I’ve been reading through Joe’s old work in my free time, and I came across a construction detail I found a little odd when he described how he re-renovated his barn: https://www.buildingscience.com/documents/insights/bsi-056-leiningen-versus-the-ants-redux

During the renovation, he adds a layer of structural sheathing and applies a peel-and-stick membrane to it. So far so good. He proceeds install four layers of faced polyiso, taping each layer as he goes. This seems excessive — after all, he already has an air and water control layer under all that foam, so why bother with taping each layer?  He then leaves a rainscreen gap for the siding, and by all indications seems to treat both the exterior face of the foam and the membrane on the structural sheathing as water control layers, connecting and draining where appropriate. Presumably, a small gap was left between the structural sheathing and the foam to allow for drainage, though his descriptions do not explicitly note that. I’d also assume that windows were flashed to the exterior of the foam, but he does not describe the window flashing process.

I’m failing to see the benefits of including the two water control layers in the assembly. He mentions in the article he was going for bragging rights on his air tightness — is this the reason? Is the peel-and-stick membrane intended to primarily act as an air control layer, and he is just forced to treat it as a second water control layer to guard against hydrostatic pressure? And if that is the case, wouldn’t having to leave a drainage gap between the foam and the structural sheathing eliminate the air-barrier benefits of taping all that foam? The whole set up seems a little confusing, so I am wondering if there is some other detail or benefit I am missing, or if I am just plain misinterpreting his construction details.