Energy Efficiency and Heating in Historic Buildings: Insulation, Glazing and Heat Pumps

A conservation guide to making historic buildings warmer, cheaper to run and lower in carbon without damaging them: how traditional construction handles heat and moisture, which measures work, which carry risk, and how to plan heating, radiators and heat pumps for an old building.

Building Restoration Index — Technical Library. By Paul Stenning and Mike Walters, Historic Buildings Advisors, Tuscan Foundry Products.

How traditional buildings handle heat and moisture

Most buildings put up before about 1919 are of traditional construction: solid walls of brick, stone, cob or timber frame, bedded and finished in lime, without cavities or damp-proof membranes. They are often described as “breathable”, which really means that they are vapour-open and capillary-active. Moisture gets into the walls from rain, the ground and the air inside, and it leaves again by evaporation from both faces.

This balance is the key to every energy decision in an old building. While the walls can dry out, they stay healthy. If impermeable materials such as cement render, gypsum plaster, impermeable modern paints or the wrong insulation are added, moisture builds up, timbers decay, plaster fails and walls feel colder.

• Thermal mass. Thick masonry stores heat and releases it slowly. Rooms take longer to warm up but stay warm for longer, which suits steady, low-temperature heating better than short bursts.
• Ventilation. Old buildings were designed to be ventilated by open flues, loose-fitting windows and suspended floors. Moisture from occupants, cooking and bathing has to be removed. If air leakage is cut without providing ventilation, condensation and mould follow.
• Real performance. Research for Historic England and the Society for the Protection of Ancient Buildings has found that the measured heat loss of many solid walls is lower than standard calculation methods assume. Energy assessments can therefore overstate how poorly an old wall performs and push owners towards unnecessary or harmful wall insulation.

Start with a whole-building approach

Historic England recommends a whole-building approach: understanding the building, its significance, its condition and how it is used before choosing any measure. A measure that makes sense in one building can be harmful in another, and measures interact. Draughtproofing, for example, reduces ventilation, so it may need to be paired with extract fans or trickle ventilation.

A sensible order of work is:

1. Understand the building: its construction, significance, condition and any damp problems.
2. Understand how it is used: occupancy, heating patterns and which rooms really need to be warm.
3. Repair and maintain, so the fabric is dry and sound.
4. Reduce heat loss with low-risk measures: draughtproofing, curtains, shutters, secondary glazing, loft and floor insulation.
5. Improve the heating system, controls and hot water.
6. Consider higher-risk measures, such as wall insulation, only with specialist assessment.
7. Add renewable energy where it can be fitted without harm.

Historic England’s guidance Adapting Historic Buildings for Energy and Carbon Efficiency sets out this approach in detail. In the retrofit industry, PAS 2035 requires an assessment and coordination process for funded retrofit work, and its assessors should take account of traditional construction. Make sure whoever assesses the building has experience of old buildings.

Repairs and maintenance first

A wet wall loses heat much faster than a dry one, so keeping water out of the fabric is one of the most effective energy measures in a historic building, as well as the most important conservation measure.

Leaking gutters, blocked hoppers and cracked downpipes soak the walls below them, sometimes for years before damp shows inside. Defective pointing, cement renders that trap water, raised ground levels against walls and blocked drains all do the same. Damp walls feel cold, encourage condensation and mould, and make rooms harder to heat.

• Clear and repair rainwater goods before spending money on insulation.
• Lower external ground levels and clear soil and vegetation from the base of walls.
• Replace cement pointing and render with lime where it is trapping moisture.
• Repair roofs, flashings and leadwork.
• Find and fix the cause of any damp before adding anything to the walls. Our guide to damp in historic buildings explains how to diagnose it.

Draughtproofing and ventilation

Draughtproofing is usually the most cost-effective and least harmful way to reduce heat loss in an old building, provided enough ventilation remains.

