I've read that room air should be turned over a minimum of about twice per hour in a greenhouse. In my zone 6a with below-freezing temperatures most of the winter it seems very wasteful to be pumping heated air right out the window. Are there any standard approaches to conserve heat while getting fresh air? Like using a heat exchanger to heat the incoming air with the outgoing air?

I have seen Millet's approach with painted-black, water-filled drums to retain heat but I could not do this as my greenhouse will be above ground and I could not have much water before exceeding the load rating of the structure. However there is a material used for thermal storage that stores more heat for its weight as it melts and freezes at 64.4F. Seems to be cheap and safe to store. http://en.wikipedia.org/wiki/Sodium_sulfate#Thermal_storage

This page http://www.agrisk.umn.edu/cache/ARL01480.htm has some other good ideas and mentions that 2.5gal of water per sq foot of glazing is enough, which for me is only 940gal which is well under the load limit of the floor.

I researched this more and there is a device known as a Heat Recovery Ventilator that uses outflow air to warm inflow air using while circulating. Depending on how well I am able to insulate the greenhouse I may consider this.

I you want to minimize costs (as we all want to do I guess) I would start with the best possible isolation. Styrofoam plates seem to be very usefull resulting in less heat needed from an external source. It's also light weight so that shouldn't bother you like the water barrels.
I think styrofoam plates are a very good choice compared to polyester panels because eve on sunny days the temperature won't go up to much (unlike plasticlike materials).

If you manage that I guess an electrical heating device won't be costing you all that much to keep the greenhouse warm.

You could also choose a standard greenhouse warmer which runs on petrol. Intended for keeping the greenhouse frostfree though combined with isolation could garantuee a nice temperature. DOwnside: petrol exhaust (hough minimal, can't be all that good for citrus).

I must say I see good things coming from your 'Heat Recovery Ventilator'. Just considering the fact that it blows fresh air inside is a very positive thing!

Not sure what you mean about styrofoam plates... you mean for wall insulation?

My plan is to get rolled fiberglass for non-glazed walls and tyvek or some sort of vapor barrier to help keep drafts out, and an electric heater + heat recovery vent if heating costs are high enough to justify it.

I'm kind of torn on choosing heat absorbing materials (minimize cost) or light reflecting ones (maximize growth).

I noticed a commercial combined aquaponics/greenhouse heaped their compost up next to their aquaponics tanks (perch) to let the heat of the compost pile heat the tank and the greenhouse. I thought that very clever.

Obviously one only has so much space, but using old out of circulation municipal garbage cans for composting in the greenhouse in winter may be of use in both cutting heating bill and making greenhouse more thermostable.

Of course where I am at cooling the greenhouse, not heating, is the problem.

While I suspect the containerized compost idea isn't practical I started doing a little research. This site http://www.spur.org/publications/library/report/critical_cooling/option13 seems to suggest that the *best case* energy potential from a ton of compost is 1300 kilowatt hours per ton, or 0.65/lb, or 2250 btus/lb. I forget how many btus I need.

Am I correct in assuming that there may be no benefit, or a negative benefit, of using thermal storage (i.e. water jugs or similar) if temperatures are allowed to drop below 55F at night, but are still above freezing?

Right now my little heater isn't sized to maintain 55F at night with <32F outside temperatures despite its thermostat being set at 55F. This will be fixed, but not immediately. I have my pots sitting on sealed 5gal water buckets but I am thinking this is preventing the roots from quickly heating up to 55F+ in the sun when they need it.

Also, I'm trying to find a source of bulk glauber's salt. Most places only sell food or lab grade in small quanitities.

My joists measure 9" thick with 16" on center spacing. I think the 9" is actually 9.25" which means 2x10 lumber.

I am having a really hard time finding a calculator that tell me load capacity for a span, instead of the reverse. But it seems that 2x10 construction can handle at least 20lbs/sq ft static load over a 10ft span. 20ftx24ft is 480sq. ft. which is 9600lbs.

9600/8.3 (lbs/gal water) = 1156gallons of water or 21 55gal drums or 231 5gal buckets.

Uh, I must have done some math wrong either way earlier or just now as that is more than enough. I'm glad you mentioned this.

I have been looking into phase change materials and they look too expensive anyway. Glauber's salt and similar materials are supposedly unstable over time and separate, losing effectiveness. I think water will have to do after all.

EDIT: I think the limitation is the middle support beam and not the joists themselves. I don't think adding unnecessary weight is a good idea.

Brian,
Why can't you put a large insulated water container in the garage, and a small pump to move water through pipes in the room during the day to cool it, and return heat at night.

I saw a system like this for a solar hot water system. You can build the container out of cement blocks and styrofoam.

Since you don't need to control temperatures nearly as much, it would be much simpler to install.