I have more thoughts....
I am worried about the gh heating too quickly since my solar vents and exhaust fan will be bypassed with this new ceiling in place. How else can I hold heat in other than lining the inside with a platic tent? I thought about putting a steel 55 gallon drum with water in front of the heater to soak up some heat. That thing gets toasty on your pant leg if you stand in front of it. I will line the North wall/door wall with film of platic and cut a slit in it like meat lockers have.... I want to insulate the North wall but wonder if it will really help.... that is the wall that faces the house and I don't want it to look bad.

Why don't you contact you local packaging house and buy a roll of 1" bubble wrap as insulation? Avoid Charlie's as he charges several times the price for the same stuff. My Rion is 8 X 16' and we did everything but the vents and south wall. The small HFGH we only did the north wall and roof section.

I insulated my north wall with some of that polyisocyanurate foam sheathing...foil faced. I have read that most of the heat is lost through the roof, but I started with the north wall since I didn't get much light through it anyway (close to our house) and I figured any improvement in holding in heat was worth doing...and I'd deal with the roof later. It's permanent and stays up all year. Some folks have insulated the bottom half of the north roof too, and I may do that this year (I now know I have TONS of light in there so I'm no longer worried about adding a bit more opaque material.)

The polyisocyanurate foam insulation I bought (from Lowes) is foil on one side and had big ugly blue and white lettering on the other side. I did not want that ugliness showing on the outside of the greenhouse (like you) so I just rolled on a coat of flat white paint over the advertising, and now the north wall looks white from outside the GH. (The foil side should not be painted.) I have pics of how I installed it about 1/3 of the way down this page, if it helps.

You're in a bit cooler climate than I am, but I had also considered blocking off the peak in the winter to reduce heat needs. After going through my first winter, I've abandoned that idea, because (here) it would get way too hot on sunny winter days to do that. I need to have that extra room for hot air to gather up there, and I also need to be able to let my exhaust fan run on the sunny days. Your mileage may vary, though!

I may also add bubble wrap to the whole interior roof for next winter...I think the roof is probably the place I'm losing the most heat, now that I have the gaps all sealed and the north wall insulated.

I am totally clueless about misting set ups!

Thanks for the replies so far....
I plan on taking the false roof out as things warm up. That is why I was just going to put a sheet of plastic there. You know how it is with these small ghouses. It's freezing at night and 100 degrees when the sun gets up. I just need to get through these really cold couple of months but I need to get my seedlings out there. My basement is overrun with flats....
I don't plan to heat it all winter. I store my elephant ears and bananas in my crawl space. My dryer vents in there and it never freezes. I actually have new growth on some plants in there and the leaves are all white and blanched due to the lack of light. I am hoping to move those outside and get a jump on things. Daffodils are starting to poke up around town.

darrinky You have a lot of options and all of them would be better than a piece of plastic hanging from the roof.

I have a 10x12 HFGH I put a pool cover on the roof and have it installed permanently.
The pool cover will save a lot of heat I also put bubble insulation on the inside roof and walls including the south wall.
The North wall has foil back foam board and the foam board is half way up the east wall and to the roof on the west wall
I have every inch of the interior insulated

I use those large trays and set Styrofoam cups full of soil in the tray with holes in the bottom of the cups pour water in the tray and it will water you plants from the bottom.
Pour a couple inches in the tray and they will be fine for 24 hours so NO need for the auto watering system
I have my heaters and inlet vents and exhaust fans all on thermostats so it is completely automated.

I can leave it for 24 hours without worrying about the temp you need to do the same anything other than a reliable auto system will result in less than optimum results.

UPDATE!!
I put a layer of plastic and all four walls and put a curtain of plastic over my North side doors. I put one sheet at head level and sealed it up with duct tape. I turned my heater to its lowest setting and it seems to be working very well. We have an inch of snow and it is still coming down. Temps outside are in the upper 20's and the greenhouse was reading a toasty 65. Trying to dial the heater down a few notches to keep it from coming on as often. Will try and post some pics tomorrow.