chicken coop greenhouse

This year I wanted to try something new for my chickens.  The door they use to move from the coop into the run is on the south end of the coop with pretty good sun exposure from morning until about 3 in the afternoon, when the sun moves beneath the large white pines that are in the run.  My chickens don't seem to be bothered much by cold, but they won't walk in snow unless they are forced to.  This year I decided to build a greenhouse-type structure that would keep an area free of snow so they could get out of the coop and walk around.  I used 2 cattle panels bent over into a hoop house design.  The bottom is 8 ft wide, and the cattle panels are 52" I believe, for a total of 104" in length.  I pushed it up against the end of the coop, framed a 1 ft door at the bottom of the doorway and another 5 ft door above that so I could get in and out.  Either one or both of the doors can be opened at the same time.  I covered the entire structure in either 4 or 6 mil (can't remember right now).  My expectations were that I would get some solar heat gain and more importantly, and area free of snow for the chickens to play in.  The first day the structure was in use, the outside temp was 20 degrees F.  The inside temp maxed out at 78 degrees, much higher than I expected, and that is with pretty significant gaps around the door and the area where it is against the coop.  On sunny days I get 40 to 50 degrees of gain, regardless of the outside temp, including days that it was been well below zero.  I would call it a resounding success in that department.  The next step is figuring a way to store some of the heat so I don't lose it immediately when the sun goes down.  I can't imagine temp swings from 50 degrees down to 0 in a few hours is necessarily beneficial, and may even do more harm than good.  I have a number of blue plastic 55 gal drums that I could fill with water and put in the greenhouse, but I fear that we may have several days in a row without sun and I'll end up with 55 gal barrels, probably split open, full of ice, having the opposite effect that I am trying to achieve.  Anyone have any thoughts on storing the heat and tempering the extremes somewhat?  Keep in mind that below 0 temps are the norm here rather than the exception.

Why are you worried about the 50 degree temperature swing?  Is the swing affecting the temperature inside the coop?  You could just increase the gaps around the sides so you only have a 30 degree gain.  That would maintain the snow free area and still give the girls a warmish place to play..

Hey Mike.  I thought about doing that, but I would rather figure out a way to maintain  some of the heat throughout the night.  You know how cold it gets here, I would love it if I could keep the temp above 20 or so.

The heat from the greenhouse doesn't really affect the temp inside the coop much, since only the small coop door is open to the greenhouse.  If I could figure a way to sustain the heat a bit, I may open a vent at the top of the coop inside the greenhouse to create a thermosiphon effect.  I was thinking that barrels of water would absorb some of the heat during the day so it doesn't get so warm, and then release some of the heat during the night to moderate the temps.  If I can't figure some way to do that, the thermosiphon won't help, it will just cause the coop to overheat during the day.

As you already know, you won't be able to keep the area under the hoop above freezing [for a greenhouse of that size, single glazing, your climate, etc].  If you could, then you'd be the hero greenhouse guy for the northern part of the US.  

Water barrels will definitely hold some heat but they will probably freeze.  If you can keep the barrels from splitting open, you may be onto something though.  The phase change from liquid to solid takes a ton of energy.  So as the water freezes it holds the temperature around it at 32F for a long time.  Then when it melts, it absorbs heat for a long time keeping the temp down at 32 until it's done. Whenever the day to night swings align with the tank freezing and thawing, it would do a lot to keep the temps around 32.  So maybe in Dec and Jan you'd have frozen thermal mass, but for Nov and Feb they'd start phase changing most days to help keep you at 32.

Anti freeze would keep them from splitting so you'd have split free thermal mass.  But since they wouldn't be freezing you wouldn't get the phase change advantage.

Just a few thoughts, probably a bit off topic from where you were headed

Dumb question, but if the temperature swings under the hoop don't affect the coop much, why do you care about them?  The chickens aren't out at night so they won't mind that it's cold out there.  I don't have chickens yet so I may be missing something...

My thought process is that if they spend time during the day outside where it is say 70 degrees for example (it has hit 78 once and 79 once), it may keep them from acclimating well to the really cold temps they have to deal with at night.  Since the coop temps aren't affected by the greenhouses temps, they can easily see a swing from 50 degrees when they are in the greenhouse on a sunny day, to -20 on a cold night.  I'm not sure that is more healthy for them than to just stay in the range of say -5 during the day to -20 at night.  I just don't know.

Todd,
Yep, water barrels are a very common tactic for smoothing out temperature swings in a greenhouse. They don't freeze in most greenhouses (but most greenhouses are somewhat closed up). Not sure what your starting structure is like. If interested in water barrels I wrote about it in our recent book called The Year Round Solar Greenhouse... if you go further down the grower route, may be worth a read! Otherwise search 'water wall greenhouse' and you'll find plenty of info.
Can't help much on the chickens' preferences, sorry.
https://www.amazon.com/Year-Round-Solar-Greenhouse-Design-Net-Zero/dp/0865718245/ref=sr_1_1?ie=UTF8&qid=1483061084&sr=8-1&keywords=year+round+solar+greenhouse

Lindsey Schiller wrote:Todd,
Yep, water barrels are a very common tactic for smoothing out temperature swings in a greenhouse. They don't freeze in most greenhouses (but most greenhouses are somewhat closed up). Not sure what your starting structure is like. If interested in water barrels I wrote about it in our recent book called The Year Round Solar Greenhouse... if you go further down the grower route, may be worth a read! Otherwise search 'water wall greenhouse' and you'll find plenty of info.
Can't help much on the chickens' preferences, sorry.
https://www.amazon.com/Year-Round-Solar-Greenhouse-Design-Net-Zero/dp/0865718245/ref=sr_1_1?ie=UTF8&qid=1483061084&sr=8-1&keywords=year+round+solar+greenhouse

Thanks Lindsey.  I ordered your book.  I'll be building a greenhouse soon, and I'm pretty sure I'll go with a geo-air system ala "Citrus in the Snow", but I'm open to other ideas.  I would like to make it big enough that the chickens can live in half of it thru the winter while I use the other half, and then I can switch sides for the following winter.