I'm planning on putting my figs in the greenhouse this winter - at least what fits. It's new so I've never heated it. It's only 8" x 12". The roof is 9" at its highest point. It has electricity to it. Gas isn't an option and I'd like to stay away from anything that would need vented (for now at least) So electric is probably easiest, though most expensive. Anyone have any ideas? I was thinking I'd look into an oil filled radiator. But I really don't know much about this topic. Thanks

I have a 7x7x9 greenhouse, my hydroponic guys will sell me an oil filled 120v heater, with a single zone thermostat, I expect that will be more than satisfactory for me, and for you as well. It also is relatively inexpensive, less than $100, can't remember the exact price.

Growing figs in a GH may not be the best idea - unless you want to early-start/extend fruit production.
Any GH gets too warm in winter and figs will leaf out like crazy.
Best to winter/dormant store them figs in a cooler place (25-40*F).

The only figs that go in my GH are some little ones that I need to keep them growing.

Oh yeah, you will need some venting in your GH.
I started with only a couple of those expensive automatic (wax) openers for my 2 top roof vents (~2x3').
Very soon I learned I needed forced air too.
I used cheap 2x2' Lasko box-fans with a built-in thermostat and hung them from the top of the vents,
and added a piece sheet plastic to act as a baffle/air-director.
A bottom (intake) for cooler air was needed; all I have now is still manually operated.

As for a night-time heater, I just use an electric Vornado (the Vortex kind)  - $50-100.

Try googleing passive heating a greenhouse.Might give you some cost saving ideas.

Tony, how many plants are we talking about? I agree that you need them to get cold, stay cold and dormant throughout the Winter season. You don't want them heating up drastically during any January or February thaws. Do you plan on insulating your greenhouse from cold and covering to keep it relatively dark? Most important thing i think is not let them freeze or warm up prematurely. You want to make sure that you have access during dormancy so you can periodically hydrate them a bit. Send us a pix of your GH so we can see the operation. Good luck this Winter. We all need a bit of that. Cheers

from experience it's very costly and I live in SC.   gorgi   is right, keep them dark and cold and you won't be sorry.  Good luck.

There are several post on this forum about this. Electric is probably the best option. You have to decide what's the objective of the heater. Like George said, a GH will heat up fast on a sunny day.

In winter, I use my GH strictly for winterization. Again like Geroge said, they will leaf out and start growing early. So, your main goal should be trying to keep a constant dormant temp. In my GH during winter, I try to keep mine no lower than 33 and no higher than 45. I have a thermocube that's connected to my radiant heater. But I have to open the vent and door daily. This works for me in the South. I keep about 150 to 170 tree in my GH and come March, they are all leafed out.

I have a 6x8 GH and I used an oil radiator space heater last winter on the coldest nights.  I needed about 8 feet of extension cord and the electrical safety advice I received in forum was to "Use a heavy duty, short cord or cut one down to the actual length needed."  I cant remember the amps.  The shortest, heavy duty I found was 25 feet on Amazon. I didnt know how to cut it down so I used all 25'.  I also used a thermal cube that turnes on at 35F.  Most of my plants were 1 gal figs and they survived. 
   I dont have a back-up plan if the electricity goes out. yikes.

I just want to use it to store figs this winter. I'd keep it about 35 degrees. That will be a chore in pittsburgh. I don't expect it getting too hot will be an issue. It has a paver floor so I was hoping that would help retain some heat. Here is a photo. It has power so electric would be most convenient.

Just me but I won't build a greenhouse and then not use it. The figs probably won't start growing until February. That depends on how warm you let it get by day. You've got the power exhaust vent. Run that on warm sunny days and they won't start growing too early. Run the heater at night at that 35 after they start growing. You could safely lower that to 28, probably lower, when they are dormant. At those temperatures heating won't be that expensive. If it is adjust next winter. This winter see what she'll do.

Starting figs off early in spring has a huge upside. It could double your harvest season.

Putting them in the GH would just be a 1 year thing - I hope. We turned our garage into a family room. A new detached garage will come next year and give me a lot of space - I really hope. Then I would just use the GH In the spring to jump start select figs. Time will tell. Thanks for advice everyone.

Tony, Here is what I have in my garage and my greenhouse. http://www.rezspec.com/en/na/products/product-unit-heaters-ws This one is a hot water type, but they make them in electric also. Reznor is the company and they are local, from Mercer, Pa.

Here are the electric models. http://www.rezspec.com/en/na/electric-heat

We will warm our garage again with a forced air tube type heater and kerosene. This year I'll put it on a thermostat.

Thanks Art. I'll take a look.

I've received some good advice about heating options. But now I'm wondering if I should insulate the side walls. I posted pictures before. My back wall is framed and so is 1 side. The others have mostly glass. Should I insulate the walls as much as I can? Or is it a waste since the whole top is twin walled polycarbonate (8mm) and doesn't have much of an r-value. Thanks.

Hi ajv73,
If you don't want a killer bill, consider the insulation as a higher priority than heating the greenhouse.
Insulating the roof and the walls - even the door is important, especially in Zone6 - Just saying.

Do you know how they would heat the houses before ? They had the cows living/sleeping under the houses.
We even know of one child that had a cow and a donkey to heat his room at birth :) .
Do you have 3 sheep?, and some space at the greenhouse for them ? :) ...Just giving ideas ...