Loft insulation depth: how thick should it be?

Why 270mm

The Energy Saving Trust’s roof and loft guide says: “Current building regulations recommend a minimum depth of 270mm for mineral wool insulation.” It describes 270mm as the minimum recommendation for new builds, and uses it as the target depth in all its cost and saving figures. That is why 270mm appears on nearly every scheme specification and installer quote in Great Britain.

The figure is specific to mineral wool, the glass or rock fibre rolls that fill most British lofts. The EST notes that other materials need different depths to reach the same performance, because what matters is how well the layer resists heat flow, not its thickness as such. Rigid foam boards, for example, can reach similar performance in a thinner layer, which is why they are used where space is tight, such as between rafters.

Why you sometimes see 300mm

You will find 300mm quoted too. Some of that is installers rounding up. Some is legitimate: Northern Ireland’s Affordable Warmth Scheme lists “installation or topping up of loft insulation to 300mm” among its priority measures. Both figures are defensible; neither is a reason to rip out a sound 270mm layer.

How the depth goes in: two layers, crossed

Joists are much shallower than 270mm, so insulation laid only between them leaves the tops of the joists exposed, and the EST notes that between-joist insulation alone may not meet modern recommended levels. The EST describes the standard method:

1. The first layer of mineral wool rolls is laid between the joists, filling the gaps without compressing the material.
2. A second layer is laid at right angles, across the joists, to cover them and bring the total up to the required depth.

Crossing the layers also covers the gaps where rolls meet. The finished surface should be a continuous blanket with no bare joists showing, stopping short of the eaves so air can still move through the roof space.

How to measure what you have

You need a torch, a ruler or tape measure and somewhere safe to kneel (a board across the joists, never the ceiling between them).

• Measure in three or four places, away from the hatch and the eaves, where the insulation has not been disturbed.
• Push the ruler straight down to the plasterboard and read the depth at the top of the fluffy layer, not a flattened patch.
• Note whether the joists are covered. If you can see the tops of the joists, you almost certainly have less than 270mm.
• Look for wet or discoloured patches, which point to a leak or condensation that must be sorted before anything is added.

Depth and the funded schemes

Ofgem’s ECO4 delivery guidance treats loft insulation as two measures, split at 100mm of existing insulation. Before any loft work, the installer and occupier sign a declaration of what is already there, and a copy stays in the loft. Measuring beforehand means you know that declaration is accurate. The schemes’ interest drops sharply once there is more than 100mm, because the saving from a top-up is small: the Energy Saving Trust puts a 120mm-to-270mm top-up at an extra £17 or so a year for a semi-detached house. See topping up loft insulation.

Building regulations, in plain terms

The 270mm figure comes from building regulations for new work, which is where the EST takes it from. Insulating an existing loft at joist level is a routine improvement that homeowners and installers do every day. If you are renovating the roof itself, converting the loft or changing its structure, different rules apply and you should get professional advice. Installers who self-certify building work belong to a Competent Person Scheme, which you can check on GOV.UK. For roof-level choices, see warm roof vs cold roof.

When depth is not the problem

A full 270mm can still underperform. Gaps around the loft hatch, insulation pulled back by someone running a cable, squashed patches under stored boxes and blocked eaves all undo some of the benefit. When you are up there with the ruler, look for those too. And if the loft is dry, clear and easy to reach, the DIY guide shows how to finish the job yourself.