Join Date: May 2016
Location: Deltona, Florida
This is opinion, based on looking at videos about concentric intake/exhaust systems and my own 57 years of experiences ... it appears there is only need for the one concentric pipe.
Intake air is pulled in through the outer pipe, which cools the exhaust while heating the intake. If you're using the PVC of the video, there shouldn't be enough condensation on the outside to worry about.
With a 25' run, I suppose it would be somewhat possible to have the exhaust tube too cool, by the time you get to the wall to the outside, to warm the intake air. You might get some distance of cold air coming through, and a little condensation. Only then, would I worry about insulating. And you'll also know how far it's required.
I am not sure if this is what your talking about, though. It sounds as if you're actually running THREE pipes. One that will be the concentric pipe of two, and a third pipe. I am unsure of your intent.