The first step is to check that your walls are definitely of cavity construction. Solid brick walls (225mm thick) were the norm for Victorian houses but some were built with early cavity walls.

Filling the space with insulation introduces the danger of rainwater penetrating across to the inside leaf, particularly where the leaves are less than 50mm apart. There’s also the chance of condensation and mould forming on indoor plasterwork near cold spots (thermal bridges) caused by unfilled pockets in the cavity.

On top of this will be the risk of localised condensation arising from the cold bridges inherent with early cavity walls, especially when the walls layers are tied together with bricks or ceramic tiles.

Although good ventilation in areas such as kitchens and bathrooms will lower the risk, it may still be inadequate.

Where the ties between the two layers of a wall are of a porous material or the gap is less than 50mm, cavity insulation is not going to be an advisable option. In other instances, cavity fill may be suitable, but should only be carried out by a certified installer after a detailed appraisal has been made of the house and its performance characteristics.

Adding insulation to any wall, regardless of whether or not it is constructed with a cavity, always needs to be carried out with great care. For free advice speak to the Energy Saving Trust on 0300 123 1234.

Douglas Kent, technical and research directory at SPAB