VCL’s (vapour control layers) or vapour barriers are important in a flat roof to stop damp forming in the insulation area.
A vapour barrier is always needed in any form of construction where you are insulating. The vapour barrier goes on the warm side of your insulation.
The only installations where a VCL isn’t needed to be installed is where you are installing insulation over an old roof covering that is still waterproof. This is because the old waterproofing acts as the vapour control layer.
The other instance is in an inverted roof where an XPS or EPS insulation boards are installed over the waterproofing system, this then acts as the vapour barrier and the waterproofing. Inverted roofs are used in ballasted roofs such as a green roofs or where concrete paving is installed for a roof garden to be created.
The extra importance in a flat roof over a pitched roof is that in most flat roofs there is no ventilation in this area unlike pitched roofs. Most pitched roofs have a breather membrane below the tile or slate waterproof layer. This allows any moisture that does occur in this layer to evaporate out. Whereas most flat roof solutions don’t have ventilation and this put extra need for a high quality VCL such as Alutrix which is a self adhesive vapour barrier that created a fully sealed solution and can be bonded onto. This allows products such as Kingspan TR27, INSTA-STIK and fully adhered EPDM’s such as HERTLAN or RESITRIX to be used. This is because they can be bonded to the vapour barrier rather than having to be mechanically fixed. This means that you have a fully vapour tight solution as the waterproofing is bonded to the VCL encasing the insulation completely and there is also no punctures from mechanical fixings.