My house is listed so the outside walls cannot be insulated. The stone walls are about 3 feet thick with some dampness inside the building. Will it be possible to insulate the inside to a high enough level to use air source heat pump (ASHP) heating successfully?

Anne Netherwood

If there’s any issue with damp, this needs to be addressed before installing any insulation. Based on our expert’s experience, three common causes, apart from leaking roofs, are:

1) defective drains or rainwater goods

2) external ground level higher than internal floor and/or impervious paving next to the wall and 

3) concrete ground floors with damp proof membrane installed within solid walled buildings – these trap water beneath them and so it can only evaporate via the walls.  This is often thought to be “rising damp” and treated with chemical injection, which is never very effective and sometimes does more harm than good.

Once the cause of the damp problem has been dealt with, you could then look at potentially installing internal wall insulation. It is important that the walls remain breathable, and it’s also important not to have too much internal wall insulation. 

The more insulation you put inside a solid wall, the colder the wall will be outside, and you don’t want to make the walls so cold that moisture condenses inside and freezes, or rain penetrates the outer surface and freezes. Water expands as it freezes and this damages the stone. So it’s best to use a fairly thin lime-plaster based insulation – not more than 50mm – and make sure it’s firmly adhering to the stone. If the walls have already got dry lining this would need to be taken off first. 

Ecological Building Systems market Diathonite which is a lime-based cork insulated plaster, and various systems which combine this with wood fibre board. Lime Green markets a lime plaster with foamed glass beads, and they also have a combination system, Warmshell, which can be used on the inside or outside of walls. (Warmshell is a completely different system to Warmcell which is based on recycled cellulose).  There’s also a hemp fibre insulation on the market which can be mixed with lime as hempcrete. The beauty of an insulated plaster for IWI is that it gives you a complete vapour barrier without having to apply a membrane, and because it’s integral with the wall you don’t lose thermal mass.  If you install one of the systems which include wood fibre you would lose some of the benefits of thermal mass but they are still closely bonded to the wall so the effect won’t be large.

Improved insulation makes significant impact on the energy efficiency of the building, and reduces heat loss through the building envelope (external walls, roof, floor), so improved heating systems could be considered with greater confidence.