Expert: Dan Griffin - 3/8/2009

Question
I have a concrete slab that was built on with 2x6 and insulated with foam no vapor barrier. i found old dry rot on a 4x8 section there are water pipes going thru the floor they are encapsulated with the foam. there is a toe kick heater that is blowing hot air in the winter thru the one section of the floor the air travels thru the space left over by the foam and subfloor and comes out of a square hole made in the floor. where the hole is the subfloor is full of moisture.there was tile on the floor i took it up . put hardwood floor down on the subfloor and the hardwood floor cupped,i removed the floor. there is no cross ventilation or vents between the floor slab and the height of the 2x6.'s what should i do to prevent moisture

Answer
Chris, I'm having some trouble grasping the problem.

Concrete floor, 2x6 stud wall, water pipes coming out of the floor that have been foam sealed.  Somehow you have an HVAC grille and duct?

Where are the water pipes going?  Are there sanitary sewer line(s) also?  Where did this toe kick heater come from?  Are you under a kitchen sink?

YOu obviously have a large condensation  or plumbing leak if you have floor and rot problems.  The only way you get heavy condensation is having hot air hitting cold surfaces.  Plumbing leaks should be fairly easy to diagnose/eliminate.

I suspect that you are pumping forced hot air against a cold concrete slab and making it sweat like a banshee.  I think you need to work at providing an insulated duct from its source to its grille.

The amount of cold that transfers through a concrete foundation/slab is tremendous.  Good builing practice usually uses rigid foam on the building side of the foundation and under the first 2 feet of the slab perimeter.