We recently purchased a 1920s house and would like to install Hardiplank siding over the existing/original siding. The siding is in decent shape and lays relatively flat. I have found a couple of similar threads on the internet with differing opinions. One opinion was it would be ok, but to add building paper on top of the existing siding. What is the purpose of this? A picture of the siding can be see here:

http://i1318.photobucket.com/albums/...psdaf7f2b0.jpg

We will blow insulation into the walls, but as an additional insulator, we were thinking of also adding a radiant foil barrier at least on the East & West sides, as it is hot as the dickens in SE Texas. Here's a link to what we are thinking: http://atticfoil.com/applications-a-...rd-siding.html

Do you think this will help? The website states that there needs to be an air gap, but can't quite tell if the gap needs to be between the foil and the siding or behind the siding.

Also, we may want to change out some of the old windows to more efficient ones, can this be done after the siding, or should we do it before?

Thank you for any and all comments and advise.