Please use the search function (the one on ArcicadWiki works best for me vs the one here). For example:
http://www.archicadwiki.com/results.html?q=screen%20porch
You'll find lots of posts, including this recent one with a solutions from Fuzzy, David and Rick:
http://archicad-talk.graphisoft.com/viewtopic.php?p=219134
But, I guess you're asking for the alpha map and not how to do it. I'm sure one of those posts has one. If you'll not be close enough to see the screen, the no-alpha translucency options mentioned are the most efficient for rendering speed.
