Warm Roof Installation

Warm Roof Installation  

Between Rafter Insulation 

Insulation placed between the rafters, under the roof tiles creates a warm space within the loft space. This is known as a warm roof installation. Depending on the type of underfelt used and the method of fixing, the space can be full filled or partially filled with sheep wool. 

Supported Breathable Underfelt (low vapour resistance membrane) 

To create a supported membrane the breathable underfelt is pulled taught and fixed in place with counter battens which run parallel to the rafters. This method of fixing stops the underfelt/membrane from sagging. In this situation the space between the rafters can be filled with sheep wool. No ventilation space is required between the felt and the insulation. 

Unsupported Breathable Underfelt (low vapour resistance membrane) 

An unsupported membrane is fixed in place by the tile carrying battens which run across the rafters, this results in the felt sagging between the rafters to create troughs with a depth of approximately 10mm.These ‘troughs’guide any collected moisture away from the tile battens and into the rainwater gutters. In this situation, even with a breathable membrane, a space of approximately 20mm should be maintained between the tops of the rafters and the insulation to ensure the troughs are not compromised 

High Vapour Resistance Roofing Felt 

Where a non-breathable under-felt such as the traditional bitumen type has been used, a 50mm continuous airgap between the felt and the insulation should be left, there should also be a 25mm gap at the eaves and a 5mm gap either side of the ridge. This will encourage ventilation and help to reduce condensation. 

Timber Sarking Boards 

Timber sarking boards are considered to have low vapour permeability so treat in the same way as a high vapour resistance roofing felt. Leave a 50mm continuous airgap between the felt and the insulation, a 25mm gap at the eaves and a 5mm gap either side of the ridge 

 Increasing thermal Levels 

To increase thermal levels,a further layer of wool can be fixed under the rafters between battens. The battens and secondary layer of insulation should run in the opposite direction to the rafters to help reduce thermal bridging. A vapour control membrane should then be fixed below the battens before closing with a plasterboard or similar. 

A wood fibre board can alsobe used to increase thermal levels. Place the wool between the rafters and then fix the woodfibre board under the rafter. The woodfibre board can be finished with a suitable plaster or alternatively you can install a vapour control layer under the woodfibre and fix a plasterboard to counter battens. 

"I" Beam Rafters 


Sheepswool can be installed between engineered "I" beam rafters supported on a breathable racking board, OSB or Plywood sheets which are fixed to the flange of the lower "I" beam component. 

When installing from above, insert the insulation between the "I" beams before fixing the sarking board / racking board. A breather membrane should then be placed over the sarking boards before fixing counter battens in place. 

A vapour control layer should be applied to the warm side of the insulation unless a suitably sealed vapour-controlling plywood or OSB is used. 

Services should be installed on the warm side of the Vapour control layer to ensure the VCL is not punctured. Care is required where dormer windows, roof lights, or other fixed objects disrupt the ventilation space.