Our house is 35′ x 50′.  We have a steel I-beam frame spaced at 16′, so a total of 4 on the 50′ length that run vertically.  The roof is similar with I-beams welded directly to the vertical I-beams.  This skeleton was held together with c-purlins.  The inside was then framed out with standard 2×4 construction for both floors. Spray foam was sprayed against the inside of the metal siding before finishing off the interior 2×4 wood construction with drywall (cellulose inside of the 2×4 wall as well).

Unfortunately, no J channel was used around the windows and closure strips were not installed anywhere.  So to say critters are an issue is an understatement.  To fix this issue, I intend on removing all of the metal siding to replace with fiber cement.  I know the spray foam will be removed and it’s poorly done so I’m not concerned.

My plan is to install Zip panel to the 2×4 walls and add mineral wool to top off any cavities where the cellulose has settled or else replace the cellulose with mineral wool entirely where needed.  The Zip panel will give the walls some support that it lacks right now on the outside.  The issue is with the C-purlins.  To address those, I’m now wondering if I could possibly remove the C-purlins for the walls since the roof is in place and for the walls they are now essentially just there to support the metal siding.

If I don’t remove the purlins, I have 2 thoughts on how to go about doing the siding:

Option 1: Put 2″ foam board against the studs to bring the walls in plane with the C-purlins, then add Zip panels using longer screws.  This gives me a single surface to tape before adding a rain screen and siding.

Option 2: Attack Zip panels directly to the studs and then Zip Tape the entire section of C-purlin where the bottom of a panel meets the purlin and the top f the next panel down starts again under the same purlin.  I would essentially have a horizontal bump out of 1.5″ that is taped in between panels.  These are 6″ and would be at least 2 passes of Zip tape overlapped.  Then, add 1.5″ of foam board to bring everything in plane.  I would possibly do a final layer of foam board in addition so that I have exterior insulation over the C-purlins as well.

Option 3: Remove C-purlins, add Zip to the 2×4 walls, add foam board on top, then add siding.  I’m hesitant to remove them but think it should be fine since the I-beams are welded to the plates embedded in the concrete and the roof is tied together already.  Any racking the horizontal C-purlins prevented has been assumed by the 2×4 walls.