I think a 'greenhouse inside a greenhouse' is the best way to go about it and is probably how I will build my metal frame one next year. As Brain pointed out though, permits around here are a nightmare, especially for a greenhouse. Thus it has to be a 'temporary' structure which means no foundation, etc, so modular is the way to go. Plus I am not willing to spend the sort of money Forever Flowering charges for their houses. It would be nice to have it fully automated, but even a 10 x 20 like I have here would be about $20,000 (based of last quote I saw a year or two ago) where as this temporary one cost only $500. The breathable siding fabric would help a lot, maybe next year...
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I solved the automation aspect of light deprivation via the cheapest and most reliable method I could devise.
https://www.thcfarmer.com/community/threads/summer-2011-blackout-tent-needed.30866/
Automation was achieved via a single timer set to produce a 'on' state every 12 hours.
This 'on' drove a single pole relay which in turn pushed the button on a garage door opener.
The opener has an internal latching relay, so no further action takes place for 12 hours when the opener then travels to the other position.
It worked well, but there are other aspects that remain sub-optimal.
I used a single ply black plastic for my light shield. I subsequently learned that two ply works better.
Ventilation and humidity control need to be addressed. I had a fan blowing greenhouse air into the blackout tent. This turned the greenhouse into a dehumidifier in that water condensed on the inside of the greenhouse would reduce the water in the air which then gets circulated back into the blackout tent. Perhaps a dehumidifier is required.
This summer I am running the space as a conventional greenhouse.
Three autoflowers, some vedging moms, cuts, and seedlings along with garden plants and herbs. (Having my wife occupy the space with me is a powerful political move)
I feel the need for a better shade material but have been unable to find many options other than simple black plastic.
My original 10X20 frame was only $200 and I covered it with a single piece of greenhouse plastic. It is anchored with a deadman off each end with a cable running through the peak. The ends are controlled via assorted ties and restraints that allow for manual ventilation adjustments, and a thermostatic fan ventilates the space when temps go above 80 F.
Drums are full of water which yield up 10 deg F to soften the night chill.
Black plastic on N wall is a ventilation tube.
Radio plays NPR.
Moms on left, autos on right.
Seedlings are Tre Star Dog and Alien Rock Candy.
Current plan has the blackout tent back in play for summer 2013.
