Loft insulation is the single best-value energy upgrade most UK homes can make — cheap, quick, and it pays for itself in a couple of years. But “just roll some insulation up there” misses the details that separate a good job from a damp, compressed mess. Here’s how to insulate your loft properly, whether you’re DIYing or hiring.

How much insulation do you actually need?

Current UK guidance: 270mm total depth of mineral wool (or equivalent) in the loft. Many older homes have 100mm or less — some have none at all. If you can see your ceiling joists clearly above the insulation, you don’t have enough.

The standard approach for a top-up: if you have 100mm between the joists already, add a second 170mm layer laid crosswise over the top. The cross-laying matters — it covers the joists themselves, which are thermal bridges leaking heat.

Choosing your insulation material

• Mineral wool (glass or rock): the default. Cheap (£5–£8 per roll covering ~8m²), effective, DIY-friendly. Itchy — wear a mask, gloves, goggles and long sleeves. Non-negotiable.
• Sheep’s wool: natural, lovely to handle, no itch. Costs 3–4x mineral wool. Worth it if you’ll be up there often or hate the itch.
• Rigid boards (PIR): best insulation per mm, useful where depth is limited (e.g., insulating a loft room’s sloping ceilings). More expensive and fiddlier to fit.
• Loose-fill cellulose: blown in by professionals, great for awkward lofts with lots of obstructions. Not really DIY.

For most people, mineral wool is the right answer. Don’t overthink it.

How to insulate your loft properly: the DIY method

Before you start

• Board a walkway first. Lay crawl boards across the joists so you have safe footing. Never stand on the plasterboard between joists — you will go through the ceiling. I’ve seen it happen; it’s expensive.
• Clear the loft. Move stored items out or to one end. You’ll need clear access.
• Check for problems: look for roof leaks (stains on felt or timbers), wasp nests, and old wiring. Fix leaks before insulating — trapping moisture is worse than no insulation.
• Protective gear: dust mask (FFP3), goggles, gloves, long sleeves, knee pads. Lofts are filthy.
• Lighting: a head torch plus a portable work light. Loft lighting is always terrible.

Laying the insulation

1. First layer between joists: cut mineral wool to fit snugly between the joists. It should fill the gap fully without being compressed — compressed insulation loses effectiveness. Butt joints tightly; gaps leak heat.
2. Second layer across joists: roll the 170mm layer perpendicular to the joists, covering them completely. Again, no gaps, no compression.
3. Around the hatch: insulate the loft hatch itself — stick a piece of insulation to the back of the hatch door. An uninsulated hatch is a heat chimney.
4. Around pipes and tanks: insulate around (not over) water tanks — the loft warmth actually helps stop them freezing. Lag all pipes with foam pipe insulation.
5. Leave the eaves ventilated: this is the critical one. Do not block the eaves ventilation. Push insulation up to but not into the eaves — leave a 25–50mm air gap. Blocked eaves cause condensation, which rots roof timbers. Use eaves vent cards/trays if needed to maintain airflow.

What NOT to do

• Don’t squash insulation under loft boards — if you need storage, raise the boards on loft legs (£15–£25 for a pack) so insulation stays full-depth underneath.
• Don’t insulate over recessed downlights unless they’re fire-rated and rated for covering — fire risk. Use downlight covers.
• Don’t block ventilation at the eaves or gable vents.
• Don’t forget the pipework — frozen burst pipes cost thousands; foam lagging costs pennies.

DIY vs professional: costs

DIY for a typical 3-bed semi: £200–£400 in materials plus a weekend. Professional installation: £400–£700 (see our full breakdown of loft insulation costs). DIY is very doable if you’re reasonably handy and the loft is accessible — the job is more unpleasant than difficult. Hire a pro if the loft is very low, has awkward access, or you suspect damp or structural issues.

Grants and free insulation

Check the Great British Insulation Scheme and ECO4 — many households qualify for free or subsidised loft insulation, particularly if anyone receives benefits or the home has a low EPC rating. Your energy supplier can tell you what’s available. The GOV.UK energy grants page is the official starting point. Free insulation with professional installation beats DIY on every measure — check eligibility before buying rolls.

How it fits the bigger picture

Loft insulation is step one of a sensible efficiency plan, not the whole plan. Pair it with draught-proofing and the tips in our guide to reducing heating bills, and you’ll feel the difference from the first cold evening — a house that holds its warmth instead of leaking it into the sky.

Insulating a loft with a room in it

Standard loft insulation advice assumes an empty loft. If yours has been converted (or partially boarded for storage), the rules change:

• Converted loft rooms: insulation goes between and under the rafters (the sloping roof), not on the loft floor. This needs rigid PIR boards fitted between rafters with a ventilation gap above — a more specialist job, often £1,500–£3,000. Don’t just stuff mineral wool against the roof felt; you’ll cause condensation.
• Boarded storage areas: lift the boards, insulate properly underneath to 270mm, then re-board on raised loft legs so the insulation isn’t compressed. Compressing 270mm to 100mm under boards wipes out most of the benefit.
• Loft hatch: insulate the hatch door itself and fit draught seals around the frame. An uninsulated hatch in an insulated loft is like a thermos with no lid.

Loft insulation and damp: the warning signs

Done wrong, loft insulation causes condensation. Watch for:

• Water droplets on the underside of the roof felt — the classic sign of blocked ventilation.
• Mould on roof timbers — black spotting means persistent damp.
• Damp insulation — mineral wool that’s wet has lost most of its insulating value and needs replacing once the cause is fixed.
• Musty smells in the loft or rooms below.

All four usually trace back to blocked eaves ventilation or warm moist air leaking from the house into the loft (unsealed hatch, gaps around pipes and lights). Fix the ventilation and the air leaks, and the damp resolves. Insulation doesn’t cause damp — bad installation does.

The takeaway

Insulate to 270mm, lay the top layer crosswise, never block the eaves, board on raised legs if you need storage, and lag the pipes. Do it in autumn before the cold arrives, check for grants first, and wear the mask — your lungs will thank you. It’s a dirty weekend’s work for decades of lower bills.