Loft insulation is one of the cheapest ways to cut a heating bill, but only if it is deep enough. Heat rises, and a poorly insulated roof can let a quarter of your home's warmth escape straight through the ceiling. Get the depth right and you trap that heat where you want it. The good news is that the recommended figure has barely moved in years, so once you know the number you can check your own loft in about five minutes with a tape measure.
The short answer. Loft insulation should be 270mm thick if you use standard mineral wool (the soft glass or rock wool rolls). That is the figure the Energy Saving Trust and Building Regulations both point to for a typical cold loft. If your insulation uses a higher-performance material such as rigid PIR board, you can hit the same level with roughly 120mm.
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The short answer: 270mm of mineral wool
For the rolls of soft mineral wool that most lofts use, the target depth is 270mm. That is about the height of a standard house brick stood on its end, plus a little more. It is the depth that gets a typical cold loft to a U-value of around 0.16 W/m2K, which is the level Building Regulations work to for new and upgraded roofs in England.
It was not always this deep. Homes insulated in the 1980s often got 25mm or 50mm. Through the 1990s and 2000s the standard crept up to 100mm, then 200mm, and 270mm has been the recommendation for well over a decade now. So if your loft was done a long time ago, the depth that was correct then is almost certainly too shallow today.
The number changes with the material. The 270mm figure is for mineral wool specifically. Denser, higher-performing boards do the same job in less space:
- Mineral wool (glass or rock): 270mm
- Sheep's wool or recycled fibre: roughly 270mm, similar to mineral wool
- Rigid PIR or PUR board: roughly 120mm
- Expanded polystyrene (EPS) board: roughly 220mm
Most people use mineral wool because it is by far the cheapest per square metre and it is easy to lay yourself, so 270mm is the number worth remembering.
Why 270mm and not more
Insulation works on diminishing returns. The first few centimetres do most of the work, and every centimetre after that saves a little less than the one before. Going from nothing to 100mm makes a big difference. Going from 100mm to 270mm makes a useful difference. Going from 270mm to 400mm makes a small one that almost never pays for the extra material.
That is why 270mm is treated as the sweet spot rather than a hard ceiling. It captures nearly all of the easy savings without wasting money on depth you will barely feel. There is no harm in laying more if you happen to have it, and some installers go to 300mm to be comfortably inside the regulations, but you should not lose sleep over the extra 30mm.
One practical limit worth knowing: piling insulation higher than your joists is fine, but you must never crush it. Squashing mineral wool flat under boards or boxes destroys the trapped air that does the insulating, so a compressed 270mm performs far worse than a fluffy 200mm. If you want a boarded loft for storage, raise the boards on platforms or joist risers so the full depth of insulation can breathe underneath.
How to measure what you already have
You do not need a survey for this. Go into the loft with a torch and a tape measure, push the tape down through the insulation until it touches the ceiling below, and read off the depth in a few different spots. The middle of the loft tends to be deepest; the edges near the eaves are often thin or missing.
If the layer sits flush with the top of the joists, you probably have around 100mm, because that is a common joist height. If it rises well above the joists in a second crossing layer, you may already be near 270mm. If you can see bare ceiling between sparse strips, you have a job to do.
The standard way to reach 270mm is two layers. The first goes between the joists, usually 100mm, sitting snug against the boards but not jammed in. The second layer, around 170mm, goes across the joists at a right angle so it covers the timber as well. That cross-layer matters because the joists themselves leak heat, a problem called cold bridging, and laying over them stops warmth sneaking out through the wood. Topping up an existing 100mm loft to 270mm is exactly this: one fresh cross-layer rolled over what is already there.
For the wider picture on materials, U-values and installation, see our main loft insulation guide.
What topping up to 270mm saves
The savings depend almost entirely on how much insulation you start with and how exposed your roof is. A detached house has a large roof and no neighbours sharing heat, so it loses the most and saves the most. A mid-terrace, sandwiched on both sides and often below another flat, loses far less through the roof and saves the least. The figures below are Energy Saving Trust estimates for going from a completely uninsulated loft to the full 270mm.
| Property type | Typical saving a year | Rough install cost (DIY) |
|---|---|---|
| Detached house | around £355 | £300 to £500 |
| Semi-detached house | around £225 | £250 to £400 |
| Mid-terrace house | around £135 | £200 to £350 |
| Detached bungalow | around £390 | £350 to £550 |
Savings are Energy Saving Trust gas-heating estimates at 2026 prices (gas 7.97p per kWh, Ofgem cap October to December 2026). DIY material costs are typical mid-2026 retail ranges for mineral wool rolls; figures are illustrative and vary by loft area.
Those numbers are for a loft starting from scratch. If you already have 100mm and you are topping up to 270mm, expect roughly a third to a half of the saving above, because the first 100mm has already done the heavy lifting. Even so, the materials are cheap enough that most top-ups still pay for themselves within a few years. A bare loft pays back fastest, often in two to four years.
The bits people get wrong
Depth is the headline, but a few details decide whether the insulation actually works.
- Do not block the eaves. Your roof needs a gap at the edges for air to flow in, or moisture builds up and rots the timber. Stop the insulation short of the eaves or fit ventilation trays so the airflow stays open.
- Insulate the loft hatch. An uninsulated hatch is a cold hole in an otherwise warm ceiling. Stick a piece of board or a wool offcut to the back of it and add draught strips around the frame, the same trick covered in draught-proofing.
- Keep water tanks and pipes above the insulation, not under it. If you have a cold-water tank in the loft, insulate around its sides and lid but leave the area directly beneath it uninsulated, so a little warmth from the house below keeps it from freezing. Lag any exposed pipes.
- Mind the downlights. Older recessed ceiling lights can overheat if you bury them in wool. Use fire-rated covers or leave a clearance, depending on the fitting.
- If your loft is a room, the rules differ. A converted or boarded-out loft room needs insulation in the sloping roof and walls instead of the floor. That is a separate job covered in room in roof insulation.
Grants and the bottom line
Loft insulation is the lift-and-shift of energy saving: cheap, quick, and it works the day you finish. For a standard cold loft, lay enough mineral wool to reach 270mm, in two crossed layers, without crushing it. That single number covers almost every home.
If money is tight, you may not have to pay full price. Several government schemes help with insulation, including the Great British Insulation Scheme and ECO4, which are aimed at lower-income households and homes with poorer energy ratings. Eligibility and the amounts on offer change from year to year, so check the current rules on the official GOV.UK and Ofgem pages before you assume you do or do not qualify.
Once the loft is sorted, the next biggest wins are usually your walls and your draughts. If you have empty cavity walls, cavity wall insulation is the natural follow-on, and a good seal on doors, windows and the loft hatch keeps the warm air you are now trapping from leaking out elsewhere. Insulation and a sensible heating setup work as a pair: the better the loft, the lower you can run the thermostat for the same comfort.