Depth does the work
Insulation performance is roughly proportional to thickness, and the recommended depth has risen steadily over the decades. Lofts insulated to the standard of twenty years ago are typically at 100 mm; current guidance in most temperate climates is around 270–300 mm of mineral wool.
That is normally achieved in two layers rather than one. The first goes between the joists, snug but not compressed. The second goes across them at right angles, covering the joists themselves. That second layer matters more than it looks: timber joists conduct heat far better than the insulation around them, and a single layer between joists leaves a repeating cold bridge across the whole ceiling.
Compression destroys performance. Mineral wool works by trapping still air, and squashing it into a smaller space removes exactly the property you paid for. Never force a thicker batt into a shallower space.
| Location | Typical target | Method |
|---|---|---|
| Loft, cold roof | 270–300 mm | Two layers, second crossing the joists |
| Stud wall | Fill the stud depth | Friction fit, no compression |
| Suspended timber floor | 100–150 mm | Supported from below by netting |
| Flat roof | Design it properly | Condensation risk — get advice |
The gaps that undo the job
Insulation is unforgiving of small omissions. A gap covering 5% of the area does not cost you 5% of the performance — heat concentrates through the gap and the loss is disproportionately larger.
The usual offenders are the edges where the loft meets the eaves, the area around the hatch, and the boxed-in sections around pipework or a water tank. Each is fiddly and each is where people stop.
One place you should leave a gap: the eaves themselves need ventilation. Stuffing insulation hard into the eaves blocks the airflow that keeps the roof space dry, and the result is condensation and rot. Ventilation trays exist for exactly this.
Area is not the room footprint
The calculator works on the area you give it, and for a loft that should be the area between joists, not the floor area of the room below. If your joists are wide and closely spaced, the timber can account for 8–10% of the plan area.
For a stud wall the same applies. Measure the openings between studs and multiply, rather than taking the wall area, or you will over-order by roughly the fraction the timber occupies.
Common questions
How deep should loft insulation be?
Around 270–300 mm of mineral wool in most temperate climates, laid as two layers with the second crossing the joists.
Can I compress insulation to fit?
No. It works by trapping still air, and compressing it removes most of the performance you paid for.
Should I insulate right into the eaves?
No — leave a ventilation path. Blocking the eaves traps moisture and causes condensation and rot in the roof space.