I just installed a portable air conditioner. It's one of those ones that hang out your window. Anyway, to say the least I don't know a lot about air conditioners or insulation, but I put a piece of cardboard to fill in the gaps around the air conditioner (it was the easiest thing I could find) and I'm wondering that the cardboard will just let all of the cold air out, and if it will, what would be better to use to fill in the gap? A piece of wood maybe?

Hi! MissRissa

Wood would be good :)
But it's a little hard to work with.

If you knew anyone in the construction trade
That had some leftover closed cell insulation (like styrofoam)
That comes in sheets, they may give you enough to make a curtain.

Are you sure you checked the sides of the A/C unit?
They are made with slide-out curtains..

In lieu of all that, a double or triple layer cardboard would be better.

K

Thanks KUXJ!

Your welcome.