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Advice On Cooling a Greenhouse Wanted

Joined
Jul 27, 2008
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I am running out of room in my house to grow Nepenthes. I've been needing to get a greenhouse for them for ages but I always talk myself out of it due to the absolutely horrific weather we have here in North East Oklahoma. We literally have everything you don't want, from tornadoes & earthquakes to desert-like heat in the summer and ice storms in the winter that will put inches of ice on everything and then cover it up with 12 - 24" of snow (not every year but it has happened a few times in the past decade). A few winters back we hit -28F at one point....

However, freakish natural disasters aside, the main problem I can't seem to figure out how to address is the zone 7b heat that we are plagued with. The vast majority of my plants are highland Nepenthes that I grow indoors under lights and I really want to move them outdoors so they can enjoy natural sunlight (through shade cloth of course), but cooling seems to be a real dilemma. I can't see temperature-controlled vents doing me much good if the outside temperature is 105F, which it frequently gets to from July to the end of September. As of right now on October 3rd at 3:25pm it's 90F outside.

Also, I don't see swamp coolers being cool enough as I understand they usually can only lower the temperature inside a greenhouse by around 10F, and they may not work too well since we have really high humidity here most of the time and swamp coolers are evaporative coolers.

Misting alone won't be adequate because, again, we are dealing with extreme heat here, and it would also be counter-productive if using a swamp cooler due to raising the humidity.

So here is the only solution that I can come up with that I think might keep the highs during the day time to around 85F:
  • First, cover the entire greenhouse in 50% shade cloth
  • Second, install an automatic, timer-controlled misting system
  • Third, install a large 220v window unit air conditioner that runs during the day when the temperature exceeds, say, 82F

I think it might work at least during the day, though I have no idea how I would get the temps at or below 60f at night without spending $1,000.00 per month on electricity. Sometimes the low overnight temperatures in Oklahoma don't go below 80F, and they usually never go below 70F.

I really am at a loss as to what to do outside of spending a ton of money to cool the greenhouse.

Any ideas? Would be greatly appreciated....
 
Joined
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Misting alone won't be adequate because, again, we are dealing with extreme heat here, and it would also be counter-productive if using a swamp cooler due to raising the humidity.



I think it might work at least during the day, though I have no idea how I would get the temps at or below 60f at night without spending $1,000.00 per month on electricity. Sometimes the low overnight temperatures in Oklahoma don't go below 80F, and they usually never go below 70F.

I really am at a loss as to what to do outside of spending a ton of money to cool the greenhouse.

Any ideas? Would be greatly appreciated....

One issue with misting, you need the water to be evaporating in the air for it to create a cooling effect. I had misters set up when i was raising rabbits, and they did help a ton. That was the year we had 115 degree heat, and they did lower the temps around my hutches, but again only because of the evaporative cooling effect, which you won't get in a green house.

Also, don't underestimate the summer lows here, several times it has still been 100 degrees at 10pm.

Best bet for a full time outside growing area would probably be an insulated shed. I am basing this off of my rabbits, and the fun it was to keep them alive here, the plants have different needs obviously but the lethal extremes are the same. With an insulated shed, and a window unit for summer, and a space heater for winter, I bet you could do it. You get the space you need, but lose out on the benefits of the sun, and have to rig up lighting, and use a humidifier, but i bet the shed could be built or bought fairly cheap.

I get used sheds from time to time here in the park btw, if interested let me know.
 
Joined
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Are you sure you can't grow in your house anymore? It really sounds like what you need is a basement grow area that's moisture-proofed.

I can, and have been for a long time. I just want to expand outdoors to take advantage of the natural sun and free up space in my home.
 
Joined
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One issue with misting, you need the water to be evaporating in the air for it to create a cooling effect. I had misters set up when I was raising rabbits, and they did help a ton. That was the year we had 115 degree heat, and they did lower the temps around my hutches, but again only because of the evaporative cooling effect, which you won't get in a green house.

Greenhouse misting systems serve 3 purposes. 1.) Increase humidity 2.) water plants naturally 3.) cooling via evaporation. Evaporation from surfaces effectively cools said surfaces, which I'm sure everyone knows. That said, MANY greenhouses have misting systems installed to provide cooling, but those same greenhouses exhaust air at the same time causing the inside of the greenhouse to reach equilibrium with the outside, venting out humidity which promotes evaporation, effectively cooling the inside. I believe this would work great even here in Oklahoma with lowlanders, Sarracenia, VFTs, and other warm-temperate CPs. But in Oklahoma, venting a highland greenhouse during the summer would be unwise since the outside air gets 20F+ higher than the recommended maximum highs for highlanders. So misting an unvented greenhouse here would provide humidity and water but not cooling, since evaporation would occur very slowly.

I can counteract that by using an A/C unit. A/C units dehumidify the air which promotes evaporation & therefore increases the effective cooling achieved by misting. The problem here lies with cost. And if I'm using an A/C I don't greatly need misting for cooling purposes, but the two in tandem will increase the effectiveness of the other by a small margin.

Also, don't underestimate the summer lows here, several times it has still been 100 degrees at 10pm.

Precisely. My original comment = "Sometimes the low overnight temperatures in Oklahoma don't go below 80F, and they usually never go below 70F." Lows being the key words here, which is normally reached between 4 - 7 am. Highland Nepenthes need a temperature drop at night to thrive long term. It's something I've been able to provide in my grow tent using a window unit A/C that blows into the tent. Mine do fine with a drop down to 60F every night but some do better with an even lower drop.


