Hoop House

I'm building a small plastic hoop greenhouse (very small, 5'w x 20' long) o ver an existing garden bed. Using the gray pvc conduit, painted where it w ill touch plastic, with rebar stakes. Structurally I'm basically good. But I have questions about types of plastic, keeping it heated, problems to wat ch for, all of that.
There's lots to read online but i'd rather have a conversation about it. Lo oking for any advice, tips or warnings, thanks!
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Use 6 mil greenhouse film, not "vapor barrier plastic" or other hardware store plastics. The greenhouse product is treated to resist UV with a rated life of 4 years, while the vapor barrier will go to heck in less than a year in the sun.
For an extra penny per square foot, "anti-drip" or AD coating and IR (helps keep heat in) are worth it to me. That's one penny for both (not each) at my usual supplier.
If you run power to the greenhouse, using a double-layer of plastic with a small (50W or less) blower to keep it inflated helps plastic life (it does not flap against the structure in the wind as much) and is slightly better insulated.
Don't know about heating, I don't - well, I do know about heating, which is why I don't, more accurately. You might try the "Eliot Coleman" technique of putting cold frames/row covers inside a hoop house (double-layer coverage) - basically greenhouse heating is simply expensive - it's an inherently poorly insulated structure. One source gives an R-value of 0.83 for greenhouse plastic, and R1 or so for IR-coated plastic (roughly double that for two layers, inflated) - if your house is a cramped 5 feet high, you have at minimum 176 square feet of surface area - if it's 0F outside and you'd like to maintain 40F inside, you'll need 7040 BTUs per hour or roughly 2 KW electric heat (or about half that for double-layer plastic.) How expensive that is will vary with how harsh your local climate is, your electric rate or other fuel source, and what temperature you attempt to maintain overnight (daytime with the sun shining will be different.) VERY roughly it might cost me nearly $1000/year to heat your little greenhouse in my climate, with electricity from my provider as the fuel source.
If you get some solar heating during the day, unless you can figure out a way to remove it to "storage" VERY effectively, you soon hit a point where you need to vent it or the plants will cook. I have thought about solar PV panels running fans that would pull air from the peak down into pipes in the soil, but never actually implemented the idea, so I'm not sure how well it would work, if at all. I know for long stretches in certain months it would have no useful sun to speak of so it would not be much of a heat source. At least the coldest weather here is usually associated with sunny days, but they are also pretty short days.
Have you thought about ventilation?
Depending how far along you are, if your BED is 5x20, you may want to provide some room so you can walk, not on the plants.
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Thanks so much for the thoughtful reply. Lots of good ideas to consider. I've actually ordered a custom cover (tarp) made of woven UV-resistant gree nhouse plastic (similar to "Bob's super strong" but from a less expensive s upplier).
Our garden spot gets full sun, but we're in a canyon setting so sunset come s early as far as the plants are concerned. It will get ragingly hot in the re on the warm days - I plan to build end flaps that can be opened and then once we're past the heavy frost danger, roll up one or both sides to allow some ventilation that way. At some point in the season, we'll roll it up c ompletely or even replace with a sun blocking tarp and it will become summe r shade. Currently it is a freakish 69 but we'll get some more snow and fre ezing here (our safe last-frost date is May 8th)
As for heating on the coldest nights... not sure. I like your suggestion of little hoops inside the big hoop. That could be all that is needed here. A lso wondered about water jugs next to each plant, to get hot during the day and radiate some back at night. I've read about doing this with milk jugs painted black.
Wind is a big concern. I imagine I'll be fighting that for a while no matte r what. I love the pipes in the soil idea... maybe next year : )
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It turned out pretty good - got our superstrong clear tarp clamped on yesterday under cloudy skies - even transplanted some garlic from last year's garden. This morning I stood in my dry little hoop hut enjoying a cup of coffee in a snowstorm.
https://lh6.googleusercontent.com/-2NXwXmBt-GQ/VOkakFgk4BI/AAAAAAAALFU/t3iTT5yRj5Q/w506-h899-no/20150221_165348.jpg
I still need to put on the ends and finish stretching and clamping the tarp but i'm really pleased with the results so far. Here it is before we put the cover on
https://lh6.googleusercontent.com/-0ChgKMigfCg/VN_r5NDy_sI/AAAAAAAALBg/I3ZP4a0if5s/w1598-h899-no/20150214_174317.jpg
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On 2/22/2015 11:15 AM, Michael Evangelista wrote:

Good looking hoop house Michael, what's the berm looking thing behind it in picture 2? Looks like a nice windbreak to me but I don't live out where you do. Had a friend who built a camp that looked a lot like that, was cozy in the winter and cool in the summer. Unfortunately heavy rains in E. Texas ate it after one year.
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That pic suggests hilly terrain.
Look at the houses in the background.
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hey sorry i never answered this - been busy outside when i don't absolutely have to be at my computer. Our yard is carved out of a hillside so whereve r we haven't created flat space with retaining walls, we have a steep red d irt slope. The hoop house is built on top of what used to be a big raised b ed. So far results are excellent.
We've had mild spring weather here, and it looks like we're going to get of f with no late freeze (woohoo cherries!) but the temps in the greenouse hav e been great for the tomato plants
here's a photo from a week or so ago: https://www.amazon.com/clouddrive/share/Ocs5WBwHMNlmq2cY7sBrW9r0vNKBCwg9vUo NYXny-cI?ref__ph_share_link_copy_flash
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