Insulating a Period Home Without Ruining It: Breathable Insulation, Moisture and PAS 2035

The idea in one minute

Insulating a period home is worthwhile, but the order and the materials matter more than the thickness. Historic England warns that retrofit can change how moisture, heat and air move through old walls, which can trap water in the fabric: surface condensation can lead to mould, and interstitial condensation can corrode metal and rot timber out of sight.^[1] The safe route is to repair first, treat the building as a whole, use breathable materials and keep ventilation.

This guide is about insulating a period home without damaging it: the order of work, which materials suit old buildings, what the moisture risks are, where standards such as PAS 2035 fit, and when you need consent. England only, and general information rather than a design.

If you are still planning repairs, read our guides to lime and gypsum plaster and sash windows first.

Why old houses are different

Most traditional houses have solid walls without a cavity. BSI’s PAS 2035 brochure describes traditional buildings as generally solid-wall buildings built before 1919.^[2] The SPAB says standard theoretical U-value calculations appear to underestimate the thermal performance of traditional walls, in some cases by up to a factor of three, and that 77% of traditionally built walls in its sample performed better than predicted. It warns that overstated heat loss could lead owners to adopt unnecessary interventions that are invasive and harmful to historic fabric.^[3]

That does not mean doing nothing. It means measuring before spending. The SPAB advises understanding the building first, including how it performs, how it is used and how occupants live in it, and basing decisions on accurate figures.^[3]

Step 1: repair before you retrofit

Historic England says well-considered maintenance and repair should always be the first step, starting with rainwater goods, because water penetration can cause deterioration.^[4] Its risks page lists ‘repair the building before retrofitting’ as the first mitigation step and says wet materials conduct heat better, so heat loss increases.^[1] For walls it is blunt: the building must be free from excess moisture before wall insulation is installed.^[7]

Diagnosing damp comes first. Historic England warns that time and money can be wasted if the cause of damp is not properly diagnosed.^[8] See our guide to lime plaster vs gypsum plaster in period homes for what the conservation bodies say about damp and plaster.

Step 2: understand the whole building

PAS 2035:2023, published by BSI and sponsored by the Department for Energy Security and Net Zero, treats a home as a system of building fabric, services, renewable systems and occupants, and plans improvements across the whole dwelling. Its sequence is ‘fabric first’: repair defects, address low-cost measures, improve insulation and airtightness, upgrade heating, then add low- or zero-carbon technologies.^[2] Every domestic retrofit project under it must have a Retrofit Coordinator, and BSI says traditional buildings and protected buildings (listed or in conservation areas) require an assessment of significance under BS 7913.^[2]

BSI says PAS 2035 ‘can be applied to all domestic retrofit activity’ and that its 2023 edition takes effect on 30 March 2025.^[2] Whether it applies to your project depends on how the work is funded and procured, which we cannot judge from here; ask your installer or retrofit coordinator. Historic England adds that retrofit should consider the external environment, services and occupants together, with an options appraisal that weighs operational and embodied carbon and durability.^[1] Its energy efficiency hub points to Advice Note 18 for consistent decision-making.^[4]

Step 3: draught-proof, but keep the air moving

Draught-proofing is the cheapest step. The Energy Saving Trust, in a page updated in May 2026, says draught proofing windows, floors and doors could save around £85 a year in Great Britain, and that sealing an unused open chimney could save around £60 a year, with a chimney pot cap from around £150 or a removable excluder.^[10] It also warns against blocking fixed ventilation, sealing rooms with open fires or flues, or sealing a home that has no trickle vents or mechanical ventilation.^[10] Historic England says a ventilation strategy (natural, mechanical or hybrid) is needed in every case.^[1] See our guides to sash windows and period fireplaces.

Step 4: loft and roof

Insulating at ceiling level is simpler than at rafter level. Historic England says if the top floor is open to the rafters and used as living space, rafter-level insulation is needed; otherwise insulating at ceiling level is simpler. It prefers vapour-permeable, hygroscopic materials such as wood-fibre board and sheep’s wool or hemp fibre between rafters, and describes vapour barriers, foil products, sprayed foam and loose-fill cellulose as generally unsuitable at rafter level.^[5] It notes that impervious roofing felts can trap moisture, that sarking adds roughly 75 to 100 mm to the roof depth, and that listed buildings will usually need listed building consent for changes to roof levels, materials or historic plaster ceilings.^[5]

The Energy Saving Trust says rafter-level insulation is fundamentally less efficient than insulating the loft floor, advises against spraying foam under a damaged roof without fixing the leak first, and says some mortgage lenders are reluctant to lend on properties with foam in the roof.^[12]

Step 5: floors

For suspended timber floors, Historic England recommends breathable, moisture-buffering natural fibres such as wood-fibre, sheep’s wool, hemp and cellulose, says closed-cell foamed plastics are generally unsuitable, and says higher-status floors should usually stay in place with insulation fitted from below. Floors with a U-value above 0.7 W/m²K are usually cost-effective to insulate, it says, and vapour barriers should be avoided unless their effects are properly analysed.^[6] The Energy Saving Trust gives a typical cost of roughly £1,400 to £2,500 and an average saving of about £55 a year, and says not to block air bricks.^[11] Our period floors guide covers the floor itself.

