Breathable membrane or vapour barrier: which goes where

07.10.2026

A breathable membrane and a vapour barrier look alike but do opposite jobs. When they are mixed up or fitted on the wrong side, the insulation gets damp and the roof loses heat. This guide explains the difference, where each one goes and how to choose a roofing membrane. The examples come from the Foliarex STROTEX range that Ymalko offers in Montenegro.

1. The difference in short

Breathable membrane. It goes outside, above the insulation and under the roof covering. It keeps rain and snow out and lets water vapour escape from the insulation. It is also called a roofing underlay.
Vapour barrier. It goes inside, on the warm side, between the insulation and the interior lining. It stops water vapour from the room before it enters the insulation.

The rule is simple: closed to vapour on the inside, open to vapour on the outside.

2. What happens when it goes wrong

Water vapour from the kitchen, the bathroom and breathing moves towards the colder side of the roof. Without a vapour barrier it enters the insulation and condenses there. Damp insulation insulates less, and rafters and boards start to grow mould.

The same happens when a foil that does not let vapour through is fitted on the outside: the moisture stays trapped in the roof.

3. How to choose a roofing membrane

Roof with full boarding. For boarded roofs, four-layer membranes with high resistance to mechanical damage are recommended, such as STROTEX-Q Supreme, Nextreme or Strong Premium.
Roof without boarding. In an open system the membrane is laid directly on the insulation, over the rafters. STROTEX Basic, Toples and V are made for this, and the universal STROTEX-Q Medium suits both roof types.
Metal roofing. Under copper and aluminium standing-seam roofing, STROTEX-Q TPU is recommended; it withstands high temperatures, briefly up to 120 °C.
Clay tiles and solar panels. STROTEX-Q Performance is recommended for clay tile roofing and for roofs that will carry solar panels.

One foil in the range is not breathable: STROTEX Aktiv PI is a low-permeability roofing foil with a reflective layer. A ventilated gap is left under such a foil, above the insulation, so it does not replace a breathable membrane.

4. How to choose a vapour barrier

The range includes STROTEX AL vapour barriers with an aluminium layer, at 90, 150 and 180 g/m². A higher weight means a stronger foil that is harder to damage during installation.

The aluminium side faces the room.
Joints are overlapped and taped, as are pipe and cable penetrations.
A gap of about 2 cm is left between the aluminium-faced foil and the lining.

A vapour barrier only works when it is continuous, so every tear is taped before the lining is closed.

5. Order of layers in a pitched roof

From outside to inside:

Roof covering on battens
Breathable roofing membrane
Insulation between the rafters
Vapour barrier on the room side
Interior lining, for example plasterboard

Checklist before ordering

Boarded or unboarded roof
Type of roof covering
Roof area in m²
Whether the attic is a heated space

Roofing membranes in Montenegro

See the range of membranes and foils:

Models mentioned in this guide:

Send us the roof area, the type of covering and whether the roof is boarded, and you will get a foil recommendation and a quote.

Contact Ymalko Montenegro for a quote