I don't grow veggies so I'm probably not the best person to answer you, but I have a few thoughts.

My greenhouse is 10x12, so not much larger than yours. (Twinwall polycarbonate walls.) I've learned that smaller structures are harder to keep cool. I use shadecloth on my greenhouse roof, and many fans, to keep it cool. In my climate (zone 8, so warmer than yours) there is no one material that will keep the interior cool in the summer; I have to use a combination of shadecloth, fans, and ventilation. I suspect this is true for most of us, although I know climates vary a lot.

Having a greenhouse is one giant endless learning experience, but it can be a fun one. One suggestion would be to buy a thermometer so you can monitor the temps inside (so you can learn what happens.) If your greenhouse is not too far from your house, you could buy an inexpensive remote thermometer. I have a sensor in the greenhouse, and the digital display unit is in my kitchen. This way I can always keep an eye on the GH temps, even if I'm not out there. Over the years it has helped me learn what to expect, in various seasons. And it can give me a heads up, if it's getting too cold/too hot in there, so I can figure out what to do.

I think temperatures will definitely fluctuate throughout the day (and heck, throughout the year!) no matter what you do. That is OK, as long as the temps in there don't go too high or too low. What you have to do is learn what temperature ranges your particular plants will tolerate, and what your temperatures your particular greenhouse and climate will create for you. Then, you can hop back to this forum, and elsewhere, to learn what you can do to raise temps or lower temps. Vents, fans, shadecloth, misting systems, and so forth.

Nah, you haven't bitten off more than you can chew...it's just a learning process, and over time you will learn what works for you. I think most of us have frozen or roasted a few plants in the process of learning! The best thing you can do, I think, is try to find ways to learn exactly what is happening inside your new greenhouse. That's why I think an inexpensive remote thermometer might be helpful. (I'm using one from Walmart, although there are better quality ones online I'm sure.)

Maybe someone else with knowledge about growing veggies in Zone 6 will chime in. Hang in there, keep reading in this forum, and have fun.

Very insightful! Thank you for your input! Once I got it built using it was a bit daunting! I'll do like you said though and monitor temps throughout the year to see how it gets inside. Sometimes our summers can get super hot and humid, so part of me was afraid I would cook all my plants! haha but as you said, it's a learning experience!

Thank you for taking the time to answer my questions!

You're welcome. I know my post was really general (sorry not to have a good answer on the particular product you asked about!) You're very right that you need to buy plastic made for greenhouse use, with the UV protection.

I mostly just wanted to reassure you that most of us have been where you are, figuring out things a step at a time. I think a lot of it is good old trial and error, because we all grow different things, in different climates.

Our summers can be awfully hot here too. Cooling is a bigger problem than heating, for me. I keep cacti and succulents in my GH year round, even when our summer temps are above 100, so I couldn't do it without shadecloth, fans to exhaust the hot air, and occasionally hosing down my paver floor. The triple digit days are my least favorite greenhouse days.

Another thing I've learned is to simply watch my plants, and let them tell me what is going right or wrong. I can tell if they're getting too much sun, or not enough, for example. Sometimes it means adding shade to the greenhouse, or it might just mean moving them off the top of a sunny bench on the south side to a shady spot underneath a bench on the north side. Even in my little greenhouse there are different zones of varying light, so it's an ongoing process of learning which kids are happy in which spot. Fiddle, fiddle, trial and error! I hope you enjoy it.

Since the plastic is shot anyways, remove it and put the shade cloth on until fall when it starts cooling down. No need to fight the heat all summer. I'm about to take my plastic off and shade cloth on myself.