, and clearly distinguish the roofing underlay from the vapour barrier on the inside. In sarking systems, check water discharge to the eaves and wind-tightness around the perimeter (see also sarking insulation).
Installation direction and detailing that prevent failures
Unrolling direction and overlaps: avoiding reversals that trap moisture
| Underlay type | Sd (m) | Consequence in an insulated roof |
|---|---|---|
| HPV | ≤ 0.10 | Direct contact with insulation possible, subject to technical assessment and substrate |
| Non-HPV | > 0.10 | Ventilated air gap on the insulation side mandatory |
Unroll from the bottom upward. The upper strip overlaps the lower strip. This prevents any leak from running beneath the lap. Respect the minimum overlaps required by the manufacturer. The HPV roofing underlay is installed with the marked face toward the roof covering. Do not confuse it with the vapour barrier, which works on the inside.
Wind and water tightness: joints, tapes, penetrations and edges
The roof underlay does not tolerate improvisation. Ensure continuity with compatible tapes and connection pieces for the membrane. Treat each penetration (MVHR duct, aerial, flue) with a sleeve or collar. At verges, valleys and eaves, create a drainage path to the outside, with no low points or water backflow.
Managing counter-battens and roof ventilation: what must remain ventilated
Counter-battens ensure ventilation beneath the covering. Maintain a continuous air path from eaves to ridge, without compressing the membrane. Do not block air inlets with insulation, ridge closures or underlays folded incorrectly. The air gap above the underlay must remain ventilated, even with HPV membranes. To size and understand the issues involved (air renewal, types, impact), you can refer to the ventilation types covered in the EPC.
Common site mistakes (and their consequences for the insulation)
Confusing HPV underlay with a roof vapour barrier: the classic mistake that causes condensation and mould
Two opposite sides. The HPV roofing underlay is installed on the outside side, with high vapour permeability. The vapour barrier, by contrast, is installed on the inside side, continuous and airtight. Reversing their roles, or adding a vapour retarder on the cold side, blocks drying. Vapour condenses within the insulation layer. The result is persistent moisture, mould and degraded performance.
Poor compatibility with the insulation: moisture, settling and loss of performance
Unintended contact. Not all underlays are designed to be in direct contact with the insulation. When the underlay is not HPV, a ventilated air gap remains necessary. Without that buffer space, moisture stagnates, batts or rolls settle, and thermal resistance drops. Add perforations and poorly handled fixings, and you create recurring damp zones.
Missing continuity: junctions, discontinuous underlays and stray air infiltration
Real continuity. Problems often start at singular details such as valleys, verges, eaves and penetrations. An underlay with poor overlaps or unbonded joints allows air and water in. The insulation gets wet locally, then convection creates diffuse heat losses. On site, document the overlaps, taping and tie-ins before closing the roof. To go further on the mechanisms involved, make a dew point calculation to anticipate condensation in insulation build-ups.
2026 checklist to secure the installation and explain it clearly to the client
Checks before closing up: stated Sd, markings, condition of the underlay and accessories
Record the Sd on the label and technical data sheet. For an HPV roofing underlay, verify a very low Sd (around 0.1 m). Check the CE marking and the declared standard. Inspect the membrane: no tears, no permanent creasing, no contamination by plasters or solvents. Confirm the compatibility of tapes, closers, collars and accessories with the membrane.
Checks during installation: tension, stapling, counter-battening and continuity
Install without over-tensioning. The underlay must follow the pitch, with compliant overlaps and fixing points protected by the counter-battens. Ensure continuity at eaves, ridge, valleys and penetrations. If the membrane is not HPV, keep a ventilated air gap on the insulation side.
Documents to keep: photos, technical sheets and simple explanations to avoid disputes
Archive 6 to 10 dated photos. Roll marking, overlaps, connection details and penetrations. Attach the technical data sheet and installation instructions. On the quote and handover report, write it clearly: underlay type, Sd, installation direction, and the difference from the vapour barrier on the inside side.
