Question: Will installing closed cell spray foam over the ventilation for a basement French drain significantly affect the sump pump system's ability to remove water from the basement?

Background: I recently had a sump pump installed. The installers left an air gap for weep holes in my cinder block foundation with some thick plastic-type sheeting over it similar to what is shown in this picture. At the end of the job, the installers told me to make sure that if I painted the walls, that I should make sure to go along the top of the plastic sheeting with an exacto knife to allow air to flow down into the French drain and prevent a vacuum.

I would now like to use closed cell foam to insulate and provide a vapor barrier on my basement walls as part of finishing the basement. This puts me between a rock and a hard place on advice for using closed cell foam:

  • If I use closed cell foam without cutting along the top of the plastic sheeting of the French drain, I will be violating the advice of the installer and significantly impeding the transmission of air into the French drain.

  • If I use closed cell foam and put a cut along the top of the plastic sheeting of the French drain, then I will satisfy the French drain installer's advice while reducing the foam's effectiveness as a vapor barrier and defeating one of the purposes of using it.

  • While not an answer: I invited a basement finishing company to my home. It turns out that the representative is the owner of the basement finishing company as well as the basement waterproofing company. The method his finishing company uses to finish basements includes putting a 2" layer of extruded polystyrene (XPS) foam board over the foundation wall (including the plastic sheeting). The XPS board creates an effective vapor barrier (though not quite as tight against the concrete as closed cell spray foam). (comment continued below) – statueuphemism May 13 '15 at 21:13
  • (comment continued from above) I now doubt that adding closed cell foam would significantly impact performance of the sump since his company would also be responsible for the bill in the event of a water issue per the his company's waterproofing warranty and they do a lot of business in the area. – statueuphemism May 13 '15 at 21:13

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