Granite and stone cottages are part of what makes Cornwall look like Cornwall, but they can be cold, damp-feeling and expensive to heat. Adding insulation is tempting, and done well it can make a real difference to comfort. Done badly, it can trap moisture in the walls and cause the very problems you were trying to avoid. This guide explains why old stone walls behave differently, the materials that suit them, and what to check before you start.
How solid stone walls work
Modern cavity walls keep rain out by using a gap and a waterproof barrier. Traditional granite and rubble stone walls work differently. They're thick, often 500mm to 900mm or more, and they manage moisture by absorbing it and releasing it again as the weather changes. Lime mortar, lime plaster and lime render allow that moisture to evaporate, which keeps the wall dry over time.
Cornwall's wind-driven rain makes this even more important. Walls on exposed coastal and moorland sites get wet regularly, so the wall needs to be able to dry out. If you seal the wall with materials that don't let moisture pass, it can stay wet inside, leading to damp patches, decayed timber ends and frost damage to stone and mortar.
Fix the basics first
Before you add any insulation, it's worth making sure the wall is in good condition. Common issues to deal with first include:
Blocked or leaking gutters and downpipes that soak the wall below.
Ground levels built up against the wall, or hard paving that sends water towards it.
Cement pointing or cement render, which traps moisture and can often be replaced with lime.
Failed pointing, open joints or cracked render letting rain in.
Poor ventilation inside, particularly in kitchens and bathrooms.
Often the biggest improvements come from repairing and repointing with the right materials. Our guides to lime rendering on Cornish stone homes and renovating an old Cornish cottage cover repairs in more detail, and our stone, rendering and lime work page explains the service.
Breathable insulation materials
For solid stone walls, the usual advice is to choose vapour-open materials that let moisture move through the wall. Options often considered include:
Wood fibre boards, which are vapour-open and add some thermal mass.
Lime-hemp (sometimes called hempcrete), applied as a lining or plaster.
Cork and sheep's wool, which handle moisture well.
Calcium silicate boards, which can move moisture by capillary action and are often used internally.
Insulating lime plasters, which add a modest improvement while keeping the wall breathable.
Rigid foam boards such as PIR or foil-faced products, and gypsum plasterboard fixed over a plastic membrane, are generally not recommended for traditional walls because they can stop moisture escaping. Always follow the manufacturer's guidance and take advice from someone experienced with traditional buildings.
Internal or external insulation?
Internal insulation is applied to the inside face of the wall. It's usually easier to carry out room by room and doesn't change the look of the outside, which matters in conservation areas and for attractive granite fronts. The downsides are that rooms get a little smaller, the wall itself gets colder and may be more prone to frost, and details around floors, window reveals and joist ends need careful design to avoid condensation.
External insulation goes on the outside and is usually finished with a breathable render. It keeps the wall warm and reduces thermal bridging, but it changes the appearance of the house, may cover original stonework, and needs careful detailing around windows, roof edges and eaves. It's more likely to need planning permission, particularly on a house in a conservation area, in the Cornwall National Landscape (formerly the AONB) or on a listed building.
Neither approach is automatically better. Sometimes a mix of measures, such as roof, floor and draught proofing combined with a thin internal lining, gives the best balance.
Planning, listed buildings and Building Regulations
Check the position before you start:
Listed buildings will usually need listed building consent for insulation that affects their character or fabric, internally or externally.
Conservation areas and protected landscapes can limit what you can do externally, so you may need planning permission for external insulation or changes to render and windows.
Building Regulations may apply when you're altering walls, and energy efficiency requirements (Approved Document L) can apply when work involves a thermal element. Special rules can apply to historic buildings, so check with Building Control.
Our guide to building in an AONB or conservation area explains how those restrictions work, and our Building Regulations guide covers the basics. It's also worth reading about weatherproofing coastal homes.
Planning a whole-house approach
Insulation works best as part of an overall plan. A good approach is to:
Start with a survey that checks the condition of the walls, roof, floors and ventilation.
Look at roof and floor insulation and draught proofing, which are often more straightforward and less risky than walls.
Choose materials that suit the existing fabric, and avoid mixing breathable and non-breathable layers.
Plan ventilation so moisture from daily living has somewhere to go.
Think about heating at the same time, such as heat pumps, which can work well in well-insulated, steadily warmed homes.
A broader property renovation is a good moment to do this, because walls are often open and services are being replaced anyway.
Insulating stone houses FAQs
Can I insulate a granite house?
Yes, but the method and materials matter. Breathable products and careful detailing are generally recommended so the wall can still dry out.
Is cavity wall insulation suitable for stone houses?
Many traditional stone houses are solid-walled, so there's no cavity to fill. Walls with a cavity should be assessed by a specialist first.
Will insulation stop damp?
Not by itself. If damp comes from leaking gutters, high ground levels or cement-based repairs, those problems should be fixed first.
Do I need permission to insulate a listed cottage?
Listed building consent will usually be needed for works that affect the character of a listed building. Check with Cornwall Council before starting.
Talk to us about your project
If you're thinking about renovating a stone house, we can talk through repairs, lime work and the wider project. Call Cornish Building Company on 01208 816991 or contact us to discuss your plans.
