I was in need of an outdoor place to keep a few pineapples alive through the winter so I decided to build a greenhouse. It is a 4'x5' structure that is about 5.5' tall. I built it with a 2x4 frame and covered it with two layers of 3.5 mil plastic, on on the inside and one on the outside of the frame. The problem I am having is that it is not retaining any heat throughout the night and the temperatures are equaling the outside temps which was 26 last night. I have made sure all seams are taped and do not see any obvious holes in the plastic. I also added some foam insulation board the the north wall and the floor. I also have four 5 gallon buckets filled with water to act as a thermal mass. I have watched the temps come up as high as 98 inside but they soon drop and just does not seem to retain any heat. I am not concerned with it staying 65-70 degrees throughout the night but would like it to stay 45+ and really protect the plants from frost. Any suggestions to retaining heat would be greatly appreciated.

Many of us use various forms of heaters: electric, propane, etc. A thermostat helps keep heat at even temperature too. Dont forget, your pineapple needs lots of sun, too. We harvested our pineapple in early Dec. It sat under growlights indoors.

@handrew ~ the cold air enters at ground level. If you try to insulate around the base of your greenhouse it will help prevent the temperature from dropping as quickly inside. I have heard of people using straw or hay bales around the perimeter base. Some folks stack bags of leaves or other types of barriers. Worth a try...

Just curious as to your location?

You will need much more insulation to retain the heat in the greenhouse. I read a thread awhile back where they placed some type of insulated blanket over their little greenhouse at night to help keep the heat in. I think they said it was made for protecting concrete or something like that.

I have wrapped my trees with a heating cable in the past to protect them from really cold nights. That worked pretty well.