Description
Every warm roof needs a vapour control layer under the insulation, and it isn't optional detailing — it's the layer that stops warm, moist air from inside the building reaching the cold side of the build-up and condensing there.
Skip it or lap it badly and the problem shows up years later as a saturated deck.
This is virgin-grade polythene VCL in a translucent green tint, so you can see your fixing lines through it as you lay.
- Virgin-grade polythene vapour control layer, 4m x 25m roll covering 100m²
- Two gauges: 1000 gauge (250 micron) and heavier-duty 1200 gauge (300 micron)
- Translucent green tint — see through to the deck for accurate fixing
Specification
| Roll size | 4m x 25m (100m² per roll) |
|---|---|
| Gauges | 1000 gauge (250 micron), 1200 gauge (300 micron) |
| Material | Virgin-grade blown polythene |
| Colour | Translucent green tint |
| Position in build-up | Above the deck, beneath the insulation (warm roof) |
Frequently asked questions
Which gauge should I choose? 1000 gauge is the usual specification for domestic warm roofs.
Step up to 1200 gauge where the membrane will see more foot traffic during the build, or where the specification calls for a heavier-duty layer.
Where exactly does it go? Directly on the deck, beneath the insulation.
The order is deck, then VCL, then insulation, then waterproofing — that keeps the vapour barrier on the warm side of the insulation, which is the whole point of it.
How much do I need? Each roll covers 100m². Allow generous laps at every joint and turn the membrane up at perimeters and upstands, so order comfortably more than the bare roof area.
How are the joints sealed? Lap the sheets and seal the laps with a suitable vapour barrier tape. A VCL only performs if it's continuous, so laps, penetrations and perimeter upstands all need sealing.
Is a VCL the same as a breather membrane? No, and they aren't interchangeable. A VCL resists vapour and goes under the insulation on a warm roof.
A breather membrane is designed to let vapour out and belongs in a ventilated pitched roof build-up.
Building a warm flat roof? This sits directly beneath our foil-faced PIR insulation board, with 1.2mm EPDM membrane bonded over the top using our EPDM contact bonding adhesive.
If the specification calls for a hot-air-welded, contractor-installed finish instead, our PVC single-ply roofing membrane is a suitable alternative top layer over the same VCL and insulation build-up.
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