Loft Insulation Calculator

How This Is Calculated

The extra depth needed is simply your target depth minus what's already there — if that comes out at zero or below, the loft already meets the target and no top-up is needed.

Packs needed is the loft area divided by the coverage stated for your chosen product, rounded up to a whole pack. If you're starting from bare joists rather than topping up existing insulation, that figure is doubled, reflecting the standard practice of laying two crossed layers to cover the joists themselves and avoid a cold bridge running the length of each one.

Common Questions

Why 270mm?

270mm of mineral wool (or the equivalent in another material) is the depth long recommended by the Energy Saving Trust and used as the reference point across the UK insulation industry for existing homes. It reflects a genuine point of diminishing returns - going deeper keeps adding cost for progressively smaller heat-loss savings, so 270mm is generally where value for money levels off rather than a hard legal minimum for existing lofts.

Do I need to remove old insulation before topping up?

No, as long as it's dry, undamaged and not compressed flat. Simply lay the new insulation on top, up to the combined target depth. If the old insulation is wet, mouldy or has been squashed by stored boxes, it should be replaced rather than built on.

Why would I need two layers instead of one?

If you're insulating bare joists with no existing insulation, the standard approach is to lay one layer between the joists and a second layer directly over the top, running at right angles to the first. This covers the joists themselves, which would otherwise be a cold bridge letting heat escape. If you're just topping up existing insulation, a single extra layer on top is normally enough.

Do I need to leave a gap at the eaves?

Yes. Insulation must not block the ventilation gap at the eaves where the roof meets the walls, since that airflow keeps the roof structure dry and prevents condensation building up in the loft. Proprietary eaves vents or simply holding insulation back from the eaves by the recommended gap (usually at least 50mm) keeps that airflow clear.