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new greenhouse planning

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brian:
Next spring I'm getting married.  Along with that, we are looking at moving to a house with more land (in the same area - Southeast Pennsylvania).  Wife-to-be fully supports my citrus hobby and I am planning on constructing a free-standing greenhouse at the new property.  I am hoping to be able to ventilate well enough to keep my trees inside year-round, planting most directly into the ground.  I'm not sure how realistic this is with 90+F summer high temps here.  I have just begun my preliminary research into this, so I still have a ton of unanswered questions.  I am thinking of something like this http://www.greenhousemegastore.com/product/ventmaster-30-commercial-greenhouse/commercial-greenhouses as the frame/ventilation solution.   I have an initial budget of ~$15-20k.  I don't have a particular size in mind, but I'm thinking at least 25'x50', which is about quadruple my current floorspace, and 15'+ tall, which is twice my current height.  I don't need to move my plants in the first winter as I will still have access to my existing winter greenhouse for the next year at least. 



I know that Millet has a substantial greenhouse in a temperate climate that I can use for inspiration.  I imagine others on this forum have some experience with these things also.  Some of the big questions I have are...

Glazing material:  If I remember correctly, inflated PET sheeting is the best in terms of cost and insulation, but isn't very durable.   Will PET stand up to snow load in temperate states?  I've been extremely happy with the dual-layer 8mm rigid polycarbonate that I use for my winter greenhouse, though I can feel the heat loss through it in winter.  I'm thinking 10mm+ if I go this route.   It isn't cheap, but is far cheaper than something like glass.  I haven't priced glazing a structure this large yet.

Ventilation: My winter greenhouse seems to be fine when buttoned up all the time.  That is, I have zero ventilation beyond whatever leaks out from cracks in the floors, walls, etc.   It doesn't seem to be starved for CO2 as I get lots of new growth each winter.  Is this reasonable to expect for a larger greenhouse or will I need to either ventilate in winter or provide a supplemental CO2 source?  I haven't had an problems with condensation yet.

Heating:  I believe natural gas is the cheapest heat source at the moment.  Would make sense to allow some of the exhaust gasses to flow inside the greenhouse to add C02?  Obviously you would need some kind of sensors to detect dangerous pollutants or CO2/CO levels.  But my understanding is that natgas & propane can be very clean burning if properly maintained.   

Cooling:  my winter greenhouse gets above 115F in the summer when it is empty and the plants are all outside.  Is it possible to keep temperatures reasonable with only natural+fan ventilation?  It would certainly be possible to have a shade cloth either year-round or in the warmer months only.  I'm not sure how much air conditioning would cost but I imagine it would blow my budget entirely and I'd have to rethink the whole project.  It gets humid in summer here so evaporative cooling may not be effective.

Extra insulation: I can feel that the main source of heat loss in my winter greenhouse is the polycarbonate glazed face.  I wonder if it is realistic to have some kind of insulation blanket can roll down at night over the roof of the whole structure - assuming there's no snow?  I could also have grow lights running at this time as supplemental light. 

Water: it looks like most greenhouses can support gutters, which means rain barrels are an easy source of water for plants.  In addition, I would run a water line & spicket out to the greenhouse.  I know that others are using water barrels for heat storage also, I should be able to do the same.

Millet:
I'll add some comments a little at a time rather than all at once.  The north wall of a greenhouse does not add any light to the greenhouse,  in fact north walls lose greenhouse light.  On my greenhouse, I covered the inside of the north wall with 1.5 inch silver coated polyurethane insulation board. I also taped the seams of the boards using the 2" silver tape.  This reduces heat loss during the winter and provides additional light by reflecting the sun's light back into the greenhouse.
Millet

cory:
Congratulations Brian on your upcoming marriage, new house and new greenhouse!  Very exciting.  I would love to have a large greenhouse like you are planning too.  My greenhouse is only 12' × 16' so I can't provide much help beyond the basics which you have probably learned from your current greenhouse, but that experience you have gotten will help you with planning your new one.  From what I have read it is actually easier to keep the climate of a large structure more stable than a smaller one.  Good luck.  It will be interesting to read what others will recommend for your plans, and I am hoping to learn from them also.  One thing I would recommend you think about is how you could provide different environments within the structure in case you want to grow plants that need different temperatures and humidity levels.   I really can't do much with that in my small structure besides finding small micro climates.  I grow orchids and citrus in mine.  I would love to have at least a cool and a warm section to better accommode the orchids I grow that grow best cooler and warmer than I keep the greenhouse for the majority of my plants.  I look forward to reading about the plans as well as the structure's construction progress and startup.

