You are correct, the air space must be well sealed (I caulked mine). It's about 1/2" deep. Be wary of fiberglass, unless it uses an acrylic binder. Off gassing of the "pink" stuff can be fatal to bees (the binder is formaldehyde based). Fiberglass is also prone to mold growth. Your best bet is rigid foam, foil bubble duct wrap, thick wood or plain old air.
We usually have winter here, not much this year. I'm certainly not in the FAR North, but I've had bees a lot of places and this can be pretty cold here. Usually a couple of weeks of -10 or -20 F. The Top Bar Hives have wintered just fine. No better and no worse than my horizontal Langstroth hives nor my vertical Langstroth hives.
Here is a link to the necroscopy:
As you can see there was plenty of honey. Of 20 bees I examined only 4 had visible mites. The pictures progress from the rear of the hive forward, starting with the gap behind the follower board. I wasn't able to locate the queen, she's probably in one of those moldy clusters. It was getting late in the day so the lighting is a bit funny. I closed thehive back up and I'll try and get out for a more thorough examination this weekend.
Just my two bits on this great discussion. I started with three top bar hives. Located just at the border of Ohio and Michigan. We have lots of wind here on the prairie, but not the extremes that Dennis gets in Wyoming. the great lakes do temper it that way. This year January was pretty warm, then back to lows of 0 in Feb.
Anyway, I started with 3 top bar hives. Only one made it through the first winter. It made it through this past winter as well.
I did do a split last summer (in July) and harvested a little honey off it.
Anyway, to get to the point, I think the top bar hive can winter very well IF you do two things. First make it big, as Dennis suggests. I believe he has hit on the optimum size for colder climates. When you first start it, make a feeder and use it as a follower board in the middle of the hive so they will have a good guide to make their comb. Move the feeder back every few weeks as they build out the comb.
Second, do as Kris suggests and insulate the top. The cupola would not help IMO. THere is a good article in this month's AM. Bee Journal that shows how to insulate the top of a Lang. The same procedure can be used for the TBH.
I do think there are fewer mites with the Top Bar. The split I took off the TBH last year overwintered very very well and is my strongest Lang this year.
I plan to try to make 2 splits off my surviving TBH this summer, probably in June and July.
I do like the TBH, it has taught me more about the hive and the bees than the Lang. And I think it does have a place in low impact low cost beekeeping. I hope to be able to always have a top bar hive around. It is extremely low maintenance too.
I looked at your photos and the bees did starve. But not because of lack of feed. Their cluster was just too small to generate enough heat to utilize it. They were doomed before their wintering problems.
T mite can cause this problem. Most of the bees will die off before the hard part of winter comes. The small surviving cluster will rapidly move up and get trapped at the top bar. The hive will be relatively free of dead bees. Most of them will be outside the hive, in a pile, by the entrance. That is, unless some critter comes along and eats them.
Another possibility is a very late season queen failure. The results would look and behave the same, except for the pile of dead bees.
It's always hard to loose a hive. But try again. Some animals and colonies just don't make it.
Thanks for the input, I agree the clusters were very small. There weren't many dead bees in the bottom of the hive, I'll try and get photos of it. I am getting another NUC for it, Do you think I should reuse the comb that's already there? I don't want to spread any diseases.
Unless you see signs of AFB I would reuse the comb.
If you went with a package or shook bees into your tbh last season, there would only be a very remote possibility that foulbrood killed them.
The combs should be safe to use.
the hive was started from a nuc (reused comb). funny you should mention foulbrood. while i didn't see any capped brood, or dead larvae per see ou can see in one of the photos that there is a small patch of brrod comb with about 10 or so dead bees in them, all facing "butt-out". i'll take some better photos this weekend. like i said, this was my first hive so i'm not very experienced with all the diseases out there. if it's afb should i try and treat? could it spread to a second hive if placed near it (is so how far away should another hive be)?
Bees butt out are not in any way a sign of AFB. That's generally a sign of starvation.
Look through this: