How to Install Loft Insulation — Step-by-Step UK Guide

What You'll Need

Before You Start

• Check for any electrical cables running across the joists — they must sit on top of the insulation, not under it, to prevent overheating.
• Inspect for damp, leaks or signs of condensation. Fix any roof leaks before insulating.
• If your home was built before 2000 and has loose vermiculite insulation, it may contain asbestos — have it tested before disturbing it.
• Ensure you have safe access — use a proper loft ladder, not a stepladder. Walk only on joists or crawl boards, never on the ceiling plasterboard.
• Use our Loft Insulation Calculator to work out how many rolls you need, and our Insulation Thickness Calculator to check the required depth for Part L compliance.

Step-by-Step Instructions

Step 1: Clear the Loft

Remove stored items and any old, damaged insulation. If existing insulation is in good condition, leave it in place — you will lay on top of it. Sweep debris from the joist tops so the new insulation sits flat.

Step 2: Fit Eaves Ventilation Trays

Push ventilation trays (baffles) into the eaves at each rafter space. These hold the insulation back from the soffit vents, maintaining the airflow path that prevents condensation. Without these, insulation blocks the eaves and causes damp problems.

Step 3: Lay the First Layer Between Joists

Unroll 100mm mineral wool between the joists, pressing it down so it sits snugly against the ceiling plasterboard below. Cut to length with a knife. Do not compress the insulation — it works by trapping air, so squashing it reduces its effectiveness. Butt the ends of rolls tightly together with no gaps.

Step 4: Lay the Second Layer Across the Joists

Unroll 170mm mineral wool at right angles across the top of the joists. This eliminates cold bridges where heat escapes through the timber joists. Again, do not compress — let it sit at its natural loft.

Step 5: Insulate Around Pipes and Tanks

Lag all water pipes in the loft with pipe insulation — the loft is now much colder, so pipes are at greater risk of freezing. Fit a water tank jacket around any cold water storage tanks. Do not insulate under the tank — allow heat from below to help prevent freezing.

Step 6: Insulate the Loft Hatch

Glue a piece of insulation board (50-75mm PIR board such as Celotex or Kingspan) to the top of the loft hatch. Fit draught-strip tape around the hatch frame. The hatch is a major source of heat loss if left uninsulated.

Step 7: Handle Electrical Cables

Electrical cables must sit on top of the insulation to prevent overheating. Clip cables to the side of joists above insulation level, or drape them across the top of the insulation. Never bury cables under insulation.

Step 8: Board Over (Optional)

If you need loft storage, use loft legs (raised supports) to create a raised platform above the insulation. Fix loft boards to the legs. Do not compress the insulation by boarding directly onto the joists — this halves the insulation depth and dramatically reduces its effectiveness.

Common Mistakes to Avoid

• Blocking eaves ventilation: Pushing insulation into the eaves blocks airflow and causes condensation. Always use ventilation trays.
• Compressing insulation: Squashed insulation performs poorly. Use loft legs if you need to board over.
• Forgetting pipe lagging: Once insulated, the loft is much colder. Unlagged pipes will freeze in winter.
• Burying electrical cables: Cables under insulation overheat. Always lay cables on top.
• Insulating under the water tank: Leave the area directly under a cold water tank uninsulated — it needs warmth from below to prevent freezing.
• Not insulating the hatch: An uninsulated loft hatch loses as much heat as a missing window. Insulate it and fit draught strip.

Cost Estimate (2026 UK Prices)

Use our Loft Insulation Calculator and Insulation Thickness Calculator for exact quantities for your project.