Breathable membrane vs vapour control layer
Both products manage moisture, which is why their names are often mixed up. The easiest way to separate them is by location and direction: the outer membrane protects the roof beneath its tiles, while the inner layer controls vapour leaving the rooms below.
A breathable roof membrane sits beneath the external covering and sheds water while allowing water vapour to pass out. A vapour-control layer sits on the warm side of insulation and limits moist indoor air entering the roof build-up. They do different jobs and one is not a substitute for the other.
Where a breathable membrane belongs
On a pitched re-roof, breathable underlay is fitted above the rafters and beneath battens and tiles or slates. It forms a secondary drainage layer if wind-driven water gets past the outer covering and allows water vapour to move outwards. HRD’s re-roof process includes breathable membrane and treated battens after the old covering is stripped and defective timber is addressed. The membrane is not the finished roof surface, and it should not be treated as permission to ignore cracked tiles, flashings or ridge details. Its job is part of a layered construction.
Where a vapour-control layer belongs
A vapour-control layer belongs on the warm side of the insulation, behind the internal finish. Its purpose is to reduce the amount of warm, moisture-laden indoor air entering colder parts of the construction where it could condense. The site’s warm-roof and flat-roof insulation guidance places this layer below the insulation. Continuity matters: unsealed laps and holes around lights, cables or pipes weaken the control. Calling it a vapour barrier can suggest absolute sealing, but the practical issue is controlling airflow and vapour through the intended layer and detailing every penetration.
Waterproof does not mean finished covering
A breathable underlay is designed to resist liquid water that reaches it while allowing vapour to move through. That does not make it an exposed roofing material. It relies on the tiles or slates above, correct laps, support and drainage to the eaves. If a tile slips, the membrane may limit immediate ingress, but it is not a permanent repair. Likewise, a vapour-control layer manages moisture from inside; it is not intended to stop rain from outside. Asking which one is waterproof misses the more useful question: where is the water coming from and which layer should control it?
Why ventilation still matters
The existing site guidance is clear that breathable underlay does not universally replace ventilation. Roof build-up, pitch, product specification and the path available at the eaves and ridge all matter. In a cold loft, insulation pushed tight into the eaves can block the low-level air path. Adding high-level vents without an inlet below achieves little. In a warm construction, the vapour-control layer and insulation need to be designed as a system. The correct answer is not simply “fit breathable felt”; it is to establish how moisture enters, where it can dry and whether the proposed product is approved for that arrangement.
How the layers work in a pitched roof
A typical renewed pitched roof has the internal ceiling below, insulation at ceiling or rafter level depending on the use of the loft, a ventilation strategy suited to that build-up, breathable underlay over the rafters, then battens and the external tiles or slates. A room-in-roof also needs careful vapour control on the warm side because its insulation follows the roof slope. HRD’s loft-conversion guidance warns against stuffing insulation into a space without preserving the designed air gap or sealing the warm-side layer. Each component controls a different part of the moisture journey.
How the layers work in a flat warm roof
The site describes the standard warm-flat-roof approach as vapour control below rigid insulation, with the waterproof covering above. Keeping the deck and structure warm reduces condensation risk, while the inner layer limits moist air entering from the room. The outer EPDM, GRP or felt system handles rain. This is different from trying to place insulation between joists without a viable ventilation route. Door thresholds, upstand heights and outlet levels can constrain the available build-up, so insulation thickness cannot be chosen separately from drainage and perimeter detailing.
Common specification mistakes
Typical mistakes include using the product name without stating the exact build-up, leaving the warm-side layer unsealed at penetrations, blocking eaves ventilation with insulation, assuming every breathable membrane allows an unventilated roof, and using underlay as a temporary exposed covering for too long. Another is mixing old and new construction without considering where the dew point moves. A useful quote or drawing names each layer in order from inside to outside and explains the ventilation path. If two proposals use the same words but arrange the layers differently, they are not equivalent specifications.
Questions to ask before work starts
Ask whether the roof is cold, warm or a designed hybrid; where the insulation sits; where the vapour-control layer is continuous; which underlay is specified; and how air enters and leaves any ventilated void. Ask how the membrane drains at the eaves and what happens around rooflights, chimneys and pipes. For a re-roof, confirm that the covering will be stripped so timbers can be checked before the new underlay is fitted. HRD assesses insulation and ventilation together because correcting one while ignoring the other can turn a dry roof space into a condensation problem.
FAQs
It resists liquid water as a secondary underlay, but it is not the exposed finished roof covering and still needs correct laps and drainage.
No. It sits on the warm side of insulation and limits indoor moisture entering the construction; roof underlay sits nearer the outer covering.
Not in every roof. The product, pitch and complete build-up determine the ventilation requirement.
In the warm-roof approach described on this site, the vapour-control layer sits below the rigid insulation and waterproofing sits above.
Related reading
More on comparisons and the jobs that usually come with it.
- uPVC vs timber fascias and soffitsComparisons
- Dormer or rooflight loft conversion: which is right for your houseComparisons
- Warm roof vs cold roof constructionComparisons
- Flat roof drainage and falls: why ponding wrecks flat roofsMaterials & systems
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