I walked that stretch in December last year and some of the albergues were very cold. Ave Fenix and the municipal in Villafranca were not heated (very much), the municipal in Vega de valcarce had a heater which we put into one room to at least be warm there (it also had an open air kitchen, which was coooold! to cook in). All the municipal albergues in Galicia are warm, if stark, in my opinion. I would probably consider getting a warmer sleeping bag, rated to -5 degrees or so.
Also walking the Camino in January this year. I am starting from SJPDP on the 14th Jan 2011. I know i will be know where near Ponferrada by the 20th but thought I'd just say hi and wish you all the best for your walk and let you know there will be fellow pilgrims walking with you in spirit.
I have walked the Camino Frances in January for the last four years and it is fine. Not everywhere is open and there are not so many Pilgrims about however that also has its own benefits. This year I start in Pamplona on the 7th Jan and have just finished packing my backpack.
In the summer I now walk the Via de la Plata ( and leave the Frances to the tourists at that time of the year- apologies for the harshness of this comment)