Expert: Bruce E. Johnson - 2/19/2009

Question
QUESTION: Is it necessary to use a vapor barrier on the interior side of your exterior walls if your batt insulation has a vapor barrier already on it? Can you go ahead and use one for added protection even if using a batt that has v.b already? Or is that overkill? Also, if it is an expandable insulation should you use a vapor barrier? Thanks of much!
Shawn

ANSWER: Hi Shawn, I have a very distinct belief about vapor barriers.  They belong at the outside of a wall, not the inside of a wall.  The sub sheathed structure should be wrapped with a vapor barrier prior to the siding being installed.  Installing a vapor barrier on the inside of a room tends to trap moisture inside the walls which can lead to mold issues.  When using a faced insulation the faced side normally is toward the inside of the room.  Follow all insulation specifications and installation instructions.  Not sure what you are referring to when mentioning the "expandable insulation". I hope this information helps. feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com

---------- FOLLOW-UP ----------

QUESTION: When you say apply vapor barrier prior to siding, are you referring to the typical house wrap (Tyvek, etc.) or some other kind of vapor barrier? I was referring to the insulation that is blown onto the stud walls that then expands and is cut flush with the studs prior to sheetrock being applied. Thanks for the help!

Answer
Hi again Shawn, yes Tyvek or Typar are the new external vapor barriers that have taken over the older methods such as felt paper and these are good products.  Any type of expandable foam generally shouldn't need a vapor barrier installed over them on the interior.  Like I said I don't like to trap moisture in the wall I like to stop it before it can penetrate the wall system.  I hope this information helps feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com