Best bet for a full time outside growing area would probably be an insulated shed. I am basing this off of my rabbits, and the fun it was to keep them alive here, the plants have different needs obviously but the lethal extremes are the same. With an insulated shed, and a window unit for summer, and a space heater for winter, I bet you could do it. You get the space you need, but lose out on the benefits of the sun, and have to rig up lighting, and use a humidifier, but i bet the shed could be built or bought fairly cheap.

I have definitely considered this option and some form of it may end up being my long term solution as space indoors becomes more of a problem. I'm not sure which would be more expensive to upkeep though, an insulated shed or a well-cooled, shade-cloth-covered greenhouse, since in a shed I will have to run a BUNCH of lights on top of heating, cooling, & humidifying. In the greenhouse I'll have no need to provide light or humidity, but I'll have more of a problem keeping it cool for 8 months out of the year. Heating a greenhouse in the winter around here won't be a problem during the day, but will be a big problem at night from mid December through the end of February. Heating a shed that gets no sunlight, on the other hand, will present heating problems day and night, though since insulated probably won't be that bad. Remember in my OP, the second reason for me wanting to relocate to a GH is to take advantage of sunlight for the plants. So the shed idea will end up being a more expensive version of what I'm already doing but will free up space in my house so it's definitely an option going forward.

Is there anyone in a warm-temperate area successfully using a greenhouse to achieve HL conditions? How they are doing it? I'm assuming some form of cooling like what I listed in my OP would have to be used(?)
 
Joined
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oh hmm.. lots to think about here. You will need shading during the Summer without a doubt. The direct sun will be too much on the plants and the shading will help reduce the heat buildup. The aluminet reflective type are best for reducing the heat but are more $$ vs the typical black shade cloth. You will need to vent the greenhouse. It's impossible without a massive air conditioner system to combat the amount of heat sunlight will pump into the greenhouse. You will be effectively trying to cool a solar oven otherwise. So an air conditioner is really only effective for cooling during the night. During the day you would need to go with a combination of venting and cooling by rehumidification. Either using evaporative pads or misting/high pressure fogging. I don't know what the relative humidity is out there on a typical Summer day but a good evaporative pad system could provide more than a 10 degree drop over ambient air temperatures if the existing humidity isn't too bad.

I guess my first question is what temperatures day/night are you trying to obtain? I suspect from prior discussions with hobbyists that you are overly concerned with how warm the day temperature will be vs the night. Highland Nepenthes are much more critical of their night time temperature and don't really mind it in the low to mid 80s during the day. So I think if you can just keep the day time temperatures moderate with venting/evaporative cooling of some sort then you will be fine for most highland Nepenthes. Perhaps then an AC unit for night temperatures when you are not trying to beat the sun.. Although I suspect that your night time temperatures are reasonable for many highland Nepenthes also other than the real demanding highland/ultrahighland species.

Personally I would be less worried about the Summer cooling than I would be about the Winter heating... Have you seen the heating calculator? http://www.greenhousemegastore.com/greenhouse_btu_calculator

Tony
 
Joined
Jul 27, 2008
Messages
189
oh hmm.. lots to think about here. You will need shading during the Summer without a doubt. The direct sun will be too much on the plants and the shading will help reduce the heat buildup. The aluminet reflective type are best for reducing the heat but are more $$ vs the typical black shade cloth. You will need to vent the greenhouse. It's impossible without a massive air conditioner system to combat the amount of heat sunlight will pump into the greenhouse. You will be effectively trying to cool a solar oven otherwise. So an air conditioner is really only effective for cooling during the night. During the day you would need to go with a combination of venting and cooling by rehumidification. Either using evaporative pads or misting/high pressure fogging. I don't know what the relative humidity is out there on a typical Summer day but a good evaporative pad system could provide more than a 10 degree drop over ambient air temperatures if the existing humidity isn't too bad.

I guess my first question is what temperatures day/night are you trying to obtain? I suspect from prior discussions with hobbyists that you are overly concerned with how warm the day temperature will be vs the night. Highland Nepenthes are much more critical of their night time temperature and don't really mind it in the low to mid 80s during the day. So I think if you can just keep the day time temperatures moderate with venting/evaporative cooling of some sort then you will be fine for most highland Nepenthes. Perhaps then an AC unit for night temperatures when you are not trying to beat the sun.. Although I suspect that your night time temperatures are reasonable for many highland Nepenthes also other than the real demanding highland/ultrahighland species.

Personally I would be less worried about the Summer cooling than I would be about the Winter heating... Have you seen the heating calculator? http://www.greenhousemegastore.com/greenhouse_btu_calculator

Tony

Tony,

Thanks for the link to the calculator. Love that! Also, thank you for the advice about using aluminet reflective material over standard black shade cloth. That is something I had not considered.

As for the night time temps with the A/C, I definitely believe that is doable like you said. My concern is daytime cooling when it gets above 100F. A monster A/C may be the only way to keep things below 85F. That combined with both misting and the shade cloth you mentioned. To keep that affordable I will probably have to settle for a small GH, something like 8'x10' or so.

To answer your question about the ambient or relative humidity here, it usually get into the 70-80% range overnight where I live. During the day it doesn't usually dip below 55% unless its really hot in which case it might get down into the 40s. So yeah it's pretty muggy here when it's hot.

Thanks for your help!
 
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iv always been told that heating and cooling small greenhouses is just as hard as heating and cooling larger ones. higher surface area to volume of space ratio leads to faster losses
 
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