Setup the Penthouse

[TheHoneyPump]

What would be your standard, preferred and proven, boards arrangement for wintering a small colony above a bigger one?

Small colony is in one 10 frame deep box and occupying 6 frames.  New August queen. 2F mating nuc colony that is steadily growing.

Bigger colony is in two 10 frame deep box and occupying full. New June queen. Wall2wall bees in both deeps.

I am bouncing between boosting the small from extra resources in the rest of the apiary or just putting it on top of the strongest hive and see what happens.  I have not done the penthouse wintering before.  Apparently it works well.  Am looking for inputs as to how best to do it.

More specifically:
- screen board between them or solid but very thin board
- what about ventilation, so the upper is not always wet from moist stale air coming from below.
- how to achieve upper and lower entrances on both

[Acebird]

I have never done it but some do it by double screening between the hives.

[Michael Bush]

Whatever you do, do NOT put a double screen between them.  Put a sheet of 1/8" luan between them.  The moisture from the lower hive will kill the upper one if any moist air is allowed to go up.  The luan will make a barrier to the air (convection) while allowing heat to get through by conduction.  The screen might work in Alabama.  It does not work in cold climates.  Even a screened hole in an inner cover between them will kill the top colony.  Moisture is the problem.

[TheHoneyPump]

Perfect.  Thank you MB.  That is the guiding insights I was needing. .

[beepro]

This is a very interesting topic.   This winter I'm putting 2 late season mated queens in the
same hive.  They were separated by a QE before and now the QE removed.   The last time I check about 2
weeks ago I still saw the 2 queens, one in the upper level and the other at the bottom.   These are strong sister
queens that I put them together.    I'm not sure if they will keep both or just one strong queen.    For the breeder queens I'm
using a QE on. 

[Michael Bush]

>I'm not sure if they will keep both or just one strong queen.   

In my experience there only be one in the spring.

>For the breeder queens I'm using a QE on.

In my experience if you leave the queen excluder on during winter one of the queens will get left behind when the bees move up and she will die from cold.  If you have only one queen below the excluder, she will get left behind to die in the cold.  But I'm not in California...

[TheHoneyPump]

>I'm not sure if they will keep both or just one strong queen.   

In my experience there only be one in the spring.

>For the breeder queens I'm using a QE on.

In my experience if you leave the queen excluder on during winter one of the queens will get left behind when the bees move up and she will die from cold.  If you have only one queen below the excluder, she will get left behind to die in the cold.  But I'm not in California...

I agree with Michael Bush on this.  Seen it.

[Beepah]

Whatever you do, do NOT put a double screen between them.  Put a sheet of 1/8" luan between them.  The moisture from the lower hive will kill the upper one if any moist air is allowed to go up.  The luan will make a barrier to the air (convection) while allowing heat to get through by conduction.  The screen might work in Alabama.  It does not work in cold climates.  Even a screened hole in an inner cover between them will kill the top colony.  Moisture is the problem.

How are upper and lower entrances handled?  My first thought is to add an upper entrance to the bottom booming hive (in case of snow cover and for moist air outlet). Would the heat loss at the lower-hive upper-entrance be great enough that stacking would no longer be useful?  I'm also thinking just an upper entrance on the penthouse hive, or do they need a small lower entrance as well for air-flow/moisture-control?

I think I'm going to be in the same situation, a booming colony and a weak colony, and might give the stacking approach a go.

Thanks for your help,
Kevin
-first year

[TheHoneyPump]

That was part of my original question; how to setup the board(s) properly.  We've now got as far as understanding that the board must be solid.  Thin for heat to emanate through it, but solid nonetheless.  Next, we need to understand how to achieve entrances and proper ventilation for both colonies.

To some degree, particularly around insulation, will depend on your climate and what you need to pull them through for winter conditions.  Here I've got a few days of mid winter lows as deep as -45 degC.  The average of the rest of winter is -18 degC to -25 degC.

At these temperatures the bees huddle in really tight cluster.  They will not do much if any movement by way of fanning.  Ventilation of stale moist bee breath air in these conditions is mostly passive.  Therefore a fresh air bottom entrance and a upper vent/entrance are must haves for both colonies.

