Building Greenhouse - Polycarbonate choices driving me crazy

[orchidfan]

I'd like to build an energy efficient lean-to greenhouse - however, the higher the mm for the clear polycarbonate the lower the light transmission numbers. I have a mix of cattleyas, laelias, mixes of same, zygopetalums, dendrobiums (mix), miltoniopsis/miltonias, phals, paphs and a few cymbidiums. 32mm has a light trans of 59%, 20mm @ 62%, 25mm @ 61%. R value vs. Light transmission is the issue. For the product I'm looking at - For 32 mm, I'd get an R value of 3.57 with light trans of 59%. For 16mm with a 4 M wall type, I'd get 2.7 R value with light trans of 67%. For 16mm, triple wall type, I'd get 72% light trans and R value of 2.5. I just don't know what light trans the plants need - and I know that with my selection of orchids it will vary. Any one have any suggestions or experience about which light transmission level to go with? Help!!!

[Ross]

It's actually a process of determining how much you are willing to pay for heating in winter vs. light transmission. I personally would opt for the highest R value, but I live way north (translate colder) than you. Triple wall is in vogue right now in these days of energy crunch, but you are right - there is a price to pay (transmission of light). Is 59% a really big deal? Based on full sun, probably not. But if you have 100% transmission on a day when light is dismal (say 1500FCs) then what's the big deal? You'd need supplemental lighting anyways. I think you need to analyze this a bit more before springing for a greenhouse. Big bucks to purchase and way big bucks to heat.

[orchidfan]

Thanks. I've been heading toward the R value over the light trans. It will be on the south side of the house with minimal shade. Didn't think about the option of adding lights. The greenhouse will be built starting in May next year so I'm in the planning (and being driven crazy by) the details. It won't be a kit greenhouse. I'm working with my family (who all built their own years ago) and they either still have glass or older polycarb and are at a loss about which to choose for mine. Adding plant lights in the greenhouse isn't something we've done in the family - but that might just be the ticket for me. Thanks. (I've accumulated too many plants to try to keep them all in the house so I have to go into a greenhouse.)

[John D.]

I agree with Ross. Higher R value should (if the house is tight) lower heating costs. You will probably need shade cloth in the summer anyway so the small loss of light should not be a big deal.

[Misting Systems]

I just notice your post.
Did you consider include misting system in your green house to control humidity? Let me know I can help with design.

Home

[Learner]

How big a greenhouse are you planning? The size of the area (read: volume) you need to heat could be as critical as the R factor. Obviously, the smaller the volume, the less total heat you will have to provide. Also, the shape of the GH is important because a short, more square, structure will be easier to heat than a long skinny one with the same volume

Also, is the polycarbonate you are looking at really over an inch thick or am I missing something? I would love to look at a web site for this material if you don't mind sharing.

[orchidfan]

The greenhouse will be 8' (from the house) x 16' (along the house) against the south side of the house. So the north side of the greenhouse is definitely covered - from an insulating point of view. 8' is as far as I can go - then I run into a property line problem. (I'm in a very old neighborhood with close homes.) Height against the house will be 9', decreasing to 5' at the outer wall. Thinking I'm going to have to run air ducts along the outside wall to keep the temp even. No problem sharing the polycarb site: Greenhouses, Sunrooms, Misting System Kits & More!. They have a 32mm clear that's 1.25" thick, R-value 3.57. Their product name is Macrolux. They are actually Ecologic Technologies Inc. in Pasadena, MD.