Housewrap over foam vs behind foam—what yields better drying potential in your climate?

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Housewrap over foam vs behind foam—what yields better drying potential in your climate?

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The drying potential of housewrap over foam vs behind foam depends heavily on your climate, especially whether it’s cold, mixed, or hot-humid. Here’s how to think about it in terms of moisture management and drying potential:

  1. Cold and Mixed Climates (e.g. most of the northern US, parts of Europe, Canadian regions)

Best Practice: Housewrap over foam

Reason why it is better is that In these climates, the primary drying direction is to the exterior. Placing the housewrap on the outside of the foam allows any moisture in the wall to escape more freely through the foam and housewrap.

Drying Potential: Better drying to the outside with housewrap over foam

Foam (like XPS, polyiso, EPS) is somewhat vapor resistant

Putting housewrap behind the foam traps moisture between sheathing and foam

Exterior housewrap helps protect seams, and flashing is easier to integrate

  1. Hot-Humid or Southern Climates

Best Practice: Housewrap behind foam (optional, depends on assembly goals)

Reason Why is that Inward vapor drive can be a concern in hot-humid zones, especially with air-conditioned interiors. Moisture typically wants to move into the house from the outside.

Drying Potential: Less about drying, more about managing inward vapor drive

Foam can act as a vapor barrier, so vapor from outside doesn’t enter the sheathing

If housewrap is used behind foam, it mostly serves as a drainage plane or WRB

May not be necessary if foam is foil-faced or sealed well

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In most climates, placing housewrap over foam improves outward drying and water shedding, while behind foam offers better inward drying but can trap moisture if interior vapor control is poor.

The optimal choice depends on your climate’s wetting and drying balance, with over-foam favored in wetter regions.