Loose fill asbestos is raw fibre blown into a loft or a cavity with nothing binding it, and it is the most hazardous asbestos material found in ordinary homes. Disturbing it puts fibres straight into the air, which is why a loft containing it stops being a storage space and why removal is licensable work under a sealed enclosure. It is rare, and where it exists it matters more than everything else in the building put together.
What it looks like
A loose grey or blue grey fibrous layer between the joists, sometimes under later mineral wool, sometimes lying alone. It has no backing paper and no rolls: it was poured or blown. Pictures narrow the question and do not answer it, because cellulose insulation, which is shredded recycled paper, looks very similar in a dark loft, and mineral wool in an old loft can be grey with dust.
Cellulose, vermiculite and loose fill asbestos
Cellulose is shredded paper, usually grey, often with visible print fragments, and it is not asbestos. Vermiculite is a mineral that expands into light golden granules and is not asbestos itself, though some of the world supply carried contamination from the mine it came from, which is why vermiculite is sampled rather than assumed. Loose fill asbestos is fibre rather than granules or paper. None of those three is reliably told apart by eye.
What to do if you suspect it
Come out of the loft, close the hatch, and leave everything where it is. Do not sweep, do not vacuum, do not move stored boxes, and do not let a trade into the loft until it is identified. Ordinary vacuum cleaners pass fine fibres straight through. Get a sample taken by an accredited organisation, and if it is positive get quotes from HSE-licensed contractors.
Cavity wall insulation
Blown cavity wall insulation is normally mineral fibre or polystyrene bead and is not asbestos. Where a survey raises the question, the cavity is sampled through a borescope hole by a surveyor rather than opened up. It matters most before cavity work, because drilling and injecting into an unknown cavity is a disturbance nobody has assessed.
Questions people ask about loose fill asbestos insulation
Is all loft insulation dangerous?
No. Modern mineral wool and cellulose are not asbestos. The material to identify is loose grey fibre, and vermiculite is sampled because of possible contamination rather than because it is asbestos.
Can I insulate over the top?
Covering it is not a decision, it is a delay. Get it identified first, because insulating over friable material without recording it leaves a trap for whoever opens the loft next.
Is removal licensable?
Loose fill asbestos removal is licensable work: an HSE-licensed contractor, advance notification, a sealed enclosure and an independent four stage clearance.