Do loft insulation first. It costs £300-£600 versus £500-£1,500 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 saves up to £270 per year from loft insulation and £295 per 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?
| Feature | Loft Insulation | Cavity Wall Insulation |
|---|---|---|
| Typical cost | £300-£600 | £500-£1,500 |
| Annual savings (semi-detached) | Up to £270 | £295 |
| Savings per £1 spent | £0.45-£0.90 | £0.20-£0.59 |
| Installation time | 2-3 hours | 2-4 hours |
| Disruption | Minimal (loft access only) | Minimal (external drilling) |
| Heat loss addressed | 25% (through roof) | 35% (through walls) |
| Payback period | 1-2 years | 2-5 years |
| DIY possible | Yes | No (specialist equipment) |
| Guarantee | Lifetime of material | 25 years (CIGA) |
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, you spend £300-£600 and save up to £270 per year. That is a payback period of 1-2 years and a return on investment that no other home energy measure matches.
Three practical reasons support this priority order:
1. Cost efficiency: At roughly half 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 £300-£500 and still saves £100+ per year.
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)
| House Type | Annual Saving |
|---|---|
| Detached | Up to £355 |
| Semi-detached | Up to £270 |
| Mid-terrace | Up to £165 |
| Bungalow | Up to £270 |
Cavity wall insulation savings
| House Type | Annual Saving |
|---|---|
| Detached | Up to £505 |
| Semi-detached | £295 |
| Mid-terrace | Up to £165 |
| Bungalow | Up to £270 |
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), the priority order still stands: top up the loft first, then do the cavity walls. Topping up is cheaper and faster than a full cavity wall installation.
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 £5,000-£14,000 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 up to £85/year) — windows, doors and chimneys
2. Floor insulation (£500-£1,000, saves £70-£80/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?
Yes. Topping up from 100mm to the recommended 270mm costs £300-£500 and saves £100+ per year. The additional 170mm significantly reduces heat loss because insulation performance improves with depth. Payback is typically 3-5 years.
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 the lifetime of the building if undisturbed. 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?
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