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Double poly film - optimizing the layers  RSS feed

 
pioneer
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I'm about to buy 6 mil poly film for my greenhouse.  I'm aiming for a double wall glazing with a space between the sheets for insulation.  Here's a slightly longer write-up for reference (Mike's Passive Solar Greenhouse)

I've looked as a few online sources and it seems like there are two commonly available films:
  • A more affordable 92% transmissivity, UV resistant and Anti Dust film with about 10% diffusion
  • A fancier 88% transmissivity, UV resistant, sometimes Anti Dust film with Infrared Resistant, Anti-Condensation and about 25-50% diffusion

  • The price isn't that much higher to use the fancy stuff for both layers but I don't want to give up that much light.  Is it reasonable to use the simpler film on the outer layer and the fancier film on the inner layer?  Then I should get about 81% overall transmissivity.  If I used the fancy stuff for both layers it would knock me down to 77%.

    Am I correct in thinking that the cheaper stuff would then be in the sacrificial position against the worst elements so I might be able to replace that twice before I have to redo the inside.

    And most importantly, is that the best order for the films?  I'm thinking you want the IR coating on the inside to hold in the heat.  And you want it on the inside to control condensation on the inner surface of the glazing. 

    Thanks for any input!
     
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    Unless you are going to raise orchids I don't think you would gain enough benefit from the "fancy" material to bother with it.
    poly film this is what I like for my greenhouse.
    I use it for the exterior layer then I just use visquene for the inside for winter time. (my house frame is 2x4s so I have 3.5" space between the layers.
     
    Mike Jay
    pioneer
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    Thanks Bryant!  I'm not doing orchids but I am aiming for tropical plants in northern WI.  So my winter light is very borderline.  What characteristics would the orchids need that normal growing wouldn't?  Is anti-condensation only important for drips on precious plants?  Or does the condensation block lots of light as well?  I'm hoping to have moveable insulation at night in winter so maybe the IR isn't important to me.  I just don't know what I could be missing out on cuz I don't know enough about this stuff.
     
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