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Doug Frank FCSI CCS
Senior Member
Username: doug_frank_ccs

Post Number: 126
Registered: 06-2002
Posted on Wednesday, October 12, 2005 - 03:48 pm:   Edit PostDelete PostPrint Post

I was asked an interesting question by one of my project architects regarding moisture content of concrete slabs. Apparently the project, under construction, has shown to have moisture in the concrete significantly in excess of the allowable 3 lbs per 1000 sq. ft. (as much as 8 lbs in some areas). In order to ameliorate the potential problem the contractor has applied a topical surface treatment to the concrete slab in suspect areas. Other construction has continued and metal studs and drywall are in place in many of these areas. Consequently. the water repellant surface treatment does not extend under the stud track. The metal track, and the open stud cavity, and the drywall, are now subject to potential, and maybe significant, wetting.

The interesting question: How much water / moisture is Too Much under a metal stud and drywall partition? Itís pretty apparent that 3 lbs does not pose a threat since we ignore that much moisture as a general rule. But what is the limit? Anyone have any experience or suggestions?
Doug Brinley AIA CSI CDT CCS
Senior Member
Username: dbrinley

Post Number: 131
Registered: 12-2002
Posted on Wednesday, October 12, 2005 - 04:42 pm:   Edit PostDelete PostPrint Post

Well, the empirical approach (eg opening walls up later) implies that IF a sane person (find one) sensed dampness of concrete, there would be risk of corrosion in the framing elements, cavities, fasteners and so forth. The tests are useful but your body can tell you what you need to know if you're paying attention.

What I've seen in remodels has been no deterioration in many instances, but extreme and almost unbelievable corrosion in others. 'Where did the studs go?!' The key seems to be drying potential. In areas like back-of-house areas in a foodcourt (retail mall), there is little drying potential. In an office environment with reasonable HVAC there is a lot more drying potential.

For really tough situations where we know there is risk for (lack of) drying potential, we are specifying products (and application) like Ardex MC. This is very expensive - $7.50/sf installed, so it's simply not available to most projects.
Phil Kabza
Senior Member
Username: phil_kabza

Post Number: 140
Registered: 12-2002
Posted on Wednesday, October 12, 2005 - 04:51 pm:   Edit PostDelete PostPrint Post

Our two coastal offices routinely finish projects where new concrete slabs never read below 6 lb/1000 sq.ft. When we have to apply impermeable floor finishes, we sometimes use one of the newer top of slab liquid vapor retarder treatments (Doug - I think this is what your contractor used, or I hope so). These treatments aggresively adhere to shot-blasted concrete slabs and resist water vapor pressure. Water vapor at this location doesn't necessarily translate into moisture - it has to meet dew point first, and often, but not always, does.

The underlying question is whether the excess concrete slab moisture is a temporary condition due to slow emission of excess construction water, or whether the slab is not properly decoupled from wet subsoils. If the latter, the topical treatments may tend to concentrate continuing water vapor movement in the untreated slab areas such as under the partitions, and your concerns over long term potential for condensation and wetting of the framing and gyp bd are justified. I'd be inclined to use a moisture and mold-resistant gyp bd and absolutely keep it up min. 3/8 inch above the slab to prevent wicking.

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