• Windows and doors. Sash and casement windows can be draughtproofed with discreet brush or compression seals routed into the frames, and sashes can be eased and repaired at the same time. External doors can have seals and threshold strips. See our guide to historic windows and doors.
• Floors. Gaps between floorboards and at skirtings can be filled with timber slips, flexible fillers or a breathable underlay.
• Chimneys. An open, unused flue can lose a great deal of heat. A removable chimney balloon or a register plate with a small opening reduces the loss while keeping some ventilation. Never fully seal a flue that is in use, and keep some ventilation to a disused one.
• Loft hatches, service holes and pipe penetrations are easy to seal.

Keep ventilation where moisture is produced. Kitchens and bathrooms need extract fans or other controlled ventilation, and rooms with open-flued appliances must keep the air supply the appliance needs.

Windows, shutters and secondary glazing

Historic windows are often blamed for heat loss, but repaired and draughtproofed, with shutters, heavy curtains or blinds in use at night, they can perform far better than their reputation suggests.

• Shutters. Many Georgian and Victorian houses have working internal shutters that have been painted shut. Closed at night, they significantly reduce heat loss through the window.
• Curtains and blinds. Lined curtains and blinds that fit the reveal are cheap, reversible and effective.
• Secondary glazing. A discreet internal secondary window, ideally with slim frames lining up with the meeting rails and glazing bars, keeps the historic window intact and reduces heat loss and noise. It needs careful fitting so it can be opened for cleaning and ventilation and doesn’t damage shutters, panelling or plaster.
• Slim double glazing in existing sashes is sometimes accepted where the historic glass has already been lost, but it means heavier sashes and new glazing bars, and it isn’t normally acceptable where historic glass survives.

Replacing historic windows is rarely justified on energy grounds and is unlikely to be acceptable on a listed building. Leaded lights and stained glass need their own approach; see our guide to stained glass and leaded lights.

Roofs and lofts

Insulating a loft at ceiling level is one of the most effective measures in any building, and in a historic one it is usually low risk if ventilation is kept and the right materials are used.

• Use vapour-open, natural insulation such as sheep’s wool, wood fibre or hemp, which can absorb and release moisture.
• Keep eaves and ridge ventilation clear so the roof space stays dry.
• Insulate and draughtproof the loft hatch, and lag pipes and tanks in the roof space.
• Don’t block access to historic roof timbers, which need to be inspected.

Where the roof space is lived in, insulation has to go at rafter level, which is more complex. Avoid spray foam applied to the underside of roof coverings or timbers. It hides the timbers, can trap moisture and cause decay, makes slates and tiles hard to reuse, and is a known problem for surveyors and mortgage lenders. Insulation added during reroofing should be designed to suit the roof covering and keep the roof ventilated. See our guide to historic roofs, coverings and leadwork.

Walls: the highest-risk measure

Insulating solid walls can reduce heat loss, but it carries the greatest risk of trapping moisture and of harming the character of a historic building, so it should only be done after specialist assessment.

• Internal wall insulation changes the temperature of the wall behind it. The masonry becomes colder and wetter, and moisture can condense between the insulation and the wall. It also covers historic plaster, cornices, panelling and skirtings. If it is used, it should be vapour-open and capillary-active, such as wood fibre boards with lime plaster, or insulating lime plasters, and the wall should be dry and protected from driving rain first.
• External wall insulation changes the appearance of the building completely and is rarely acceptable on listed buildings or in conservation areas.
• Hygrothermal assessment. For significant schemes, a moisture risk assessment based on the actual wall build-up and exposure, rather than a simple condensation calculation, gives a far better picture of risk.
• Thin, reversible measures, such as replacing failed gypsum plaster with lime or insulating lime plaster, or fitting insulation behind window reveals and in shutter boxes, can be worthwhile where full insulation is not.

Our guide to lime plaster and decorative plasterwork covers the plaster finishes involved.

Floors

Suspended timber floors can usually be insulated between the joists from below or when boards are lifted; solid floors need much more caution.

• Suspended timber floors. Insulation such as sheep’s wool or wood fibre held between the joists, with a breathable membrane, reduces draughts and heat loss. The void below must stay ventilated: airbricks must be kept clear, and in some cases more are needed.
• Solid floors. Historic floors of stone flags, brick, clay tiles or lime are often laid directly on the ground. Replacing them with concrete and a damp-proof membrane drives ground moisture into the base of the walls and destroys historic fabric. Where a solid floor has to be relaid, a breathable insulated limecrete or similar floor is a better choice, but the original floor is often significant in its own right.

