Quick answer: For mineral wool loft insulation, the recommended depth is 270mm, the figure the Energy Saving Trust gives in line with building regulations guidance. It is normally laid in two layers: one between the ceiling joists and a second across them at right angles. If you measure much less than that, a top-up is usually the answer.
Many lofts were insulated decades ago, when a thin layer between the joists was the norm. That old layer still helps, but it is well short of today's recommendation. This guide explains where the 270mm figure comes from, how to measure your own loft, and what to do next.
Why 270mm?
Warm air rises, and the roof is one of the main ways heat leaves a house. The Energy Saving Trust says that in an uninsulated home around a quarter of the heat can be lost through the roof. Mineral wool slows that loss by trapping still air between its fibres. The deeper the layer, the more still air there is and the slower heat escapes.
270mm is the recommended depth for mineral wool laid at ceiling level, which is how most lofts are insulated. Adding more beyond that gives smaller and smaller gains, so 270mm is the sensible target for most homes. It is what we work to on a standard loft insulation job, using Knauf Earthwool mineral wool loft roll.
The figure is for mineral wool. Other products, such as rigid boards or multifoil fixed to the rafters, are measured differently, so you can't compare them by thickness alone. Our SuperFOIL insulation page explains when a multifoil may suit a loft better.
How to measure the insulation you already have
You need a torch and a ruler or tape measure. Only stand or kneel on the joists or on boards. Never put weight on the plasterboard between the joists, because it will not hold you.
- Pick a spot away from the hatch. Insulation near the hatch is usually the most disturbed.
- Push the ruler gently down through the insulation until it touches the ceiling below. Try not to flatten the wool as you go.
- Read the depth at the top of the layer.
- Repeat in a few places, including towards the eaves and anywhere boxes have been stored. Edges and storage areas are often thinner or squashed.
- Note the condition. Wool that is damp, dark, flattened or soiled won't perform well, whatever it measures.
A quick check: if you can see the tops of the joists, you almost certainly have less than 270mm, because ceiling joists in most houses are much shallower than that.
Between the joists, then across them
The standard method uses two layers. The first fills the space between the joists. The second is laid across the joists at right angles, bringing the total up to the recommended depth. Cross-laying matters for two reasons:
- It covers the joists. Timber insulates far less well than mineral wool, so bare joist tops act as cold lines across the ceiling. A top layer covers them.
- It closes the gaps. Where the rolls in the bottom layer meet, small gaps can open up. A layer running the other way overlaps those joins.
What if you have 100mm or less?
This is common in older lofts. What we recommend depends on what we find.
Dry and in decent condition
Keep it and add a second layer across the joists to reach 270mm. There is no point throwing away insulation that still works. Our loft insulation top-up service is built around exactly this.
Damp, crushed or contaminated
Insulation soaked by a roof leak, flattened under old boards for years, or soiled by mice or rats should come out first. Covering it only hides the problem. We would find the cause, put it right, then insulate. Our guide to the signs your loft needs a top-up covers what to look for.
Nothing there at all
Then the loft needs the full two-layer build-up. It is a straightforward job, but it uses the most material.
Things to get right while you insulate
- Keep the eaves clear. Insulation should not be pushed hard into the eaves so that it blocks the air that comes in there. Ventilation keeps the roof timbers dry. Our guide to loft condensation and ventilation explains why.
- Insulate pipes and the cold water tank. A well-insulated loft is colder above the insulation, so pipes and tanks up there need their own lagging. The area directly under the tank is usually left uninsulated so a little warmth reaches it from below.
- Treat the hatch. An uninsulated, undraughtstripped hatch leaves a weak spot in an otherwise well-insulated ceiling.
- Allow for lights and cables. Recessed downlights and electrical cables need appropriate clearance or protection rather than being buried. We check these during the assessment.
- Don't squash it. Mineral wool works because of the air it holds. Laying boards straight onto the joists over a deep layer crushes it. See can you board over loft insulation? for the right way to add storage.
When 270mm of wool is not the right answer
Some lofts don't suit thick rolls across the joists: very shallow roof pitches where the eaves are hard to reach, rooms in the roof with sloping ceilings, or lofts with damp problems that need fixing first. In those cases a different approach may make more sense, and we will say so on the assessment rather than force a standard job onto an awkward space.
Want someone to check for you?
If you would rather not go up there, a loft inspection will tell you the depth and condition of what you have, whether ventilation is adequate, and exactly what is needed. We carry out free assessments for homes across Teesside and the North East.
Every job starts with a free assessment and a written quotation that sets out the scope, materials and price before anything is agreed. The work is carried out by The Insulation Company or an approved installation partner. Call 0800 229 4668 (Monday to Saturday, 8am to 6pm) or request your free assessment online.