higher light or lower u-value?

lisa_z5wmi(Z5)May 17, 2011

I am trying to finally put in the lean-to greenhouse I have been dreaming of for ten years. I have a 30' x 8' south facing basement porch that is perfect. Now, I just need to decide what type of polycarb to put on the top. I grow mostly phals now, but that is just because they tolerate my low winter humidity. I also have cattleyas and various oncidiums, but would love to branch out a bit to some other stuff- maybe some vanda hybrids- you know the whole obsession deal..So my question is this; with a ceiling space of almost 500 square feet, and given that I live in Michigan zone 5 and need to keep the thing at least 60 F, do I go for the 16mm 5 wall Uvalue .33, but only 67% light transmission, or do I go for the 8mm Uvalue .5 with 77% light transmission? Solar gain for both are equal. I am really stumped here, guys. Please help me out!

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westoh Z6


Not a gh expert by any means, but logically...

I'd go with the 16MM. I think most people use shade cloth for south facing gh's??? so the 67% light would still be OK for almost all plants. The thicker poly will just help hold winter heat *and summer :-( * but still let in plenty of light.

Just to keep things rolling: Is it the amount of light or the heat/lack of heat issue you are contemplating?


    Bookmark   May 17, 2011 at 3:43PM
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It is both. I want to be able to bloom my orchids, but I don't want to use any more heat than I have to. I am sinking the thing two feet into the ground for insulation, and maybe using subterranean heating and cooling- all to use as little fuel to keep it warm as I can.. but all comes to naught if the plants don't bloom because there is not enough light.

    Bookmark   May 17, 2011 at 5:26PM
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arthurm(Sydney, NSW AUST)

I'd seek help from the local orchid Society members.

Just curious, there must be a point where an indoor plant room with lights becomes the best option.

    Bookmark   May 17, 2011 at 10:30PM
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highjack(z6 KY)

Lisa 67% would be about right for your location. You should be able to bloom almost anything. If you need additional shade you can always attach it inside to grow more low light plants. I have two different areas inside with additional coverage that remains in place after the outside cloths come down.

Arthur an indoor plant room is great but it isn't a g/h, it doesn't *feel* like a g/h, particularly when you want a g/h.


    Bookmark   May 18, 2011 at 6:38AM
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Great. Knowing things will actually bloom with the higher U-value is what I needed. I also think I will get to my orchid club meeting early (tomorrow, actually) to talk to the two members I know who have greenhouses and see what their transmissions are- that was a good idea. I do also know that the local phal grower for our grocery stores has 50% shade on top of 50% shade cloth starting in May or June and just 50% for the winter- so I'll do fine with those. It is just such a big investment in both money and all my daydreams from the last ten years that I want to get it right.

    Bookmark   May 18, 2011 at 9:34AM
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highjack(z6 KY)

Lisa I take both of my roof shade cloths off in the winter. The first 50% cloth comes off mid-September-ish and the second 50% cloth comes off early to mid November. I live in KY and I need all the light I can get during winter to keep me off the heater. You are farther north and much colder so I don't think you will need anything but your polycarb during the shortest and coldest days of winter.

During the winter I do have to run my vent fan(s) sometimes on really sunny days and sometimes I just prop the vents and door open to cool it off.

I do have two shade cloths up inside for both my Phal and my Masdie area year round.


    Bookmark   May 18, 2011 at 11:20AM
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I'm reading this thread because it was found under 'greenhouse insulation'; I'm looking at that because I'll be spending winters in North Carolina, Zone 7a, moving my orchids down from Maine (Zone 4) where they grow pretty well in a basement fluorescent light room. I figured on getting a lean-to greenhouse in NC and showing them what the sun looks like. Then I noticed minimum winter temps average about 25 for 3 months, with several days last winter to 11 to 13 degrees F. Not as harsh as Maine, but not what epiphytic orchids are meant to experience.

Highjack' post talks of dealing with winter cooling - how about winter night time heating? Insulation would help, but it still would require several thousand BTU's per hour. Then there are those short days... Is the greenhouse worth the cost to buy, operate, and the increased risk of plant loss from freezing? I'm still considering a lean-to, but another grow-room built around a large window sounds more practical.

    Bookmark   May 30, 2011 at 5:22PM
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richardol(Santa Royale CA)

"Is the greenhouse worth the cost to buy, operate" I don't think about that question. Having healthy orchids is worth quite a bit to me, but I still don't want to know the actual cost.

    Bookmark   May 31, 2011 at 8:10AM
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I have down-scaled my plans due to cost, but after doing the indoor growing for eight years now and never getting it even close to right, I am moving forward. I might have to bring them all in for the coldest months of the year, but I think with good insulation I will be able to double the amount of time they spend out of my house and greatly improve their conditions. Not to mention, stop filling up every sink in the house on watering day.

    Bookmark   May 31, 2011 at 5:46PM
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