Ever seen this?

I decided to replace my basement exterior door in my house with a patio glass sliding door. I bought this house a few years back and the exterior walls are extra wide not like your average 2x6 construction but never thought much about it. Come to find out when I opened up the wall there is actually two 2x4 walls built inside the wall. Each 2x4 wall has the cavities insulated plus the two walls are spaced apart 3 and 1/2 inches and that space has insulation running horizontally filling the space. In a sense it is a super insulated house. Each individual wall has poly between them and creates two vapor barriers. has anyone ever run into this? Let me know at my web page at www.finishcarpentryhelp.com I would be interested to hear and learn a little more concerning this.