Is your loft insulation enough?

1The figure the regulations aim for

Approved Document L Volume 1 covers energy use in homes in England; the 2021 edition with its 2023 amendments has applied since 15 June 2022. Table 4.3 deals with existing roofs and gives two numbers: 0.35 W/(m²·K) as a threshold, and 0.16 as the improved standard. Table 4.2 sets 0.15 for a roof element that is newly built or replaced.

The U-value expresses how readily warmth leaks out through a building element, so a smaller number means a warmer house. It reflects the material, the thickness and the quality of fitting, which is why there is no single depth in millimetres that suits every loft. Mineral wool, rigid board and other products reach the target at different thicknesses, and the maker's data sheet states which.

Those numbers apply when work is carried out. Nobody is ordered to retrofit a loft they are not touching. They do show what a well-insulated roof looks like today, which is a fair yardstick for your own.

2When roof work brings insulation into the job

Paragraph 11.2 lists a rebuild down to the structure among the works that renovate a thermal element, and re-covering a flat roof is on the same list. Under paragraph 11.3 the whole element must be upgraded once more than 50% of its surface is renovated. A major renovation, meaning more than 25% of the external building envelope, has the same effect.

A full re-roof can therefore bring insulation with it, while mending a handful of tiles does not. Cases in between depend on the share of the roof the work touches, and your building control body, a registered building control approver or the Adur & Worthing team, has the say.

Mention insulation at the survey: where work is notifiable, the consent explanation and the Building Regulations notification are part of our job. If you are unsure how often the roof deserves a look before any of this arises, read how often to inspect a roof, and see our roof inspections and surveys overview for what a visit covers.

3What to look for when you go up the hatch

Take a torch and keep your weight on the joists or a board laid across them. A foot between joists goes through the ceiling below.

Judge the depth first. If the wool barely covers the tops of the joists, it is probably well short of today's target. Then judge coverage. Gaps round the hatch, pipes and cables, or along the eaves, let heat escape and let warm damp air reach the cold roof, where it can condense on timber and on the underside of the covering.

Check the eaves too. Insulation stuffed into them can shut the ventilation gap that keeps the roof space dry. Marley's account of BS 5250 gives, for high-resistance underlay, 25 mm of continuous opening at the eaves for pitches up to 15° and 10 mm for steeper ones. Whatever the figure for your roof, the gap has to stay open.

Last, look for damp: dark patches on timber, a musty smell, rust on nail tips. Those point to a moisture problem that extra wool will not cure.

4Where the VAT relief fits

HMRC's VAT Notice 708/6 sets a zero rate for fitting certain energy-saving materials between 1 May 2023 and 31 March 2027, after which the reduced rate of 5% returns from 1 April 2027. Insulating a roof or loft is on the list, and the relief covers homes and charitable buildings. Buying the materials without installation stays at the standard 20%.

Combined jobs are less simple. When insulation is fitted with other work, the question is whether it is one supply or several, and one supply takes a single rate. HMRC's own illustration is insulating a roof space during an extension build, which HMRC treats as one supply charged at the standard rate.

Roofing work is charged at that rate, and the insulation can be treated separately only in some arrangements of the job. Put the question to us and the quote will state the position. With any firm, read how VAT is treated before accepting.

5Foam sprayed under a leaking roof

Some firms propose curing a leaking roof by foaming the underside without tackling the leak. The Energy Saving Trust advises against it, noting that open-cell foam lowers the risk. The RICS consumer guide points out that poorly fitted insulation can cause damp through uncontrolled condensation, and that condensation checks need a qualified person.

It also advises checking with your own mortgage provider before anything is installed. As of November 2024, reports said some lenders declined homes with spray foam while the rest looked at each application individually.

Our order of work is to trace and close the entry point first, then deal with insulation and ventilation on a dry roof. A dated photograph of what is already in the loft, with a tape against the depth, makes that conversation useful. For what to gather see preparing for a survey visit and asking a roofer for photos, and who should survey if a valuation is involved.