Quick question about loft insulation.

I currently have 4" of 'loose' fibre glass insulation in a 1930's semi. I
may add some extra joists at 90 degrees to add another 4" of insulation. As
I understand it, its the air that gets trapped in the insulation that
provides the insulating effect. Therefore, there's no point in buying the
thickest pack of insulation around and squash it into 4", rather get one
which is closest to the thickess I require. Should be cheaper (per m^2) and
provide similar insulation to a thicker one squashed to 4". Or am I wrong?

Thanks

D

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