Loft insulation in 2026:is the existing one enough?

How much loft insulation is enough in 2026?

The Energy Saving Trust says current building regulations recommend a minimum of 270 mm for mineral wool loft insulation, and uses that depth for its cost and savings figures. Approved Document L itself sets a performance target rather than a depth: a roof being upgraded should reach a U-value of 0.16 W/m²K. A U-value measures how much heat escapes through a square metre for each degree of temperature difference, so lower is better.

Its worked examples show what that means in practice: at least 250 mm of mineral fibre or cellulose fibre, laid between and across the ceiling joists. The 2026 edition of Approved Document L, which takes effect in England from 24 March 2027, keeps the same 0.16 target for upgraded roofs.

In England, the English Housing Survey found 42% of homes had 200 mm or more of loft insulation in 2024, up from 39% in 2014. So most lofts are below today's recommended depth, but many are not far off.

How do you check what is in your loft?

1. Take a torch and a tape measure or ruler. Step only on the joists or on secure boarding, never on the insulation between joists: the ceiling below will not hold your weight.
2. Measure the depth in several places, away from the hatch, where insulation is often thinner or disturbed.
3. Look for gaps, lumps and areas that have been pulled back for wiring or pipes.
4. Note whether it is squashed. Boards laid straight onto insulation compress it, and compressed insulation insulates less.
5. Check the eaves. Insulation should reach the edges of the loft without blocking the ventilation gaps at the eaves.
6. Look for damp: stained timbers, mould, or wet patches under the roof. The Energy Saving Trust says damp problems must be fixed before any insulation is added.

Your Energy Performance Certificate often records loft insulation depth, but it may be out of date, so trust your own measurement.

Is topping up an existing layer worth the money?

The first layer of insulation does most of the work. Each extra layer saves less than the one before, which is why going from nothing to 270 mm is well worth it and going from 120 mm to 270 mm barely moves the bill. The Energy Saving Trust expects correctly installed loft insulation to last around 40 years, so even a slow payback is eventually recovered, but £17 a year is not a reason to spend £600 on its own.

Smart Energie's typical range is from £450 for a standard loft top-up and £650 to £1,200 for a full re-lay, confirmed at survey. On the Energy Saving Trust's £17 a year, even the lower figure is a long payback, so if your existing layer is sound, a good survey should tell you to wait.

What should a good loft insulation job include?

• Two layers: one between the joists, and a second laid across them at right angles to cover the joists and reach full depth.
• Clear eaves ventilation. Approved Document L's examples require the condensation risk in the roof space to be assessed and provided for, in line with Part C of the Building Regulations and BS 5250.
• Pipes and the cold water tank insulated, because they are now in a colder space. Leave the area directly under the tank uninsulated so a little warmth rises to it.
• An insulated and draught-stripped loft hatch.
• Storage on raised platforms rather than boards pressed onto the insulation.
• Care around electrical cables and recessed downlights, which your installer should check before covering.

Installation of insulation in homes is zero-rated for VAT until 31 March 2027. The Energy Saving Trust suggests asking any installer about ventilation, guarantees and building regulations before hiring, and recommends a TrustMark-registered business.

Can you do it yourself?

Often, yes. The Energy Saving Trust says a straightforward loft is a reasonable DIY job if it is easy to reach, free of damp and not a flat roof, with regular joists that suit rolls of mineral wool. Wear gloves, long sleeves, eye protection and a dust mask.

Get a professional if the loft is a living space, the roof is flat, access is awkward, or there is any damp, condensation or structural concern. Insulating at the rafters to make a warm loft also needs professional design, because it changes how the roof space is ventilated.

What about spray foam under the roof?

Spray foam applied to the underside of the roof is sold as an alternative to loft insulation, but it is rare: the English Housing Survey counted about 231,000 homes with it, around 1% of homes with loft insulation. The Royal Institution of Chartered Surveyors warns that foam covering the roof timbers means they cannot be fully inspected, that incorrectly installed insulation can cause uncontrolled condensation, and that the presence of spray foam could affect a surveyor's valuation.

RICS advises getting independent advice from someone with no commercial interest before installing it. For most lofts with an accessible floor, mineral wool between and over the joists is the simpler, well-understood choice.

Common questions