Extensive upgrading of insulation: To roof/attic space, external walls, ground floor, windows/external doors. (Which I will be discussing over the next couple of week’s blogs)

In order to tackle this very important aspect of insulation, I will need to comprehensively cover this for many specific house construction types and illustrate their various technology options.  I will therefore leave this topic to my next blog to describe in detail some of the various solutions now available.

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.

Sealing against draughts and wind infiltration:

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 breathe.  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 week I will return to discuss Controlling Vapour diffusion and dampness from condensation.

-Duncan Stewart