Of course, I say, "Why couldn't they have been both?" http://io9.com/5977029/archaeologists-mistake-viking-brewhouses-for-bathhouses
Archaeologists can be very obtuse people, a while back the had this heated debate about the function of stone slabs found in front of hearths on the orkney's, many wild and somewhat elaborate theorys were postulated. A local was near during one of them and pointed out it is still in use today in many homes, its to dry your boots by the fire.
At least in these latitudes sauna was only place where necessary temperatures could be achieved needed to make decent beer. And making beer in traditional way is not exactly rocket science. So when the Vikings "migrated" southern I think they would have used old proven methods there as well.