Heating a historic building

Thick-walled historic buildings respond best to steady, low-temperature heating that keeps the fabric gently warm, rather than short periods of high heat.

• Heat loss calculation. Any new system should start with a room-by-room heat loss calculation using realistic values for solid walls and the measures already taken.
• Controls and zoning. Thermostatic radiator valves, room thermostats, weather compensation and separate zones for rooms used at different times save energy without any change to the fabric.
• Pipe runs. New pipework should follow existing routes where possible. Avoid notching historic joists and chasing into panelling, plaster or stone.
• Rapid heating. Heating that raises temperatures quickly can dry and crack timber, panelling, furniture and organs, and increases condensation on cold surfaces such as glass and stone.
• Hot water. An insulated cylinder, insulated pipes and sensible controls reduce losses.

Churches and other large spaces have particular needs; our guide to caring for church buildings covers heating them.

Cast iron radiators and low-temperature heating

Cast iron column radiators suit historic interiors and low-temperature heating well, but at lower flow temperatures any radiator gives out less heat, so it must be sized for the system it will run on.

• Output and temperature. Radiator outputs are usually quoted at a temperature difference (ΔT) of 50°C between the water and the room, which matches a traditional boiler system. A heat pump typically runs at much lower flow temperatures, so the same radiator may produce around half its rated output or less. Ask for outputs at the system’s design ΔT, often ΔT30 or lower for a heat pump.
• Size and number. Low-temperature systems usually need larger radiators, more columns or sections, or additional radiators. Cast iron radiators can be built to length and specified with more columns to suit.
• Mass and water content. Cast iron holds heat and releases it steadily, which suits the slow, continuous operation of a heat pump and the thermal mass of solid walls.
• Existing radiators. Historic cast iron radiators can often be refurbished, pressure-tested and reused. They may be significant fittings in their own right and could need consent to remove.
• Valves and pipework. Traditional-style valves, including thermostatic types, are available, and pipe sizes may need to be checked for the higher flow rates of a heat pump system.

Underfloor heating is sometimes proposed with heat pumps, but in historic buildings it usually means lifting and disturbing historic floors, so radiators are often the less harmful choice.

Heat pumps in historic buildings

Heat pumps can work well in historic buildings, but only when the building’s heat loss has been reduced where possible, the system has been designed for its actual heat loss, and the external unit or ground works can be accommodated without harm.

• Air source heat pumps need an external unit with free airflow. Siting is the main issue: a unit on a principal elevation, on a roof or against a historic wall can harm the building’s appearance. Rear yards, outbuildings, garden locations screened by walls or planting, and enclosures are usually better. Noise for neighbours and vibration through historic walls should also be considered.
• Ground source heat pumps avoid a visible outdoor unit but need trenches or boreholes, which can disturb archaeology, gardens and tree roots. Specialist advice and sometimes archaeological input are needed.
• Design. Use an installer certified under the Microcertification Scheme (MCS), who should carry out a full heat loss calculation and size the heat pump, radiators and pipework accordingly. Ask whether they have worked on solid-walled buildings.
• Space. A hot water cylinder and possibly a buffer tank need room inside the building.

Permitted development rights for heat pumps generally don’t apply to listed buildings, and installing one on a listed building normally needs Listed Building Consent. Historic England has published guidance supporting heat pumps and solar panels on listed buildings where they don’t cause unacceptable harm. Talk to the conservation officer early.

Solar panels and renewables

Solar panels are increasingly accepted on historic buildings where they can be sited discreetly, but they need careful placement and fixing.

• Rear or hidden roof slopes, valley roofs, outbuildings, garages and ground-mounted arrays usually cause the least harm.
• Fixings must not damage historic slates, tiles or roof timbers, and the roof structure should be checked for the extra load.
• Integrated solar slates or tiles can be less visible than panels on some roofs.
• Cabling routes, inverters and batteries need space and should avoid historic interiors.

