I am remodeling a bathroom and will have an acrylic tub. Is it OK to foam(low expansion) the voids in the wall areas of the tub? It will be set on a mortar bed, with poly between the tub and mortar. I am thinking it would be good to insulate the tub wall cavity. I am talking specifically about the area between the inner and outer tub wall, the room walls will be insulated with batts.

Thanks