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Can I double up my twin wall polycarbonate?

 
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Hello. I have a commercial greenhouse kit I put up around 6 years ago in my backyard. It's 22 x 38, and 15' to the peak. It's skinned in 6mm twin wall polycarbonate. I have an above-ground swimming pool in the greenhouse that does a great job of keeping the greenhouse warm in the winter,  and also adds humidity which the plants love. There cam be two feet of snow on the ground and it can be 0 degrees farenhite outside, and it will be a nice 65 degrees inside.
The only drawback is my natural gas bill skyrockets in the winter as the pool heater tries to keep up. I considered replacing all of my polycarbonate panels with the 16mm triple wall ones, but the cost for a greenhouse this size would be enormous.  Then I thought,  I've already got the 6mm twin wall panels. I even have several spare sheets from the build. I was considering adding some slim wood framing to the outside structure to provide another gap of air, and then installing a second layer of the 6mm panels.
I know it will greatly increase my insulation factor, and at a fraction of the cost of upgrading the existing panels. What I don't know is how it will affect the plants. Will enough sunlight still get through and will the plants still thrive? Has anyone done this before, or does someone smarter than me, probably most of you, have any input?

Thanks,
Ryan
 
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Location: UK
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Haven't done it but you probably reduce light in the greenhouse by 10-15% for each sheet of polycarbonate.  This will affect your plants, how much will depend on how much sun you get, what you are growing and the quality/age of the polycarbonate.  Watch the sun, if it's so low as to only really shine through the walls in winter then putting a big insulating hat on the outside of the greenhouse might be best (removing it once the sun is higher and temperatures are rising).
 
pollinator
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Location: Near Philadelphia, PA
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Can you cover and insulate the North and hip walls?  You often lose more heat from those than you gain in light, so blocking them off with a high R wall would be beneficial - you might even move some of that twinwall over to the other sides.  Also a windbreak outside that doesn't obstruct the light might help too!  Let us know what you do, pictures would be great!
 
pollinator
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Doubling up the thinwall will cut your heat loss in 1/2.

To make an even larger improvement, convert the north wall (or south wall if you're in the southern hemisphere) and back half of the side walls and roof into an insulated walls, they don't have to be clear since the back side of the greenhouse doesn't contribute much light to the interior.

Google "Solar Greenhouse" for more information.
 
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