Quick answer: For mineral wool loft insulation, the widely recommended depth is 270mm, which is the figure the Energy Saving Trust quotes from building regulations guidance. If you measure 100mm or less, your loft is well short and is worth topping up. The usual way to do it is to keep a layer between the joists and lay a second layer across them at right angles.
Plenty of homes in Cardiff and across South Wales were insulated decades ago, when 50mm or 100mm was considered generous. That old layer is better than nothing, but it is a long way short of what is recommended today. This guide explains how to check your own loft and what to do about it.
Why 270mm?
Heat rises, and an uninsulated roof is one of the biggest routes for it to escape. The Energy Saving Trust says homes can lose around a quarter of their heat through an uninsulated roof. Mineral wool slows that loss by trapping air in its fibres, and the more depth of trapped air you have, the slower heat gets through.
The 270mm figure is the minimum depth the Energy Saving Trust gives for mineral wool loft insulation, and it is what we install to on a standard job. Going beyond it gives smaller and smaller gains, so 270mm is the sensible target for most homes rather than a starting point for piling on more.
The figure applies to mineral wool laid at joist level, which is the most common loft set-up. Other materials, such as rigid boards or multifoil fitted to the rafters, work differently and aren't measured in the same way. We compare those options in our guide to SuperFOIL vs mineral wool.
How to measure the loft insulation you already have
You only need a tape measure or ruler and a torch. Take care in the loft: only stand on the joists or on boards, never on the plasterboard between them.
- Find a representative spot away from the hatch, where the insulation is usually most disturbed.
- Push the ruler straight down through the insulation until it touches the ceiling plasterboard. Don't press the insulation flat as you do it.
- Read the depth at the top of the fluffy layer.
- Check a few more places, including near the eaves and around anything stored up there. Insulation is often thinner at the edges and squashed under boxes or old boards.
- Look at the condition as well as the depth. Dark, damp, flattened or nibbled insulation won't perform properly even if it measures well.
If your joists are still visible above the insulation, you definitely have less than 270mm, because joists in most lofts are well under that depth.
What to do if you have 100mm or less
This is the most common situation we find. Here is how we decide what to do.
If the existing insulation is dry and in decent condition
Keep it. We lay a new layer of Knauf Earthwool loft roll across the joists, on top of the old layer, to bring the total up to 270mm. Topping up is quicker and cheaper than starting from scratch, and there is no reason to throw away insulation that still works.
If it is damp, flattened or contaminated
Old insulation that has been soaked by a roof leak, crushed under boards for years, or soiled by mice and rats needs to come out. Insulating over it just buries the problem. We remove it, find out why it got that way, and put that right before laying new insulation.
If there is nothing at all
Then you need the full depth: a layer between the joists and a second layer across them. This is the most straightforward job, but it uses the most material.
Cross-laying: why the second layer goes the other way
The Energy Saving Trust describes the standard method: the first layer is laid between the joists, and another layer is then laid at right angles to cover the joists and bring the insulation up to the required depth.
Laying the top layer across the joists matters for two reasons:
- It covers the joists. Timber insulates far less well than mineral wool, so bare joist tops act as cold strips running across your ceiling. A cross-laid layer blankets them.
- It closes the gaps. Any small gaps where rolls meet in the bottom layer are covered by the layer above, rather than lining up into a continuous gap.
The details that make 270mm actually work
Depth on its own isn't the whole story. A loft can have 270mm and still perform poorly if the details are wrong.
- Don't squash it. Mineral wool works because of the air it traps. Compress it under boxes or boards and you lose much of that. If you need storage, raise the boards on loft legs. See our guide to boarding a loft without crushing the insulation.
- Keep the eaves clear. Insulation pushed hard into the eaves can block the ventilation that keeps a cold loft dry. Proper installation keeps that airflow path open.
- Insulate the hatch. An uninsulated, draughty hatch is a hole in your insulation. Insulating it and fitting draught strips is simple and worth doing.
- Protect pipes and tanks. Once the loft floor is well insulated, the loft gets colder. Pipes and the cold water tank in the loft need their own insulation, and we leave the area directly under the tank uninsulated so a little warmth reaches it from below.
- Watch out for downlights and cables. Recessed lights and electrical cables need appropriate clearance or protection rather than being buried. We check these on the survey.
Do building regulations in Wales matter for a top-up?
Wales has its own version of Approved Document L, the building regulations guidance on conservation of fuel and power, published by the Welsh Government. It is mainly relevant when building new homes, extending, or carrying out larger renovations of roofs and other parts of the building fabric. A new edition has been published to take effect in March 2027. For an ordinary loft top-up, the practical target is the 270mm recommended depth. If you are converting the loft or reroofing, talk to your builder or building control about what the regulations require for that work.
When 270mm of mineral wool isn't the right answer
Some lofts don't suit thick rolls laid across the joists:
- Lofts with very low pitches where there isn't room to work or to reach the eaves
- Lofts you want to use as a room or a heavily used storage area
- Rooms in the roof with sloping ceilings
- Lofts with damp or condensation problems that need fixing first
In those cases, insulating at rafter level, sometimes with SuperFOIL multifoil, or a combination of approaches, may make more sense. We explain the options honestly on the survey.
Want us to check your loft?
If you'd rather not go up there yourself, we will measure what you have, check its condition and the ventilation, and tell you exactly what is needed. Our surveys are free, quotes are usually back within 24 hours, and most standard jobs are completed in a single day. Read more about our loft insulation service or book your free survey.