General US Sealed Crawlspace - Insulation or Vapor Barrier against the Concrete?
I'm trying to work through a standard sealed crawlspace detail, and as I look through previous projects I've noticed a lot of different layerings stemming from different research articles. Looking for opinions from those who've worked through these options in detail.
The main question: On the interior face of the stem wall, does the rigid insulation go against the concrete first, or does the vapor barrier go against the concrete first? Note I typically work in climate zone 4C (PNW).
Option A: Insulation against concrete
Option B: Vapor barrier against concrete
I've seen both in reputable sources. Building Science seems to show Option A (rigid wraps concrete). Martin Holladay's FHB sealed crawlspace article shows Option B (poly goes up the wall first, foam over it). ASIRI Designs shows a versionOption B as well.
Secondary questions:
- I also do work in 3A (Mid-Atlantic) which is warmer and more cooling load driven. Does this warrant a different strategy?
- Does it come down to which layer you're designating as the primary air barrier - taped rigid foam vs. taped poly?
- How far should the ground vapor barrier extend up the stem wall? I've seen some details where it laps up only a few inches, and others where it runs the full height of the stem wall. Does that choice interact with which assembly (A or B) performs better?
- Many jurisdictions require a termite inspection gap at the top of the stem wall. Does that interruption in the insulation or VB affect which assembly performs better?
A few resources I've tried to decode between:
https://www.finehomebuilding.com/project-guides/insulation/creating-a-sealed-crawlspace
https://buildingscience.com/documents/information-sheets/crawlspace-insulation
https://asiri-designs.com/f/crawl-space-insulation-strategies
Thanks in advance for your help!