I am a very lucky gardener. Many years ago, I was given a small, unheated greenhouse by our neighbors. It was on their property when they bought it and they knew they wouldn't have a use for it.
It is 6 feet wide by 8 feet long and has glass panes on the sides and twinwall panels on the roof. It also has an automatic roof-vent opener so it doesn't get too warm inside the greenhouse. I use it every year as a staging area for plants I've started indoors until it's warm enough for them to be transplanted into the garden.
Because I'm too cheap to pay to heat it (hey, I admit it), I've tried to come up with ingenious ways to keep the plants from freezing when it's really cold at night. You wouldn't think plants could be damaged or killed by frost when they're inside a greenhouse but I've unfortunately learned it is indeed possible.
While looking through a catalog from Charley's Greenhouse (www.charleysgreenhouse.com) a few years ago, I saw that they sell sheets of bubble-wrap that you attach to the inside walls of the greenhouse to provide some insulation from the cold. Since I had saved a lot of bubble-wrap from shipping boxes in the past, I decided to give this a try. While it might look a little odd (see top photo), it actually works quite well. All I do is use clear package tape to attach it to the glass panes.
But sometimes, if the temperatures are particularly chilly, you have to go one step farther. And that is to cover the seedlings with a layer of floating row cover. That has solved my problem. For those of you unfamiliar with floating row cover, it's a woven fabric that looks like the interfacing that folks use when sewing collars on clothing. It lights in air, light and moisture while providing a few extra degrees of frost protection.
Now, if you look at the 2nd and 3rd photos, you are probably wondering what's going on there. This is a self-watering system that I'm quite proud of.
First, I bought an 8-foot length of plastic rain gutter for each plant bench -- along with end caps and brackets -- and installed it at the front top edge of each bench. Those will get filled with water and act as reservoirs. I covered the tops of each of the 8-foot-long planting benches with plywood. Then I covered that with black plastic sheeting, extending it down into the gutter.
Next, I purchased some capillary matting from Charley's Greenhouse which is a type of fabric that wicks up moisture from the gutters. The matting comes the exact depth that I need (21") and you purchase it by the linear foot. Before placing it on the benches, I soak it with water to get the wicking action started.
I'll tell you what, this little self-watering system works beautifully! I only have to refill the rain gutters about once a week which is a real timesaver. This keeps the flats of seedlings lightly moist, which in turn keeps the plants happy and unstressed.
I wanted to let you know about this method just in case it will help you in your situation. I realize not everyone has the luxury of having a greenhouse but perhaps it will give you some ideas of ways to set up a self-watering system of your own.