How to Insulate a Loft

What you need

Insulation rolls or packs, a tape measure, a sharp knife, personal protective equipment (a dust mask, gloves and goggles), boards to kneel and walk on, and the insulation calculator. Check the coverage on the pack before you order: it is the number the calculator needs, and it varies by product and thickness.

Step 1 — Measure the loft floor

Measure the length and width of the loft floor, in sections if the loft is not one rectangle — add the sections together. The insulation calculator takes the length and width and gives you the area: a loft 8 m × 6 m is 48 m² (516.67 ft²). Measure the floor, not the roof: the insulation goes on the floor, between and over the joists.

Step 2 — Work out the packs

Divide the area by the coverage per pack and round up: 48 m² at 5 m² per pack is ceil(48 ÷ 5) = 10 packs. Always round up — a pack short means a cold patch, and a cold patch is a damp patch. Buy one spare pack: insulation is cheap, and a return trip is not.

Step 3 — Prepare the loft

Clear the loft, check the joists, and lay boards to walk on — never stand on the plasterboard ceiling below, which will not hold your weight. Check for wiring and downlights before you lay anything, and find the eaves vents: they must stay clear, because a loft needs air moving through it.

Step 4 — Lay the insulation

Lay the first layer between the joists, then a second layer across the top, at right angles. Do not compress the insulation — compressed insulation insulates less, because it works by trapping air. Keep it clear of the eaves vents so air can still flow, and leave the loft hatch area clear so you can get in and out.

Step 5 — Mind the electrics

Never bury electrical cables in insulation: cables need air to stay cool, and a buried cable overheats. Keep insulation clear of downlights, or use fire-rated covers made for the purpose — recessed lights generate heat, and insulation over them is a fire risk. If you are unsure about any wiring or light fitting, stop and ask a qualified electrician before you lay another roll.

Worked example

A loft floor 8 m × 6 m is 48 m² (516.67 ft²). At 5 m² of coverage per pack, that is ceil(48 ÷ 5) = 10 packs. Those are exactly the numbers the insulation calculator gives you. For the heating saving, the room heating calculator estimates the heating a room needs — insulation lowers that figure, which is the point of the job.

Common mistakes

• Blocking the eaves ventilation. A loft with no airflow grows condensation, and condensation grows rot and mould. Keep the vents clear.
• Compressing the insulation. Squashed insulation traps less air and insulates less. Lay it loose and full thickness.
• Covering downlights or burying cables. Heat builds up where air cannot reach it. Keep insulation clear of lights and wiring, or use proper covers.

Safety

Wear a dust mask, gloves and goggles: insulation fibres irritate skin, eyes and lungs, and loft dust is worse. Work in pairs and keep the loft hatch clear — a fall through a ceiling is a serious injury, so walk only on boards laid across the joists. Keep the eaves vents clear for ventilation, and never cover downlights or bury cables in insulation: recessed lights can overheat and cause fires. If the loft has old wiring, vermiculite insulation or anything that looks like asbestos, stop and get professional advice before you touch it.

Try these calculators

• Insulation Calculator — packs for any area and coverage.
• Room Heating Calculator — heating needed for a room.
• Square Footage Calculator — area for rectangles, circles and known areas.