Solar thermal panels, biomass boilers and small-scale hydro are other options in the right setting. Biomass needs fuel storage and a suitable flue; see our guide to chimneys, fireplaces and flues.

Consent and regulations

On a listed building, any work that affects its character as a building of special interest needs Listed Building Consent, and that can include insulation, secondary glazing, new radiators and pipe runs, heat pumps and solar panels.

• Listed buildings. Consent depends on the building and the work. Draughtproofing and like-for-like repairs often don’t need it, while insulation that covers historic plaster, new windows, external units and panels usually do. See our guide to Listed Building Consent.
• Conservation areas. Planning permission may be needed for external units, solar panels on principal elevations and changes to windows, and permitted development rights may have been removed by an Article 4 direction.
• Building regulations. In England, Approved Document L allows special consideration for historic and traditional buildings, so that energy improvements are made as far as is reasonably practicable without harming their character or causing moisture problems. Rules differ in Wales, Scotland and Northern Ireland.
• Places of worship may fall under a denominational system rather than Listed Building Consent.

Choosing advisers and installers

The most important choice is an adviser who understands traditional buildings; many energy assessors, insulation installers and heating engineers work mainly on modern houses.

• A conservation-accredited architect or surveyor can assess the building as a whole and prepare consent applications.
• A retrofit coordinator or assessor with traditional building experience can produce a plan that takes moisture risk into account.
• Heating engineers and heat pump installers should provide a heat loss calculation, radiator schedule and explanation of running costs.
• Insulation installers should use and understand vapour-open materials.

Ask for examples of similar work on solid-walled or listed buildings, and check references.

Common mistakes

Mistake	Consequence
Insulating walls before fixing damp or leaking rainwater goods	Moisture is trapped behind the insulation, causing decay, mould and cold walls
Using impermeable insulation, membranes or cement and gypsum plasters on solid walls	The wall can no longer dry out, leading to damp and damage to timber and plaster
Draughtproofing without providing ventilation	Condensation and mould, and risk to open-flued appliances
Spray foam under roof coverings	Hidden timber decay, slates and tiles that cannot be reused, and problems with surveys and mortgages
Replacing historic windows to save energy	Loss of historic fabric and character, often for less benefit than repair, draughtproofing and secondary glazing
Keeping existing radiators when switching to a heat pump without checking outputs	Rooms that never get warm, and higher running costs
Sealing up chimneys completely	Damp in the flue and chimney breast from lack of ventilation
Siting a heat pump on a principal elevation or against historic fabric	Visual harm, noise and vibration, and refusal of consent
Replacing a historic solid floor with concrete and a membrane	Ground moisture driven into the walls, and loss of historic flooring
Relying on a standard energy assessment for a solid-walled building	Overstated heat loss and recommendations that may be unnecessary or harmful

Find a specialist

Specialists in the Building Restoration Index who work on energy efficiency and heating in historic buildings, grouped by the part they play in a project:

Contractors and craftspeople

• Heating and Plumbing — heating engineers and plumbers for historic buildings
• Windows and Doors — repair and draughtproofing of historic windows and doors
• Builders — general builders experienced in traditional construction
• Plasterwork — lime plastering and insulating lime plasters
• Roofing Contractors — insulation and ventilation during reroofing

Architects, surveyors and engineers

• Architects — whole-building assessment and consent applications
• Surveyors — condition surveys and energy-related advice

Consultants and designers

• Heritage Consultants — significance and heritage statements for retrofit schemes
• Consultants — energy, environmental and retrofit consultants

Investigation and recording

• Non-Destructive Investigations — thermography, moisture monitoring and other surveys
• Materials Analysts — analysis of mortars, plasters and moisture in walls

Material suppliers

• Insulation — natural and vapour-open insulation materials
• Secondary Glazing — discreet internal secondary glazing
• Lime Mortars & Plasters — lime mortars, plasters and insulating plasters
• Solar — solar panels and integrated solar roofing
• Paints & Finishes — breathable paints and finishes

Advice and training

• Organisations — national amenity societies and trade bodies, including those that publish technical advice
• Courses, Training & Events — courses and events for owners, contractors and professionals

Further reading from Tuscan Foundry

• Cast iron radiators and heat pumps in listed and period buildings
• How to size and choose cast iron radiators for a period room
• Cast iron radiators for listed buildings
• Breathing walls: cast iron airbricks and free airflow

Related guides

• Damp in historic buildings
• Historic windows and doors
• Listed Building Consent: what needs it
• All guides in the Technical Library

The Index lists specialists without endorsement; check experience, references and insurance before appointing anyone. Specialists can add a free listing.

