I’m struggling with a newly discovered cavity wall and am looking at different insulation options. My go-to for reasons of breathability, thermal properties and environmental properties is cork. I’m mulling it in granulated form but have struck on a bigger question.
Namely, where do people go for objective information on the appropriate materials for a task? Does AECB have something?
This topic is is too complex for a simple dichotomous chart. I think understanding of the building fabric and insulation types is required.
My initial thought is that if there is no chance of water ingress the cork should be fine, but not necessarily the best for that wall. Organic matter and periods of water ingress are not an acceptable combination.
Reasonable. My quandry is that having discovered the cavity, I’ve also found the exterior render is completely impermeable - sand and cement. I’m also tackling some damp on the inner wall. My thinking is I’ll stick a damp proof course on the interior leaf, and see if I can dangle some probes in to monitor moisture and work out if it is interstitial instead of rising damp.
My thinking with the cork was that because it is fairly resistant to moisture (eg, corks in wine bottles etc) and has such low thermal conductivity but comparably very high specific heat capacity, it might introduce enough thermal lag that the dew point is never reached in the cavity.
I’m unclear on whether this is sane, though - having to do some learning and fast to work out whether that sand and cement has to come off entirely or if I can get away with making some egress points that would be made good with something like lime plaster to allow for breathability.
The quest continues…! 
In case your brain isn’t frazzled yet here is some more to think about.
If you lie face down (or use a periscope), can you see the top of a cavity from your loft? Unless you use expanding foam you are unlikely to be able to really fill the cavity unless you can top up from the loft (or through the roof, removing the tiles or slates to do so). Fill right to the top or if the consistency of the insulation allows, a little proud of it. Then arrange the loft insulation to rest on the cavity wall insulation, overlapping a little if you can, to make contiguous insulation from the footings one side, over the roof and down to the footings again. Cork insulation will not be suitable to use between footings and DPC. If you suspect damp continue the non-organic insulation to 1 metre above DPC.