gillies-
Isn't that the opposite of what you want? Materials that absorb moisture from inside the house and hold on to them sounds bad to me. Something that can get wet, and then dry out is OK (old school lime plaster and pointing etc).
Was in attic of a mates tenement last week, found the mineral wool put on the ceiling above what is now their bathroom was properly soggy. Has a new drop ceiling from the inside, left old on in situe, no vapour barrier, no other insulation, just the 300-450mm of mineral wool in the attic. Even in summer its wet with the moisture from the bathroom/shower passing up. All the lathe + old plaster underneath had fresh mould and wet to the touch (don't own a humidity probe thing). Also found their extractor that was fitted by factor's contractor a few years before just vents into the attic (in a different area) and doesn't go outside at all.
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BrickMan
External wall insulation is where its at these days. Several 60's/70's ex council houses on our street now have it, if done well you can barely even tell its there, coupled with 50-100mm PIR on inside + correct use of vapour barriers + correct use of vents = a considerably easier to heat property. Side win, also keeps bugs out.