Insulating an old house without trapping moisture

How old walls actually behave

A solid stone, brick, cob or timber-framed wall has no cavity and no damp-proof course. It takes on moisture in wet weather and gives it back in dry weather, and lime mortars, lime plasters and limewash exist precisely to let that exchange happen. The wall is not failing when it is damp; it is working.

Introduce cement render, gypsum plaster, plastic paint, foil-backed board or closed-cell foam and the exchange stops on one face. Moisture then accumulates at the interface, and it concentrates exactly where you cannot see it: behind skirtings, at wall plates, and at the buried ends of timber joists. Decay of embedded timber is the most expensive outcome of a well-intentioned retrofit, and it is usually invisible for the first decade. Our old building fabric page covers the material science in more depth.

The right order of work

1. Fix the water first. Blocked gutters, defective downpipes, raised external ground levels and cement pointing cause more "damp" than any lack of insulation.
2. Let the building dry. A wall saturated by a decade of trapped moisture needs a full drying season before anything is fixed to it.
3. Draught-proof and control ventilation. Cheapest, least intrusive, biggest immediate comfort gain.
4. Insulate the loft. Highest return, lowest risk, provided the void stays ventilated and downlighters are not creating moisture paths.
5. Insulate floors where they are already being lifted, preserving underfloor ventilation.
6. Then, and only then, consider the walls.

Solid wall insulation, safely

• Wood fibre board bedded and finished in lime plaster: vapour-open, hygroscopic, forgiving of imperfect walls.
• Insulating lime plasters including cork-lime and hemp-lime: thinner build-up, useful where losing floor area or covering mouldings is unacceptable.
• Hemp-lime (hempcrete) infill for timber frame, where the frame allows it.
• Sheep's wool or wood fibre between joists at roof and floor level.

Detailing matters more than the product. Return the insulation into window reveals to stop cold bridging and surface condensation, keep joist ends ventilated or isolate them from the insulated face, and never seal the top of a wall so that trapped moisture has nowhere to go. On a building of any significance, a moisture risk assessment before work starts is money well spent.

Roofs, floors and draughts

Loft insulation between and over ceiling joists is usually straightforward, provided eaves ventilation is maintained and the insulation does not bridge into a cold, unventilated void. Suspended timber floors can take wool insulation supported on breathable netting, but the underfloor airbricks must remain clear. Draught-proofing sash windows with discreet brush seals, adding secondary glazing where permitted, and fitting shutters or heavy curtains often match the comfort gain of far more invasive works.

What to avoid

• Injected chemical damp-proof courses on solid walls, and the cement "tanking" render that usually accompanies them.
• Closed-cell spray foam applied to the underside of rafters. It traps moisture, conceals decay, and has repeatedly caused mortgage and sale difficulties.
• Foil-backed plasterboard or PIR dot-and-dabbed to a solid wall.
• Plastic masonry paint and cement render on any traditionally built elevation.
• Sealing a building tightly without providing controlled ventilation.

Consent and grants

Internal wall insulation to a listed building generally requires listed building consent because it alters or conceals historic plaster and joinery. External insulation is rarely acceptable on a designated asset. Building regulations may also be engaged, though there are specific allowances recognising that energy efficiency requirements should not damage historic fabric. Grant support changes frequently, so check the current national schemes and your local authority's own heritage grants before committing.