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Do I need plastic sheeting (polyethylene) in a wall that has spray foam?

DenmarkDave | Posted in Energy Efficiency and Durability on

in process of having a house built in Denmark ME. The wall assembly is as follows

1. Zip R Sheeting (1″ Foam attached)
2. 2 x 6 Stud Wall
3. 1.5″ Closed Cell Spray foam in the stud bay
4. Dense Pack Cellulose to fill the rest of the bay
5. Polyethylene Plastic Sheet
6. Sheet Rock / Wall Board

From reading some of the articles, is the Polyethylene Plastic sheet needed? Will it potentially trap moisture in the wall cavity?

the house will have mechanical ventilation (Zehnder ERV) , mini splits for heat/AC.

thanks for you advice

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  1. GBA Editor
    Martin Holladay | | #1

    The type of wall assembly you are describing is called flash-and-fill. It is very similar to flash-and-batt. For more information on this type of wall assembly, see Flash-and-Batt Insulation.

    This type of wall should never have interior polyethylene. It needs to dry to the interior (similar to a wall with a layer of exterior rigid foam). For more information on the building science (and code requirements) behind this type of wall, see this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.

    For more information on interior vapor barriers, see this article: Do I Need a Vapor Retarder?

    If you are struggling to explain these issues to an ignorant building inspector who is insisting that you install interior polyethylene, you might be able to resolve the dilemma by explaining to the inspector that you will install vapor-retarder paint. Vapor retarder paint (or primer) on the drywall satisfies code requirements for a vapor retarder.

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