How much loft insulation do I need?
The current recommended depth is around 270mm. Here's why, and how to check what you've got.
The recommended depth for mineral wool loft insulation in the UK is around 270mm (about 27cm). That's a fair bit more than older standards — lofts insulated decades ago often have 100mm or less.
Why 270mm?
Insulation works by trapping still air, and more depth means more trapped air and less heat loss. The gains taper off beyond 270mm, which is why it's the sweet spot for cost versus benefit.
How to check yours
- Pop your head into the loft and measure the depth of existing insulation.
- If it's level with or below the joists, you almost certainly need a top-up.
- If it's old, flattened or patchy, it's underperforming regardless of depth.
Topping up vs starting fresh
If you have some insulation, we can often lay more over the top to reach 270mm — cheaper than a full replacement. Damp or damaged insulation should be removed first.
Book a free survey and we'll measure and advise.
Different materials, different depths
The 270mm figure applies to standard mineral wool. Other materials reach the same thermal performance at different thicknesses — some rigid boards and spray products are thinner for the same effect, though they cost more. For the vast majority of standard lofts, mineral wool at 270mm is the practical, cost-effective choice.
What about boarded lofts?
If you use your loft for storage, standard boarding laid directly on the joists usually leaves too little room for full-depth insulation. Raised loft boarding lifts the boards above a full 270mm of insulation, so you keep both your storage space and your warmth. It's worth asking about if you need the space.