How to Insulate a 100 Year Old House
Short answer: How to insulate a 100 year old house? Start with the roof and the draughts, fix any damp first, and only then tackle the walls with breathable materials. A house built around 1926 almost certainly has solid brick or stone walls, not a cavity to fill, so the modern quick fixes do not apply. Done in the right order, insulation makes an old house warmer, drier and cheaper to heat. Done in the wrong order, it can trap moisture inside walls that were designed to breathe, and that damage is slow, hidden and expensive to undo.
A century old house is not a badly built modern house. It was built with different materials and a different idea of how moisture should behave. Lime mortar, lime plaster, solid masonry and timber all absorb small amounts of moisture and release it again as conditions change. That is what heritage specialists mean when they say an old building breathes. Modern insulation that seals a wall with foam boards and plastic membranes can block that drying route. Moisture then collects inside the wall, salts crystallise on the surface, plaster fails and timbers decay. Historic England’s guidance on internal wall insulation makes the same point in practical terms: insulation and finishes must be compatible with the existing wall, must not cause moisture to accumulate, and must not hinder the wall’s ability to dry. Keep that principle in mind and most of the decisions in this guide become straightforward.
First, work out what kind of old house you have
Before you buy anything, answer three questions. First, are the walls solid or cavity? Homes built before the 1920s in the UK usually have solid walls, a single thickness of brick or stone, while later homes usually have two skins with a gap between them. You can often tell from the brickwork outside: an alternating pattern of long and short brick faces usually means a solid wall, while a regular repeating pattern usually means a cavity wall. If you are unsure, a surveyor or an experienced installer can confirm it in minutes, and it is worth confirming, because cavity wall insulation injected into a solid wall is not possible, and solid wall insulation applied to a cavity wall is the wrong job.
Second, is the house listed or in a conservation area? Listed building consent is almost always needed for internal wall insulation in a listed building, according to Historic England, and external wall insulation changes the appearance of the building, so it needs consent and careful design in sensitive areas. Check with your local planning authority before you commit to any wall work. A short conversation early on saves a refused application and a wasted deposit later.
Third, where is the damp coming from? Insulation never fixes damp. It hides it. Rising damp, a leaking gutter, a failed flashing, a blocked drain or condensation from everyday living all need to be dealt with first. Walk the house after rain, look for staining, peeling paint, salt marks and musty smells, and get gutters, downpipes and roof coverings repaired. If you insulate over a wet wall, you are simply trapping the problem behind an expensive layer.
The order that works: roof, draughts, windows, walls, floors
The cheapest, lowest risk measures come first, and they also deliver a noticeable improvement while you plan the bigger jobs. This is the sequence heritage and energy advisers consistently recommend for traditional buildings.
Start with the loft. Heat rises, and an uninsulated roof is one of the largest single sources of heat loss in an old house. The Energy Saving Trust recommends 270mm of loft insulation as the standard depth for mineral wool, and its guidance puts the saving at up to £135 a year for a typical home. In an old house, use a breathable material such as sheep wool or wood fibre where possible, keep insulation away from recessed lights that need ventilation, and never block the eaves ventilation that lets the roof structure dry. A cold, ventilated roof space is a healthy roof space.
Next, draught proofing. Old sash windows, ill fitting doors, loft hatches, letterboxes and gaps around floorboards all leak warm air. Draught proofing is cheap, often DIY, and the Energy Saving Trust puts the saving at around £25 a year, which understates the comfort gain: a draught free room feels warmer at the same thermostat setting. For sash windows, brush seals fitted into the staff beads preserve the window and cut the draught without replacing anything historic. For doors, seals plus a letterbox brush and a keyhole cover do most of the work.
Then look at the windows. Replacing original timber sashes with modern double glazing is expensive, often needs consent, and loses historic fabric. Secondary glazing, a discreet second pane fitted inside the existing window, is usually the better answer for a 100 year old house. Historic England notes that secondary glazing can reduce heat loss through windows substantially while keeping the original window intact, and it also cuts outside noise. It is removable, which matters for consent, and it can be fitted room by room as budget allows.
