Why an Old Cottage Needs a Different Approach

A stone or brick cottage built before the twentieth century usually has solid walls rather than a cavity, lime mortar rather than cement, and timber that has spent a century or two quietly absorbing and releasing moisture. That isn't a flaw; it is how the building was designed to work, drying outwards and inwards as conditions change.

It is also why insulation that transforms a modern loft can cause real damage here. Foil-faced foam boards, cement renders and plastic vapour barriers trap moisture. Trapped moisture in a solid wall means damp patches, blown plaster and, eventually, rot in joist ends and roof timbers. Choose your material first for how it behaves with moisture, and second for its thermal performance.

Breathable Insulation Explained

Breathable, or vapour-open, insulation lets water vapour pass through rather than holding it back. Every material has a vapour resistance figure, and the guiding principle in an old building is simple: keep materials progressively more vapour-open towards the cold side, so moisture escapes outwards instead of condensing inside the wall or roof.

In practice, that means avoiding foil facings and integral vapour barriers on the warm side of a solid wall, and choosing insulation that can take up a little moisture and release it again. It also means being airtight where it matters — around loft hatches, along skirtings, at rafter junctions — without making the fabric itself vapour-tight. Air leakage and vapour movement are separate problems with separate solutions.

Natural Materials Worth Considering

Happily, the materials best suited to a cottage are also pleasant to live with.

• Sheep's wool — around 0.038 to 0.040 W/mK, hygroscopic and easy to cut around pipes and cables. Excellent between rafters and joists.
• Wood fibre — batts or rigid boards at roughly 0.038 to 0.045 W/mK. Denser than wool, so it adds a little thermal mass and takes plaster or render well.
• Hemp fibre — similar performance to wool, with good draught-filling qualities and a long history in British building.
• Cork — boards and granules at about 0.040 W/mK, naturally water-resistant and useful where space is tight, such as under floors.
• Cellulose fibre — recycled paper, blown into lofts and closed cavities at around 0.040 W/mK. Dense-packing takes real skill; done badly it settles and leaves gaps.

All of these cost more per square metre than mineral wool. In a cottage the difference is usually money well spent, because you are buying moisture management as well as warmth.

Start with the Loft and the Draughts

Before touching walls, take the easy wins. A well-insulated loft is the best-value improvement in most cottages. Aim for the equivalent of 270mm of mineral wool, laid in two crossed layers to cover the joists, but keep the eaves clear — cottages often have ventilation paths there that must not be blocked. Use baffles to hold a 50mm gap, and never cram insulation into the corner where rafters meet the wall plate.

Draught-proofing comes next. Chimney balloons, brush seals on sash windows, excluders on external doors and a decent letterbox flap cost little and make an immediate difference. Be careful with open fires and wood burners: they need a permanent air supply, so don't seal a room so tightly that the stove pulls smoke back down the chimney.

Solid Walls, Floors and Awkward Corners

Insulating a solid wall is the most demanding job. Internal insulation keeps the outside looking untouched, which matters in a conservation area or a listed building, but it costs floor space, creates cold bridges at internal walls and joists, and needs careful detailing at reveals and skirtings. Breathable boards such as wood fibre or cork, fixed to battens or bedded on a lime adhesive, work well; keep gypsum plaster and cement render away from the cold side.

External insulation wraps the whole building and removes cold bridges, but it changes the appearance, adds thickness at windows and doors, and normally needs consent on a listed building. Any external system must be vapour-open, finished with lime or mineral render rather than cement.

Suspended timber floors are often overlooked. Lift a few boards, deal with any rot or damp first, then fit breathable batts between the joists supported on netting or a breathable membrane, leaving the void below ventilated through the existing air bricks.

Details That Decide Whether It Works

Two things sink otherwise good insulation jobs in old cottages: moisture and ventilation. Fix the causes of damp before you insulate anything — clear the gutters, repair downpipes, sort out drainage, and check that external ground levels sit well below any damp-proof course. Insulation laid over a wet wall simply hides the problem.

Then think about air quality. A cottage that was once draughty is now sealed, and cooking, bathing and breathing still produce moisture. Background ventilation, extract fans and trickle vents will keep humidity in check; a small hygrometer is a worthwhile investment.

Finally, find an installer who works on old buildings and can discuss vapour openness without reaching for a foil-faced board. Ask what they will do at the eaves, at joist ends and around the chimney. The answers tell you more than any brochure.