I too have a 6x8 Harbor Freight greenhouse, I use it to overwinter many tropicals that won't survive our harsh Oklahoma winters, plus my house is just too dark to grow many plants successfully. I too used bubblewrap on the sides as well as the ends. For the front entry & sides & roof I used clear bubblewrap, & then I used a silver reflective wrap for the north wall were I also have a natural gas heater installed. Use a fan to circulate the air & have never had a problem usually keep the thermostat set on 1 maybe 2 at the most. I also use a black shade cloth to keep things from getting too hot. I absolutely love mine & am giving serious consideration to buying another to possibly see about adding on to the front of mine. I got mine from a Harbor Freight store in Tulsa, using a sale coupon I got off eBay for a couple of bucks & ended up paying around $270 for it.. I am sure it would have been higher had I paid for shipping, but it was only an hour and a half drive that I make often anyway. Oh yes wanted to mention too that the bubblewrap is held in place by little plastic clips that snap into the rails that make up the roof, these also double as plant hangers. Perhaps I can post a pic later.
