09/28/2026
A conditioned crawl space isn’t just about adding spray foam. It’s about changing how the entire crawl space functions as part of the building enclosure.
When converting a traditional vented crawl space to a properly conditioned crawl, the existing fiberglass insulation should be removed, not simply abandoned between the floor joists and sealed into the newly conditioned space.
Why? Fiberglass is air-permeable, holds dust and contaminants, and unfortunately makes an excellent nesting material, and hidden traveling highway for rodents. In an existing crawl space, that insulation may contain years of urine, f***s, nesting material and associated odors. Encapsulating the crawl without removing it can trap those contaminants and potentially even active rodents inside the new building enclosure. If odor problems already exist, you may simply be sealing the source into the structure.
Removal also allows the floor system, rim/band joists and penetrations to be properly inspected before the crawl is converted.
The wall detail matters just as much.
It’s also not the best practice to simply run ground v***r-retarder/poly all the way up the foundation wall to the sill plate and spray foam over it.
Spray polyurethane foam should be installed over a clean, sound, compatible substrate, with good mechanical adhesion, in accordance with the foam manufacturer’s installation requirements. Covering the entire foundation wall with polyethylene and then spraying SPF over it can create unnecessary adhesion and durability concerns while also potentially creating an avenue for condensation/moisture to weaken the condition of the masonry behind it.
A better approach is to detail the ground v***r retarder properly at the perimeter, then use the SPF as part of the crawl space’s thermal and air-control layers, directly adhered the to the wall substrate, with careful attention to the foundation wall, and rim/band area, penetrations.
The goal isn’t simply to make a crawl space look encapsulated, pretty and clean, although those are benefits.
The goal is to intentionally move the crawl space inside the home’s conditioned building envelope and make sure every component of that assembly works together properly.
Good spray foam starts with good building science.
FOAM IT RIGHT! 👊🏻
