Why Older Roofs Behave Differently

If your home was built before the Second World War, your roof almost certainly works on a different principle to a modern one. Traditional roofs were designed to get a little wet and then dry out again. Slates and tiles overlap rather than seal, there is often no underlay at all, and the mortar, plaster and lime finishes throughout the building are vapour-open. Moisture from cooking, bathing and breathing migrates upwards, passes through the roof structure, and escapes.

Modern insulation and modern membranes can interrupt that. Foil-faced rigid boards, sealed vapour barriers and impermeable underlays turn a roof that used to breathe into a sealed box. Water vapour then condenses on the cold side of the insulation — usually the underside of the rafters or the back of the sarking — and stays there. The result is damp timbers, black mould on plasterboard, rusted fixings and, in bad cases, wet rot in the rafter feet. Breathable insulation is essentially a way of working with the building rather than against it.

What "Breathable" Actually Means

The word gets used loosely, so it helps to separate two properties. Vapour-open means moisture can pass through the material as gas. Hygroscopic means the material can absorb and release small amounts of moisture without losing performance. The best natural insulants do both, which is why they suit period roofs so well.

This does not mean you can ignore ventilation. Even a breathable build-up needs a route for moisture to escape — typically a continuous 50mm air path above the insulation, running from eaves to ridge. Breathable insulation slows and buffers moisture movement; it does not make it disappear.

Materials Worth Considering

All of these are available in the UK and behave well in older roof structures:

• Sheep's wool — naturally hygroscopic, easy to fit around irregular rafters and services, and pleasant to handle. Typically supplied in batts or rolls.
• Wood fibre — comes as flexible batts for between and below rafters, and as rigid sarking boards that sit above the rafters. Excellent moisture buffering and good summer overheating resistance.
• Hemp and recycled cellulose — dense, vapour-open and well suited to awkward, uneven roof spaces where cutting to fit is unavoidable.
• Cork and recycled cotton — useful for thinner build-ups and tight rafter depths where you cannot afford much extra thickness.

Vapour-open boards such as wood fibre or calcium silicate perform a similar job internally, and they pair naturally with lime plaster and clay or breathable paints instead of vinyl emulsion. On the warm side of a period roof, that combination is often more appropriate than foil-backed plasterboard.

Detailing That Makes or Breaks It

The material matters less than the way it is installed. Get these details right and the system works:

• Never block the eaves gap. Baffles or trays should hold the air path open where insulation meets the wall plate.
• Replace failed bitumen felt with a breathable membrane if you are stripping the roof anyway. Do not leave old felt in place and add insulation beneath it.
• Avoid double vapour barriers. If you fit a sealed barrier on the inside and old felt remains outside, moisture is trapped in the middle.
• Keep rafter depth realistic. Adding insulation below the rafters is usually the only way to reach a sensible U-value in an older roof without losing head height.
• Check the roof first. Slipped slates, failed flashings and blocked gutters need sorting before insulation goes anywhere near the structure.

If your property is listed or in a conservation area, breathable solutions are often looked on more kindly by conservation officers, because they are reversible and do not trap moisture in historic timber.

Attic Storage Without Creating a Damp Problem

Most people insulating a loft also want somewhere to put things. In a cold loft, insulation sits between the ceiling joists, and the golden rule is simple: do not squash it and do not cover it with boards laid directly on top. Compressed insulation loses much of its value, and boarding over it traps moisture against the ceiling.

Use loft legs or a raised platform system instead. This keeps the full insulation depth intact and maintains the ventilation path above it. A few practical points:

• Store items in lidded plastic crates rather than cardboard, which absorbs damp and feeds mould.
• Keep stored goods back from the eaves so air can still move around the roof void.
• Leave a clear gap around water tanks and pipework so you can inspect them.
• If you are converting the loft into a habitable room, storage shifts to the eaves cupboards — line these with a vapour-open board and keep them ventilated.

Getting the Balance Right

The aim in an older roof is not to make it airtight and sealed, but to make it warmer while still allowing it to dry. Breathable insulation, vapour-open boards and careful detailing of vents and air paths usually achieve that better than a fully synthetic, sealed alternative. Take advice from someone who understands period construction, measure your rafter depths before ordering anything, and treat ventilation as part of the insulation job rather than an afterthought. Done properly, a breathable roof is warmer, drier and far more forgiving of the British weather.