4 February 2026 · 6 min read

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

If you can only do one measure now, which delivers more? We compare cavity wall and loft insulation on savings, cost and disruption to help you choose.

If you can only fit one insulation measure first, the answer depends on what your home already has. Loft insulation should come first if your loft has less than 100mm of existing insulation, because it is the cheapest measure (£300 to £500) and delivers the highest annual saving for the money, around £355 a year. Cavity wall insulation should come first if your loft is already well insulated and your walls have unfilled cavities, since it saves around £295 a year and stops the largest remaining source of heat loss. In an ideal world both are done together, often as a single grant-funded package under ECO4, which combined saves a typical Black Country home around £650 a year. The two measures are complementary rather than competing: heat escapes through both the roof and the walls, so a fully insulated home needs both. A free survey measures your current loft depth and confirms whether your walls have a fillable cavity.

Cavity wall and loft insulation are the two most common measures, and homeowners often ask which to prioritise if budget or time means doing one before the other. Here is how they compare.

The quick answer

Do loft insulation first if your loft has less than about 100mm of insulation. It is cheaper, faster and delivers the highest saving for the money. Do cavity wall insulation first if your loft is already well insulated and your walls have unfilled cavities. If you can, do both together, which is often possible under a single grant.

Savings compared

The Energy Saving Trust estimates a typical semi-detached home saves around £355 a year from loft insulation and around £295 a year from cavity wall insulation. Loft edges ahead, partly because around a quarter of a home's heat escapes through an uninsulated roof.

Cost compared

Loft insulation costs £300 to £500, cavity wall insulation £450 to £700 for a semi. Loft is the cheaper and quicker job, usually finished in a few hours. For full pricing see our insulation cost guide.

Disruption compared

Both are low-disruption. Loft insulation is fitted entirely in the roof space. Cavity wall insulation is done from outside through small drill holes, with no internal mess. Neither requires you to move out or redecorate.

Grant availability

Both measures are funded under ECO4 and the Warm Homes Local Grant for eligible homes, and they are commonly installed together as a funded package. If you qualify, the cost question disappears and the answer is simply to do both.

Why both, eventually

Heat leaves a home through every surface: roof, walls, floor and windows. Insulating only the loft still leaves the walls losing heat, and vice versa. The two measures are complementary, and a home with both loft and cavity wall insulation is far warmer and cheaper to run than one with only a single measure. A free survey tells you what your home already has and what it needs next.

Frequently asked questions

Which saves more, cavity wall or loft insulation?

Loft insulation saves slightly more for a typical semi (around £355 a year versus £295 for cavity wall) and costs far less, which makes it the better first measure if your loft is under-insulated. Cavity wall insulation saves more in homes that already have a well-insulated loft but lose heat through unfilled walls.

Can I get both done at once?

Yes, and it is often the best approach. Cavity wall and loft insulation are frequently installed together as a single package, and under ECO4 both can be grant funded in one go. Combined, they typically save a Black Country home around £650 a year.