Loft, Cavity Wall and Floor Insulation Costs Compared

If your home is losing heat through the roof, walls and floor, the honest starting point is: loft insulation pays back fastest, cavity wall insulation is usually next, and solid wall or floor insulation take longest but can still be worth doing — the right order depends on what your house already has and how it's built.

This guide compares typical UK costs, payback times and disruption for the main types of insulation, so you can prioritise sensibly rather than guessing. Costs below are broad, indicative ranges from typical UK installer pricing — always get 2-3 quotes for your actual property, since wall construction, loft access and property size all move the price.

Loft insulation

Most UK homes lose a significant amount of heat through an under-insulated roof, which is why loft insulation is usually the first job worth doing. Government guidance has long recommended a depth of around 270mm of mineral wool (or the equivalent in other materials) for a typical pitched roof — many older homes still have far less than this, sometimes only 50-100mm from decades-old original insulation.

• Typical cost: roughly £300-£600 to top up an average semi-detached loft with easy access; more for a larger loft, restricted access, or if old insulation needs removing first.
• Payback: often the fastest of any insulation measure, frequently within a few years, because materials are cheap and installation is quick.
• DIY or professional: topping up an easily accessible loft with roll insulation is a realistic DIY job for a competent homeowner. See our guide to topping up loft insulation for exactly when and how much to add.

Cavity wall insulation

If your home was built from roughly the 1920s/1930s onwards, there's a good chance it has cavity walls — two layers of brick or block with a gap between them. That gap can usually be filled with insulation, injected through small holes drilled in the outer wall and made good afterwards.

• Typical cost: roughly £500-£1,500 for an average semi-detached or terraced house, depending on wall area and material used.
• Payback: generally strong — often within 3-6 years — because the work is relatively quick and non-disruptive compared with wall insulation done from inside or outside.
• Suitability matters: not every cavity is suitable — exposure to driving rain, cavity width, and existing damp issues all affect whether it's a sensible job. See our guide on cavity wall insulation suitability before booking a survey.

Solid wall insulation (internal or external)

Homes built before around the 1920s typically have solid walls with no cavity to fill, so insulating them means adding a layer either to the inside face (internal wall insulation) or the outside face (external wall insulation, sometimes called EWI or render-and-insulate).

• Typical cost: internal wall insulation is often cheaper per square metre than external, but eats into room space and is disruptive room-by-room; external wall insulation commonly runs from roughly £8,000 upwards for a full house, sometimes considerably more depending on size and finish.
• Payback: the slowest of the measures here, often a decade or more, purely because the upfront cost is so much higher — but the comfort and heat-loss improvement can still be substantial, especially in a draughty older solid-wall property.
• See our full comparison in internal vs external solid wall insulation for the trade-offs in detail.

Floor insulation

Ground floor heat loss is easy to overlook. In a house with a suspended timber floor, insulation can often be fitted between the joists from below (if there's crawl space) or from above when floorboards are lifted for other work. A solid concrete ground floor is much harder to retrofit — it usually only makes sense to insulate it if you're already replacing the floor for another reason.

Costs vary enormously depending on access, so it's genuinely a case of getting a survey rather than relying on a generic figure — but as a rough steer, insulating a suspended timber floor with good crawl-space access is often a similar order of cost to loft insulation for an equivalent area, while lifting floorboards throughout a whole ground floor is a much bigger job.

What should you do first?

As a general order of priority for most UK homes: top up loft insulation first if it's below modern recommended depth, then investigate cavity wall insulation if your walls are cavity-built, then consider solid wall or floor insulation as longer-term projects once the cheaper, faster-payback measures are done. Draught-proofing doors and windows (see draught-proofing costs and methods) is also worth doing alongside any of the above — it's cheap and makes an immediate, noticeable difference to comfort.

Once your fabric insulation is in reasonable shape, it's also worth looking at how you heat and power the home — see our guides to air source heat pumps and solar panels. Your current EPC rating is a useful starting point for seeing where your home currently stands and which improvements would move the needle most.

Common mistakes

• Insulating without addressing ventilation. Reducing draughts and adding insulation without ensuring adequate ventilation (extractor fans in kitchens/bathrooms, trickle vents) can increase condensation and damp risk in some properties.
• Assuming cavity walls when they're solid, or vice versa. Get this checked properly — insulating a wall the wrong way can cause real problems.
• Chasing the cheapest quote without checking installer accreditation. For most insulation grant schemes and for genuine quality assurance, look for installers registered with a recognised industry body (such as CIGA for cavity wall guarantees, or equivalent schemes for external wall insulation).

Do grants cover any of this?

Government energy efficiency schemes for insulation do exist from time to time and eligibility criteria change, so rather than quote a specific scheme name or amount that may be out of date by the time you read this, the reliable approach is to check current government guidance (gov.uk) or ask a reputable local installer what you may currently be eligible for, particularly if you're on a lower income or receiving certain benefits.