Is a two layer poly roof for commerical greenhouses standard or....
Printer-friendly versionGreetings I am new to the forum, but have a questions regaurding greenhouse heating.I am growing Ornamental grasses as a second business and I am a Landscaper by trade. I have recently required some old Tobacco greenhouses to use for my grasses and propagation. 30ft by 75ft with a three foot sidewall drop down curtains and elevin foot 4 inch height. I am in the process of making truss systems for each bow, for snow load and better support for a high tunnel house. My question is can I install one layer of poly covering over the hoops then inslall a second layer about 7ft up from the bottom of the greenhouse supported by plastic coated cables running accross the greenhouse from bow to bow, and one cables running down each side of the greenhouse between the bows and layer of poly to hold the poly on the sides. Creating a open "attic" area with the wasted space overhead, thus elimating the need for an inflation blower.Instead of having a one to two inch warm air layer having a four ft warm air space. I already have two louvered automantic shutters in the top of the bow at each end three ft by three ft, to allow hot air to be moved thru the top of the Green house. Should I attenpt this design?
I see no problem with this configuration as long as the layers are secured properly. I'm no physicist, so I'm not entirely sure that the increased air mass between the layers will necessarily translate to better insulating value. It makes sense, but I would think after a certain distince the effect is minimal, kind of like a law of diminishing returns. I would also be careful about allowing too much heat to escape through the louvered windows in the "attic."
In a house there are shutters like this because the attic space is normally kept at outside air temperature and insulation is placed above the living space. Your shutters could act in a similar manner, when in fact I assume you want the temp up there to remain elevated to help with heating costs.
Those are my thoughts, but like I said I'm not a physicist so take it all with a grain of salt.