quilterj, You could make one with just one piece of insubrite and test it out to see if you think it would be OK or if it needed another piece. You could even test it out without sewing it first, then sew it if it is enough, or add more then sew.
I haven't used Insulbrite before, but that's just because I've never found any, yet.
Your questions are not stupid. Ask all the questions you want. There are tons of quilters/sewers on here that will be more than happy to help you out. AND...In most cases more than one person learns from one person's question.