If you use normal poly go 6 mil, 4 year, UV resistant, IRAC. IR means it's somewhat resistant to passage of infrared radiation. AC equals anti condensate. It's not clear but rather hazy which reflects some IR radiation. I think in a double layer instulation they recommend IR on the inner layer and clear on the outer. I've done both layers IR and it works well.

Right now I have woven poly which is even more effective at keeping the greenhouse cooler by day in summer and warmer at night in winter. If you could afford 8-12 mil woven that would be good. I'd probably put that on the outside layer and 6 mil normal poly on the inner.

You should also consider both corrugated and double layer polycarbonate. The Solarsoft 85 and 90 material looks good, see below. Over the long run it's probably just as cheap as double layer poly because it should last 2-4 times longer than poly.

There's also the Solexx material. I've seen that on a local greenhouse.

Here is a link that might be useful: greenhouse coverings

I had a question about using double layers. If we do this do we need a blower. I was going to use a double cover and I was going to leave about 5 inches in between the two. Would that work without the blower?

Yes that works from an insulation standpoint if there is something, like the frame, in between to maintain the gap. But what the inflation does for you is control wind flapping. Just tacked on each side of a wooden frame and the wind will beat it up way more than inflated. If you don't have high winds like here that may not be a concern.