I finally finished the greenhouse on the roof of my apt building here in Brooklyn, measuring 10' X 20'. I attached all the plastic using Wiggle Wire, which turned out to be strong enough to make it through Hurricane Sandy, so W.W. has my endorsement forever;) My landlord asked me to secure the GH frame using pallets, so I ended up bracketing the pallets to the baseboards and then cutting out the interior top boards of the pallets to form planting beds. We lined the pallet/beds with landscape fabric and threw in potting mix.
I don't have access to electricity on the roof and want to supply additional heat if possible. I've been tracking the temperatures so far this Fall and have had pretty typical internal/external temp. fluctuations during the day and typically similar internal/external temps at night-- I get warm internal temps during the day, and much cooler temps at night. I investigated passive solar heaters using the basics of convection, but feel like this might be redundant for a greenhouse already trapping heat when the sun is out only. The GH plastic is already serving this function.
I'd love to keep the temp up at night as much as possible and started investigating passive solar heating, using water to trap heat. I found this site to be helpful in terms of calculations for how much water you would need to keep internal temps up. But I don't have enough space to stack up a bunch of barrels, so I want to experiment with either water bladders or hanging containers from the hoops inside. I'd be open to any ideas folks might have on how to do this inexpensively, or any links people could send on about how to make this work best. Thanks in advance. I'll post my experiments here as the Winter progresses, and maybe post a summary and tool recommendation after tweaking the design.
This looks very cool! Looking at things, I understand you space issue. Bladders or something to store water beneath the pallets seems a good idea. Hanging something from the hoops inside might block the sun for your plants though?
Maybe you could design a system to circulate water outside during the day to capture sunlight and heat.
Water weighs about 8lbs/gallon, so a full 250 gallon container would weigh at least a ton, and a lot of roofs can't support that kind of weight in a small area.
Another thing to consider is surface area - the more surface area you expose the water to, the more heat loss there'll be. So on a cold night, you may lose all of your heat by midnight instead of 6am! Using a variable number of bladders that can be easily filled from a larger water mass (tank) is probably best. Good idea Louis!
Thanks for the input, gentlemen. I was doing some rough calculations (based on the U. Mizz site), and I determined I'd need about 1,000 gallons of water to keep the temperature up through the winter. So that would be about 18 55-gallon drums. And yes, jbd, water weight would normally be an issue for a roof, but our roof happens to be concrete-finished and reinforced--overbuilt to say the least. Good for me as a gardener... I did add about 100 gallons of water in various buckets/containers, etc but haven't noticed any real temperature changes.
An Orange County, NY farmer recommended using drums of water as props for planting benches, thus using the space the drums are taking up to my advantage. I would have to pull up the planting beds and source the barrels, which is more work than I can put in right now. But it's definitely on my mind.
One other point, another farmer in Saratoga Springs, who is dealing with the cold a lot more than us, said water could be HARMFUL in the winter if it ends up freezing. He said if it freezes, it might end up radiating colder temperatures than the air. It takes a lot of energy to thaw the water out, so the water may be staying colder longer than the air. Just a thought, but I'm definitely game for experimentation.
We're going to be working on a similar hack for this greenhouse with FarmhackNYC: http://www.farmhack.net/tools/solar-powered-greenhouse-ventilation#wiki
Maybe using the FIDO alert system...