This work should be carried out on the outside of the insulation layer, of external walls, roof and ground floor, especially at critical junctions, such as at roof eaves, attic trap doors, chimneys/fireplace, dormer roofs and attic conversions, caulking and taping joints between block work walls and window frames. Also, opening sashes of windows and doorframes not draught-sealed, gaps around pipes, cables and cavity block walls that are dry-lined on the inside.

It is important that draught-sealing membranes and boards fitted on the outside of insulation should always be ‘breathable’ ‘vapour diffuse’ and adequately ventilated to the outside air. An example where difficulties can arise is where insulation is fitted on the inside of a bituminous roofing-felt, which typically does not breath.  This condition can create a build up of condensation and dampness in the insulation and roofing timbers, reducing the insulation’s effectiveness and worse, placing the structural timbers at risk of decay. In this case, a clear ventilation cavity will be required directly inside the layer of felt. However, this method can also be defective, where the ventilation to the outside is inadequate or the cavity can cause its own problems of draughts and further heat loss through wind infiltrating through the insulation.  In all conditions an airtight layer is required on the warm side of the insulation, to prevent in the first place.

Next Weeks blog topic: Controlling vapour diffusion and dampness from condensation.