Builders have reported excessive shrinkage in Medium Density Fiberboard (MDF) interior trim. Gaps between splices and joints as large as ¼-inch can occur after installation from moisture loss. Building materials are often installed at a higher moisture content than the typical values reached in heated and air conditioned buildings. To minimize excessive shrinkage, MDF trim should be protected from exposure to high humidity and water during shipment, storage, and on the job site. Some conditions are beyond the control of builders, such as exposure at the warehouse and during shipping. Precautions should be taken to isolate MDF trim from damp surfaces such as concrete or subflooring that has been rained on. The moisture produced by unvented construction heaters can also cause condensation to form on stored materials.
One way to determine the potential for shrinkage is to assess the trim’s condition with a moisture meter before installation. Five to ten percent moisture content is a typical range for trim that has acclimated to indoor conditions. If trim has a higher initial moisture than its final acclimated moisture content, it will shrink in proportion to the amount of moisture loss. Solid wood lumber also expands and contracts with moisture change, however, its length change is five times less than that of MDF. For example, in trim that has acclimated to moisture in air at 80% and 40% relative humidity, wood trim shrinks 0.05% whereas MDF shrinks 0.25%. For a 15 foot run of trim, wood trim shrinks by about 3/32-inch, and MDF trim shrinks 7/16-inch. This shrinkage can be distributed among multiple joints, but if a single length of MDF trim is used the end joints will develop a ¼-inch gap.
One method to accommodate shrinkage is to use shorter lengths and multiple splices. This requires leaving a gap between each splice and use of a low modulus sealant. Typical painter’s caulk will not work because it hardens and will crack when the trim shrinks. A low modulus sealant will stretch like a rubber band. Using butt joints or painter’s caulk will undoubtedly result in service calls to repair "cracking" joints.
Source: ToolBase Services.
