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Texture is like those little white balls of polystyrene that packing is made from and it's easily compressed.
There's drifts of it all over the floor of the attic.
What is it? Is it 'safe'?
House is 15th century so all old materials that need to breathe - assume this stuff is ok with that?
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That looks like the stuff that's retrospectively "blown" into old cavity walls to me.
Looks like a form of vermiculite insulation
Yes, does look like vermiculite.
So, we've no idea how long it's been there.
Just add further insulation over the top (it's mostly only level with the top of the rafters) or get it tested and removed if it has asbestos in?
It's either popcorn or Sugar puffs.
I'd need to taste it to be sure.
Most likely vermiculite.
For the price of a test, ( about £30-ish) you should get it sampled for peace of mind.
I'd remove it and replace it with something a bit more insulationey.