A foam layer would still be good for charged particle radiation because assuming it's a polystyrene (or other oligameric hydrocarbon), it's a hydrogen rich material. You'd need a much thicker layer until the areal density approximated that of water to get an equivalent shielding layer.

Problem is (and I know the igloo is not without problems) it that you'd have to haul the orcanics with you or have a source from which to synthesize it on the moon. At least there IS a known (probably anyway) source of water on the moon. Location would be key though.

It's like homesteding by building a cabin with a sod roof as opposed to draging a trailer (with foam insulation) out to the plains. It assumes you have both a source of sod and a way to cut it but the point is you don't have to haul it with you.