DIY cavity insulation

[trebor1]

I have a gable end wall that I will gain access to, the cavity of, when I install a loft window. Whilst I have the cavity open, is there any insulator I can pour into the cavity (eps beads, vermiculite) to give me DIY cavity insulation.

[KARL]

There is unlikely to be any reason why you cannot fill the cavity (between two layers of brick, is it ?) with any material which neither absorbs or transfers moisture. Either of the ones you mention would be suitable plus one or two others. Insulation is of enormous benefit and the building trade here has been very slow to catch up with other developed countries and been very ready to issue warnings of dire consequences of its installation. If you have access to a good library, check up on a book or two, but you are not really likely to go far wrong. A full cavity does not need the draught that is so often suggested as a must (hangover from old ideas which worship the "bracing" cold.