For about two years now I have been wanting to build a green house for my fig trees, well last week I finished it 10`x 10` and now to heat it I used snow melting cable the same you put on a roofs and it works very well so well that today we had a temperature +9 c and and I  had to leave the door open and unplugged the cable.

Just thought I should share with all of you.
Jose
Ott,Can

IMHO a used natural gas furnace is the way to go.
My father had a home made swamp cooler and a used furnace to heat and cool his 2 car garage sized green house full of orchids in the 60's and 70s without a problem in SE Michigan when we lived there.

I heated mine last year and the power bill sucked. this year I'm going to let them ride without heat. I'm in zone 7b/8 though, it's not anywhere near as cold as you get. 

Hello Jose,
 How much snow melting cable did you use and how did you lay it out? 
Richard

I use a small radiant heat heater with a thermocube thermostat. It turns on at 35 off at 40. Works pretty good.

Richard....I had 160` in it today I bought an 60` and tomorrow I will change them 160` is too much, I just lay it down on the ground at the base of the walls.

snaglpus...that`s what I use in my solarium where I keep my younger plants being attached to the house it does not get as cold.
Jose

i have a 250 watt red heat lamp in mine.  When I got home the green house temp was 30.  about an hour later is was at 32 and stayed there.  Still supposed to get colder tonight.  For some reason, my wireless weather station stops reporting the outside temp around 11pm and starts working again around 7am.  Of course, this is only the second night i've had it.  Don't know why it would do that though...

I'm pretty sure everythings going to freeze in there tonight though.. :( 

I agree with Hungryjack ,
Wait till it goes to -20 c , you will need two 1500 watt heaters at night time to keep it above 0 c . Very pricey .
In Canadian winter , fall and spring season extension is when I heat my greenhouse , otherwise too expensive for a hobby .

John

I already had the -20c nights and it did ok I keep a water bottle half filled if it stays liquid is good.
Jose

My HFGH is 12x10 and I have a 1500w oil filled electric heater next to a 30 gal. water barrel. Also a 1500 watt electric fan heater plugged in as a backup. I'm very happy with my Home Depot Acurite "Wireless" Backyard Weather Thermometer. Measurement range for -40f to 158f and a wireless range of 165 ft and keeps a high and low temp reading for 24hrs. Sell for about $15. and can now watch temp very closely and adjust as needed. The model#00822HDSBA3.
Richard

Jose , i don't know how many watts your strip heaters are rated for . But if they pump out enough heat to keep the temp at 5C on a -20c day then they are high wattage . My 6x8 glass greenhouse would need a 1500 watt heater going full out to keep up at night or a cloudy day .
1500 watts at our electricity rates full out would be apx 8 dollars a day , thats 240 a month . No thanks for simply storing figs that don't need any sun while dormant . 
Your greenhouse is twice the size of mine , so even if your heaters are not going full out its a lot of expense . you are better off putting them in a shed or garage where there is less thermal loss .
You also didnt mention if you monitor the night time temps with a thermometer , it might get a lot colder than you think .
This is my setup :

http://figs4funforum.websitetoolbox.com/post?id=5040799

John

My wife freaks out about me turning my heaters in March in in my 10x12 glass greenhouse to wake the kiwi up....  boy she would definitely not like me trying to keep it above freezing ALL winter lol.

This lacrosse weather station I bought is crap. It only reports the outside Temp about 5% of the time. While it did work, it said 30f when TWC said 27 for my zipcode. I don't know how accurate TWC is. But it did not freeze water inside the GH. So the 250 watt heat lamp helped a little. Don't think it will tonight when it gets down to 16f. I trying a fan heater now to see how it does.