Expert: Dan Griffin - 3/30/2008

Question
QUESTION: Hello, I live in North New Jersey with approx 7500 heating degree days, and I am in need of your expertise.  I've searched your Q & A's  but still couldn't find the answer to my question. I'm a little bit confused due to all the various opinions on where to use a vapor barrier.  Some say use, some say don't and both arguments have validity to them.  And the problem is most of the discussions I see are about finishing a basement.  My home is a raised ranch on a slab. The block foundation (stem wall)continues approx 40" above the slab, then the upper half is framing of the first floor. Presently the interior side of the block is furred w/ 2x4 to the wall, cavity filled insul., foam board, vapor barrier & 1/2" sheetrock. Which I did approx 16 years ago and has developed mold at the base of the sheetrock.  I am redoing this floor with XPS on the above ground block wall to correct the thermal gaps. I would like to know if the XPS is usable/advisable for my above ground block wall? Also after I frame out on top of the foamboard do I fill the cavity w/insulation with kraft face or no face and then a vapor barrier? Or do I forgo the vapor barrier to allow the vapor to migrate out?  Sorry for the lengthy question but I do thank you for any advice.  Believe me I've read many online articles and still don't know the best way. Thanks, Zig

ANSWER: Zig,  I certainly share your frustrations on vapor barrier location theory.  The fact that you have mold says you have a damp/wet condition, though it would seem not enough to deteriorate the drywall or furr strips.  The standard answer is to use the vapor barrier on the warm side, this becomes a problem in mixed climates where there may be substantial air condition days AND heating days as the "warm side" becomes a moving target.  There is one site that I feel good about recommending
http://www.buildingscienceconsulting.com/resources/walls/air_barriers_vs_vapor_b
I particularly appreciate his signature block:  
Joseph Lstiburek, Ph.D., P. Eng.
"Rocket-Scientist"

My thoughts on your problem.  I guess that XPS is expanded polystyrene as neither I nor Google have any idea what it is.  Several things are unclear.
Is the outside of the block wall below grade?
Is it damp proofed, water proofed, French drained (if so to daylight or sump), is there a sump system?
Is there good exterior drainage - gutters, downspouts, planting beds?
Is there a vapor barrier under the slab, continuous under the footing, is there a capillary (gravel) break under the vapor barrier?
What is on the outside of the wall? Siding, brick, stucco, EIFS, is there a drainage plane between the exterior and whatever sheathing you have?

There is non paper, mold resistant drywall:
http://www.gp.com/build/product.aspx?pid=4659

There are inexpensive dehumidifiers that can remove surprising amounts of water.

I think I would avoid polyethelene sheeting to allow some natural air movement/migration to dry surfaces encouraged by fan, air vents.  Kraft faced insulation functions as both R value and vapor barrier.  I like a slight air separation between the masonry face and framing/wall sandwich which you won't be able to do with the furr strip install.  I also like a poly sheet loose fitted to the masonry to keep any potential moisture from entering the wall sandwich, it might get wet on the floor, but not in the wall, with a treated base plate and deliberate air gaps.  Again, a bit of natural air convection  sounds best to me, kind of a suspender and belt approach in case one idea wasn't perfect, provide a backup system.

You didn't tell how bad or extensive the mold was.  If there is light, no food, or no moisture mold cannot grow.  Mold exists and cannot be eliminated, only controlled.

I know I've not given you a definitive answer.  You already know I cannot.  I have tried to limit my comments and questions to facts, not hearsay.  If any of it helped, great!

 




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

QUESTION: Thanks for speedy answer!!, As a follow-up, you are somewhat correct about the XPS.  It is an extruded polystyrene. My apologies for not being more clear with regards to some of the variables I omitted.First the block wall is not below grade, and is stucco finished with a good drainage plane. The downspouts are all drained away to daylight. The beds around the house all slope away at a good grade. No french yet, will be installed this summer on the high side of the house, as a safety for any accumulating water.As for under the slab, I don't know since I nor the town have any plans to review, since they were lost to water damage, kind've ironic. As for the mold, it was only at the base of the sheetrock on the interior(room)side not behind the rock. If you feel this may alter your answer I would appreciate any additional assistance as I am actually working on these walls right now. Regardless, your answer makes the most sense of all the items I've looked into.  Again a big THANKS, Zig

Answer
Zig,  if all you had was some light surface mold on the bottom outside, I think I would just close it back up just the way it always was, put a little mildewcide in the paint, and call it good.  If you have the wall ripped open for other work,you might use the paperless drywall.

I suspect warm meeting cold or floor moisture being attracted to a slightly colder wall.  It can probably be handled completely by increased air movement.