In response to Ribac's note on maintaining an observation hive:
 
     I keep a 2 frame hive going all year here in south-central Pa. on a
sun porch that gets as low a 45 degrees F. at night.  As it never
freezes, the situation is a bit different that that in an unheated
building.  Even at 45 I use styrafoam covers over the glass to preserve
heat.  I find that they don't readily take feed during the cold weather,
so I like to have a full frame of honey going into the Winter - and I
have to force them to take that in the Fall because they would rather
raise brood up until Nov.   I keep a top feeder on all of the time from
Sept until the frame is full.  With our warm weather this year I am
concerned at the usage.
 
     I have not tried it, but have you considered heat tapes of the type
used for plant starter boxes.  I believe they have a thermostat pre-set
at about 70 degrees and if that could be fitted under styrofoam covers it
should keep the bees "toasty warm" as long as the covers are on.
 
Jerry in Pa.