How Much Loft Insulation Do I Need?

How deep should loft insulation be?

The recommended depth in the UK for mineral wool loft insulation is 270mm. Many older homes have far less – often 25–100mm laid decades ago, sometimes flattened. If your insulation is level with or below the top of the ceiling joists, it's almost certainly worth topping up.

How much could you save?

Energy Saving Trust estimates for insulating an uninsulated loft to 270mm run to a few hundred pounds a year, with the biggest savings for detached houses and less for terraces and flats. Topping up from 100mm saves less, but it's cheap to do. Insulation typically lasts for decades, so it pays for itself many times over.

How it's laid

Loft insulation usually goes in two layers:

First layer: 100mm (or the depth of your joists) laid between the joists.

Second layer: usually 150–200mm laid across the joists at right angles, covering the tops of the joists and the gaps.

Laying the top layer across the first stops heat escaping through the timber and any gaps.

How many rolls do you need?

Measure the loft floor (length × width, minus any area you can't or won't insulate). Check the coverage on the roll packaging – it varies a lot with thickness. Thinner rolls cover more area per roll than thicker ones. Divide your loft area by the coverage per roll for each layer and round up. The Loft Insulation Calculator asks for your existing depth and works out the rolls you need to reach 270mm.

Doing it safely

• Wear gloves, long sleeves, goggles and a dust mask – mineral wool irritates skin and lungs.
• Only step on joists or on boards laid across them. Stepping between joists goes through the ceiling.
• Use good lighting and take your time.

What to avoid

• Don't block the eaves. Leave a gap at the roof edges for air to circulate, or you risk condensation and damp in the timbers.
• Don't squash it. Compressed insulation loses much of its effect. Raise storage boards on loft legs above the insulation.
• Don't insulate under the cold water tank. Leave that area bare so warmth from below stops it freezing; insulate the tank and pipes instead.
• Lift cables above the insulation where you can. Cables buried in insulation can overheat.
• Keep clear of recessed downlights unless they're rated for it or fitted with proper covers.
• Insulate and draught-proof the loft hatch.

Help with costs

Some households can get free or discounted insulation through government or energy supplier schemes, usually depending on income, benefits or the home's energy rating. Check GOV.UK for current insulation grant schemes and ask your energy supplier.