Insulating a loft, cold or converted

The steps, in order

1. Read the loft before you carry anything up

   Cold loft or convertible, cut roof or trussed, headroom, the state of the ceiling joists, whatever insulation is already there and what condition it is in. That survey decides the method, and it is done with a lamp and with boards laid across the joists so you do not go through the ceiling.

   Tools: work lamp, tape measure, crawl boards

2. Fix what hides under the insulation

   A slipped tile letting water in, a tired junction box, an undersized cable or beetle in the timber gets dealt with now. Under three hundred millimetres of wool those faults are invisible and expensive to find again. A pass with the workshop vacuum also stops you blowing insulation onto a carpet of dust.

   Tools: work lamp, wet and dry vacuum

3. Protect the flue, the downlights and the eaves

   A rigid non-combustible casing keeps insulation off the flue at the clearance the manufacturer and Approved Document J require. Recessed downlights get fire-rated loft caps unless the fitting is stated as safe to cover. At the eaves, ventilation trays keep the air path open: a roof that stops breathing starts condensing.

   Tools: drill driver, utility knife, stapler, tin snips

4. Fit the vapour control layer on the warm side

   The vapour control layer goes on the warm side, the room side, never against the roof covering. It has to be continuous: laps sealed with the membrane maker's tape, careful returns at the walls, collars around every penetration. A layer punctured everywhere does nothing at all, continuity is the whole point.

   Tools: stapler, vapour control tape, utility knife

5. Blow it in, for a cold loft

   Depth pegs go in before the pour so the thickness can be checked as the work moves along. The blowing machine, usually lent by the merchant with the bags, throws loose fill from the far end back towards the hatch. Spread it evenly and never press it down: the quoted performance assumes an uncompressed depth. The loft insulation calculator gives the number of bags for your area, target depth and the bag you buy.

   Tools: blowing machine, depth pegs, FFP3 mask, coveralls

6. Insulate the slope, for a room in the roof

   Two crossed layers beat one thick layer: the first between the rafters, the second under the frame, with joints staggered. The wool has to fill without being squashed, and the ventilation gap under the roof covering stays clear the whole length of the slope, from eaves to ridge. The same calculator counts layers and rolls from the area printed on the label.

   Tools: insulation knife, tape measure, resilient hangers, drill driver

7. Line it, insulate the hatch, then check

   Metal frame, plasterboard, tape and joint: the lining goes up like a stud wall. The hatch gets its own insulation and a draught seal, or it undoes part of the work on its own. If you want storage, fit raised loft legs and board over the insulation, never straight onto it.

   Tools: drill driver, tin snips, jointing knives, circular saw

Consumables

• Insulation: loose fill for blowing, rolls or batts for a sloping ceiling, at the thickness that reaches the target performance.
• Vapour control membrane and the tape its maker specifies.
• Eaves ventilation trays to keep the air path open.
• Fire-rated loft caps for downlights, unless the fitting says it can be covered.
• Non-combustible casing around the flue.
• Depth pegs to check blown thickness.
• Hangers, channel and plasterboard for lining a slope.
• Insulation and a draught seal for the hatch.
• Raised loft legs and boards if part of the loft is for storage.

Safety on this job

• Airborne fibres: FFP3 mask, goggles, gloves and a hooded coverall, in a space that barely ventilates.
• Falling through the ceiling: walk on the joists or on boards laid across them, never on the ceiling itself and never on the wool.
• Heat: a loft gets brutal in summer, so work early and keep drinking.
• Electricity: isolate at the consumer unit before touching a junction box or a downlight, and prove it dead.
• Flue: no insulation in contact with it, and the manufacturer's clearance kept without exception.
• Unidentified old material: leave it alone and get it surveyed.
• Cold water tank: insulate the tank and its pipes, and leave the insulation out from under it so warmth from below keeps it from freezing.