Step 6: walls, last and with care

Historic England says solid walls can be insulated externally, internally or both, that each approach is technically complex and affects appearance, character and hygrothermal performance, and that not every wall is suitable or will benefit.^[7] It warns that unsuitable insulation can trap moisture, hinder natural drying and push moisture inward, raising indoor humidity and mould risk, and lists salt crystallisation and condensation cycles at the junction between old walls and new insulation among the hidden risks.^[7] It also notes that internal insulation can cut off a wall’s thermal mass and buffering capacity.^[7]

The numbers are sobering. Citing the DEEP research, Historic England says installing internal wall insulation always led to increased moisture risk in the case studies, and suggests that targeting a U-value of about 0.8 W/m²K rather than 0.3 can reduce condensation risk while still delivering about two-thirds of the energy savings.^[1] The Energy Saving Trust says internal insulation makes the wall colder so vapour entering from inside may condense within the wall; solutions are breathable insulation or a continuous vapour barrier, designed by an experienced specialist with a moisture control strategy. It advises a 25-year guarantee through SWIGA or an independent insurance-backed scheme.^[9]

Our view: for most period houses, do every other step in this guide before walls. If you plaster, lime plaster is part of the answer: Historic England notes traditional lime and earth plasters can achieve significant airtightness.^[7]

Windows

Windows are covered in detail in our sash windows guide. In brief, Historic England describes draught-proofing as the best-value measure and secondary glazing as an efficient, low-risk option, and notes that in many older houses windows account for a relatively small share of heat loss.^[13]

Consent and who to ask

Listed buildings usually need listed building consent for changes that affect their character. Historic England says to check with the conservation officer first, that applying costs nothing and that unauthorised work is a criminal offence.^[14] Changes in conservation areas may need planning consent, says its roofs guidance.^[5] BSI’s PAS 2035 asks for a Retrofit Coordinator and, for protected buildings, outside specialists where needed.^[2] Not sure whether your house is listed? See how to find out if your house is listed.

Common mistakes

• Insulating before repairing. Repair first.^[4]
• Using impermeable materials in old roofs and floors. Historic England calls them generally unsuitable.^[5][6]
• Sealing the house without ventilation. Every case needs a ventilation strategy.^[1]
• Starting with walls. Highest moisture risk.^[7]
• Trusting a generic U-value. Traditional walls may perform better.^[3]
• Using spray foam to hide a leak. Fix the roof first.^[12]

Quick-reference table

Glossary

What to do next

Start with the cheap, reversible steps: repair, draught-proof and keep ventilation. Then get a specialist survey before any wall work. For the decorative side of a warm period room, our curtains guide shows how lined curtains help, and our hand-cast plaster ceiling roses and wall lights are decorative additions that do not affect the thermal fabric.

Keep reading

• Lime plaster vs gypsum plaster in period homes
• Sash windows: repair, draught-proof or replace
• Period fireplaces and reopening a chimney breast
• Period floors: parquet, encaustic tiles and floorboards
• How to find out if your house is listed
• How to hang curtains

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Frequently asked questions

Can I insulate an old house with solid walls?

Yes, but with care. Historic England says not every wall is suitable, the wall must be free from excess moisture first, and unsuitable insulation can trap moisture. Treat walls last and get specialist advice.

What insulation is best for a period home?

Historic England prefers vapour-permeable, hygroscopic materials such as wood-fibre, sheep's wool and hemp for roofs and floors, and calls closed-cell plastics, foil products and sprayed foam generally unsuitable there.

What should I insulate first?

Repair first, then draught-proof with ventilation kept, then loft and floors, with walls last. This follows the fabric-first sequence in PAS 2035 and Historic England's advice to repair before retrofitting.

What is PAS 2035?

A BSI specification for retrofitting dwellings. It takes a whole-dwelling, fabric-first approach, requires a Retrofit Coordinator, and asks for a heritage assessment under BS 7913 for traditional and protected buildings.

Is internal wall insulation risky?

Historic England, citing the DEEP research, says it always increased moisture risk in the case studies, and suggests a U-value of about 0.8 rather than 0.3 can reduce condensation risk while delivering about two-thirds of the savings.

How much does draught-proofing save?

The Energy Saving Trust says around £85 a year in Great Britain for windows, floors and doors, and around £60 for sealing an unused open chimney. Averages vary by home.

Do I need consent to insulate a listed building?

Usually yes for changes that affect character. Historic England says listed building consent is free, unauthorised work is a criminal offence, and the conservation officer should be asked first.

Do old walls really lose that much heat?

Less than calculations suggest, according to the SPAB: standard U-value calculations can underestimate performance, in some cases by up to a factor of three, and 77% of its sample performed better than predicted.