Vapor retarder needed? – GreenBuildingAdvisor

Greetings GBA! I have read everything I can find on vapor retarders here on GBA and want to confirm that I am doing the right thing by NOT including a vapor retarder over fiberglass or rockwool before I close up my walls. No permits or inspectors here – I want to do the right thing without any consideration of codes.

TLDR: Should I add a vapor retarder (like CertainTeed Membrain, etc) on the interior over fiberglass or rockwool in a 2×4 wall assembly that includes exterior air barriers and 2″ foil-faced polyiso (roughly 50% of total wall R value)? No building inspectors/permits/etc.

I am renovating a 1973 Vermont (climate zone 6A) home with 2×4 walls. I am replacing original R11 fiberglass in 2×4 walls with a new wall assembly (from interior to exterior): Drywall, cavity insulation, OSB sheathing, Henry Blueskin VP100, 2″ foil-faced Polyiso taped with Nashua 324A foil tape, Tyvek taped with Tyvek tape, 3/4″ battens for rainscreen, LP Smartside siding. The original fiberglass has a kraft-faced vapor retarder, with no air barriers or exterior insulation. My understanding from GBA articles on vapor retarders is that I do NOT need to – and perhaps should not – install any interior vapor retarder due to the exterior air barriers (Blueskin VP100) and vapor barriers (foil-faced polyiso). Specifically, in 2010 Martin wrote that an interior vapor retarder often does more harm than good, and the experts he asked all implied that they are harmful or unnecessary in most situations, even if they are still required by building codes in cold Northern climates (not a concern my case as we have no inspections).

In 2013 Martin wrote that “the main reason to install an interior vapor retarder is to keep a building inspector happy.”

In his 2021 update to vapor retarder code requirements, he shared that “if you install a continuous layer of exterior rigid foam in zones 3 through 8, use a smart vapor retarder on the interior, or else you’ll have to hire an engineer to perform a hygrothermal analysis that gives the thumbs up to your plan.” I do not need to worry about hiring an engineer but still want to do the right thing.

In 2025 he wrote that “if you install an adequately thick layer of continuous rigid insulation … on the exterior side of your wall sheathing, you don’t have to worry about condensation problems in walls … if you choose this type of wall assembly, remember that you should never have an interior vapor barrier.” I know the difference between a vapor barrier and retarder, and I think this implies leaving out both. Is there a general consensus about best practice these days?

Thank you all!