Loft Insulation vs Cavity Wall Insulation: Which Should You Do First? (2026)

Do loft insulation first. It costs about £750 versus about £2,200 for cavity wall insulation, installs in 2-3 hours versus half a day, and delivers the best return per pound spent. A semi-detached house with an empty loft saves about £190 a year from loft insulation and about £200 a year from cavity wall insulation (Energy Saving Trust). Combined, the two measures cut up to 60% of your home’s heat loss. Both are often available free under ECO4 for eligible households. If you can only afford one measure, start with the loft.

How Do Loft and Cavity Wall Insulation Compare?

Sources: Energy Saving Trust, Energy Saving Trust

Why Should Loft Insulation Come First?

Loft insulation delivers the highest return per pound of any insulation measure in a UK home. The maths is straightforward: for a semi-detached house with an empty loft, you spend about £750 and save about £190 a year (Energy Saving Trust figures). That is a payback period of about 4 years and a return on investment that no other insulation measure matches.

Three practical reasons support this priority order:

1. Cost efficiency: At roughly a third of the price of cavity wall insulation, loft insulation frees up budget for the next measure sooner

2. Speed: A professional installs loft insulation in a standard three-bedroom house in 2-3 hours. You see savings on your very next energy bill

3. Simplicity: Loft insulation is the one major measure that a competent DIYer can do themselves, reducing the cost to materials only (£150-£250 for mineral wool rolls)

The recommended depth is 270mm of mineral wool — the first layer between the joists, the second layer laid across them at right angles. If your home already has 100mm of insulation (common in homes treated in the 1980s or 1990s), topping up to 270mm costs about £600 but saves only tens of pounds a year: the Energy Saving Trust puts a top-up from 120mm at about £17 for a semi-detached house.

How Much Heat Does Each Measure Save?

An uninsulated home loses 25% of its heat through the roof and 35% through the walls (Energy Saving Trust). Walls account for more total heat loss, but the cost to address roof heat loss is dramatically lower.

Here is how the savings break down by house type:

Loft insulation savings (from zero insulation)

Cavity wall insulation savings

Sources: Energy Saving Trust, Energy Saving Trust

The key insight: loft insulation saves almost as much as cavity wall insulation in absolute terms, but costs a fraction of the price. Pound for pound, the loft wins every time.

What If Your Loft Is Already Insulated?

If your loft already has 270mm of insulation, skip straight to cavity wall insulation. Many homes treated under previous government schemes (Warm Front, CERT, CESP) already have adequate loft insulation. Check the depth yourself by looking in the loft — if the insulation sits well above the joists, it is likely at or near 270mm.

If you have partial insulation (50-100mm), topping up is cheaper and faster than a full cavity wall installation, but it saves much less: the Energy Saving Trust puts a top-up from 120mm at about £17 a year in a semi, against about £200 for cavity wall insulation, so the cavity walls may deliver the bigger improvement.

If your home has solid walls rather than cavity walls, loft insulation is even more important as your first step, since solid wall insulation costs about £12,000 (internal) to £15,000 (external), according to the Energy Saving Trust, and has a much longer payback period.

Can You Get Both Measures Free?

ECO4 runs until December 2026 and can cover the full cost of both loft insulation and cavity wall insulation (GOV.UK). Under this scheme, energy companies fund energy efficiency improvements for qualifying households.

You may be eligible if you:

• Receive a qualifying benefit (Universal Credit, Pension Credit, Income Support, JSA, ESA, Housing Benefit, Child Tax Credit or Working Tax Credit)
• Have a household income below your council’s LA Flex threshold (typically £31,000-£38,000)
• Live in a property rated EPC band D, E, F or G

The Great British Insulation Scheme (GBIS) closed in March 2026 after delivering 139,200 measures across 100,900 households, with cavity wall insulation (37%) and loft insulation (28%) as the two most common installations (GOV.UK).

ECO4 funding remains available but is approaching its final allocation phase. Apply as soon as possible.

Visit our insulation grants page for full details.

What Should You Do After Both Are Installed?

Once you have both loft and cavity wall insulation in place, you have addressed up to 60% of your home’s heat loss. The next steps in priority order are:

1. Draught-proofing (£100-£300, saves about £55/year, plus about £45 for an unused chimney) — windows, doors and chimneys

2. Floor insulation (£1,400-£2,500, saves about £55/year) — if you have a suspended timber floor

3. Upgrading windows — if still single-glazed, double or triple glazing reduces heat loss further

4. Heating system upgrade — a modern condensing boiler or heat pump maximises savings from your newly insulated home

A well-insulated building envelope makes every heating system work more efficiently and reduces the size (and cost) of any future heat pump installation.

Frequently Asked Questions

Is it worth doing loft insulation if I already have 100mm?

It can be, but the saving is modest. Topping up from 100mm to the recommended 270mm costs about £600 and saves only tens of pounds a year: the Energy Saving Trust puts a top-up from 120mm at about £17 for a semi-detached house and £35 for a detached one. The additional 170mm significantly reduces heat loss because insulation performance improves with depth. Payback is long, though: around 17 years in a detached house and 35 in a semi, within the roughly 40-year life the Energy Saving Trust gives loft insulation.

Can I install loft insulation myself?

Yes. Loft insulation is one of the most straightforward DIY jobs in home improvement. You need mineral wool rolls, a dust mask, gloves and goggles. Lay the first layer between joists and the second layer at right angles across them. Materials cost £150-£250 for a typical semi-detached house from any builders’ merchant.

Does cavity wall insulation work in all homes?

No. Cavity wall insulation requires a clear cavity of at least 50mm between the inner and outer walls. It is not suitable for homes with solid walls (typically pre-1920s), homes with very narrow cavities, or properties in severely exposed locations where driving rain could penetrate. A pre-installation borescope survey confirms suitability.

How long do loft and cavity wall insulation last?

Loft insulation (mineral wool) lasts around 40 years if undisturbed, according to the Energy Saving Trust. It does not degrade, rot or lose performance over time. Cavity wall insulation lasts 25+ years and comes with a 25-year CIGA guarantee. Blown fibre insulation in cavities may settle slightly over decades but retains most of its performance.

Will insulation make my house too warm in summer?

No. Insulation works both ways — it keeps heat in during winter and heat out during summer. A well-insulated home with good ventilation stays cooler in heatwaves than an uninsulated one, because less solar heat penetrates through the roof and walls.

Ready to insulate your home in the right order?

Planning this upgrade? Estimate your costs with the free savings calculator or request a free quote from up to three approved installers who cover your area.