Windfall, I feel your pain……
I found this today in one of my double decker foam nucs. That was a 5 frame deep + 5 frame medium nuc with 2” thick foam walls, bottom entrance, and small top vent. Weighed the dead bees; that’s 2 lbs of bees. No signs of brood and the bees were still mostly in the bottom box. The cluster was spanning the gap between the deeps and medium frames. I got out my magnifying glass to check for varroa; none seen. Bees all looked healthy, except for being dead.
It looks to me like they got stuck against the front wall of the nuc and they hadn’t packed the sides of the frames full with honey. My diagnosis is they starved. I’m a little surprised this many bees in a nuc got too cold to move to more stores (There were lots of stores to their sides and above them.) 2lbs of bees should be able to generate at least 5 watts of heat and in this super insulated nuc, it really shouldn’t have gotten colder than 32F/0C inside the nuc. We had a few days of bitter cold weather that seemed to do these bees in. It got down to 0F for a few nights.
As noted before; moisture has been a concern in my foam nucs. The dead out had a fair amount of mold (white mold) growing on the sides away from the bees. I’ll copy some photos to my photobucket page later for those wanting to see a bunch of dead bees.
My other nucs and hives looked fine today.