"growroom" need advice

this is a pick of my growroom
it's outside in my back yard 3x4x5.5, 400w Metal Halide
seedlings were given toused black & white poly no other
insulation think it might be to cold and does a space heater mess
with the photoperiod

i've heard lots of people using space heaters. negatives of them are safety and electricity.
what are you temps in there?

60 to 70 degrees

Try to heat it up a bit more, I keep mine between 73-80. Where is the ventilation? Fresh air in and ventilation out is needed. A standard oscillating fan would suffice if thats all you have. I would lower the lamp a tad until the plants get some hight. Other than thay looks good.

its in my back yard outside, i just open the front piece of poly i used to cover
the way in, the same opening on the picture my tent
its covered by other trees (guava tree)
so its stashed no beef

thanx u all for the help