What this calculator works out
This calculator divides the installation cost by the estimated annual saving to give a payback period, and shows the net saving across a chosen assessment period.
Loft insulation has one of the shortest paybacks of any home energy measure, largely because it is cheap and the heat loss through an uninsulated roof is substantial.
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Before you estimate
- The current recommended depth is 270 mm of mineral wool. Many houses have 100 mm or less, and topping up is much cheaper than starting from nothing.
- Savings estimates vary enormously with house type, existing insulation and how warm you keep the house. Treat published figures as indicative.
- Check for grants. Schemes such as the Great British Insulation Scheme and ECO have covered loft insulation for eligible households.
- The assessment period is not the material's life — mineral wool lasts decades if kept dry and uncompressed.
Most of the benefit comes from the first 100 mm. Going from nothing to 270 mm saves far more than going from 100 mm to 270 mm, so payback is much faster on an uninsulated loft. Both are usually worth doing; the arithmetic just looks very different.
How this calculator works
A payback period and a net figure:
Payback in years = installation cost ÷ annual saving
Net saving = (annual saving × assessment period) − installation costThe calculation ignores energy price changes. If prices rise, the saving rises with them and the payback shortens — which makes this a conservative estimate rather than an optimistic one.
It also ignores comfort, condensation risk and the effect on an EPC rating, all of which have value that does not appear as a number here.
Worked example: £800 to install
Using the default figures — £800 to install, £180 saved a year, assessed over 20 years:
- Payback: 800 ÷ 180 = 4.4 years
- Net saving over 20 years: (180 × 20) − 800 = £2,800
A 4.4 year payback is an effective return of about 22% a year, tax-free and guaranteed, which is better than any savings account and most investments. That comparison is the useful framing: insulation is not really a home improvement decision, it is a financial one — and one of the few where the arithmetic is this favourable.
Common mistakes
- Using a saving figure for an uninsulated loft when yours has 100 mm. The top-up saving is much smaller.
- Not checking for grants. Eligible households have had this done free or heavily subsidised.
- Compressing the insulation with boarding afterwards. It undoes much of the benefit you just paid for.
- Insulating over recessed downlights without checking clearances. Some are a fire risk if covered.
- Forgetting the loft hatch. An uninsulated, undraughtproofed hatch is a significant hole in the insulation.
Frequently asked questions
How much should loft insulation cost?
A straightforward DIY job with mineral wool rolls is materials-only, often a couple of hundred pounds for an average loft. Professional installation varies with access, area and whether the existing material needs removing. Grant schemes have covered it entirely for eligible households, so it is worth checking before paying.
Can I do it myself?
Usually yes, and it is one of the more accessible DIY jobs. Wear a mask, gloves, long sleeves and eye protection — mineral wool irritates skin and airways. Work off boards or joists rather than the ceiling, which will not take your weight. Lay the second layer at right angles to the first and do not compress it.
What about a warm roof instead?
Insulating at rafter level rather than joist level keeps the loft space warm, which suits a loft in use or one containing a water tank or pipework. It costs considerably more and is usually done as part of a conversion. For storage-only lofts, insulating at joist level is more cost-effective.
Do I need to insulate pipes and tanks in the loft?
Yes, and it becomes more important once the loft is well insulated, because the space is then colder. Lag pipes and insulate the tank sides and lid but not the floor beneath the tank, so a little warmth rises from below to reduce freezing risk.
Is what I enter stored?
No. Costs and savings figures are processed in your browser and never transmitted or retained.
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References
- GOV.UK — home energy efficiency grants, Approved Document L and insulation guidance
- Health and Safety Executive — handling mineral wool safely and working at height
Sources are checked at publication and can change — how I choose and check references.