I’ve been a fan of Wachuwannano for years. Now I have a question that maybe you can help me with. I’ve looked through all my technical data and can’t find an answer.
I’ve been a commercial drywall contractor for more than 20 years, and I’ve run into this before but can’t find any literature to settle this.
On a commercial job (a hospital) that we are currently working, the contractor has requested that we install the drywall a 1/2-inch off the floor. On a previous job, a contractor requested that the drywall be installed 5/8-inch above the floor, and then at the end of the job, the owner wanted us to fill it in with mud. On both jobs the concrete was new, poured within 20 days prior. The drywall is installed on metal studs.
I know there can be an issue with moisture migrating into the drywall from the concrete.
Some walls are fire rated, some are not. Some are sound rated, some are not.
I am looking for technical data that might provide a recommendation for this.
It is always good to know that people do read the column; it helps prevent writer’s block.
On to your problem: After searching the library, ASTM standards, as well as Gypsum Association and AWCI technical documents, I can find nothing definitive about a space between the drywall and the floor. USG’s The Gypsum Construction Handbook suggests starting at the ceiling line and working downward. In your situation you could have an open joint at the bottom of the board that caused the floor to be out of level. The Handbook also suggests installing gypsum ceiling panels first and then installing the wall panels. Again, the floor being out of level could cause a gap between the drywall and the floor. In most cases, a gap of some dimension will in all likelihood occur.
I think the problem you are having directly relates to the concern of water vapor being transferred to the drywall and producing mold. If the concrete is in fact cured and properly sealed with a curing compound, the building is closed in and the HVAC system—temporary or permanent—is in operation, it is highly unlikely that water vapor will transfer to the drywall. In most cases, mold will occur because water has entered the wall system from another source and is providing food for the mold to grow. If in fact the floor system being used is subject to heavy maintenance or the presence of water, a material other than drywall should have been specified by the designer.
When dealing with sound-rated and fire-rated assemblies, the drywall must fully cover the floor track and ceiling runner to produce the desired sound and fire rating. In fact, in some cases a sealant is specified in the assembly design, to be applied around the perimeter of the gypsum panels and around penetrations to protect the integrity of the assembly.
Your problem points out a lack of knowledge on the part of the general contractor in two areas: the steps needed to protect against the occurrence of mold, and the correct methods needed to build out tested assemblies to reduce sound transmission and the prevent the spread of fire and smoke.
About the Author
Donald E. Smith, CCS, is AWCI’s director of technical services. Send your questions to him at firstname.lastname@example.org, or give him a call at (703) 538.1611.