vapor transmission through concrete planks

[housarchitect]

I'm designing an auditorium using hollow core precast concrete planks for an elevated floor. There will be a crawl space with a maximum height of 3'-0" underneath the floor. The project is in Texas so the outside weather is hot and humid, and the auditorium will be air conditioned. My concern is that the underside of the floor will develop condensation. How do you find the water vapor transmission through the hollow core concrete panels? Will there be a problem with the carpet on the interior? Should the underside of the concrete floor planks be coated with a water resistant coating to avoid any concrete deterioration? Any help will be appreciated!!

 

[Ron]

You have several things working in your favor. First, hollow core panels do not allow vapor transmission in the same manner as solid concrete. The hollow spaces and their vertical orientation usually keep moisture near the bottom of the slabs. Allowing air circulation in the cores will help this. The concrete in hollow core slabs is relatively dense, though is often contains lightweight aggregate which is porous and will accumulate moisture, though it also releases it readily.

The fact that you have carpet on top is good also. This does not create a barrier on the surface and allows consistent dissipation of accumulated vapor. Use partial attachment of the carpet rather than full glue down for better results. The dehumidification from the HVAC system will usually pull the vapor out without much trouble.

Having said all that, you are still likely to get condensation on the bottom of the slabs in the crawl space. This is better handled by ventilating the crawl space than by providing a coating to the underside of the hollow core panels. I would suggest force-air ventilation tied to a humidistat.

Also, be sure to put sealant between the panels to prevent a moisture migration "short circuit".