Roof/Attic: To be well insulated, roofs or attics should be fitted with about 400mm of mineral or glass wool insulation or other high performance, rigid insulation boards to thickness of say 250mm, that achieve similar performances in thermal transmittance (U-value – measured in watts per sq metre, per degree Kelvin). I hope to deal with this topic in a later blog.

Insulation to attic floors:

The insulation should be laid on a continuous, vapour resisting, airtight membrane that is sealed at all joints and junctions and to adjoining external walls. This is to prevent warm vapour laden-air in the habitable rooms of the house from driving through the permeable ceiling plaster, where it seeks to stabilise pressure with the colder, lower vapour pressure air above in attic space or outside the building. It is critical that this membrane should be fitted on the inside, warm surface of the insulation to prevent condensation and dampness forming within the insulation and where the temperature of the airtight membrane will always be maintained above the dew point. The trapdoor to the attic should also be draught sealed like an external door. More recent houses may have a foil-backed plasterboard ceiling. This helps, but in my opinion is not sufficient, as un sealed gaps occur at joints, allowing warm vapour-laden air to drive into the insulation and cold attic space.