Insulation types explained

Where insulation fits into an EPC

The EPC efficiency rating is cost-based: it reflects modelled running costs under standard assumptions rather than the energy a particular household happens to use. Because the rating is built on cost, carbon and cost outcomes can move differently, and a measure that lowers emissions does not automatically move the letter. Insulation works on the demand side by reducing the heat that escapes through the roof, walls and floor, so it generally pushes the rating in the right direction and keeps doing so whatever heating system is fitted later.

That is the reasoning behind the “fabric first” habit of dealing with the building envelope before changing the boiler or adding technology. If you are weighing up which measures to tackle in what order, our guide to improving your EPC rating sets out the wider picture, and the recommendations section of a certificate shows how the software presents these measures for a specific home.

Loft and roof insulation

In a conventional cold loft, mineral wool or a similar quilt is laid between the ceiling joists and then across them in a second layer to break the line of the timber. Guidance for lofts is commonly expressed as a depth in mineral wool, and the equivalent depth differs from one material to another, so the current figure is worth checking rather than assumed. Two practical points matter more than the exact number: the insulation should not be squashed, because compressing it removes the trapped air that does the work, and it should not be stuffed tight into the eaves, because the loft still needs to ventilate.

A room in the roof is a different problem. Loft conversions, dormer bungalows and chalet-style houses have sloping ceilings, low dwarf walls and small void spaces behind them, and the insulation has to be worked into the slope rather than laid flat. It is more disruptive, more likely to be done from inside the room, and more exposed to condensation risk if the build-up is wrong. Assessment methods treat rooms in the roof as their own element, so an unimproved one can weigh noticeably on a certificate.

Cavity wall insulation

Cavity walls have two leaves of masonry with a gap between them, a form of construction that became common through the twentieth century. Where the cavity is empty, it can be filled by injecting blown mineral wool, polystyrene beads or foam through small holes in the mortar joints. It is one of the least disruptive measures available, because nothing changes inside the rooms and the work is often completed quickly.

It is not universal, though. A proper suitability survey should check the width and cleanliness of the cavity, the condition of the wall ties, the state of the pointing, and how exposed the wall is to driving rain. Filling a cavity in a severely exposed or defective wall can carry moisture across to the inner leaf. More recently built homes were generally constructed with insulation already in the cavity, so the question is often whether it has been recorded rather than whether to add it.

Solid walls: internal or external

Older homes commonly have solid brick or stone walls with no cavity to fill, so insulation has to go on one face or the other. External wall insulation wraps the outside of the building in insulation and a render or cladding finish. It keeps the masonry on the warm side, deals with junctions in a continuous run and costs no internal floor area, but it changes the appearance of the house, needs the roof overhang and window reveals reworked, and may raise planning or conservation questions.

Those questions matter most where a building is listed or sits in a conservation area. It is a common assumption that listed status automatically rules out insulation work, or automatically exempts a property from efficiency rules; neither is safe to rely on. The position depends on the individual building, the consents required and the measure proposed, so check the current guidance on GOV.UK and speak to your local authority conservation officer before planning work. Our guide to EPC exemptions explains why these cases need to be looked at individually.

Internal wall insulation can be done room by room and leaves the outside untouched, which suits terraces, flats and properties where the frontage must not change. The trade-offs are real: you lose some floor area, skirtings, sockets and radiators have to be moved, and the original wall now sits colder and wetter behind the new lining. Older masonry in particular usually needs a vapour-open, breathable approach rather than a sealed one. This is the measure that most deserves proper design and specialist advice, and our guide to EPCs for older homes covers the traditional-construction side in more depth.

Floors and party walls

A suspended timber floor can be insulated between the joists, either from below where there is a cellar or crawl space, or by lifting the boards from above. The airbricks that ventilate the void underneath must stay clear, because blocking them to stop draughts causes rot in the timbers. Solid concrete floors are harder: insulation goes above or below the slab, which raises floor levels and means adjusting doors, thresholds and skirtings, so it is normally done as part of a wider renovation rather than on its own.

Party walls are easy to overlook. Where a cavity party wall is open at the top, air can move up through it and draw heat out of the house, an effect known as party wall bypass, which can be addressed by sealing and filling the cavity. On a mid-terrace or semi, the junction where internal wall insulation meets the neighbouring wall is also a classic cold bridge, and a short return of insulation along the party wall is often used to soften it.

How insulation is recorded, and why “assumed” matters

Existing homes are assessed using RdSAP by an accredited domestic energy assessor, a standardised method based largely on what can be seen and measured during the visit, and the finished certificate is lodged on the national register. New builds are assessed using full SAP instead. Where insulation is hidden inside a wall or under a floor and no evidence is available, the software falls back on a default for the age and construction of the property. On the certificate this appears as an assumed entry, and it can understate a home that has quietly been improved over the years.

The remedy is paperwork. Keep installer invoices and guarantees, building control paperwork, product specifications and photographs taken while the work was open, and have them ready for the assessor rather than mentioning them afterwards. Our guide to understanding your EPC explains how the fabric entries are set out on the certificate itself.

Landlords, and where you are in the UK

Insulation is often the practical route to meeting minimum standards in rented housing. In England and Wales, since 1 April 2020 a property may not be let below band E unless a valid exemption has been registered. A minimum of band C has been proposed and consulted on rather than settled in law, so it should be treated as a proposal without a confirmed date, and landlords should check the current position on GOV.UK before committing to a programme of work. Our guide to EPC rules for landlords covers the detail.

Energy performance is a devolved matter, so Scotland and Northern Ireland run their own arrangements and the rules, forms and registers differ from those in England and Wales. If your property is outside England and Wales, read our guide to EPCs across the UK and follow the guidance for that nation. EPC Checker is a research aid built on public EPC and property data – it is not an assessment, a survey, or professional, legal or financial advice, so for anything involving solid walls, damp or breathability, take advice from a qualified professional before committing.