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The honest guide toInsulation.
Cavity, solid wall, loft, internal vs external: which measure suits which fabric, and where the moisture risks sit.
Insulation
Who this is for
For anyone who wants a warmer home and lower bills and needs to know which insulation suits their walls and loft, what it costs, and where the moisture risks sit.
The questions these guides answer
- Does my home have cavity walls or solid walls, and how can I tell?
- Is the insulation already in my loft enough?
- Solid walls: insulate inside or outside?
- What does each measure cost, and which pays back first?
Worth knowing first
The Energy Saving Trust puts an empty loft at about £750 to insulate, saving about £190 a year on a gas-heated semi, and filling empty cavity walls at about £2,200, saving about £200 a year. Solid wall insulation saves more but costs far more, so it is usually done for comfort and alongside other work. Installation is zero-rated for VAT until 31 March 2027.
Reading order: beginner first
Start at the top.
04 guides
- 01
Cavity wall vs solid wall: which does my home have, and how to tell
Check the brickwork and the wall thickness. Bricks all laid lengthways and a wall 260 mm to 350 mm thick usually mean a cavity wall; short brick ends mixed in or a thinner wall usually mean solid. Render, timber or steel frames and narrow early cavities need a professional check before anyone insulates.
7 min read
- 02
Loft insulation in 2026: is the existing one enough?
If your loft already has 250 mm to 270 mm of insulation in good condition, it is enough. If it has little or none, insulating is one of the best-value jobs in the house. Topping up from 120 mm saves only about £17 a year, so do it alongside other work.
6 min read
- 03
Solid wall insulation: internal vs external, and when each makes sense
Internal wall insulation goes room by room and leaves the frontage alone, but takes floor space and carries more moisture risk. External wall insulation keeps rooms intact and handles cold spots better, but changes how the house looks and often needs planning permission. Getting moisture right matters most.
7 min read
- 04
How much does insulation cost? Loft, cavity and solid wall prices
Typical 2026 costs for loft, cavity wall, internal wall and external wall insulation, what each saves on a typical bill, and which to do first for the biggest return.
8 min read
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