Why Old Walls Need to Breathe

Most period homes — Georgian, Victorian and Edwardian alike — were built with solid walls, usually nine inches of brick or stone, held together with lime mortar and finished inside with lime plaster. None of these materials pretend to be waterproof. Instead, they work as a system: moisture from the air, from driving rain and from the ground is absorbed, held briefly, and then released again as conditions change. The wall dries outward and inward, and the building stays comfortable.

Introduce a modern, impermeable layer — cement render, gypsum plaster, vinyl silk paint, or a sheet of foil-backed insulation board — and you interrupt that cycle. Trapped moisture has nowhere to go. It accumulates inside the wall, and the first sign is usually a musty smell behind a skirting board, a patch of mould in a cold corner, or paint blistering near a window reveal. Insulating a period home safely means choosing materials that let vapour pass through rather than blocking it.

Where Damp Really Comes From

Before you touch a roll of insulation, it helps to know what you are dealing with. Damp in old houses usually falls into one of four categories:

• Interstitial condensation — warm, moist indoor air migrates into a cold wall and condenses inside it, often out of sight.
• Penetrating damp — failed gutters, cracked render, blocked hopper heads or defective flashings letting rainwater in.
• Rising damp — ground moisture travelling up through brickwork where a damp course is missing or bridged by raised paths and soil.
• High internal humidity — cooking, showering, drying laundry indoors and simply breathing, all adding litres of water vapour to the air every day.

Insulation changes the temperature profile of a wall, which shifts where the dew point falls. Get that wrong and you can create condensation in a place that was previously dry. The good news is that a warmer wall is a drier wall, so the aim is to keep the structure warm while allowing it to dry.

Breathable Materials That Suit Old Buildings

Happily, the market now offers genuinely vapour-open options that perform well and feel at home in period interiors:

• Wood fibre boards — rigid, easy to plaster onto, and excellent at buffering moisture. Ideal for internal wall insulation on solid brick.
• Sheep's wool, hemp and cellulose — flexible quilts for lofts, floors and studwork, with good hygroscopic properties.
• Cork and calcium silicate boards — useful where space is tight or where a wall has a history of damp.
• Lime plasters, clay paints and breathable membranes — the finishing layers matter as much as the insulation itself.

Rigid foam boards such as PIR, PUR and XPS are superb insulators but effectively act as a vapour barrier. They can work in the right assembly, with a ventilated cavity or a smart membrane, but they need careful calculation and detailing. If in doubt, keep the wall able to dry.

Room by Room: Where to Start

Prioritise the easy wins before the complicated ones. Lofts are usually the cheapest place to save energy: top up to at least 270–300mm, but keep the eaves ventilated and never block the airflow at the wall plate. Suspended timber floors can take a breathable quilt supported on netting between the joists — just don't obstruct the air bricks that keep the void dry.

For walls, internal insulation is often the only option, particularly on listed buildings or in conservation areas where external changes need consent. Build a stud frame or fix wood fibre boards directly, then finish with lime plaster. Accept that you will lose 60–100mm of room width and plan radiators, sockets and picture rails accordingly. Secondary glazing, heavy curtains and draught-proofing around sashes and doors deliver a surprising amount of comfort for modest cost, and they are reversible.

Remember that joist ends, party wall edges and window reveals are the classic cold bridges. Insulating the middle of a wall while leaving these unaddressed simply moves the condensation elsewhere.

Ventilation: The Other Half of the Equation

You cannot insulate and seal a house and expect it to stay healthy. Old buildings relied on draughts and open fires for ventilation; when you close those routes, you must replace them deliberately.

• Retrofit trickle vents to windows, or fit humidity-controlled extract fans in bathrooms and kitchens.
• Keep chimney flues ventilated with a cap and vent, even if the fireplace is unused.
• Consider MVHR (mechanical ventilation with heat recovery) only after specialist advice — ducting in period homes is rarely straightforward.
• Use a cheap hygrometer to keep relative humidity between roughly 40% and 60%. Above that range, mould and dust mites thrive.

Getting the Details Right

Sequence matters. Fix leaks, clear gutters, repoint with lime mortar, lower external ground levels and sort out drainage before you insulate. Any active damp should be diagnosed properly rather than covered over. Choose contractors who understand old buildings, and check whether listed building consent or building regulations approval is needed — Part L has specific allowances for historic fabric.

Finally, be patient. A breathable scheme dries out slowly, and a wall that has been sealed for decades may take months to settle. Monitor it, adjust as you go, and let the building do what it has always done: manage its own moisture, quietly and constantly.