How Much Loft Insulation Do I Need?

Why 270mm, and where the number comes from

270mm is the recommended depth for mineral wool at ceiling level. It is not an arbitrary figure. It is roughly the thickness of standard mineral wool needed to bring a typical ceiling down to the U-value that current building standards expect of new work, which is in the region of 0.16 W/m²K.

That distinction matters. If you are working on new builds, building an extension converting a loft or replacing a roof, building regulations set a required standard and your building warrant will specify it. If you live in an existing home with 100mm of insulation laid in 1985, nobody is going to make you upgrade. 270mm is the target worth aiming for, not a legal obligation.

The depth also depends on the insulation material. 270mm applies to standard mineral wool. Rigid PIR boards achieve the same thermal performance in roughly half the thickness, which is why they get used at rafter level and in loft conversions where depth is scarce. Sheep's wool and other natural fibres sit close to mineral wool.

Is 100mm or 200mm of loft insulation enough?

It is far better than an uninsulated loft, and it is not enough.

Most of the benefit of loft insulation comes from the first 100mm. Going from bare joists to 100mm delivers the single biggest reduction in heat loss you will ever get from this work. But the next 170mm still earns its keep, and because the labour and access costs are already being paid, topping up from 100mm to 270mm is usually the cheapest performance you can buy in a house.

A rough guide to where your loft stands:

One practical point: existing insulation settles and compresses over time, and anyone who has walked across a loft has flattened it. What looks like 100mm may be performing like 70mm. Measure your current insulation with a ruler rather than eyeballing it.

The reason increasing the thickness is worth the trouble is simple arithmetic on your heating bills. A poorly insulated attic leaks heat all winter, and in Scotland that winter is long. Getting to full depth is the cheapest way to insulate your loft properly and save you money every year afterwards.

Can you have too much loft insulation?

You can, though the problem is rarely the insulation itself.

Thermally, you hit diminishing returns. The step from 270mm to 400mm buys far less than the step from 100mm to 270mm did, and at some point the extra material costs more than it will ever save. For most homes 270mm to 300mm is the sensible stopping point.

The real risks of over-insulating are practical:

• A colder loft. The more you insulate the ceiling, the colder the roof space above becomes, which raises the risk of pipes and tanks freezing unless they are lagged on the cold side.
• Blocked ventilation. Insulation pushed hard into the eaves stops the airflow the roof relies on to dry out, and that produces condensation and mould.
• Compression. Piling depth in without planning wastes it, because compressed insulation performs worse than the same material laid loose.
• Lost storage space. Above about 270mm you are into the depth where boarding needs a raised system to avoid crushing the layer beneath.

So 500mm is not dangerous, but it is generally money spent for very little return, and it makes the ventilation detailing harder to get right.

How to work out how many loft rolls you need

The calculation is straightforward. To calculate how much insulation material you need, you need the area of your loft and a decision on the two layers.

Step one: measure the loft space

Measure the length and width of the loft floor and multiply them. A typical three-bedroom semi-detached house has a loft floor area of roughly 40 to 50m². Do not deduct for the joists; you will insulate between and over them.

Step two: plan the two layers

Ceiling joists in most older homes are around 100mm deep, so a single layer cannot reach 270mm. The standard build-up is:

• A base layer of 100mm laid between the joists, cut to fit the gaps snugly without squashing.
• A top layer of 170mm laid across the joists at right angles, which also covers the timber and stops the joists acting as thermal bridges.

If your joists are deeper, adjust the base layer to match and top up to 270mm total. Some suppliers sell a 200mm top-up roll, which gets you to 300mm with a 100mm base.

Step three: convert area into rolls

Each roll states its coverage on the pack. As a rough guide, a 100mm loft roll covers around 11m² and a 170mm top-up roll covers around 6.5m², though this varies by brand. Divide your loft area by the coverage figure and round up.

The bits people forget to allow for

Working out how much insulation you need is only most of the job. These details are where a loft installation succeeds or fails.

• The eaves. Leave a clear air gap where the roof meets the wall so the space can ventilate. Fit eaves trays if the gap is tight. Never force insulation into the wedge, because a blocked eaves is how a dry loft becomes a damp one.
• The loft hatch. An uninsulated hatch is a hole in the layer you just paid for. Insulate and draught-proof it.
• Pipes and tanks. Lag them, and do not insulate underneath a cold water tank. It needs a little warmth from below.
• Electrical cables. Cables buried in insulation can overheat. Lay them on top of the layer where possible, and get an electrician to look at anything that concerns you.
• Recessed downlights. Many types must not be covered. Check the fitting or use proper covers.

Should you do it yourself?

Loft insulation is one of the more achievable DIY jobs, and the Energy Saving Trust publishes a good step-by-step guide for anyone minded to insulate a loft themselves. Loft insulation rolls from a DIY store for a typical semi will run to a few hundred pounds, which is a modest cost to insulate a whole house against a winter of heat loss.

The honest case for a professional installer is not that the laying is difficult. It is that a surveyor who goes into your roof space will spot the compressed base layer, the blocked eaves, the cable buried under 100mm of wool and the downlight that should not be covered. Those are the things that decide whether the insulation performs.