What I propose to do is:
... the main penthouse divider board will be a 3/4" thick shim that has a 1/8" thick ply sheet centred in it, in a dado groove.  Much like a standard inner cover board but no hole in it.  That gives a 3/8" gap above the top bars of frames below and 3/8" gap below the bottom of the frames above.  A 3/4" wide notch will be cut into on the bottom side of the rim at one end, this is the upper vent and entrance for the bottom colony.  A 3/4" wide notch will be cut into the top side of the rim at the other end, this will be the bottom entrance for the top colony.  On top of the upper colony will be a standard inner cover board with the notch down and a thin ply covering the feed hole, the notch is the upper vent and entrance for the top colony.  On top of the inner cover will be 5" of foam insulation, and the lid will go on top of that.  The sides are also insulated, 2" foam.  I do not use any absorbent type box or pillow or fibreboard or such on top.  Such things only turn to solid blocks of ice in my climate, not helpful. 

The bottom colony will have its lower and upper entrances on one side of the hive, the upper colony will have its lower and upper entrances facing the other side of the hive stack. I am not fond of having the entrances opposite because that makes it difficult to have wind protection of all of the entrances.  Dominant winds here are from the West and some from North, so I setup my hives with entrances facing mainly east with a slight tilt to the south, so the cold winter wind is not buffing in the entrances.  That will not be possible with opposing top and bottom colony entrances.  That is the hurdle I have to figure out to solve.  One set of entrances will be well protected from wind ingress, the other will be fully exposed.  There is the conundrum with this setup.  (scratching bald head some more).   I've been thinking about having only a top entrance for the penthouse colony, but the problem arises with that need for some passive ventilation around very tightly clustered bees mid winter.

With regards to:   "Would the heat loss at the lower-hive upper-entrance be great enough that stacking would no longer be useful?"      What I have observed, on my standard setups with the upper entrance, is that on even the coldest days there is a lineup of bees just inside that upper entrance.  It seems like they line up their bodies to block and slow the outflow of air through it, to keep air and heat in.  They do not get cold because they are in a constant warm air flow stream.   I would expect the same to happen in a stacked hive with a penthouse colony.  Meaning my opinion is that they will lose limited heat through that upper entrance, so the plan does indeed maintain merit and useful for helping the penthouse colony stay warmer with less effort.

[Acebird]

With regards to:   "Would the heat loss at the lower-hive upper-entrance be great enough that stacking would no longer be useful?"      What I have observed, on my standard setups with the upper entrance, is that on even the coldest days there is a lineup of bees just inside that upper entrance.  It seems like they line up their bodies to block and slow the outflow of air through it, to keep air and heat in.

There still will be some heat loss from the lower colony going into the upper colony.
The plugging of the bees at the top entrance could also be for keeping the moisture in not just heat loss.

[blackforest beekeeper]

concerning upper entrances...
...nobody ever thinks about that in Germany. Nor in Europe, I assume. Never heard of it except from Northern America.
So far nobody seems to have needed this entrance on this continent.
Though on very old hives there were sometimes, so bees could directly enter the honey supers. NOT for winter.

[TheHoneyPump]

.... likely because of some significantly different winter conditions and how the hive is prepared to endure those conditions.

Also, while people tend to call it an upper entrance; I prefer to look at it as the main purpose that is needed most of all, a vent.

In my climate, insulating the hive is a must, which seals off cracks and crevices from drafts entering the hive.  Once wrapped, without the vent the bees are dead in short time.  Because they are clustered so tight in the cold, there is no air movement.  To be frank, they asphyxiate in the stale air around them as well as the build up of moisture chills and kills them.  A small upper entrance/vent provides for passive ventilation to keep them dry and getting fresh air.

I am really curious, In Germany: What is your typical winter?  When is the first snow that stays on the ground, what at the overnight lows and daytime highs typical of January/February, when does winter break and warmings of spring occur?

[blackforest beekeeper]

.... likely because of some significantly different winter conditions and how the hive is prepared to endure those conditions.

Also, while people tend to call it an upper entrance; I prefer to look at it as the main purpose that is needed most of all, a vent.

In my climate, insulating the hive is a must, which seals off cracks and crevices from drafts entering the hive.  Once wrapped, without the vent the bees are dead in short time.  Because they are clustered so tight in the cold, there is no air movement.  To be frank, they asphyxiate in the stale air around them as well as the build up of moisture chills and kills them.  A small upper entrance/vent provides for passive ventilation to keep them dry and getting fresh air.

I am really curious, In Germany: What is your typical winter?  When is the first snow that stays on the ground, what at the overnight lows and daytime highs typical of January/February, when does winter break and warmings of spring occur?

