How to insulate a loft yourself

First: is your loft a DIY loft?

The Energy Saving Trust (EST) says you can insulate a loft yourself only if it is:

• easy to access,
• free of damp problems, and
• not a flat roof.

It adds that if you want insulation at the sloping roof, use the loft as a living space, or have concerns about damp or structural issues, you should use a professional. If damp is present, the cause must be fixed first: insulation makes the loft colder, which makes condensation more likely and can worsen an existing problem.

What you need

• Mineral wool rolls: enough for a first layer between the joists and a second layer across them, to 270mm in total. The roll width should match your joist spacing for the first layer.
• A ruler or tape to measure the joist spacing, the existing depth and the loft floor area.
• A board or crawl board to kneel on, and a good light.
• The protective clothing, gloves and mask the product’s packaging recommends.
• A long-bladed knife or saw for cutting rolls.
• Pipe lagging and a tank jacket if there is a cold water tank or pipework up there.

The job, step by step

1. Clear and inspect. Move stored items out of the area you are insulating. Look for damp, staining, daylight through the roof, damaged timber and old or unsafe wiring. Stop if you find any of them.

2. Measure what is there. If there is existing insulation that is dry and clean, it can stay; the new layer goes on top. See loft insulation depth.

3. Lay the first layer between the joists. Unroll mineral wool into the gaps between the joists, pressing it gently into place without squashing it. If there is already insulation between the joists, skip to step four.

4. Lay the second layer across the joists. Run the second layer at right angles to the first, covering the tops of the joists, until the total reaches about 270mm. Butt the rolls together without gaps.

5. Keep the eaves clear. Stop the insulation short of the eaves so air can still enter at the edges of the roof. Blocking them traps moisture; see loft ventilation.

6. Deal with the cold spots. Lag pipes and insulate the top and sides of any cold water tank, but not underneath it, where warmth from below keeps it from freezing. Insulate and draught-strip the loft hatch.

7. Keep storage off it. Do not put boxes back on the insulation. If you need storage, build a raised platform; see loft boarding and insulation.

Mistakes to avoid

• Squashing the insulation. The EST warns that compressing mineral wool reduces its effectiveness.
• Blocking ventilation. Stuffing insulation into the eaves stops the air flow that keeps the roof dry.
• Covering a damp problem. Fix leaks and damp first, always.
• Covering recessed lights and cables carelessly. Leave clearance around light fittings that produce heat, as the manufacturer specifies, and do not bury cables you are unsure about.
• Spray foam as a shortcut. The EST describes foam sprayed under an old roof as one of the highest-risk options. Read spray foam problems first.

What it costs, and the VAT point

Before you buy anything

Check the free routes first. If your household is on a qualifying benefit or a modest income and the home is rated EPC D or below, a scheme may do the job for nothing: who qualifies for free loft insulation has a one-minute checker.