Expert: Stephen Major (Owner--Major Design Group) - 5/31/2007

Question
QUESTION: Stephen,

I live in a two-year old house in Minnesota and I recently had a T.V. installed on an exterior wall.  In order to install the T.V., an electrical outlet box had to be installed and the installers had to cut two more holes in the drywall to run the cables behind the wall.  I have now been informed by friends and others that when cutting the drywall for the electrical box and other holes the contractors most likely tore the vapor retarder installed on the wall.  What are my risks for getting mold from this torn vapor retarder?  Can I fix the torn vapor retarder or do something to minimize the risk?

ANSWER: Chris,

I assume that you are talking about a polyethylene plastic sheet installed just behind the drywall.  It is true that they did breach it, but it is pretty likely that every hole in the exterior walls (from outlets, switches, windows) represents an imperfection in the vapor barrier.  Not to mention the entire lower edge of the plastic sheet, unless that was adhered to the wall plate with sealant.

These vapor barriers are designed to limit the migration of warm moist air into a cold dry cavity (mainly during the cold months)  They are almost never perfect, but they are usually pretty effective.  A problem arises when the indoor  air is SO moist that it can carry enough moisture into the wall, through the various imperfect pathways, to cause condensation within the insulation, and eventually mold.  

Try to keep wintertime indoor relative humidity below 50 percent.  Be sure to use effective point ventilation at high moisture sources (kitchen, showers).  Avoid using a humidifier unless you are able to precisely control it at all times.

If you want, you can install thin foam gaskets at the boxes, or squirt expanding foam sealant at their perimeters.  Be sure to caulk the wire holes within the boxes as well.  But unless you have an excessive moisture problem, this detailing will provide more peace of mind than any really necessary protection.

Please let me know if you need additional information.

--Steve Major

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

QUESTION: Steve,

Thank you so much for your quick reply.  I will definitely caulk the wire holes within the boxes.  Does it make any sense to try and repair the vapor barrier by removing the boxes and taping new vapor barrier to the holes in the existing vapor barrier, or should I just focus on the foam gaskets and caulking?

Answer
Chris,

I would not attempt to repair the vapor barrier.  Dealing the the holes in the boxes and the box perimeter is adequate.

By the way, there are a lot of expensive homes being built that use only the "kraft" paper on the fiberglass insulation as a vapor barrier (i.e. a linear break in the VB every 16"), and they generally don't have problems unless excess indoor moisture becomes an issue.

You should be fine with the repairs we discussed.

--Steve Major