Hi there,

it`s a bit to be differentiated, though not as strongly as in the US:
Down south, Rhine Valley: wet winters, rarely snow, can get cold nights to -10 C. Mostly I guess it would hover around 0 C nights. And days just above. They live in perpetual fog down there in the winter...
Black Forest, just an hours drive into the hills: On higher grounds (1000 m above) snow may last till April, sometimes May. Not a must, though. I live at 500 m. It becomes green only beginning of May. In the Rhine-Valley Apple-Trees will be in full bloom or over then. We have had a hot and dry summer. Now it`s still prett warm, but where I live, a chill is in the air already. It?s 6 pm and 15 C. Nights are still about 10 C, though they are usually colder now. Daytime high was 19 in the shade. October usually brings first frost, November first snow, can stay on the ground then already. Some years it will be without frost till Christmas. But it has been heard of that snow came end of September and lay there till April. Hard frost and winter is mostly Jan/Feb. Usually the last winters are some spells of snow-fall. From 0 cm in the Rhine-Valley to over 1 m and more around our place. I got a snow-thrower. Usually it gets a bit milder in between with rainfalls (just above freezing), so the snow tends to melt away 3 or 4 times a winter and then fall again. When the weather is from the east (continent) it gets to freezing all day. "cold" is considered under -10  C. Some rare nights it will be -20C. But not every winter. "cold" I?d say -5 during the day and -15 nights. Higher up it gets colder.
In western Germany, more northerly, it`s usually milder from the Atlantic. Not much snow, wet.
In eastern Germany the climate is continental. October or November brings winter suddenly with frost and very little snow. Night-temps get around -20 and sometimes below there. Days are pretty darn cold then, too.
The alps are a bit different again... lots of snow.

Where I winter the bees, it`s almost as mild as in the Rhine-Valley. They have wine-yards there. First flow will usually be beginning of April. In the black forest: End of May. End of last flows most places: Mid-July (this year beginning of July). Black Forest: Pine`s still going.... but not to be harvested, rather to be avoided. Will load the last batch of bees tonight and haul them off.

There are beekeepers in Scandinavia. Winters are longer there. never heard of upper entrances there, but I am not into that. Maybe some reader from there among us? Please say!

[TheHoneyPump]

Excellent, thank you BFB.  Whenever discussing winter, it is very very VERY important to know the climate that the each beekeeper's experience and methods are coming from.  The opinion, experience, and/or method put forth by one beekeeper may or may not be applicable to another beekeeper, because of drastic differences in climate.  Therefore always use rational to your conditions and caution when one says, "do it like this".

For example, your winter conditions described are what I would call spring ;) . I am in NorthWest Canada.  By contrast here first frost can be before end of August.  First snow can be September, here is a picture of my yard just a few days ago, September 12.  It is currently -6C overnight and +2C day.  Hope to see a warming next week as high as +10C so can get some final feeding done and then setup the penthouse and insulation.
Here are a couple more pictures of mid winter, one showing the bee breath icicles formed over the upper vents, and one showing a sunny warm day at -12C of bees getting out for cleansing flights but not making it very far.

Last winter was particularly brutal.  Main snow that stuck came on October 1 2017 with temperature drop to -15C and pretty much stayed there or colder at -22C from December all way through to end of April 2018.  Throw in a 7 to 10 day deep freeze dip to -38C mid February; and there is a good picture of our typical winter.   The bees come through, although that last stretch in April of not being able to get out to cleanse was really tough on them.  My 2017/2018 losses were 11%, but the colony strengths were really down across the whole apiary.  They all bounced back and had a great summer.   Btw I raise my own queens from the survivors, my bees are primarily Caniolan traits with just a bit of Italian in the mix.  (Italians do NOT survive here).

Expecting similar this winter.  Hence the Penthouse for the one box.  They would have zero chance of surviving otherwise.

[TheHoneyPump]

btw, my hive packs are in pairs.  That is why they look so rectangular there under the winter wrap.  Here is an example of a pack.  Two hives mid summer.  The pictures show a few of the hives in the yard.  There are many many more in the apiary located at outyards.

[Ben Framed]

btw, my hive packs are in pairs.  That is why they look rectangular there.  Here is an example of a pack.  Two hives there.

The pictures show the few hives in the yard.  There are many many more in the apiary located at outyards.

