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Why older homes need a different approach

Insulating a Victorian terrace or a sixteenth-century timber-framed cottage is not the same job as insulating a modern new build. Older walls are usually solid, with no cavity to fill, and they were designed to manage moisture by allowing it to move through the fabric and evaporate away. Wrap that wall in a vapour-tight material and you can trap damp inside the structure, which is how you end up with rotting timbers and blown plaster.

The goal, then, is not to seal the building up like a plastic bag. It is to slow heat loss, keep the building able to dry, and improve comfort without creating new problems. Choose materials that are vapour-open, fit them carefully, and let the house keep breathing the way it always has.

Start with the building, not the product

Before you order anything, spend a morning finding out what you are actually dealing with. The right insulation depends on the wall build-up, the age of the frame, and where the moisture is coming from.

• Check for existing damp. Fix leaking gutters, blocked air bricks and rising damp before insulating, not after. Insulation hides problems rather than solving them.
• Identify the wall type. Solid brick, stone rubble, cob and timber frame all behave differently. A rubble stone wall with an uneven face may need a different approach from a flat brick wall.
• Note any listing or conservation area status. Internal wall insulation and changes to windows will often need consent, so check early rather than undoing work later.
• Look at existing ventilation. Chimneys, vents and leaky windows currently dry the house out. If you seal them, you must replace that air movement deliberately.

Natural insulation for solid walls

Solid walls lose heat quickly, and internal insulation is usually the only realistic option unless you are re-rendering externally. The key is to use a vapour-open system so the masonry can still dry inwards, and to avoid a cold, impermeable layer pressed against the wall.

Good options include:

• Wood fibre boards and batts. Typically around 0.038 to 0.045 W/mK, so slightly thicker than rigid foam for the same performance, but they buffer moisture beautifully and pair well with lime plaster.
• Hemp batts. Similar thermal values to wood fibre, easy to cut and friction-fit between studs, and made from a fast-growing crop.
• Cork board. A good choice where space is tight, as it achieves a decent U-value at a lower thickness than softer natural materials.
• Sheep's wool. Excellent for uneven surfaces, handles moisture well, and is pleasant to work with. It needs to stay dry and free from moth attack, so detail carefully.

A typical internal lining might be 60mm of wood fibre board against the wall, a stud frame filled with 100mm of hemp or wool, and a further 40mm of wood fibre board under a lime or clay finish. In England, the benchmark for a renovated wall is generally a U-value of around 0.30 W/m²K, though an older property may not reach it without losing too much room.

Timber frames and suspended floors

Timber-framed buildings are the most demanding. The frame needs to stay dry and ventilated, and any insulation you add must not trap moisture against the beams. Rigid foam boards with foil facings are usually the wrong answer here, because they create a vapour barrier in the wrong place.

Instead, use vapour-open insulation such as wood fibre, hemp or sheep's wool, fitted snugly between the studs with no gaps. Always leave a ventilated cavity behind any external cladding or render, and make sure air can move from the base of the wall up to the eaves. If you are filling an old lath-and-plaster wall, consider a breathable membrane on the warm side rather than a plastic sheet.

For suspended timber floors, lift the boards and drape a breathable quilt between the joists, supported on netting. This is often one of the cheapest comfort wins in an old house, particularly over a cold, draughty cellar or a ventilated void.

Lofts and roofs: the easy wins

The loft is usually where you get the most heat loss reduction for the least disruption. If the joists are exposed, simply lay insulation between them and then cross-lay more at right angles to cover the timber.

• Mineral wool or recycled glass wool. Around 270 to 300mm gives a strong result and remains the best value per pound spent.
• Cellulose fibre. Made largely from recycled paper, blown in to fill awkward shapes around chimneys and trusses.
• Wood fibre or wool batts. Better where you want moisture buffering or are working close to a breathable roof.

Whatever you use, keep the eaves ventilation clear. Push insulation right into the corner of the roof and you will block the airflow that keeps the rafters dry.

Detailing, ventilation and where to spend your budget

Good insulation badly fitted performs like poor insulation. A 2mm gap around a batt can cut its effectiveness dramatically, so cut materials slightly oversized and press them firmly into place. Tape or seal junctions at party walls, around joist ends and behind window reveals.

Once the house is tighter, it needs controlled ventilation. Trickle vents, extract fans with humidity sensors, or a mechanical ventilation system with heat recovery will all help, and they matter more in a sealed old house than in a draughty one. If you are working to a budget, prioritise the loft, then draught-proofing, then floors, then walls.

Finally, work with someone who understands old buildings. A contractor who usually fits foil-faced boards into new-build cavities may not appreciate why a lime-rendered stone wall needs to dry inwards. Ask about breathability, moisture movement and airtightness detailing before you accept a quote, and take your time. A careful, sympathetic retrofit will keep an old house warm and comfortable for decades without costing it the character that made you love it in the first place.