propane catalytic heater as described might be OK for marginal nights but for a 25x40' structure and 0*F nights you had better invest in a unit capable of 80K BTU or better just to be safe. Since yoiu don't count on more than 3-4 nights like that you might want to have a propane turkey fryer base handy or even a gas grill that you could set in just for those nights. I've never had any night below 20*F to contend with but I have scrambled everything available on some nights. Also get a couple of cheap box fans, or better versions if you can, so that you can move the air to avoid cold spots within the greenhouse. When I experimented with Fruitnuts suggestion years ago (plants under row cover under a tunnel within the tunnel) I lost about 10% of my tomato plants because I was monitoring temperatures at on spot and other spots just got too cold. Even the best of setups fail if the temperature drops below capabilities of your system. A Minn. researcher reported on a project using solar panels to heat water pumped under greenhouse soil during a very cold winter period. Although the circulated water reached very high temperatures during many days the extreme nights were just too cold for the setup to succeed. A few things you might consider are to develop a backup plan which may include moving your plants into your basement, etc. Also consider starting them in your house anyhow since 50*F is much too cold to germinate seedlings. From a comfortable vantage point you can look at weather forcasts and determine if conditions are favorable to move plants into your greenhouse. I have 3 greenhouses yet my house setup of 28 shoplights is maxed-out even as late as April 1st. when my greenhouses are either planted full or reaching capacity.

North wall would be a zillion times better, as you'll get more winter sun, when you need it most. Good to hear you're going for a big greenhouse. If you can afford it you'll be thankful for going bigger. A door directly into the greenhouse is fantastic as you can go out there and read a book in the middle of winter! You will have 24m^2 space. People have lived comfortably off veggies grown in 50m^2 without any extra food in temperate climates. You might be able to do this, with the bonus of some of the more hardy sub tropical crops (perhaps grow some ginger?). domeman: analysis looks good. Depending on what your goal is, you might even use 93% transmission PC (clear) and 50% shade cloth inside. This will give the lower light, but collect more heat. In summer you would either vent, or move the shade cloth outside. PC is also half the price of colourbond (but only lasts 1/3rd as long). If you put the shade cloth on the inside, two things to note: a) it would be good to have a light clear film of something like PE (say drop sheets) directly under the PC (like double glazing) to protect the PC from hot humid conditions (apparently PC breaks down quickly in hot humid conditions, but not when just hot) b) keep the shade cloth say 20cm from the surface to allow good air movement. A large diameter fan such as a ceiling fan at the top of the greenhouse could then move heated air down when required (thermostat).

I think it would be all right. The harmful stuff in asphalt is the light aromatics, but they are probably gone by now. I replaced the roof on my garage using colourbond (zincalume with some kind of epoxy paint over the top) with a strip of polycarbonate. I used the clear stuff, and the light in spring/autumn lands exactly on my back at my workbench. I've ended up stringing some shade cloth on fence wire to absorb half the light (but keep the warmth). On a cloudy day, or in winter I can just pull the shade cloth to one side. You won't have enough light with a single piece of 4x8 for any real growing.

John, A ghetto greenhouse! Seriously! The corner and center posts are 4x4 treated posts set in concrete. In between are 2x4s. One one side I have a 77" glass door and same size window that neighbors set out on garbage day. On the other side are a couple of 2'x5' windows (about 8x12" glass panels). The bottom foot or so on the sides and front will be 7/16" particle board insulated with R-30 insulation, as will be the back wall. The front will be the poly and since it is only 16 ft' long and the walls will have 4' of poly, I'll have two panels left over for the sides. (Rough Brothers sell the poly in 24' sections.) The top is 13 mil woven plastic but I'm dazzled by the light it lets in. Near the top of the back, it's over 90 percent on a sunny day (according to my lux meter). Seedling at the bottom of the back wall and in the middle are doing much better than they did this spring under 35,000 lux, but when I'm done, the back wall will be lined with Mylar. Yeah, www.roughbros.com. For the plastic, I talked with a guy at a paint supply store. He works part-time at a large greenhouse and the GH uses the same six mil plastic Pat sells to painters. What's more, it is far cheaper than what GH supply places charge! If my experiment works - that is I can raise close to 50 tomato plants that produce ripe fruit as they should, once the plastic needs replaced I will with the twin wall poly. As for the space between the poly and plastic, it will be 3.5" - the width of a 2x4 or 4x4. IME, as long as the layer isn't too wide, it works fine. I live in a very old house with old, drafty windows. I cover them with plastic in winter and the glass inside stays warm - there is about 2" from the glass to the plastic. Mike