Does mylar trap heat?

If I made a grow tent out of mylar by just putting it on a wooden frame would this trap heat and humidity?

some. but how much? well, that depends. is it going to be airtight? if so, you’d need some air exchange, or supplement co2. and you need fans. which might make thin film flappy & maybe noisy? id consider something like thick landscaping cloth, as an outer layer? tarps? you gain a little strength, if stretched tight.

i built a wooden frame & enclosed it with 1/2” styrofoam (or polystyrene?) that has foil lining. it’s easy use. i cut with a box knife, which made for easy exhaust holes. and it gave me some solid structure. the stuff was fairly cheap.

Think about the mylar survival blankets. That answer your question?

yes it will trap heat and be very reflective.
the outer covering mentioned above will not only add extra strength, it will also protect the delicate mylar and help it not get ripped