The use of cob and stone in traditional properties and external insulation

Typically in Cornwall, very old properties are constructed of stone on the ground floor with cob on the first floor. This was probably due to the weight of lifting stone to the upper elevations but may also have been due to materials being readily available and costs of transportation.

These materials have stood the test of time but more and more are being found to deteriorate faster than would have been expected.The use of cob and stone This has probably been caused at least in part by the use of modern mortar in both pointing and render. The modern mortar is harder than the stone to which it is bonded which, when subjected to thermal exposure expands and can cause fracturing of the stone. The old stone walls would also have minimal foundations and do tend to move with seasonal variations as soils shrink and expand with moisture. Lime pointing does however allow for slight movement and self repairs to some extent and does not cause fracturing of the stonework. The Ordinary Portland Cement (OPC) also tends to trap moisture into the construction which can increase levels of internal humidity and dampness. Accordingly, we always recommend that stone walls are repointed with Lime. The cob material should also be rendered with Lime and decorated thereafter with a Lime wash or modern microporous paint.

Recently, we have come across a small number of traditional properties which have been insulated externally and the walls dry lined internally. The external insulation extends the outside walls by approximately 100mm and normally comprises of a rigid foam block which is bonded to the external walls and then overboarded with weatherboard and rendered to finish. In some cases the insulation protrudes past the verge and eaves projections which is then capped off with aluminium. The aluminium over the top of the insulation material is vulnerable to water penetration which will almost certainly allow water to penetrate between the insulation and external walls causing internal dampness.

More recently, we surveyed a property whereby the insulation was tucked underneath the verge and eaves which would appear satisfactory. However, these walls were also dry lined internally so the thicknesses are now around 900mm as compared to an average thickness of random rubble fill to be 600mm. This has caused some darkness within the property.

Our concerns here are that the cob material will overdry which has the potential to make it unstable. It should be appreciated that cob material relies on a certain amount of dampness to maintain its structural stability and therefore overdrying is not desirable. Conversely, if water is trapped between insulation and cob material it can cause it to saturate and turn into mud. There would be no internal or external signs of this and cob has been known to fail without warning.

dscf0158In our opinion therefore, the best way to treat cob and stone walls is by traditional means and that is by Lime pointing and rendering externally along with Lime wash or microporous paints and Lime internally. If damp proofing works are needed then vertical membrane systems are the best solution and we would not recommend tanking slurries or chemical injections. As for thermal insulation, this type of construction acts as a heat resevoir and as such is normally found to be thermally fairly efficient.