Warmth Without Harm: Insulating an Old Wessex House in the Right Order

The pressure on owners of old houses has rarely been greater. Energy prices have settled above their historic norm, EPC ratings have moved from an abstract certificate to a negotiating point in sales and lettings, and grant schemes continue to favour measures that were designed with twentieth-century cavity walls in mind. The result is a great deal of retrofit activity in Wessex's older housing stock, some of it well judged and some of it quietly destructive.

The difficulty is not that old houses cannot be improved. It is that they were built to a different logic, and measures applied without regard to that logic can turn a sound building into a damp one within a few winters.

Why traditional fabric behaves differently

A Georgian townhouse in Dorchester, a cob cottage in north Dorset and a thatched farmhouse in the Wiltshire downs have little in common structurally, but they share one characteristic: their walls are solid and, in most cases, intended to be breathable. Moisture that enters the fabric, whether from driving rain, rising damp or the ordinary vapour produced by cooking, washing and breathing, is expected to migrate through the wall and evaporate from the surface. Lime mortars, lime plasters, breathable paints and unsealed stone all support that movement.

Modern construction works on the opposite principle. A cavity wall with a vapour barrier keeps moisture out and relies on ventilation to remove what is generated inside. Insulation systems developed for that model, notably rigid plastic foam boards bonded to a foil or plastic facing, can be disastrous when applied to a solid wall, because they place a vapour-resistant layer on the cold side of the existing fabric. Moisture arriving from within the room is stopped, and the wall behind it becomes steadily wetter. In a cob wall, which is essentially earth stabilised by straw and dependent on staying dry, the consequences can be structural.

Thatch adds a further consideration. The roof covering is designed to shed water and to allow the straw beneath to dry; anything that traps moisture in the coat, or reduces ventilation to the loft space above the ceiling, shortens its life and increases the fire risk.

The mistakes that keep surveyors in work

• Cement render and impermeable masonry paint applied to walls that previously breathed, trapping moisture and forcing it to find another route, often inward.
• Spray foam insulation applied to the underside of roof slopes, sealing rafters and preventing inspection and drying.
• Rigid board insulation fixed directly to the inside of solid walls without a hygrothermal assessment.
• Blocking airbricks and sub-floor vents in the name of draught proofing, allowing timber floors to rot.
• Sealing every gap in an old building without providing designed ventilation, so that moisture accumulates rather than escaping.

The common thread is the pursuit of a single number, usually a U-value, at the expense of how the whole building manages water.

Begin where the gains are cheap and reversible

A conservation-minded retrofit starts with the measures that cost least, disturb nothing and can be undone. Draughts come first. Chimneys are frequently the largest uncontrolled opening in an old house; a chimney balloon or a well-fitted damper, used when the flue is not in use, will change the feel of a room overnight. Letterboxes, keyholes, gaps at skirting and the junctions between floorboards and walls all merit attention, as do the windows discussed elsewhere on this site.

Heating controls follow. A modern programmer, thermostatic radiator valves and sensible zoning will often reduce consumption more than fabric work costing twenty times as much. Curtains with a proper lining, closed shutters at dusk and a well-fitted curtain across a draughty door are unglamorous and effective.

Loft insulation is the next step, and generally the best-value fabric measure available. In an older roof, however, the detailing matters. Mineral wool or a breathable natural fibre such as sheep's wool or wood fibre should be laid between and across the ceiling joists without being pushed tight into the eaves, and without obstructing ventilation paths. Where the loft has been boarded or converted, the insulation must go between the rafters with a clear ventilated gap above, and the works should be specified by someone who understands the roof's construction. Spray foam should be avoided.

Walls: the last measure, and never a generic one

Insulating solid walls is worthwhile in principle, but it is invasive, expensive and unforgiving of error. External insulation, finished in lime render, has the advantage of keeping the wall warm and dry and avoiding disruption inside, but it changes the appearance of the building and will almost always require consent in a conservation area or on a listed property. Internal insulation reduces room sizes, complicates the re-hanging of doors and joinery, and must be detailed so that the original wall remains able to dry and no cold bridge allows condensation to form behind the new lining.

Breathable materials help. Wood fibre, hemp and mineral boards with a lime or clay finish are far more forgiving than foil-faced foam in a solid wall. But the right specification depends on the wall: a Cotswold limestone, a chalk cob and a flint-and-lime panel all behave differently, and a proper assessment should consider orientation, exposure, rainfall and the existing condition before any decision is taken. On listed buildings, a specification of this kind should be discussed with the conservation officer at an early stage rather than presented as a fait accompli.

Ground floors and roofs

Suspended timber floors over ventilated voids can be insulated from below, using a breathable quilt held in place by netting, provided the sub-floor vents remain open and the void stays dry. Solid floors are more difficult and are usually left alone. Roofs over thatch need specialist advice; the priorities there are ventilation and the ability to inspect.

A phased programme for 2026

1. Commission a condition survey that includes moisture readings and identifies any existing damp or decay. Insulating a wet building simply hides the problem.
2. Eliminate draughts and improve controls. Record what you have done.
3. Insulate the loft to a sensible depth with breathable material, protecting ventilation.
4. Address windows and doors with repair, draught proofing and secondary glazing where appropriate.
5. Only then consider walls, with a written specification and, where required, consent in place.
6. Use competent, insured trades working to recognised retrofit standards, and keep documentation for future surveys, sales and insurance.

An EPC is a crude instrument when applied to a sixteenth-century cottage, and it should be treated as one input among several rather than a verdict. The objective is a house that is comfortable, dry and sound in another fifty years, not one that chases a certificate and pays for it in decayed timber. Done in the right order, retrofit and conservation are not in conflict; done in the wrong order, the repair bill will arrive long before the savings do.