Expert: Ted Barnhill (Principal, Design45 LLC) - 8/15/2008

Question
QUESTION: Before I purchased my home, the previous owner had closed in the car port.  The area was divided into a small study and a laundry room.  A portion of the wall for the laundry room was not insulated at all.  This portion adjoins a storage room.  Also new plumbing was added to handle the washing machine.  When they added the plumbing they removed the fiberboard and the insulation from a portion of the kitchen wall that adjoins the storage room.  So I have added insulation the two exterior walls of the storage room and also the wall for the laundry room wall.  All the insulation is paper faced.  My question is, I would like to add 3/4 inch foam insulation sheets as wall covering instead of plywood, drywall or some other type of wall facing.  Would you think there would be any issues or should I just install regular wall board?

ANSWER: David,

A layer of foam board on the interior of your walls will help your energy efficiency and will act as a vapor retarder - a good thing.  However, leaving it exposed is a fire hazard.  Code requires a noncombustible wall covering; 1/2" drywall is the most common choice.  

If your budget allows both (a good solution), installation is simple: just nail the foam board in place with a few washer-head roofing nails, tape the joints, then hang your drywall with long screws - 2" or 2 1/2" course thread.  Openings like electrical boxes and windows will need to be extended with box extenders or jamb extensions.

Good luck, and feel free to post any follow-up questions you may have.  -Ted

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

QUESTION: So to clarify, there is no issue with having kraft faced insulation between the studs and adding the foam sheets on top?  I have heard that having two vapor barriers is a no no.  That is why I am confused regarding this.

Answer
David,

Ideally, your walls will:

1. Be built air-tight.  This is often not achieved; leaks around windows,  electrical boxes, at wall/ceiling joints, and other locations can be sources for major energy loss and points of condensation accumulation.

2. Prevent most water vapor from permeating directly through the wall.  This should be accomplished with a low-permeance (less than 1.0 perms) layer on the warm-in-winter side.  Note: if your home is not built sufficiently tightly (like many homes; some would say most homes) there is some disagreement in the industry about the wisdom of a vapor retarder.

All building materials have some resistance to water vapor.  The key in avoiding condensation within your wall is for the wall construction to layer materials so that materials towards the outside of the home have higher permeance than interior layers.  (Thus, any vapor that gets into the wall from the interior should be able to escape to the exterior.)

Kraft-faced batts are rated at about 1.0 perm, while taped XPS board is rated at about 0.3.  This should be fine in theory; to ensure that you don't trap any moisture between the layers, cut slits into the kraft facing before adding the foam board.  Again, the critical thing is to seal your innermost vapor retarder tightly: tape foam boards together, caulk where you can, use foam spray at electrical boxes and other openings.  -Ted