Greenhouse Roof Vents?

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Hello all and thanks for letting me join!

I built my first greenhouse last year and used it with some mediocre success, now that it's almost January I would like to start plants.

I came here because last year I determined that part of the reason I wasn't very successful was because of my ventilation.
My greenhouse plans didn't call for any roof ventilation, only the windows on the side. This wasn't a problem when it was warmer out, but if it was colder outside, say around 28°F and sunny and I needed to open the windows, I ran into frostbite problems on the plants in front of the windows that opened.

I think I would have had better luck if I had had roof vents to allow the warm air to escape through the roof.

Now, as I mentioned above my plan didn't come with any roof ventilation, the entire roof - except for my propane heater exhaust - is covered in ribbed wall polycarbonate.
So, my question is: are there any easy ways to install roof vents in a greenhouse that didn't come with them?
I don't care if I have to remove polycarbonate panels or cut them, I'm just looking for a solution that doesn't require me building a new greenhouse.

I'm also open to making roof vents, if someone can provide me with good instructions or plans.
 
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I saw those threads, but I think that is different than my situation. My plan greenhouse has zero roof vents. So, I'm not sure how easy it would be to install on of those.
 

Oliver Buckle

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Hi, welcome to the forum.
I have a small greenhouse with a roof like that, I was using a small heater I had made using a candle under a large flower pot and it was making lots of condensation, so I drilled a 3/4 inch hole in the roof and used a length of copper pipe as a chimney. The first couple of feet of the pipe get nice and warm and condensation disappeared completely, there had always been some before, even with out the heater. Candles are expensive, I have now substituted a large jam jar full of paraffin (kerosene) with a tube holding a wick, it only gives me a bit of warmth, but it keeps the frost off the delicate things in there for the winter. I am thinking about putting another one the other end and having an almost heated greenhouse.
I described the jam jar to a friend, "Sounds like a Molotov cocktail to me" :)
 

pepper2.0

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Now, as I mentioned above my plan didn't come with any roof ventilation, the entire roof - except for my propane heater exhaust - is covered in ribbed wall polycarbonate.
So, my question is: are there any easy ways to install roof vents in a greenhouse that didn't come with them?
I don't care if I have to remove polycarbonate panels or cut them, I'm just looking for a solution that doesn't require me building a new greenhouse.

I'm also open to making roof vents, if someone can provide me with good instructions or plans.

Oliver Buckle's idea sounds good. I was informed this as another option, if you want more detailed info like measurements I can get that for you, although I assume it would depend on the size of the greenhouse as to what length and width of vent would work best for it.

Ribbed wall polycarbonate (also known as twin-wall or multi-wall polycarbonate) can add a bit of complexity, but it's definitely manageable. Here are some ideas tailored for this material:

PVC Pipe Vent:​

  1. Cut the Opening: Use a fine-tooth saw to cut an opening in the polycarbonate panel. The ribbed structure means you'll need to take extra care to make a clean cut.
  2. Install PVC Pipes: Fit PVC pipes vertically or horizontally across the vent opening. Use PVC end caps and vented end caps to control airflow. Seal around the pipes with silicone to ensure a tight fit.

Flexible Ducting:​

  1. Attach Ducting: Secure one end of the flexible ducting to the vent opening. Use metal or plastic brackets to hold it in place.
  2. Seal the Edges: Apply silicone around the duct to prevent leaks. Extend the other end of the ducting to where you want the air to flow.

Aluminum Air Duct:​

  1. Cut and Fit Duct: Cut the aluminum air duct to the desired length and fit it into the vent opening. Use a louvered vent cover to finish the outer end.
  2. Seal and Secure: Ensure a good seal around the duct with silicone and secure it with brackets or screws.

Old Fan Grills:​

  1. Repurpose Grill: Find a grill from an old fan and fit it into the vent opening.
  2. Secure and Seal: Use screws to secure the grill in place and seal the edges with silicone.
These methods should help you create effective ventilation while accommodating the ribbed structure of your polycarbonate panels. Let me know if you need more specific details or have other questions!
 

