Thirsty_Dirt - Welcome to my world !!! Zone 3 - that is. Light bulbs (many) will work in the early spring when the plants are small - with a frost blanket over a protection cage so it doesn't touch the plants or the light bulbs - but that isn't practical this time of year! We have found the least expensive method is to run a couple of ceiling fans 24/7 to keep the warm air down in the GH and put a space heater or two on a timer. You will have to check the GH temp constant (a remote thermometer is great to track GH temps) to figure out when to run your space heater - we try to keep our GH above 40 degrees minimum. Our GH is sealed but we have to supplement a space heater when outside temps get below 32 degrees (depending how many hours the temp is below 32 degrees) - but have the heater come on at midnight - 3:00 am and go off at 7:00 or 8:00 am this time of year. We only supplement the GH until we get the produce from the plants we want - then shut the GH down for the winter (usually the end of October). We have not had to supplement heat yet - which is unusual for this time of year - we still haven't had our first fall frost!! We have not found an affordable method (although Cole_Robbie's wood burning stove appears to be a great option) to heat the GH from November-March. Please let us know if you come up with one!

Thanks for the link -- good discussion. However, I really would like to find a good "how-to" article tht explains exactly what material they're using (and why) and how to install it. Pics would be a huge bonus.

The weather stripping appears to be placed correctly, however, it also appears that the bottom guide may be displaced.

Try Conservation Technology in Baltimore, MD They have a good selection of weatherstripping of different sizes and styles. Here is a link that might be useful: Conservatin Technology