Insulation between and below rafters with NON-breathable membrane

This method of insulation is used when roof is being insulated and there is an existing roofing felt in place that is bitumen based or otherwise non-breathable. These membranes cause issues by trapping moisture into an otherwise breathable construction, which results in condensation forming on the underside of the cold membrane. To manage the condensation risk a ventilated void must be created between the insulation and the non-breathable felt, typically this is 50mm, but this should be checked with the building inspector for your project.

With the ventilated space allowed for, the remaining depth of the rafters is filled with an appropriate flexible insulation such as the Thermafleece Cosywool or the Schneider Flex50 to make the most out of the rafter depth. Below the rafters the ProClima Intello Plus variable vapour control layer is fitted to provide air tightness and prevent draughts and manage moisture, below the VCL (vapour control layer) a plaster carrier board is fitted which can also provide addition insulation.

Suitable For: 

• Roofs with an existing bitumen felt or non-breathable membrane.
• Roof constructions required to use non-breathable membranes due to presence of bats.

Benefits:

• Insulation can be installed internally when roof coverings cannot be removed and a non-breathable felt / membrane is installed.
• New roof insulation where modern membranes are not allowed due to the presence of bats.

Considerations:

• Ventilation must be allowed between the felt and the insulation to allow the evaporation of condensation.
• The vapour control layer is important to create an airtight construction to eliminate draughts.

1. Roof covering (typically slate or tiles)
2. Non-breathable membrane or bitumen felt
3. Well ventilated cavity - when retrofitting this may require alterations such as felt lap vents.
4. Flexible Insulation between rafters (maintaining the cavity above)
5. Intelligent vapour control layer for air tightness
6. Internal Insulation and plaster carrier board
7. Lime plaster finish onto boards