An EPC records loft insulation by its depth, and the depth to aim for with mineral wool is 270mm. In most houses topping up a thin layer is the best value structural improvement there is. The one thing that trips people up is boarding, which can hide the insulation from the assessor.
How it is recorded
From the loft hatch I look at what is there and how deep it is. The method works in bands of thickness, and since June 2025 it has had a wider range of them, so the difference between a thin old layer and a full modern one shows clearly. A room in the roof is a separate case: it is measured and recorded on its own, gables included.
If the hatch is blocked, or the loft is boarded over, the depth that cannot be seen cannot be credited. Insulation topped up under boards is recorded at whatever depth is visible, unless there is paperwork showing what went in.
How much you need
Building regulations recommend at least 270mm of mineral wool, which is the minimum for a new build. The Energy Saving Trust reckons an uninsulated roof can lose around a quarter of a home’s heat, and that loft insulation, fitted properly, lasts around 40 years.
| From nothing to 270mm | About £750 to install |
|---|---|
| From 120mm to 270mm | About £600 to install |
| Saving from a 120mm to 270mm top-up | About £17 a year |
| Lifespan | Around 40 years |
Source: Energy Saving Trust, “Roof and loft insulation”, figures for Great Britain. The saving is on bills; what it does to the EPC depends on the home.
If you use the loft for storage
Boards laid straight onto the joists leave room for only one layer of insulation underneath, which may not reach the recommended depth. The better way is to raise the floor, lay a second layer across the joists, then fit the boards, leaving a small gap for air so condensation does not build up. Squashing mineral wool under boards makes it work less well.
Before you start
- Check for damp. Insulation makes the loft colder, which can make condensation or damp worse. Fix any leak or damp first.
- Keep the air moving. Do not cover vents, grilles or airbricks.
- Insulate the pipes and the water tank. A colder loft means they are more likely to freeze.
- Do the hatch too. An insulated hatch with draught strips stops cold air coming back down.
- Know when to call someone. A flat roof, a loft you live in, or any worry about damp or the structure needs a professional. A dry loft that is easy to get into is often a job you can do yourself.
The Energy Saving Trust notes that the government’s Warm Homes Plan is set to offer fully funded insulation for people on low incomes, and low and zero-interest loans for homeowners, so check what is available first.
Getting it counted
Keep the receipt or the installer’s paperwork, especially if the insulation is going under boards, and make sure the hatch is clear on the day of the visit. In a flat, this only applies if you are on the top floor with a loft above.
Common questions
How much loft insulation do I need for my EPC?
Aim for 270mm of mineral wool, the depth building regulations recommend as a minimum. The EPC records the depth the assessor can see or that paperwork shows.
Does boarding hide loft insulation from an EPC?
It can. Insulation under boards is recorded at the depth that can be seen, so keep the receipt or installer’s paperwork to show what went in.
How much does loft insulation cost?
About £750 to go from nothing to 270mm, or £600 to top up from 120mm, for a typical home in Great Britain according to the Energy Saving Trust.
Will loft insulation improve my EPC rating?
It usually helps where the loft has little or none, and in most houses it is the best value structural improvement. How much the rating moves depends on the home.
Can I insulate my loft myself?
Often, if the loft is dry and easy to get into. A flat roof, a loft you live in, or damp or structural worries need a professional.
Where to go next
Cavity wall insulation and your EPC, the cheapest ways to improve a rating, and preparing for your EPC.