Why the loft is the easiest place to start

If your home is like most in the UK, a good chunk of the heat you pay for is quietly escaping through the roof. In an uninsulated house, roughly a quarter of heat loss goes that way. The good news is that the loft is usually the simplest, cheapest and least disruptive place to make a real difference — no plastering, no scaffolding, no waiting in for a builder.

The current recommendation for mineral wool in a UK loft is around 270mm of depth. Plenty of homes still have 100mm or less, sometimes just a thin layer laid decades ago and flattened by years of storage boxes. Topping it up is one of the best-value jobs you can do, and it is very much within reach of a competent DIYer with a free weekend.

What you will need before you climb up

Loft insulation is messy rather than difficult. Take the safety kit seriously, because mineral wool fibres irritate skin, eyes and airways.

• Protection: a dust mask (FFP3 is worth the extra), safety goggles, gloves, long sleeves, tucked-in trousers and knee pads.
• Tools: a sharp retractable knife, a long tape measure, a straight timber offcut as a cutting guide, a torch or headtorch, and a broom.
• Access: a board or two to kneel and crawl on — joists alone are not a safe surface.
• Budget: a 100mm mineral wool roll covering roughly 8 square metres typically costs between £25 and £40, so a typical semi-detached loft can often be topped up for well under £200.

Have a look at the joists first. Most run at either 400mm or 600mm centres, which tells you which roll width to buy and helps you work out how many you need.

Materials that cost little but work hard

• Mineral wool rolls: glass or rock wool remains the cheapest option per square metre, is non-combustible and cuts easily with a knife.
• Recycled options: insulation made from recycled plastic bottles or glass performs well and is often comparable in price.
• Sheep's wool or hemp: pricier, but breathable and pleasant to handle, which suits older, moisture-prone roofs.
• Rigid PIR boards: higher cost, but excellent where headroom is tight because they deliver more insulation per millimetre.
• Reflective multifoil: thin and useful over rafters or in awkward spots, though it should not be your only layer.
• Loose-fill granules: handy for filling the odd gap around pipes and irregular edges.

If you already have some insulation in good condition, do not rip it out. Simply lay a second layer at right angles to the joists — known as cross-laying — which covers the timber bridging and gives a much better result than a single thick layer.

Doing the job properly

Clear the loft floor first and resist the temptation to squash the existing insulation. Compressed fibre loses much of its insulating power, so fluff it back up as you go.

• Lay the first layer between the joists, cutting rolls slightly wider than the gap so they sit snugly with no gaps at the edges.
• Cross-lay the second layer over the top of the joists, aiming for a total depth of around 270mm.
• Keep the eaves clear. Air must still flow in at the edge of the roof, so leave a gap of about 50mm and never block ventilation openings.
• Insulate the loft hatch and draught-proof its edges — an uninsulated hatch is a surprising weak point.
• Lag any exposed pipes and fit a jacket over a cold water tank, but leave the base of the tank uncovered in a cold loft so warmth from below can reach it.

Safety and the mistakes people make

A few common errors undo a lot of good work. Never bury electrical cables under insulation — they can overheat, so run them above the fibre or move them out of the way. Keep insulation well clear of recessed downlights unless they are rated for contact and fitted with protective hoods. Around chimneys and flues, maintain the clearance the manufacturer specifies and seek advice from a registered installer if you are unsure.

Insulating a loft makes it colder up there, which can encourage condensation. Check that ventilation at the eaves is clear and that any air bricks or vents are not covered. Finally, if you want to board the loft for storage, use raised loft legs rather than laying boards directly on the insulation — you will keep the depth and a usable floor at the same time.

Getting help with the cost

Insulation is one of the few home improvements where help is genuinely available. Government-backed energy efficiency schemes, delivered through energy suppliers, offer free or heavily subsidised loft insulation to households on certain benefits or with low energy performance ratings. Eligibility changes, so it is worth a quick check on the gov.uk website or a call to your local council's energy advice team before you buy anything.

Even without a grant, VAT is zero-rated on energy-saving materials installed by a trader, which can make a professional job more affordable than you might expect — particularly for awkward lofts, very old roofs or hard-to-reach areas. For a straightforward loft, though, DIY usually pays for itself within a couple of heating seasons and keeps paying you back every winter after that.