Oliver Buckle

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Cut the Opening: Use a fine-tooth saw to cut an opening in the polycarbonate panel. The ribbed structure means you'll need to take extra care to make a clean cut.
I used a hole saw, only wanting a 3/4 inch hole. I don't know for certain, but I would suggest four holes and join them with a hacksaw blade if you want a larger hole. Also that stuff can flex in high winds and cutting holes will increase that tendency, I would suggest stiffening the sides of the hole if you make a larger one, you don't want the sheet popping out with the next gale.
 

Oliver Buckle

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It occurs to me that those panels usually slide into slots and then have an end piece holding them in. Can you remove the end piece and simply slide them out a bit to create an opening?
 

GFTL

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You might try to simply put a small ceiling fan in to circulate the warm air downward. Why get rid of the heat you're paying to produce? I have an 8x32 GH and have 2 fans running 24-7. Air circulation is good for the plants too.
 

Oliver Buckle

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if it was colder outside, say around 28°F and sunny and I needed to open the windows, I ran into frostbite problems on the plants in front of the windows that opened.
It occurs to me that a frame to fit the window covered with something like nylon net might diffuse the air flow sufficiently to protect the plants, but still allow ventilation.
 
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When I was building my shed, I realized how important proper ventilation is for temperature control, especially in colder weather. For roof vents, you could try using a simple vent system by cutting out a section of the polycarbonate and installing a manual or automated vent. I had some success with this method when I needed to modify my roof, and it didn’t require rebuilding the entire structure. If you’re unsure about the installation, I’d recommend looking into companies like https://dakzorg.com—they have experience working with roofing and could give you advice on how to safely make those adjustments.
 
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big rockpile

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On mine I had two Fans found they was too strong blowing out the pilot light on the Heater so I'm using smaller fans just pushing the air.

Checked it yesterday and it was too warm. Turned the Heater on lower and I have an exhaust fan Turned it on problem taken care of.

I use my exhaust fan to take out the humidity.

But would like vents in the top for Summer time I have just been moving everything out.

big rockpile
 
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can u show us your greenhouse ??

i dont use roof vents (mine just a hoop house from cattle panels ) just open up both sides and the ends in the summer with fans moving air in the winter try to hold heat in as needed
building some frames for exhaust fans should be farily easy
 

oneeye

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No garden can survive without proper ventilation. The reason is the plant has to transpire gases through the stomata from the leaves. The best ventilation one can have is an intake and an output on opposite sides and an oscillating fan. To do that you will need 3 fans to keep the air cycling. I don't use greenhouse fans I use box-store-bought fans.

I have a high-velocity fan that pulls air in and a high velocity on the opposite side pushing it out. One is low and the other is high depending on if I want to remove hot air or bring in cool air to cool the greenhouse. The oscillating fan keeps all the air stirred up on the inside.

Now to keep costs down I use cheap homedepot timers to turn the fans on and off at different times to keep the electricity costs down. 30 minutes on and 30 minutes off is set differently and simultaneously as well on each fan. You will have to spend some time in the greenhouse getting all the fans dialed in to maintain the perfect growing conditions. Check it every hour for a weekend to get all the fans set to maintain the proper environment. Good luck friend. Keep us posted.
 

Oliver Buckle

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I have a different method, oneeye, I have a lovely big eight foot by thirteen foot cedar greenhouse that wasn't maintained for years by the look of it. The whole frame has slipped sideways and nothing quite fits any more, so the wind blows right through it. :) It still does a job, keeps the frost off, the fig in one corner gave us more figs than we could eat this year, an I grew some good tomatoes in it.
 
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can u show us your greenhouse ??

i dont use roof vents (mine just a hoop house from cattle panels ) just open up both sides and the ends in the summer with fans moving air in the winter try to hold heat in as needed
building some frames for exhaust fans should be farily easy
I can, but it will take a few days....or I could just send you pictures of my sister's, hers is very similar.
 

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