How do I board a loft without squashing the insulation?

This is the single most common way a well-insulated loft is quietly undone. The insulation goes in, someone wants storage, and boards go straight onto the joists — which compresses the insulation under the entire boarded area to the depth of the joist. Compressed mineral wool loses much of its value, so the boarded area becomes a large cold patch in the ceiling.

Safety

Step by step

1. Measure the joist depth and the insulation depth.If the insulation is deeper than the joists — and with modern depths it usually is — boards on the joists will compress it. That measurement decides the whole approach.
2. Think about weight before anything else.Ceiling joists in many houses are sized to hold up a ceiling. Boarding invites storage, and storage adds load. Light, occasional storage is one thing; a loft full of boxes is another, and worth getting advice on.
3. Use raised legs or cross-battens.Purpose-made plastic legs or timber battens run across the joists lift the deck above the insulation. Either keeps the full depth of insulation uncompressed underneath.
4. Leave a gap between the insulation and the boards.Insulation pressed hard against the underside of the deck can trap moisture. A small ventilated gap is part of why raised systems work.
5. Do not board over cables buried in insulation.Cables covered by insulation can carry less current safely, and boarding over them makes them inaccessible. Keep cables visible and clipped above the insulation where possible, and get an electrician's view if in doubt.
6. Keep clear of downlights and any flue.Recessed lights need their clearances maintained and must not be enclosed. Any flue passing through the loft has a clearance requirement of its own. Board around them, not over them.
7. Board only what you need.A defined boarded area near the hatch, with crawl boards to reach the rest, is usually more useful and much less load than boarding wall to wall.
8. Keep the eaves clear.Boarding right out to the edge encourages pushing insulation and stored items into the eaves, which blocks the ventilation path.
9. Work safely while you do it.Crawl boards across joists, good light, and never step between joists. Most loft injuries are a foot through a ceiling.

Common mistakes

• Laying boards straight onto the joists over deep insulation. — It compresses the insulation across the whole boarded area, which loses much of its performance and creates a large cold zone in the ceiling below.
• Treating a boarded loft as a room. — The joists, the access and the fire separation are all designed for a loft, not a room. Loading it accordingly is where problems start.

Questions people ask