I would open it at times to allow some air circulation or you may get mold/fungus problems.

Any guidelines? 1/2 the day open, 1/2 closed? Or closed with sunlight, open at night? Thanks,

I would actually try and get some type of fan/air flow system going. Do you have a light source for the plants (it look to be indoors so was just curious about light, direct/indirect and type) ? Irrigation/mist ?

Rob

bsbullie: the greenhouse is exposed to direct and indirect sunlight. I can direct a small fan towards the rolled-up window if necessary. Irrigation is just with my bottle spray.

Maybe leave a little bit unzipped. That way air can circulate but most of the heat can be trapped inside

Thanks Rodney.
Last year I built a greenhouse in my garage with growlight. Things grew very well, but I HATED the spider mites. So I didn't set it up this year. Opted for this instead.