Insulating an old house wrongly can damage the building

This is written to be used rather than admired. Each section below is a decision about retrofit insulation, and each one has a default.

Insulation changes where the cold surface is

Adding insulation to the inside of a wall keeps the room warmer and makes the masonry behind it colder, because less heat now reaches it. A colder wall stays wetter for longer and can be at risk of condensation forming within the construction rather than on its surface.

This is why internal wall insulation is the retrofit measure most often associated with unintended damage, particularly on solid walls. It can be done well, and doing it well requires a properly designed build-up rather than boards stuck to a wall.

Breathability is a real technical property

Traditional solid-walled buildings handle moisture by absorbing it and releasing it, and their materials were chosen to allow that. Introducing a vapour-impermeable layer traps moisture in the masonry, where it can cause decay to embedded timber, frost damage and internal damp.

Underneath it, vapour-open insulation systems, using materials such as wood fibre or certain mineral and lime-based products, are designed for this construction. The choice between a vapour-tight and a vapour-open approach is a technical decision that should be made by someone competent in building physics.

Embedded timber is the specific risk

Joist ends and wall plates built into solid masonry sit in the part of the wall that gets colder and wetter after internal insulation. Rot at joist ends is slow, hidden and structurally serious, and is one of the main reasons this work needs designing rather than improvising. Detailing at these junctions, and often leaving a ventilation path, is part of a properly designed system.

This is a case where the cost of professional advice is small relative to the cost of the failure.

Thermal bridges and cold spots

Insulating most of a wall but not around a window reveal, or stopping at a party wall junction, leaves a cold path through the construction. The insulated areas get warmer and the untreated junction becomes the coldest surface in the room, which is exactly where condensation and mould then appear. People frequently report new mould in a corner after insulating, and this is usually the reason.

Continuity of insulation, and returning it into reveals, is what prevents it.

Ventilation has to keep up

Sealing draughts is one of the cheapest and most effective efficiency measures, and it removes the uncontrolled ventilation the house was relying on. Moisture produced by cooking, washing and breathing then has nowhere to go and condenses on the coolest surfaces. Controlled ventilation — trickle vents, humidity-sensing extract fans, or a mechanical system with heat recovery — replaces what was lost.

The quieter point is that the order matters: plan the ventilation as part of the insulation work, not after the problems appear.

Where to start and what is low risk

Loft insulation at ceiling level, draught-proofing, and insulating pipes and hot water cylinders are low-risk, high-return measures in almost any house. Cavity wall insulation suits many buildings with suitable cavities and is unsuitable in exposed locations or where the cavity is bridged or damaged.

Floor insulation is disruptive and effective and needs to preserve underfloor ventilation in suspended timber floors. Any programme should start with the measures that have no moisture risk and take advice before touching solid walls.

The takeaway

Do the loft and the draughts first, plan the ventilation with the insulation, and get proper advice before insulating a solid wall.

The best rooms look like a decision, not a purchase.