Improving Energy Efficiency in Older Buildings

Older buildings are full of character, but they can also be draughty, damp and expensive to heat. The challenge is to improve energy efficiency without causing unintended problems such as condensation, timber decay or damage to historic fabric. The good news is that with a careful, incremental approach, you can cut bills and carbon while keeping the building healthy and welcoming.

Understand how your building breathes

Most pre-1919 UK homes were built with solid walls, lime mortar and plaster, and no damp-proof course. They manage moisture by allowing walls to absorb and release water vapour. If you trap that moisture with modern impermeable materials, you risk damp patches, mould and rotting joists. Before you insulate or seal anything, understand where moisture comes from and how it currently escapes. Look for misted windows, musty smells or black mould in corners. A simple hygrometer can help you monitor indoor humidity; aim for 40–60% relative humidity.

Key principles for older buildings:

• Breathability: use vapour-open materials that let moisture pass through.
• Ventilation: maintain a controlled supply of fresh air, especially in kitchens and bathrooms.
• Reversibility: choose measures that can be undone without damaging historic fabric.
• Incremental approach: make one change, monitor for a season, then decide the next step.

Insulate lofts, roofs and floors first

The loft is usually the easiest and most cost-effective place to start. If you have joists, add insulation between them and then lay a second layer across the top, ideally to a depth of 270–300mm. Use a breathable material such as sheep’s wool, wood fibre or mineral wool. Crucially, do not block the eaves. Air must circulate under the roof covering to prevent condensation on the underside of tiles or slates. Fit eaves ventilation trays or ensure a clear gap. Flat roofs and room-in-roof spaces are trickier; get professional advice because ventilation pathways are easily blocked.

For suspended timber floors, you can lift floorboards and insulate between the joists. Use breathable insulation such as wood fibre or sheep’s wool, and keep the void beneath well ventilated. Do not use spray foam or solid boards that could trap moisture against timber. If you have a solid concrete floor, adding insulation on top will raise floor levels, which may affect door thresholds and skirting boards.

Tackle walls with breathability in mind

Solid walls are a major source of heat loss in older buildings. External wall insulation can work, but it changes the appearance and may need planning permission in conservation areas or on listed buildings. Internal wall insulation loses floor space and can create cold bridges at junctions. If you go internal, use a breathable system such as wood fibre or lime-based insulating plaster, and avoid cement render or plastic vapour barriers. Cavity walls, common from the 1920s onwards, can sometimes be filled, but only if the cavity is clean, clear and suitable. Many older cavities are rubble-filled or have mortar snots that prevent proper filling. A specialist survey is essential before you drill.

Upgrade windows, doors and draught-proofing

Original sash windows are a heritage feature worth keeping. Repair and draught-proof them with brush seals, silicone seals or compression seals rather than expanding foam. Secondary glazing is often the best option for listed buildings or conservation areas: it adds a layer of still air, reduces noise and is reversible. If you replace windows, choose slim double-glazed units with a sympathetic frame profile, and ensure the wall can still breathe around the reveal. For doors, fit draught excluders, letterbox flaps and keyhole covers. Heavy curtains, thermal blinds and shutters also make a noticeable difference. Do not seal every gap without providing ventilation, or you will trap moisture.

Improve heating controls and system efficiency

You cannot control what you cannot measure. Fit thermostatic radiator valves (TRVs) on every radiator except the one in the room with the thermostat. Upgrade to a smart thermostat with weather compensation and zoning, so you only heat the rooms you use. Set a lower flow temperature if you have a condensing boiler; this improves efficiency. If your boiler is old, consider replacing it with a modern condensing model or, after improving insulation, a heat pump. Heat pumps work best with larger radiators and good insulation, so plan the whole system together. Regular servicing and bleeding radiators also help. A hot water cylinder should be well insulated with a jacket, and pipework in unheated spaces should be lagged.

Get ventilation and moisture balance right

As you make a building more airtight, you must provide controlled ventilation. Otherwise, indoor air quality suffers and condensation increases. In kitchens and bathrooms, fit extractor fans with humidistats or timers. Trickle vents on windows are simple but effective. Positive input ventilation (PIV) can help dilute moisture in a whole house. For a deep retrofit, mechanical ventilation with heat recovery (MVHR) can recover heat while supplying fresh air, but it needs a well-sealed building to work efficiently and careful design to avoid ductwork problems. Never block up original air bricks or vents without providing an alternative. Monitor humidity and adjust ventilation as needed. The goal is a healthy balance: not too draughty, not too airtight.