Why period walls behave differently

A solid stone, brick or cob wall has no cavity and no damp-proof course in the modern sense. It manages moisture by absorbing it and letting it dry, usually to the outside. Cement render, gypsum plaster, plastic paints and foil-backed insulation all interrupt that process. Trap water in a traditional wall and you get damp patches, blown plaster, rotten lintels and, in a timber frame, decayed oak that had been perfectly happy for three hundred years.

So the aim is not to seal the cottage like a new-build. It is to slow heat loss, cut draughts and keep every layer able to dry. Breathable materials are not a fashion; they are how these buildings were designed to work.

Start with the draughts, not the insulation

Cold is often moving air. Chasing U-values before dealing with a gale under the door is money wasted, and draught-proofing is cheap, reversible and quick.

• Block unused chimneys with a breathable chimney balloon or a sheep's wool draught excluder, but keep one flue open if you have an open fire or wood burner.
• Seal sash windows with brush or silicone-based draught strips set into the frame. They are hidden when the window is closed and can be removed without damage.
• Fit letterbox brushes, keyhole covers and a snug seal around the door. A heavy curtain across a wind-facing door makes a surprising difference.
• Deal with gaps at skirtings, floorboards, service pipes and the loft hatch. A sliver of sheep's wool or a bead of lime-friendly mastic is usually enough.

Lofts and roofs: the easy wins

Insulating a loft is straightforward, but the detailing matters. In a cold roof, keep a clear air path from the eaves to the ridge. Pack insulation into the eaves and you trap moist air against the sarking, and rot follows within a few winters.

Use breathable materials where they sit against old timber: sheep's wool, wood fibre, hemp batts or cellulose. Aim for 270mm or more between the joists, crossed with a second layer at right angles, and insulate and draught-seal the hatch itself. Lag any tanks and pipes while you are up there.

If you have a slate or stone roof with no underlay, do not spray foam it. Foam destroys the roof's ability to dry, traps water in the battens, and makes slates almost impossible to reuse. In a room-in-roof or attic conversion, resist squashing insulation between rafters. A warm roof build-up of wood fibre above the rafters, finished with a breathable membrane and slate, keeps the room warm and lets the structure dry outwards.

Walls: go gently, go breathable

Internal wall insulation is the most common mistake made in period cottages. Boarding the inside with foil-backed plasterboard or PIR creates a cold, unventilated zone where moisture condenses, often at the joist ends and lintels where it does the most harm.

Where you do insulate internally, use a vapour-open system: wood fibre or hemp boards fixed to the wall, finished with lime plaster and a clay or lime paint. Thin insulation on a solid wall can be worse than none — enough to cool the wall behind it, not enough to keep the room warm. If you are going to do it, allow at least 60mm of wood fibre and think hard about the awkward bits: window reveals, joist ends, wall plates, sockets and pipe runs. Every interruption is a potential cold bridge and a future damp patch.

External insulation with a lime render can work beautifully on some solid walls, but it changes the building's appearance and usually needs consent. Repointing in lime rather than cement is the quiet, unglamorous improvement that makes the biggest difference to how a wall dries.

Windows, doors and floors

Original windows are usually worth keeping. Secondary glazing is reversible, effective and far cheaper than replacement. Add shutters where they once existed, plus lined curtains and a pelmet to reduce radiant loss and draughts at the reveals. Where repairs are needed, patch the timber and re-putty the glass rather than reaching for uPVC.

Suspended timber floors can be insulated from below with a breathable batt held in place by netting or boards. Do not block air bricks or any sub-floor ventilation, or you will simply move the damp somewhere else. For solid floors, an insulated limecrete build-up topped with a lime screed lets the ground beneath breathe and feels far warmer underfoot than bare flagstones.

Detail, consent and a little judgement

Check the simple things before spending anything: clear the gutters, repair downpipes, and make sure ground levels fall away from the wall. A great many "rising damp" problems turn out to be water running down a wall from a broken gutter on the windward side.

If the cottage is listed, or sits in a conservation area, talk to the conservation officer early. Most are genuinely helpful when you explain what you are trying to achieve and why breathable materials are part of the answer. Building Regulations allow sensible flexibility for historic buildings, but you still need to show the work is well considered.

Finally, do not try to make a cottage airtight. Background ventilation, working extract fans in bathrooms and kitchens, and a willingness to open windows matter more than a sealed envelope. Improve what you can, watch for damp, and adjust as the seasons teach you what the building wants.