Polyhouse Design Considerations

Hey Guys - I have been researching naturally ventilated polyhouse for a while now and want to take the plunge soon. However, I have several questions on design that I would appreciate input on as there are no such structures in my area.

  1. Say wall height of polyhouse is 3 m and top height is 5 m. If I want to grow cucumber mainly and maybe capsicum, do I need horizontal hanging bar in the greenhouse structure? Instead I was thinking I could run steel / GI wire horizontally at wall height and then hang rope / net from this for taking the crop vertical. What do you think? The GI wire would give me flexibility on spacing and save cost of horizontal beam.

  2. Do you see any major ventilation issues if I do not go for saw tooth and instead go for round roof (like the type in the attachment) with top ventilation as shown? I assume ventilation will not be as good as saw tooth but I am hoping it will not be too bad with the top as well as side ventilation on two sides. This kind of structure would save me cost compared to saw tooth.

  3. What mesh insect net do you use? My main problem is whitefly (primary) and thrips in late summer months. 50 mesh seems best for exclusion but I am worried that the small hole size would really reduce ventilation. So I am thinking of going with 40 mesh. What do you think?

  4. The main thing I cannot figure out is how to incorporate shade net in polyhouse. One option is to have it on exterior like the picture attached. But having these kinds of extra columns for the shade net increases cost a lot. The other option would be to fix the shade net directly on top of the plastic film using the locking channel. But I read on Ginegar and other manufacturers’ website that they discourage this as it damages the film. Another option would be to install the shade net inside the polyhouse but then the heat would already enter the polyhouse first and I feel this would not reduce temperature much. I am also not sure how to fix shade net inside the polyhouse. Would welcome your views on this.

  5. What % shade net do you use? I am thinking of between 50-75% but cannot decide as no one in my area uses shade net.

Thanks a lot,
Rana





Hello rana

Let me talk point wise. 

  1. Are you talking about 2mm ss pipes that joins vertical pipes ?

  2. If it saves cost, i think its worth it.  But if you have ventilation problem later than it will become very tough for you manage your crop and control diseases. Check for air velocity data of your region for a whole year, talk to neighbour farmers. If its good wind, then you need not worry. 

  3. You are not using natural ventilated polyhouse as it is, do not put a very fine insect net otherwise it will be alot humid and hot inside throughout the year.  And IF their is not a major infestation of pests in your region i would suggest you to go for shade nets on side.

  4. Exterior i wont suggest because if the a strong gush of wind comes in the region which is generally the case during the onset of monsoon, the net will get damaged.  I prefer to put inside, a retractable net which i can control according to the  season. 

  5. Go for 50percent if your summers are not more than 35 degree celcius. But if its above that range you should go for 75% or above. Shadenet is of great use in maintaining temp. Inside ph during summers, i using shadenet only in summer time  And most the time except summers its open in daytime and closed at night. 

Aman

Hi Aman,

Thanks a lot for your helpful reply. Please see a few follow-up questions below:

  1. I mean horizontal pipes that are fixed to arches to provide support for hanging crops. E.g., if top height of greenhouse is 5 m, these bars are are 4 m. I do not see why these are required if I just use horizontal GI / steel wires for hanging the crop instead.

  2. Noted, thanks.

  3. Aman, what mesh size do you use? Would 40 mesh be a suitable balance between reducing pest pressure but keep ventilation going? I think 30 mesh size would let whitefly and thrips in easily.

  4. The shade net I would really appreciate more input from you on. I like your idea of shade net inside. But how do you fasten it? Do you simply tie the shade net to the vertical pipes of the of the polyhouse? What mechanism do you use for retracting it at night? I assume having the shade net inside has benefit of giving you extra night time temperature increase during the winter as well?

  5. Noted, thanks.

Hello rana

1.  The horizontal pipes are for the stability of the structure. Do you know about load distribution ? Its taught in engineering.  Basically the whole weight of the structure is distributed equally to all the pipes, and hence makes a better structure with equally distributed loads.  These horizontal pipes supports the structure and during the time of strong winds, it protects the structure from falling apart.  It needs to be their, gi wires cannot replace it, hence its purpose is not only to support your crop but also to support your structure. 

3 buddy i use shade net on sides but i have good air movement in my region so i will go for 40 mesh insect net next time. I wont recommend 30 mesh though, wont solve either of the purpose.

4 cannot explain it like this. Its a system based on gi wire and metal hooks plus their are ropes hanging on both sides, pulling 1 closes the shadenet and pulling other opens it. Will post the pictures, and then shall explain u better.

Aman