crittergarden said:...
I have room in the basement ONLY.
The temperature fluctuates wildly down there.
...
Is there NO system available where you can dial in what you want the temperature to BE, steadily?
...
Thoughts?
I don't think seedlings are HURT much by being a little too cold - they just germinate slower. And once they emerge, your basement might "pre-harden them off", at least a little.
You might get part-way there by making a big tent that encloses your lights and trays. That will tend to keep temps UP when the basement gets cold. I would try to use white reflective panels as part of the tent, then plastic over the top to hold warm air in. I applaud using Styrofoam to keep the heat-mat warmth from running away through the shelving.
However, if you make the tent tight enough to hold in much warmth, it will hold in almost all the humidity. That might be OK until the first few seedlings emerge - then it's a formula causing lots of damping off.
In fact, if the tent cuts down on the drafts, it might be bad for seedlings' growth phase. Maybe a small fan INSIDE the tent? Now it starts to sound like a plant research growth chamber.
And nothing here would keep the seedlings cool when the basement overheats. At best, a heat mat with a thermostat would shut off the heating pad.
Maybe enclose a few shelves with plastic, styrofoam or white drywall to keep heat in, and START your seeds on those shelves. Humidity is a plus.
Then, when a tray or half a tray emerges, move that tray (or that half) to a shelf with less enclosure and more air circulation. Give it the cooler, drier air that emerged seedlings prefer.
My only other idea is an expensive one: put a small electric heater in your basement or under the plight shelves and keep it at a constant (warm) temperature. Maybe not THAT expensive if the basement is well insulated and you don;t keep it VERY warm. Just enough to reduce the wild swings.
. Darn you, Woofie--you are such an enabler 
