Insulating Baltic Pine Ceiling

I've just spent a couple of days up amongst the spiders, removing about 60kg of dirt from the ceiling cavity (120 years of fine, but very heavy, windblown dust). And that's just from the two front rooms.

This was to allow me to properly insulate the ceiling, but there are some cracks and gaps in the original baltic pine boards. I will not be removing them or plastering up under them. I'm worried about glass fibres migrating down through the gaps. I think I need to place a layer of something on top of the ceiling but below the insulation (assume that will be fibreglass, or similar, batts). What can I use? I have some spare building foil but maybe this layer needs to be able to breathe?


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