Insulation -Interior Wall

Like exterior wall insulation, the installation of interior wall insulation typically involves building out the wall with additional insulation, creating a new wall finish and making adjustments to all the windows, doors and features affected. In many traditional buildings, the list of important features that could be affected is considerable and includes panelling, dado rails, skirting boards, cornices, plaster work and reveals around architraves. Internal wall insulation is unlikely to be viable if important historic features are in place, but may be workable where these historic features are absent or have been removed.

The installation of internal insulation is a complex process, best done as part of a major renovation. However, this depends on how much insulation you want to install and how much room space you are willing to lose.

The big problem with interior wall insulation is ‘cold bridging’. This happens at all the points where the insulation stops and the heat can get round the edges such as the junctions with windows, floors and ceilings and around sockets which have been cut into the insulation. It can also happen at a junction with an internal wall unless the insulation is taken round the corner for at least a couple of feet. At all these points, heat and water vapour escape, the temperature drops and the risk of condensation increases. This can lead to long-term problems, especially if the condensation is hidden, for example within a floor void between ground and first floor. These risks can be reduced, for example by treating timbers that could be affected, installing insulation in floor voids, and keeping indoor humidity low through controlled ventilation (see case study). However, given the risks involved, you should seek professional advice to work out the best approach. Great care is needed in detailing all the joins and junctions of the insulation with the existing building fabric, and therefore fitting internal insulation is not something you should do yourself.

Most internal insulation jobs in traditional buildings use synthetic insulation materials that are impermeable to moisture. They focus on keeping moisture out of the walls, including using a vapour barrier on the inside of the plasterboard. Most of the products on the market, such as insulation-backed plasterboard, are made with impermeable insulation. This approach is reasonable, given the problems that can arise when moisture does get into a wall, and is supported by Historic England. Nonetheless, there is a case for using natural moisture-permeable materials for internal insulation as well as external insulation and the guidance from Historic England supports the use of permeable, or moisture open materials as best practice.  If condensation does occur in your wall after the insulation has been installed, moisture-permeable materials will allow the moisture to escape to the inside of the building. We do not yet know enough about how traditional buildings perform with solid wall insulation to give the final word on this issue.