Insulating a Period Home Without Causing Damp

Why Old Walls Need to Breathe

A solid brick or stone wall has no cavity. It keeps the weather out by being thick, and it manages moisture by letting water vapour pass through it. Rain soaks into the outer face, travels a little way in, and then dries back outwards when the weather improves. The same happens inside: cooking, washing and simply breathing put water vapour into the air, and some of it moves into the wall fabric, where it must be able to escape.

Modern insulation methods were designed for cavity walls and timber frames. Rigid foam boards, foil-backed plasterboard, cement render and plastic vapour barriers all act as a stopping point. Fit them to an old wall and you trap moisture inside the brickwork or behind the board. The result is damp patches, blown plaster, rotting lintels and that unmistakable cold, musty smell. The principle to remember is simple: whatever you add to a period home should be vapour-open and breathable, so moisture carried into the structure can still get out.

Start in the Loft, Where the Gains Are Biggest

Heat rises, and an uninsulated roof void is usually the single largest source of loss in an old house. Happily, it is also the easiest place to work without upsetting the building.

Clear the joists, check for old mouse nests and any crumbling mortar, and lay insulation to a decent depth — around 270mm of a breathable mineral or sheep's wool product is a sensible target. Roll it out in layers, crossing the direction of the first layer, and take it right over the wall plates. Where the roof slopes down to the eaves, keep a clear ventilation gap of at least 50mm at the eaves so air can still move through the roof space. Use eaves trays or baffles rather than cramming insulation into the gap.

If you have a breathable membrane under the tiles, you can often insulate between the rafters as well, but leave the required air path above the insulation. And if there is a water tank or pipework up there, insulate the tank itself and lag every pipe — a frozen burst in January undoes years of careful work.

Deal with Draughts Before You Think About Walls

Old houses are not draughty because they are badly built. They are draughty because they were designed to be ventilated, and because gaps have opened up over a century or two. Chasing out draughts is cheap, quick and gives an immediate improvement.

• Fit brush or silicone draught seals to sash windows — they work remarkably well and cost very little.
• Seal gaps around skirting boards, floorboards and service pipes with a flexible, paintable mastic.
• Hang a heavy curtain across a front door, but keep curtains clear of radiators so heat is not simply drawn up behind them.
• Look at chimney breasts: an unused open fireplace can pull an astonishing volume of warm air out of a room. A chimney balloon or a simple board across the throat makes a real difference.

Where a sash window is beyond saving, secondary glazing is usually a better answer than replacement in a conservation area, and far kinder to the building.

Suspended Timber Floors and Cold Ground Rooms

Ground floors in period homes are often suspended on joists over a ventilated void. Air bricks in the external walls keep that void dry, and those air bricks must never be blocked. If you block them, the joist ends sit in damp air and rot.

You can insulate between the joists from below, fixing netting to hold breathable batts in place, or lift a few floorboards and work from above. Leave the ventilation path at the sides of the void clear. In cellars and basements, resist the urge to seal and tank everything; proper drainage and airflow solve more damp problems than any membrane.

Internal Wall Insulation: A Careful Last Resort

Once the loft, floors and draughts are sorted, solid walls are the remaining weak point — but they need a gentler approach. The safest options are hygroscopic, meaning they absorb and release moisture rather than trapping it.

• Wood fibre boards, typically 40–80mm, fixed directly to the wall and finished with lime plaster.
• Hemp-lime or insulating lime plasters, useful where you cannot afford to lose much room space.
• Cork or sheep's wool batts in a timber stud frame, provided the frame is kept away from the wall and the void behind is ventilated.

Avoid cement-based renders and gypsum skim over insulation on solid walls, and never seal the wall behind an impermeable board. If the house is listed, or sits in a conservation area, check what consent is required before starting. And do one wall at a time, then watch it through a full winter before moving on.

Let Air Move, and the Building Will Look After You

Insulation and ventilation are two halves of the same job. As you tighten a house, you reduce the incidental airflow that used to dry it out, so make sure you replace it deliberately: trickle vents on windows, extract fans in kitchens and bathrooms, and a window cracked open now and then. Never block a chimney completely without giving some thought to how the room will breathe.

Working with an old house rather than against it is slower, and it asks you to understand the materials already there. But a period home insulated in breathable materials, with its lofts and floors sorted and its draughts tamed, will be warmer, drier and far more comfortable — and it will still be standing happily in another hundred years.