Very interesting! I don't know how you survive that type winter, let alone your bees hehe!! I look forward to seeing the insulation pictures and the process of preparation as you move closer to winter, if you don't mind taking the pictures and posting as this process moves along. I will greatly appreciate it as Im sure others will also. 
Thanks, Phillip Hall

[beepro]

Mr. Bush you are correct.   They will only keep one queen over this winter.  Very often it is the
newly mated late summer queen with the strongest queen scent.   I already loss 2 production queens in this
little queen bee experiment.   The remaining ones are the strongest in the hives now.   So they don't call it "queen" for
nothing.   The hive with the QE separating the 2 breeder queens I already put one in another split hive.   Over here in our
mild CA winter environment they will never form a cluster during the winter time.   In the winter I can keep on feeding them to
grow the hives.   This winter I'm considering putting them all inside my large green house.   The temp. inside a green house should be
more stable than outside. 

[BeeMaster2]

Mr. Bush you are correct.   They will only keep one queen over this winter.  Very often it is the
newly mated late summer queen with the strongest queen scent.   I already loss 2 production queens in this
little queen bee experiment.   The remaining ones are the strongest in the hives now.   So they don't call it "queen" for
nothing.   The hive with the QE separating the 2 breeder queens I already put one in another split hive.   Over here in our
mild CA winter environment they will never form a cluster during the winter time.   In the winter I can keep on feeding them to
grow the hives.   This winter I'm considering putting them all inside my large green house.   The temp. inside a green house should be
more stable than outside. 

I recommend that you put just one or two hives in the green house. It will probably cause the bees to bee more active than they should bee. If you have good nectar sources all winter, it may well work. Make sure the bees have an easy way out of it. Glass and plastic really confuses bees.
Good luck.
Jim

[Michael Bush]

>How are upper and lower entrances handled?

You could do it any of a dozen or more ways I'm sure.  I use a flat cover with shims for all my covers and it provides the entrance.  I would just do the same except make it with 1/8" Luan instead of 3/4" CDX and put a rim on top to give beespace under the frames.

As far as way do a top entrance, a bee colony has one consistent problem to deal with all year around: moisture.  And moist air rises.  Whey they are evaporating nectar or cooling the hive by evaporation or metabolizing honey in the winter, there is always excess moisture to get rid of.

http://bushfarms.com/beestopentrance.htm
http://bushfarms.com/beeslazy.htm#topentrance

[Van, Arkansas, USA]

Honeypump, you run 5 deeps????? Is this standard for your area, Canada?  I use 2 deeps in N. Arkansas.

Black Forest: thanks for the detailed weather for Germany, both East and West.  Looking at the map, I would have guessed less fluctuations.  So your post is interesting...... flowers in April or May depending on location such as wine country and altitude.
Blessings

[TheHoneyPump]

No.  The 5 deeps picture is at end of June.  Peaking brood burst and beginning of first flow.  I am standardized on deeps for everything.  In the picture is bottom deep brood box, qe, 4 deep honey supers.


I winter the outdoor hives in 2 deeps.

I winter the indoor hives in single deep.  These are in climate controlled building Oct 20 through to March 20.


For the Penthouse, I will try to describe how it ended up as is right now and pretty much considered done until it gets insulated in about 2 weeks.

Bottom strong hive is in 2 deeps.  Standard bottom board with entrance blocked off all but 2" wide opening.  Standard inner cover on top of the second box, 1/2" wide notch facing down, centre hole (feed hole) in the cover is blocked by a piece of 1/8" ply stapled over it.  The upper hive, the Penthouse, is on top of that inner cover.  The openings of the bottom board and the upper are on same side of the hive.  Call it South facing, for now.

The penthouse is in 1 deep.  The bottom board is an inner cover with the notch facing up and cut out to a bigger notch of 1.5" wide.  The centre hole (feed hole) is blocked by a piece of 1/8" ply stapled over it.  This is placed directly on top of the inverted inner cover of the bottom hive.  The opening is facing opposite of the bottom hive.  Call it North facing, for now.  Then there is the single deep.  On top of the deep is another inner cover, the 1/2" wide notch facing down and also North facing.  The centre hole is blocked by 1/8" ply.  On top of it all is 5" of foam board insulation and finally the lid.  The whole thing, sides but not the bottom, is then covered in panels of 1" foam board insulation and finally is wrapped by building paper.  There will be 2" diameter cutouts in the foam where each of the lower and upper entrances of both hives.

