As you suggest, aged galvanized (or anodized aluminum) yield significantly different values - about the same as white paint. But paint also makes it last longer. Higher roof slope eliminates most of the heat issue.

The aged emissivity of galvanized may eventually reach that of white paint, but it would run much warmer for years.

The emissivity of anodized aluminum depends entirely on the type of anodizing. Iridite (even BLACK iridite) is low-E (<0.2), and would never be similar to paints, whereas most other types of aluminum anodizing have emissivities of 0.75-0.8 or higher on day-1, with some exceptions. (I've designed too many aluminum heat sinks, I guess! :-) )

Cathy,

Checkout the problems I've had with 12 year old SIP roof panels in a previous post. I would seriously not trust more than a 5 foot span, and do not consider it without the venting techniques mentioned above.

BadAttitude