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Radiant Floor Over Vented Crawlspace

bryanmonty | Posted in Energy Efficiency and Durability on

We are working on a renovation project in the Pacific Northwest over a vented crawlspace which is to remain unconditioned.

Proposed Floor Assembly:
– Engineered wood flooring.
– Hydronic radiant subfloor.
– (E) plywood subfloor.
– (E) joists w/ new batt insulation.
– Foil-faced polyiso w/ seams taped. 

The radiant floor installer is worried the radiant subfloor (Warmboard or similar) will act as a moisture barrier due to the aluminum conductor layer. Although the aluminum itself is vapor impermeable, the product is recommended to be installed with slight spaces between panels to allow for expansion and cannot be used as an air barrier even if desired.

The installer is worried we are creating a floor system with two vapor barriers (radiant subfloor & polyiso), and is recommending using spray foam in the floor assembly below the existing sheathing (which we want to avoid). Has anyone encountered this situation before? Are the installer’s worries valid?

Thank you!

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  1. bryanmonty | | #2

    Thanks DC!

    The article mentions the foil of the rigid insulation eliminates the vapor drive that would otherwise cause condensation to form on the underside of impermeable floor finishes like vinyl. This is similar to my concern about the radiant subfloor acting as a vapor impermeable surface.

    However, doesn't this create a situation where you have a vapor impermeable surfaces sandwiching the floor system (vinyl or radian barrier at top and foil face of rigid below)? And is so, this would not allow any moisture that did make its way into the floor system to escape?

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