In cases where existing houses require wall insulation but do not cavity wall insulation is not a viable option, the more expensive option of installing of external insulation may be considered. External insulation involves fixing insulation materials such as mineral wool or expanded polystyrene slabs to the outer surface of the wall. This insulation is covered with a special cement-based render or aluminium skin to provide weather resistance. The insulation forms a barrier to heat transfer and reduces losses through the wall space. One of the main benefits of the external insulation over internal insulation is that loss of internal space is avoided. When installing external insulation, it may be necessary to extend eaves and sills and relocate downpipes to accommodate the extended footprint of the building.
Another important characteristic of external insulation is that it will maintain the houses heavy weight slow thermal performance. This means that the houses walls will absorb some of the heat when the heating is switched on, reflecting it back into the house after the heating has been turned off. The disadvantage however is that it will take longer for the heating system to heat the house up in the first place.
Another advantage of external insulation is the chance it allows for upgrading the aesthetics of the house. The chance to re-clad an aged house is often a major attraction to external insulation. This benefit can also be essential to making the economics of a project.