Expert: Dan Griffin - 4/30/2011

Question
I want to build a 15x18 room under my house. I have a few concerns that I would like an experts opinion on.

flooring: currently it is gravel with 6mil vapor barrier. Lowes carries a handiblock that allows you to lay joists into the handiblock and it raises the floor about 6 inches off the ground. So i had planned on using those, and the joists would be 2x6 pressure treated. I would also add another 6mil vapor barrier before setting the handiblock. The subfloor would be 23/23x4x8 tongue groove OSB. self adhesive tile would cover the OSB.

walls: currently 2 of the walls I would be building would be against block walls that have blown insulation on them. I had planned to use standard 2x4 studs untreated, r19 insulation and vapor barrier on the back. The interior of the wall would have mold resistant drywall around the lowest perimeter of the room.

ceiling: drop ceiling so I would have access to gas, electric and water lines.

My main concerns are rot and mold. I want to make sure to keep that out as best I can, but I really can't afford to have that area poured with a slab.

Answer
Matthew, I don't really have any experience with a wood floor in a basement space, so I am uncomfortable to suggest you go for it.

In theory, I don't have any particular reasons that it would not work.  I do think you should create some perimeter screening or similar to allow a moisture exit and allow some air circulation.  I would also hold the stud walls away from the exterior walls.  I like using materials and designs that allow drying to the inside where any possible moisture can be dealt with by a dehumidifier or other means.

The 23/32 decking will be a bit springy, so consider 1 1/8 decking.  I do not use self stick tile unless I add additional adhesive.  I would just run the mold resistant rock on all, the cost would be minimal.  Check your heights, it is an unusual basement that will allow such a thick floor and drop ceiling.

You don't tell me if you are in a cooling climate or heating climate.  Check this site for some good advice about location of vapor barrier:
http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-bar...
you may be better served with no vapor barrier or at least one that allows drying to the inside.  This portion of the same web site gives a well studied and detailed explanation of what to do and why in basements:
http://www.buildingscience.com/documents/digests/bsd-103-understanding-basements...

On the floor I would certainly use one of the heavy mil ones like W.R. Meadow's Perminator:
http://www.wrmeadows.com/wrm00068.htm

Have you honestly priced having concrete installed?