homemade greenhouse?

I bought those little pellets of dirt that puff up when you put water on them to grow my herb garden. It came with a little plastic covered greenhouse and I can see in the afternoon that it is really hot in there…the dirt is actually hot to the touch. My questions are:
1. Can it get too hot? They haven’t sprouted yet but it’s only been just over a week.
2: Can I put something like saran wrap over my other herbs in pots for the next week or two to act like a little greenhouse or will it not work the same?

Thanks.

Question posted courtesy of: Portable Mini Greenhouses