Cory

Tom:
Congratulations on all the excitement that is upcoming in your life ! I will enjoy reading about it here because you are already way out of my league. Your situation is much different than mine but very similar to Millet. He will be an excellent resource. It sounds like you have a great start ! Bigger is better. I'd think a smaller greenhouse for plants with extremely different requirements would be better if possible. You have the citrus bug for sure ! I think the variegated minneola is the best looking citrus fruit. For my area Sasumas and Meyer lemons work best. With a year long greenhouse I guess anything is possible. Home grown pineapple would taste unbelievable I think ! Tom

brian:
Millet, insulating the southern wall is a very good idea.  I expect I will do this if practical.  I may have considerable choice in the direction of the greenhouse... would it make sense for the longer length to face north-south, or east-west?  I would think north-south would receive the most light in summer, but maybe not so much in winter (because of lower sun angle) when it is needed most.  I also may be able to build the greenhouse into a slope somewhat, so the rear wall could be partially underground and insulated in that way, or the whole thing could be a lean-to into a slope depending on the lay of the land.

Cory, yes I was thinking I may have a section for higher humidty and heat and grow some true tropicals.  I'm excited about this prospect.

Tom, rather than have multiple greenhouses I'm thinking of adding a polycarb wall or similar and having different sections as I mentioned above. 

I've done somewhat more research, I'm using GreenhouseMegaStore.com as a reference for now.  Most greenhouse kits are expandable, so I can start smaller and add space later if it isn't too difficult to expand.  However, it could be complicated if I need to get a new permit each time and add concrete footers or dig.   It is an option though.   There does not seem to be meaningful cost savings when buying larger sections at once versus adding sections over time, though perhaps freight cost if that is not included.

I ran through the heating calculators on www.littlegreenhouse.com and if I'm doing the calculations right the insulation advantage of 10mm versus 8mm polycarbonate may not be enough to justify the added material cost, especially if I may have to replace it every decade.   It looks like I definitely want polycarbonate, though, as PET film doesn't look sturdy enough and glass & acrylic are more expensive than polycarb for no clear advantage to me.

My current question that I have no good answer to is - How much ventilation is required to maintain reasonable midsummer temps?  It seems there are a few styles of ventilation:

natural: thermostatically controlled vents throughout the entire length of both walls and a ridge vent through the entire length of the ridge.  This is the design of the Ventmaster link above.
most expensive) 
fan-powered: vents on short ends of greenhouse, powered by fans.  Either a pair per end or the entire length of the short ends (minus doors)
fan-powered plus Active cooling:  (condensors, water-wall swamp coolers, heat pumps)

I am wondering how much is required in my climate.  I'm willing to do some labor twice yearly to simple remove wall segments or something to allow extra ventilation in summer if that somehow introduces huge savings versus a more complex ventilation option.  I was initially hoping that the cheapest option would suffice, but if I remember correctly Millet has both shade cloth *and* active cooling to maintain temps and he is in a similar climate to mine.  So perhaps he can offer advice on this portion.

Ventmaster design comes with PET sheeting roof but it looks like you could use PC instead


Ranger Series powered fan vents on ends instead of side/ridge vents


EDIT - found some great information on the PolyVent system the Ventmaster design uses:

"A second automated passive ventilation system is the Poly Vent from Poly-Tex Greenhouse Co. This system is a sidewall formed from a series of polyethylene tubes that are connected. When the tubes are inflated, the Poly Vent makes a tight-fitting, double-layer greenhouse sidewall. The inflation fan is controlled by a thermostat so when the greenhouse is warm, the fan switches to off and the wall opens; when the greenhouse is cool, the fan switches on and inflates the tubes thus closing the wall. This system has worked fine on research greenhouses at the South Farm for six years. Each wall is controlled by a separate thermostat, so staged control is achieved. This system is best for small greenhouses, up to 60-80 feet long; with longer greenhouses, wind and rain tend to move the tubes from their enclosure. This system is less expensive than fan ventilation, initially; but the tubes have to be replaced every 2-3 years so the system still has a recurring cost.

Contact us if you need an address for Poly-Tex. This has been a simple, reliable system in our use at South Farm. It is designed for straight sidewalls, but might work on a curved wall of a quonset. Remember, when the power goes off, the wall deflates and the greenhouse is wide open; this is quite a disadvantage during winter power outages! They do sell a battery back-up system."


On evaporative cooling:   Pennsylvania average summer afternoon humidity is 56% per http://www.currentresults.com/Weather/US/humidity-by-state-in-summer.php. 

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