Key takeaways
- 100mm was the old standard; 270mm is today’s, the difference leaks real money
- Topping up 100→270mm costs £150–£300 DIY and saves £100–£150 a year
- Lay the top-up crosswise over the joists; never compress it under boards
- Keep eaves ventilation clear and insulate the hatch while you are up there
- ECO4 makes it free for qualifying households, check before buying rolls
The short answer
270mm is the current standard and 100mm was the old one, which leaves millions of lofts half finished. Topping 100mm up to 270mm costs £150–£300 DIY and saves £100–£150 a year, a payback of one and a half to three years.
Starting from bare joists costs £300–£500 and saves around £250 a year. If a tape measure into the existing layer reads 250mm or more, close the hatch and spend the money on draughts instead.
Why "some loft insulation" is not the same as done
Most UK lofts got insulated at some point, to whatever the standard was that year. Millions stopped at 100mm (the 1990s to 2000s norm). Heat loss does not care about good intentions: at 100mm the loft still leaks roughly twice what it would at the current 270mm recommendation.
The good news is that topping up is the easy version of the job. The first layer (between joists) is already down; the top-up goes crosswise over the joists, no cutting around timbers, no crawling into eaves corners for hours.
The numbers by starting depth
| # | Starting point | Top-up cost (DIY, 3-bed) | Annual saving | Payback | Verdict |
|---|---|---|---|---|---|
| 1 | None / bare joists | £300–£500 | ~£250 | 1–2 years | Do it this week |
| 2 | 100mm (the 2000s standard) | £150–£300 | ~£100–£150 | 1.5–3 years | The classic top-up, do it |
| 3 | 170mm | £120–£250 | ~£40–£60 | 3–5 years | Worth it while rolls are cheap |
| 4 | 270mm already | – | ~£10 for more | 20+ years | Done. Spend elsewhere |
Run your own figures in the insulation payback calculator, electric or oil heating shortens every payback above.
Doing it yourself in a Saturday
- Measure first. A tape measure into the existing layer, if it reads 250mm+, close the hatch and spend the money on draughts instead.
- Buy combi rolls (170mm or 200mm) and lay them at right angles across the joists, covering the timbers. Two to three rolls does a typical 3-bed semi.
- Do not compress it. Squashed wool loses its performance; storage boards need loft legs to keep the deck above the wool.
- Keep the eaves breathing. Do not stuff insulation into the eaves gap, blocked airflow means condensation on the rafters by February.
- Insulate the hatch (a slab of insulation glued to the back, plus draught strip) and lag any pipes and tanks now above the insulation, the loft will be colder from now on, that is the point.
- Wear a mask, long sleeves and stand on the joists. The wool is itchy, not dangerous.
When to pay someone instead
- ECO4 eligibility: if anyone in the house gets qualifying benefits, loft insulation is free under ECO4, check before buying rolls.
- No safe access, or a boarded loft in full use: a pro reworks decking and insulation together (£400 to £700).
- Damp stains on the rafters: fix the roof leak first, insulating under a leak just hides it longer.
- Loft conversions insulate the slope, a different job entirely (and building regs territory).
Frequently asked questions
How much loft insulation do I need in 2026?
270mm of mineral wool is the standard recommendation (or equivalent in other materials). Beyond about 300mm, extra depth saves single-digit pounds a year, more is not meaningfully better.
Can I put new insulation over old?
Yes, if the old layer is dry and not compressed flat. Lay the new roll crosswise on top. Damp, mouldy or vermin-damaged wool comes out first, in bags, through the hatch, unpleasant but simple.
Does loft insulation cause condensation?
Insulation done right does not; blocking ventilation does. Keep the eaves airflow clear (use eaves trays if the wool wants to slump into the gap), insulate and draught-strip the hatch, and make sure bathroom extractors vent outside rather than into the loft.
What about the cold water tank up there?
Once the loft floor is insulated, the loft gets colder, so the tank and pipes must be insulated too (jacket on the tank, foam lag on pipes), but never insulate underneath the tank: the gentle heat from below is what stops it freezing.
Is spray foam a good alternative?
No. Spray foam on the roof slope has caused enough surveying and mortgage problems that several lenders refuse homes with it. Mineral wool on the floor is cheap, proven, removable and lender-friendly. There is no reason to gamble the roof for the sake of a marketing pitch.
Sources
Energy Saving Trust typical savings data, Ofgem Q3 2026 price cap. Figures are indicative for a typical 3-bed gas-heated home in England; electric heating multiplies fuel-cost savings by about 3.5.