Although the two inner covers back2back are thicker at 5/8" rather than a 1/8" ply divider would be, the fact that the entire hive is going to be quite well insulated the extra thickness should be fine for plenty of the lower heat to emanate through.  I am also well past being fed up and done building more equipment for this year.  Have expanded the number of colonies in this set by 3 and, and well ... I'm done building for 2018!   Actually, secretly, that is one reason why I wanted to do this in the first place.  I did not want to be building any more boards, pallets, lids.  The 3 inner covers are what I had in hand at the time that weather was cooperative to get this done.  Hope it works!

I will try to remember to snap a picture next time I am out there.


Yes, I could have just put the single into the wintering building along with the others.  However, I do want to try this as an experiment and see what happens.  Granted there are many many many other factors that will determine if a hive will survive the winter or not.  I believe those to be mitigated.  The penthouse hive is healthy, disease free, plenty of stores, young prolific queen.  Just low on overall population at the moment on the cusp of winter.  Without a thermal boost from the colony below, they are doomed to contract and die off as the cold hits and they are forced to abandon brood to cluster more tightly.  They do have a brood cycle, some mature capped brood, about to pop which along with some heat from below should get them over the hump.  I will monitor the entrance activity throughout the winter and should be able to tell if going well or if it is a failure within the first month of true cold.

It is a fair amount of work to wrap them properly.  Is why I do only a few that way and the rest go inside.  I find that despite the inside hives having a much easier wintering period, the outdoor hives (if they survive) tend to be more "hardened" to the large spring temperature fluctuations we can have here.  The outdoor wintered seem to be more robust and suffer fewer setbacks when the overnight temperatures drop substantially.  I haven't taken specific notes and data on this, so call it conjecture - which alot of good beekeeping is anyways isn't it?

[blackforest beekeeper]

Hey honeypump,
You are the first I hear of from the AngloSaxon beekeeping world having only one deep during honeyseason. I do it a similar way.

[TheHoneyPump]

One deep, singles, is common practice.  Some run the brood box as single deep year round.  I also prefer singles but I do allow more room with doubles, yet only for specific periods of the year.


My season in brief: dates vary by a week either way.

*Spring = May 10 - June 5
*Thunder season = June 5 - June 25 calm sunny mornings, wet windy mean thunderstorm-after 3pm, every day
*Summer = June 25 - August 5
*Fall = August 15 - September 10
*The Damper Season = september.  (Cloudy wet rain first snow)
*Winter  = Oct 15 - April 20


Outdoor wintered hives
- 2 deeps for winter, adequate stores and hopeful leftover resources for spring buildup with no feeding or minimal feed.
- Switchover to 1 deep in mid June  at the cusp of the 3rd brood cycle  when I take my full splits, nuc sales, and populate the queen rearing mating nucs
- 1 deep for throughout the rest of honey flows through July and August
- Sampling and treatments for mites mid of August when I take all the honey off.
- back to 2 deeps end of August and fill them up for winter on any late season flow or/and sugar syrup

Indoor wintered hives
- 1 deep for winter
- add 2nd deep and LOTS of syrup feed in spring, May, to build population.
- Switch back to 1 deep in mid July when I take ?late? splits and am adding stacks of supers for honey flow.
- Sampling and treatments for mites mid of August when I take all the honey off.
- stays as 1 deep for fall and winter. fill them up for winter on late season flow or/and sugar syrup
- moved into cool dark wintering building October 15, come out of the building April 15.


Yes, I use queen excluders. To keep the queens in the boxes where I want them to be. 

Hope that helps in some way, or is maybe just interesting.

[blackforest beekeeper]

One deep, singles, is common practice.  Some run the brood box as single deep year round.  I also prefer singles but I do allow more room with doubles, yet only for specific periods of the year.


My season in brief: dates vary by a week either way.

*Spring = May 10 - June 5
*Thunder season = June 5 - June 25 calm sunny mornings, wet windy mean thunderstorm-after 3pm, every day
*Summer = June 25 - August 5
*Fall = August 15 - September 10
*The Damper Season = september.  (Cloudy wet rain first snow)
*Winter  = Oct 15 - April 20


Outdoor wintered hives
- 2 deeps for winter, adequate stores and hopeful leftover resources for spring buildup with no feeding or minimal feed.
- Switchover to 1 deep in mid June  at the cusp of the 3rd brood cycle  when I take my full splits, nuc sales, and populate the queen rearing mating nucs
- 1 deep for throughout the rest of honey flows through July and August
- Sampling and treatments for mites mid of August when I take all the honey off.
- back to 2 deeps end of August and fill them up for winter on any late season flow or/and sugar syrup

Indoor wintered hives
- 1 deep for winter
- add 2nd deep and LOTS of syrup feed in spring, May, to build population.
- Switch back to 1 deep in mid July when I take ?late? splits and am adding stacks of supers for honey flow.
- Sampling and treatments for mites mid of August when I take all the honey off.
- stays as 1 deep for fall and winter. fill them up for winter on late season flow or/and sugar syrup
- moved into cool dark wintering building October 15, come out of the building April 15.