Frequently asked questions

Can you insulate a listed building?

Yes, but carefully. Loft insulation, suspended floor insulation, draughtproofing and secondary glazing are usually low risk. Wall insulation carries the most risk and often needs Listed Building Consent. Use vapour-open materials and get advice from someone experienced with traditional buildings.

What does “breathable” mean for an old building?

It means the walls and finishes are vapour-open and allow moisture to move through and evaporate. Solid walls in lime rely on this to stay dry. Impermeable materials such as cement render, gypsum plaster and some modern paints and insulation stop it and cause damp.

What is the most cost-effective energy measure for a historic house?

Usually repairing the building so walls stay dry, then draughtproofing windows, doors, floors and unused flues, insulating the loft and using shutters or lined curtains. These are cheap, reversible and low risk.

Do I need to replace my sash windows to save energy?

No. Repaired and draughtproofed sashes, with shutters, curtains or secondary glazing, perform well and keep the historic windows. Replacement is rarely acceptable on listed buildings.

Is secondary glazing allowed on a listed building?

It is often acceptable because it is reversible and keeps the original window, but it usually needs Listed Building Consent. Slim frames that line up with the glazing bars and careful fixing make it more likely to be approved.

Is internal wall insulation safe on solid walls?

It can be if the wall is dry, protected from driving rain and insulated with vapour-open, capillary-active materials such as wood fibre and lime. Impermeable insulation and vapour barriers on solid walls carry a high risk of trapped moisture and decay. A moisture risk assessment is advisable.

Why is spray foam a problem in old roofs?

Spray foam bonded to the underside of slates, tiles or timbers hides the timbers from inspection, can trap moisture and cause decay, and makes the roof covering hard to repair or reuse. Many surveyors and lenders treat it as a defect.

Can a heat pump heat an old stone or brick house?

Yes, if the heat loss is reduced where possible and the system is designed for the building. That means a proper heat loss calculation, radiators sized for low flow temperatures and an installer experienced with solid-walled buildings.

Will my cast iron radiators work with a heat pump?

They can, but at the lower flow temperatures of a heat pump each radiator gives out much less heat than its rated output at ΔT50. Check outputs at the system’s design temperature. Larger radiators, more columns or extra radiators may be needed.

What size radiator do I need for a heat pump?

It depends on each room’s heat loss and the system’s flow temperature. Your installer should calculate the heat loss room by room and choose radiators using outputs at the design ΔT, often ΔT30 or lower, rather than the usual ΔT50 figure.

Do I need consent for a heat pump on a listed building?

Normally yes. Permitted development rights generally don’t apply to listed buildings, and an external unit or ground works can affect the building’s character and setting. Discuss siting with the conservation officer before applying.

Can I put solar panels on a listed building?

Sometimes. Rear or hidden roof slopes, outbuildings and ground-mounted arrays are more likely to be accepted. Listed Building Consent is usually needed, and fixings must not damage the roof covering or timbers.

Should I block up unused chimneys?

Don’t seal them completely. An unused flue needs some ventilation to stay dry, so cap it with a ventilated cap and keep a small opening or vent at the fireplace. A removable chimney balloon reduces heat loss in winter.

Do the energy efficiency building regulations apply to historic buildings?

In England, Approved Document L allows special consideration for historic and traditional buildings, so improvements are made as far as reasonably practicable without harming character or causing moisture problems. Rules differ elsewhere in the UK.

Does keeping gutters clear really save energy?

Yes. Wet walls conduct heat much faster than dry ones. Leaking gutters, hoppers and downpipes keep walls damp, so repairing rainwater goods helps walls stay dry, warmer and cheaper to heat.