Hi All – planning a reside of my 1970’s vintage house in zone 6. House is 2×4 walls, fiberglass batts, plywood sheathing, and currently alumium siding with fiberboard backing that happens to soak up water really nice. 

I’ve ordered Alside’s Ascend siding (it is foam backed) and plan to use a layer of polyiso on the outside of the house as well, taping the boards. I plan to use raindrop 3d as a housewrap on the sheathing to allow drainage, but also apply it a second time as a capillary break between the siding’s foam backing and polyiso (cheaper ideas welcome on that). 

I’ve done a lot of reading about potential condensation on the sheathing. I am toying with the idea of applying a layer of greenguard DC14 drainage mat fanfold between the sheathing and the polyiso (on top of raindrop – or maybe in lieu of raindrop). It’s 1/4 inch thick in total, but has grooves on each side so it acts similar to a rainscreen, and also provides R-1. Logic dictates that the grooves must be 1/16 or so. My thought here is that raindrop 3d might not ventilate enough to dry any issues with the sheathing, and this would help. The interior of my house is mostly drywall, some wood paneling, and I cant guarantee what type of paint was used 55 years ago, so drying to the inside may pose some challenges.

I know rainscreens typically go outside of the foamboard. But I am wondering how this detail might work out. I read once that an airgap between insulation will drop the r-value, but I cant find ANYTHING on that now in my searches (probably because its not a common ask).