What’s the best insulation for our old redbrick?

What is the best insulation for a 19th-century house? We have bought an old redbrick which needs a lot of renovation. We want to insulate it completely but we have been told that external insulation is not possible. What are the best options for us?

Retrofitting insulation to an already complete building presents many challenges. If using externally applied insulation, one must deal with rainwater goods and inlet gullies, alarm boxes and external lighting, window sills and of course general appearance. In a semi-detached or terraced home, you must consider how your property will blend in with your neighbour. If your house is a protected structure, or if you live in a conservation area, you will need the consent of the planning authority.

If you consider internally applied insulation, you need to deal with electrical sockets, radiators, boilers, fitted kitchens and wardrobes, skirtings, curtains and window reveals. You will also lose some floor space.

Applying insulation into an existing cavity can be a good option as you do not disturb the inner or outer wall fabric or finish. There are limitations here also however, as cavity widths must be sufficient to deliver a viable insulation value. You also need to be careful to avoid trying to fill cavities in a wall constructed using hollow blocks. The absence of a cavity continuity and cold bridging caused by the block itself, means your efforts will be fruitless.

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In your case, we are dealing with a typical redbrick house, built in the 19th century. You have already been advised that external insulation will not work for you. When dealing with older properties, it is usually advantageous to use a material which will be compatible or sympathetic to the original building materials. When applying insulation to an old structure, we typically need to be concerned with the effect this will have on the existing building.

In simple terms, apart from impact on character or appearance, we need to understand where condensation may occur. The potential for condensation to occur behind the new insulation on the cold historic external wall must be eliminated. This can be done by introducing ventilation behind the new insulation, or by using a breathable material that will allow any moisture which may occur to dissipate. The aim is to avoid dampness and subsequent hazardous mould.

All things considered, the best insulation in a historic property such as yours would be sheep’s wool. Sheep’s wool is breathable and resistant to damage by dampness. It should be used in a new internally-applied dry-lining system, with careful consideration given to the protection of historic building fabric. You should seek the advice of a building surveyor with knowledge of conservation to assist you.

Noel Larkin is a chartered building surveyor and member of the Society of Chartered Surveyors Ireland, scsi.ie