Yes, I use queen excluders. To keep the queens in the boxes where I want them to be. 

Hope that helps in some way, or is maybe just interesting.

Hey HoneyPump!
I am very much interested in "different" ways of bee-keeping in different climates. Always something to learn or a thinking-starter.
4 deeps as supers... wow! I heard there were good flows possible up north. Must be the long days, too? How much average yield for a hive?

I myself run about one deep all season long. In the black forest with its rougher and longer winter, I might go for more in winter.
During honey-season, I have often less than a deep for brood. Starting with about 3 Jumbo-Frames in very early spring. Works just fine. After the flows I usually take the brood out, make a split that way. So - depending on the rest of the warm season - I winter in "half" to "three-quarter" colonies (I don`t go under 5 Jumbo-frames covered with bees). 9 Jumbos are fine for that in all cases. Can`t say I have never fed in early spring, though...mostly 7 frames.
When our bee-thing has grown up to size I might winter in larger colonies. So I`d add a shallow super to the 9-frame Jumbo-Box. For the winter only.
My nucs got a little too strong this year. I mean the brood-nests I took out after the flows together with the queens. I put them about 1 click away and thought, the foragers would return to the original hives. Well, they didn`t and had lots of brood. So I quickly added a honey-super to the 5- and 6- frame nuc-boxes, I even harvested some honey yet. i kind of like this setup and as I started having the nucs on pallets, too, no problem with the forklift. They bearded anyway. But now the first frost is at hands tonight, so ... they will more or less stop breeding. Fine with me. Very few mites, enough feed, enough bees.

By the way: Most bees are kept on an open screen all year long in Germany. Considered healthier. I don`t do that, but the boards that go underneath the screen are not air-tight in my case. Lots of circulation still. Or: Enough. No mould or such. I only open up in mid-summer when they are in bright sun and it`s way above 30 C for days and weeks on end (which usually it isn`t).

[TheHoneyPump]

Short season but very intense with long daylight hours yes.   
Average hive average season:  70 kilogram per hive.  Good hive good season, all stars aligned: 100 kilogram per hive, seen as high as 130 kilogram, once.  Average hive poor season: 40 kilogram per hive.

I will PM you some links that you may find interesting. 

[Acebird]

Hey honeypump,
You are the first I hear of from the AngloSaxon beekeeping world having only one deep during honeyseason. I do it a similar way.

BF almost everything is done somewhere in the US.

[Beepah]

So, this one has been bouncing around in my head and I'm puzzled: why would the screen matter?  Whether I have one big colony in the boxes or two smaller ones divided by a screen, moisture will happen and will need to be dissipated.  If a correct moisture control strategy is in place, everything enclosed should be okay.  What am I missing?  Thanks!  (still first-year beekeeper)

[TheHoneyPump]

So, this one has been bouncing around in my head and I'm puzzled: why would the screen matter?  Whether I have one big colony in the boxes or two smaller ones divided by a screen, moisture will happen and will need to be dissipated.  If a correct moisture control strategy is in place, everything enclosed should be okay.  What am I missing?  Thanks!  (still first-year beekeeper)

Because warm air, warm moisture laden air, warm stale moisture laden air; rises.  A screen passes all of such from the lower colony into the upper smaller weaker colony.  Not good, not good at all. 
The upper colony can benefit greatly from the warmth/heat from the colony below.  Yet will be decimated by stale moist air.  A solid board allows the heat to rise, but stops the stale moist air.  Think of same concept of having heated floors in your house, or laying on a electric heated blanket.

Want the heat, do not want the air.

[Oldbeavo]

We wintered some full depth singles by using a plastic sheet, about 1mm thick between them, there was a frame 20mmx20mm on top of the sheet with a small entrance for the top box to use.
All wintered successfully though our winters are not as severe as Nth USA. No snow but overnites down to minus 1-4 degrees C at times with daytime getting to 10-14C.