Only after the roof, draughts and windows should you tackle the walls, and only after floors are considered alongside them. Ground floors in old houses are often suspended timber, which can be insulated from below with breathable material between the joists, provided ventilation below the floor is kept clear. Solid ground floors are harder and more disruptive, and many owners sensibly leave them until a larger renovation. The Energy Saving Trust puts floor insulation savings at around £40 a year for a suspended timber floor, so it is a worthwhile job, but it is not the first job.
Insulating solid walls: internal or external?
Solid walls lose far more heat than insulated cavity walls. The Energy Saving Trust’s home insulation guidance reports that solid wall insulation saves around £210 a year for a typical home, with larger detached homes saving more and flats saving less. Other published estimates based on the same Trust figures range from around £315 a year for a mid terrace to around £540 for a semi detached house and around £930 for a detached house, all for gas heated homes. Treat every figure as an estimate for an average home, not a promise for yours: your saving depends on house size, heating fuel, how warm you keep the house, and how much of the wall area you actually insulate.
Internal wall insulation is the usual choice for a 100 year old house. It is fixed to the inside face of the external walls, it does not change the outside appearance, and it can be done room by room. The trade offs are real: you lose some room space, skirting boards, cornices and architraves need to be refitted or replaced, sockets and radiators move, and thermal bridging at floor, ceiling and party wall junctions needs careful detailing, or cold spots and condensation will appear at the edges. Historic England describes three main approaches: adhering insulation directly to the wall with a permeable lime based parge coat, fixing it within a timber or metal frame, or applying an insulating plaster in layers. Whichever approach is used, the guidance is consistent: strip impermeable finishes such as vinyl paint and wallpaper first, use permeable materials and finishes, and extend insulation returns onto adjoining walls, floors and ceilings to reduce thermal bridging.
External wall insulation wraps the outside of the house in insulation and render. It does not eat into room space, it keeps the masonry warm, which reduces condensation risk inside the wall, and it can transform the appearance of a tired exterior. But it changes how the building looks, it is usually unsuitable for listed buildings and many conservation areas, it needs scaffolding and good detailing around windows, eaves and plinths, and it costs more. Published estimates put internal wall insulation at around £7,400 for a typical home and external wall insulation at around £13,000, with wide variation by house size and condition. For a large detached period house, both figures can be considerably higher. Get at least three quotes from installers experienced with traditional buildings, not just modern estates, and ask each one how they will handle moisture, ventilation and thermal bridging.
Which materials suit a 100 year old house?
The safe materials are the breathable ones: wood fibre boards, sheep wool, hemp lime, cork and lime based insulating plasters. They allow water vapour to pass through, so the wall can still dry. They also tend to be more forgiving of the uneven surfaces found in old houses. Rigid foam boards and plastic vapour barriers are the materials to be cautious about in a traditional solid wall, because they can trap moisture against masonry that was never designed to be sealed. That does not mean foam is never used in old houses, but it means it should only be specified by someone who has assessed the moisture risk for your particular wall, not as a default choice.
Thickness matters too. Thicker is not automatically better in a traditional building. Research cited by Historic England from the Department for Energy Security and Net Zero found that increasing the thickness of internal wall insulation increases the risk of interstitial condensation, moisture forming inside the wall build up. A thinner, breathable build up that is well detailed at junctions will outperform a thick, sealed build up that traps moisture. Ask your installer what moisture risk assessment they have done. If the answer is a blank look, find another installer.
Ventilation: the part people forget
Insulation and draught proofing make a house more airtight. That is the point, but it has a consequence: moisture from cooking, bathing, laundry and breathing has fewer escape routes. Without ventilation, that moisture condenses on the coldest surfaces, which after insulation are often the remaining uninsulated spots, such as window reveals, or the inside of a poorly detailed wall junction. The result is mould in exactly the house you were trying to improve.
The fix is planned ventilation, not random draughts. Use extractor fans in kitchens and bathrooms that run on after use, keep trickle vents open, and do not dry laundry indoors without ventilation if you can avoid it. In a house that has had extensive insulation and draught proofing, a ventilation strategy should be part of the design, not an afterthought. If an installer proposes sealing every gap and says nothing about ventilation, that is a warning sign.
Grants and help with the cost
Solid wall insulation is expensive, and grants can change the maths completely, but schemes open, close and change criteria regularly, so check the current position rather than relying on an old article. The main routes for insulation help in recent years have been the Energy Company Obligation (ECO4), the Great British Insulation Scheme, and the Warm Homes: Local Grant for lower income households in lower rated homes. Eligibility usually turns on your home’s energy rating, your income or benefits, and sometimes your council area. Start with the Energy Saving Trust’s grant search and your local council’s energy pages, and be wary of cold callers offering free insulation: a genuine scheme will never pressure you to sign on the doorstep, and your council or energy supplier can confirm whether an offer is real.
If you are planning a heat pump later, insulate first. A well insulated old house needs a smaller, cheaper heat pump and lower running costs, and the Boiler Upgrade Scheme grant for heat pumps is a separate pot with its own rules. Doing the fabric first is the single most cost effective sequence in a retrofit, and it is why this guide puts walls and roofs before heating systems.
A practical plan for the next 12 months
Month one: confirm wall type, listed status and conservation area status, and fix gutters, leaks and obvious damp. Get a damp and timber survey if you have any doubts. Month two: loft insulation and draught proofing throughout, plus secondary glazing for the coldest rooms. Months three to six: get three quotes for internal wall insulation from traditional building specialists, ask each for a moisture risk assessment and a ventilation plan, and apply for any grants you qualify for. Months six to twelve: insulate walls room by room, starting with the coldest and most used rooms, refitting joinery carefully and keeping records and photographs of each build up for future buyers and surveyors. Keep the house ventilated throughout, and check for condensation at junctions in the first winter after wall insulation: early detection turns a small detailing fix into a non event, while a missed cold bridge can become a mould problem.
A 100 year old house has already survived a century of winters. The goal of insulation is not to turn it into a new build, it is to make it comfortable for the next century without damaging the fabric that got it this far. Work in the right order, choose breathable materials, respect consent rules, ventilate properly, and the old house will repay the investment in warmth, lower bills and a building that stays dry and sound.
Frequently asked questions
How to insulate a 100 year old house without causing damp?
Fix existing damp first, insulate in the order loft, draughts, windows, then walls, and use breathable materials such as wood fibre, sheep wool or lime based insulating plaster. Keep ventilation routes open, especially in kitchens and bathrooms, and have a moisture risk assessment done before internal wall insulation.
Can you insulate solid walls in an old house?
Yes. Solid walls are insulated from the inside or the outside. Internal insulation is more common for period homes because it does not change the exterior, but it reduces room space and needs careful detailing at junctions. External insulation keeps the masonry warm and avoids losing space, but it changes the appearance and is often unsuitable for listed buildings.
Do you need permission to insulate a 100 year old house?
Listed building consent is almost always needed for internal wall insulation in a listed building, and external wall insulation usually needs planning consideration in conservation areas and for listed buildings. Loft insulation, draught proofing and secondary glazing are generally lower risk, but check with your local planning authority if your home is listed or in a conservation area.
How much does it cost to insulate a 100 year old house?
Published estimates put internal solid wall insulation at around £7,400 and external solid wall insulation at around £13,000 for a typical home, with wide variation by size and condition. Loft insulation is far cheaper, draught proofing cheaper still, and grants through schemes such as ECO4, the Great British Insulation Scheme or the Warm Homes: Local Grant may cover part or all of the cost if you qualify.
What is the best insulation for old stone walls?
Breathable materials suit old stone walls best: wood fibre boards, sheep wool, hemp lime and lime based insulating plasters. They let water vapour pass through so the wall can dry. Rigid foam boards and plastic vapour barriers can trap moisture in traditional masonry and should only be used after a proper moisture risk assessment.
Should you insulate the floor of an old house?
Suspended timber ground floors can be insulated from below with breathable material between the joists, provided underfloor ventilation is kept clear, with savings of around £40 a year according to the Energy Saving Trust. Solid ground floors are more disruptive to insulate and are often left until a larger renovation.
Related reading
- How Thick Should Loft Insulation Be?
- What Temperature Should You Set Your Thermostat in Winter?
- Should You Bleed Your Radiators Before Winter?
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