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They Changed my Crawlspace Detail

bryanmonty | Posted in Energy Efficiency and Durability on


– Renovation project in Portland, OR of a 1950’s bungalow with a crawlspace. 
– Existing vented crawlspace needs help but no significant signs of moisture.
– Fully conditioning the crawlspace was not within the project budget.

Our design called for:
– 12 mil poly, overlapped, seams taped and secured to sides of the stem wall.
– Foil-faced polyiso below the joists, taped, sealed to stem wall.
– Fiberglass batt insulation within the cavity.
– Plywood sheathing.
– Finished wood flooring.

The insulation subcontractor decided they did not like this approach, and instead installed the polyiso above the fiberglass at the top of the joist bay, sealed with spray foam. They said that having the polyiso below the fiberglass would result in moisture accumulation on the top of the polyiso which would be unable to escape and lead to mold or moisture problems.

Given that the entire assembly above the polyiso is vapor open, I did not agree with this assessment. I also am not happy there is no longer a layer of polyiso to eliminate thermal bridging through the joists. My worry is that the approach the subcontractor took is simply easier to accomplish, not better.

However, I wanted to first reach out to this community to see if anyone thinks there is merit to what the subcontractor is claiming before I figure out how to respond.

Thank you!

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