Anne Netherwood

It is right to be cautious about insulating rubble stone walls, but it is possible to do it properly.

Marianne Suhr and Roger Hunt have written an excellent guide to retrofitting old solid-walled properties – Old House Eco Handbook click here– which deals with retrofitting and is a companion to their earlier volume Old House Handbook click here which deals with maintenance and repair. We have heard good things about this body of work.  

One option to consider would be Internal Wall Insulation (IWI). It’s important that the walls remain breathable, and it’s also important not to have too much IWI.  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 in Shropshire 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.

External wall insulation (EWI) is less likely to give rise to unintended consequences than IWI provided it’s properly installed.  You need to make sure there’s enough overhang on the roof (and extend the roof if not) and you need to extend the roof at the gables to cover the insulation with enough overhang to stop rain penetrating.  All the outside attachments (drainpipes, wiring, meter boxes, satellite dishes etc) need to be removed before installing EWI. Windows need to be moved out in line with the EWI. And you need to make sure that the EWI goes down to the foundation; the standard recommendation is for XPS (extruded polystyrene) below the level of the damp proof course (or ground floor level if there isn’t one).  And of course, for old buildings with solid walls the main insulation needs to be breathable (XPS isn’t). In the case of EWI, it is essential that it is installed to a high quality, as bad installations can create more problems.

If there’s any issue with damp you really do need to address this before installing any insulation.

What’s the U-value of a rubble stone wall?

There’s a straightforward formula for calculating them but you need to know the thermal resistance of everything in the wall – what kind of stone, what proportion of mortar to stone, what kind of plaster etc.  You add all that up and add the resistance of the inside and outside surfaces and then the U-value is 1/the sum of all the resistance values. 

It would be very difficult to do this accurately with a rubble stone wall as the core is likely to be much less dense than the faces – more lime and less stone.  But you could probably make some reasonable assumptions which would be near enough.  There are software programmes available for calculating U-values but it’s probably not worth investing in one unless you need to do it often. Lime Green or Ecological Building Systems will do the U-value calculation for you if you are thinking of installing one of their products.