I’m hoping to add fiber-cement shake panels on top of T1-11 siding. The T1-11 is in good shape, and the house is in the mountains of western North Carolina. It has 2×4 walls, fiberglass batts, 1/2-in. plywood sheathing, housewrap, and then the T1-11. Should I use felt under the fiber cement? Tyvek? Some other housewrap? Nothing?

—Brad421 via the Fine Homebuilding forum

Travis Brungardt, a builder at Catalyst Construction in Prairie Village, Kan., replies: It’s great to consider the risks of covering existing finishes with new ones rather than removing them. Tyvek has extremely high vapor permeance (56 perms reported) and is specifically allowed by the manufacturer to be applied over another layer without risk. Both #15 and #30 felt have high enough perm ratings to make them safe as well.

But the real question is if you need to install anything at all. In my view, the fibercement siding could be applied directly over the existing T1-11, assuming it’s in good shape, with no risk of trapping moisture or hurting performance in the middle of the walls. The risk is at the transitions, specifically at the door and window openings. It would be safe to leave the existing trim in place, but I suspect you plan to replace it to stand proud of the new siding. Pulling the trim will almost certainly damage the WRB detailing, and you’ll have to address that. Taping over the existing window flashing tape with a compatible tape is pretty safe. You have to clean the existing tape carefully to allow for good adhesion, and be meticulous in proper detailing and laps, especially at head-flashing corners.

I would also take this opportunity to air-seal from the exterior. You could add another layer of Tyvek and meticulously detail it, or seal the T1-11 panel seams to reduce air leakage. I would suggest caulking these at a minimum.

The bottom edge is probably harder. Likely, the panels will sit too far out from the foundation to create an effective air barrier with sealant. This is a good spot for a vapor-permeable tape like Siga Fentrim 430, which will adhere tenaciously to the foundation with no primer needed. Attaching it to the T1-11 is harder. Best would be to apply it to the Tyvek or plywood behind the T1-11. If there is a bottom trim board, remove it and tape to the plywood sheathing. If not, separate the T1-11 from the sheathing with a flat bar and carefully cut off the bottom inch of T1-11 to expose the sheathing. Tape, then hang the new siding low enough to cover the gap.

This is a good opportunity to introduce a rainscreen to enhance drying potential behind the new siding. You can get as fancy as you like with this, but an inexpensive and effective solution is to roll out sill gasket vertically at the stud locations and tack it up before you begin siding.

From Fine Homebuilding #299