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I make PVC greenhouses to get a couple of extra degrees for clownfish during the Hawaiian winter. I used to do the same thing in a no-snow US Zone 8.5 to keep plants and ponds from freezing.
You can cut a 10-foot length of 1/2 inch EMT conduit into three 40-inch pieces. Drive pieces of conduit into the ground about a foot or so. Then, slide a piece of 3/4-inch PVC pipe over the conduit. This helps keep the side wall straighter for more head room, and anchors the pipe bows securely in place.
I use cheap 4-mil polyethylene plastic with no insulation and plan on throwing the poly sheet away in the spring. It will last for 12 months in US Zone 8.5, but only 6 months in Hawaii. Its the UV that kills it.
I rip strips of wood lattace 1.5 inches x 0.2 inches and attach them to the two end bows with cable ties. A piece of 1x3 pine is attached to the pipe bows at ground level. This gives you something to staple the poly sheet to. I figure the cost is about one-dollar per square foot for the conduit, pipe, poly sheet, wood and fasteners. The whole thing can be removed in about an hour and the frame components are reusable.
The humidity is a curse and a blessing. Everything stays wet, but the high humidity prevents further evaporative heat losses. When the humidity is 100%, there is no more evaporation. Evaporative heat losses are usually number three after convective and radiant losses.
I once left the poly sheet on a plant greenhouse into the summer in Zone 8.5 - after the plants were removed, of course. The thing literally melted when the outside temperature reached 90F+. I have no idea what the inside temperature was, but the PVC got so hot that the center beam sagged down until it touched the ground.
-stevehop kins
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