I picked up a light/moisture/ph meter pretty cheap at my local hardware store. Inside my gh, light levels are only half of direct sun. I feel most things are growing fine full-term in this environment. Outdoor stuff is not growing at twice the rate, for example. Except maybe with the corn, where the gh plants are definitely smaller and wimpier, but it doesn't look like I will see many ears this year, no matter where they were planted.

My feelings are that the somewhat shaded covering will help your seedlings. Keep in mind you'll need to harden them off to plant in direct sun if the shade is too heavy where they are started.

Venting is important, also how you anchor your structure. I'd wonder how long-lived the covering is in direct sun, or how much of a snow load it could take over the course of several years. Do you plan to remove the cover and store it after your seedlings are planted out?

For 300 bucks, I ordered enough twinwall poly from FarmTek to cover my home-built 2x4 gh frame. Year two and it's working great, handles a 12" snow load no problem and shades just about right to avoid baking plants in the gh too much.

I've actually left my door open this year. Nuisance bugs got in anyway, even with the door closed, so now at least the bees help pollinate my squashes. At least one commercial grower I know leaves both ends open on the gh she uses to sell plants.

I grow full-season in mine. I start seedlings indoors under lights, but I did start some things directly in the gh this year.

$300 ain't bad for a 10x20, but at the same time, I'd have to think it's probably kinda "cheap" in construction, so be prepared to have to spend more to seal, anchor or beef it up to make it more reliable or suitable for your seedlings or environment.

I also needed to add things like inside watering, screened roof vents and windows, electricity on a GFI circuit, plus timers and thermostats to operate fans and heaters. And I spent major effort sealing out the critters who loved to burrow in from below.

I just don't think the setup you mention will last more than a few years unless you take it down and store it out of the weather once the seedlings are establshed. At that point, perhaps a simple hoophouse is more appropriate and less expensive. For not much more than your $300, you could upgrade to some other more-durable options that could be left assembled year-round.

In my own book, the maintenance issues when I was using a cheaper covering just got to be too much. It's so much nicer not to have to deal with stripping it all down and storing or replacing it once or twice per year.
-Ed

Ed, this is very much appreciated - it answers some questions I had and raises some. I'll post more later today - the garden is calling -

That's interesting about light transmission...The guy with the nursery comes from Quebec and said everybody there uses these car shelters, so they must be sturdy enough for a big snow load. UV-resistant in summer maybe not so much...I was wondering about pollination - and thinking about leaving the ends open for it...What I like is the instant-greenhouse (or really glorified cold-frame-ness) of this car shelter. And the fact that it's portable: we could even put it over some fall raspberries or whatever to extend the season. Plus we may be moving in the next year or two. Otherwise that twinwall poly on a wooden frame would certainly be ideal. Or triple wall. Pretty expensive though.

Thanks again for pointing out some of the drawbacks Ed.