I’m speccing a 1960s ranch near Savannah and considering wood-fiber batts with a smart vapor retarder instead of spray foam to cut embodied carbon. Target is under 25 kBtu/sf/yr with a 2‑ton heat pump and good shading. Anyone have long-term moisture/performance experiences in summers with dew points in the 70s?
With wood-fiber, a 3/8" rainscreen and dehumidifier help in Savannah; ‘dew points in the 70s’ — what’s the cladding?
The big win is interior-side air sealing with a continuous smart membrane — , leaks matter when you’ve got “dew points in the 70s.” On a 1960s ranch I did near Brunswick, enabling the 2‑ton heat pump’s dehumidify/overcool mode (about 2°F) kept RH about 50% and the wood‑fiber + Class III retarder modeled and performed fine. Caveat: skip interior poly and keep a vented cladding; BSC explains the inward drive problem well: BSD-106: Understanding Vapor Barriers | buildingscience.com. Are you staying cavity-only or adding any exterior insulation?
On a Tybee retrofit, adding a 3/4" service cavity to keep the smart membrane uninterrupted (all wiring inboard) kept sheathing MC flatter through August; @william_smi69’s air-seal point is spot on. Are you staying cavity-only or adding any exterior board?
Echoing @everodr: on a Skidaway ranch, we tied the smart retarder into window bucks and the double top plate with liquid flashing so it became the primary air layer, and with a balanced ERV at about 40 cfm trickle the OSB stayed about 12–15% through August; skip low‑perm interior paint. Are you planning balanced ventilation?