greenhouse performance?

[Wolfie2]

We just built a relatively small greenhouse, 10x12. We are on the gulf coast so we very seldom have freezing weather much less what we had last night, 18 deg. That temp used to be considered a once or twice occurrence in a generation. But we have had those temps once each year for 3 yrs. in a row now.
Was sorely disappointed this morning 18 outside and 19 inside with a 150 watt heat lamp. Mom's afraid her plumeria are gonners. Not double walled due to our high summer temps but it probably won't be used in the summer anyway. I'm sure another heat source would be more effective. The heat lamp did not put off as much heat as I'd hoped. I have the cover off one of the cheap, temp. greenhouses. Same size as the new one except it is not as tall as the new one.. Thinkin about hangin it up inside the new GH. givin it a double wall with low overhead. And maybe a propane heater of some sort. I know I need to work on a couple places to stop heat loss.
Anyway, what has been Y'alls experience with greenhouses? Keepin it above freezin? Keepin it toasty?

 

[raingarden]

Some winter greenhouses have two layers of plastic film on the outside with the edges sealed all around. Then, a small squirrerl cage fan/blower is used blow air between the two layers, The space between the two layers of plastic inflates providing insulation. They are very efficient.

There's another greenhouse type that has no frame. The plastic sheet is laid out on the ground (with some slack) and the edges are buried in the ground all the way around. Then, a squirrel-cage blower is used to pump air under the plastic sheet. The sheet rises and forms a dome that you can walk under. We once made a dome over a quarter-acre pond this way. It's really neat, but if the power goes out it deflates and the sheet slowly falls back to the ground.

 

[Sally PB]

I don't heat my green house, other than passively with sunshine. It's 7 degrees today, 30+ mph winds, and I haven't gone out to the green house today, so I don't know how cold it got out there.

Due to mice, I have one spinach plant left. I put a clear plastic cover over it to help keep it from freezing. From what they're saying about the weather, I might not get out there until Sunday.

 

[Crooked C Farm]

The performance of a greenhouse is determined by the design of the greenhouse.
Something as simple as the south facing roof can affect how much solar energy (heat) your greenhouse will produce.

For me in N.E. OH and at a latitude of 40.798946 my southern facing roof needs to 60° to receive the most winter solar energy.
My first greenhouse I built was a 10' x 12' A-frame had 60° southern and northern roof with the east and west walls along with the northern roof was wood with 3 1/2 inches of foam insulation.
On a cloudy winter day with temperatures around 20/25° outside that greenhouse would be 90 to 100 degrees inside if I didn't open the windows up.
At night the temperature would drop fast so I had to use a reddy heater with a thermostat to keep the inside temperature at 68°.
If I would have added a solar mass on the north side of the greenhouse I may have kept the nighttime temps warm enough, but I didn't want to lose floor space.

 

[BarneyNCyndi]

Oftentimes, just keeping the frost from coming down onto your plants makes a big difference. Temperatures into the teens however will kill a lot of plants.
I may adapt Crooked C Farms idea and add some insulation to our north face to slow down some of the heat loss at night. Our GH is small enough, 6x6 that I may go ahead and double layer everything with the wavy polycarbonate panels that it is mostly done with now.