The approach begins well before the work starts and carries through every step of the way. Although panel movement problems are a bit easier to control during remodel applications, in every case the techniques must be followed precisely. This begins with the way the underlayment is stored prior to the job and affects the materials used and the installation techniques. Let’s look at the different areas:

Storage: The underlayment must be stored in an environment that is similar to that of the installation. This means you remove the panels from the skids and store them on end to allow air to circulate around each panel. This allows the moisture content to increase or decrease and correspond to the humidity of that area. Keep in mind that new panels have varying degrees of moisture content due to the manufacturing process. They are then transported (in some cases over long distances) from one type of climate condition to another. Often manufacturers wrap skids of underlayment with plastic to protect them from the elements, but this can also affect the acclimation process. The retailer or installer who picks up the underlayment the day before the job is asking for an installation failure because they have   allowed no time for acclimation.

Acclimation: All underlayment manufactures recommend that their product be allowed to acclimate at the job site for 48 hours prior to installation of resilient floor covering. Many installers who routinely do this with hardwood flooring seem reluctant to do the same for underlayments. That makes no sense since underlayments are as susceptible to moisture movement as hardwood. If the underlayment is stored in a shop with a controlled environment it is almost as good as storage at the job site, providing the panels are taken out of the skids and stored on edge.

Moisture testing: Moisture testing should be conducted with a pin-type moisture meter used for testing wood. The subfloor should not exceed 13 percent and the underlayment should have no more than a 3 percent difference from the subfloor.

Spacing: Depending on the underlayment manufacturer, there are several specifications regarding spacing requirements based on the species of wood, the type of lay-up of the wood veneers or chips and the thickness of the specific underlayments. Since the spacing requirements vary so much it is imperative that the installer know and follow these requirements. If you do not know the specifications, butt the edges lightly and do not fill the joints.

Nailing patterns: There are two important factors involved in the fastening of underlayments. First is the size of the fastener. Whether you’re using a ring, a screw shank nail or a staple, the first consideration is length.  None should be longer than the combined thickness of the underlayment and subfloor combined, because when a fastener blows through beneath the subfloor, the holding power (withdrawal resistance) of the fastener is compromised.

Then there is the nailing pattern. All underlayment manufacturers have recommended patterns for fastening that are determined by the thickness of the underlayment panels. You will generally find a closer pattern on panel edges than in the field. What is even more important is that the fastener be kept to within 3/8” of the joint edge to prevent the edges from peaking. In the panel’s field (away from the edges), stagger the nailing pattern to prevent a wavy effect in low-level light. The Engineered Wood Association suggests a diagonal fan pattern. Straight line nailing patterns tend to telegraph in low-level light. The nailing should commence at the corner of the two butted sides and work across to the open sides to prevent any fullness from being nailed into the panels. Each panel should be fastened before proceeding to the next panel.

Joint treatment: There are varying opinions as to the proper way to prepare the joints of underlayment panels. Some “old school” installers will edger sand all joints prior to patching and dish them out slightly. This allows the patch to be thicker at the joints. Others will patch every joint no matter its condition. I think the underlayment joints should be butted lightly and sanded to smooth. Use no filler, unless it is necessary for the occasional joint. When patching the occasional joint, the mixture of the patching compound is critical to maintain a high compressive strength. Remember, the more water you put into a patching compound the weaker in compressive strength the patch becomes. Latex additives help increase flexural strengths and I encourage the use of latex additives either patching compounds with additives already in the patch or as a liquid additive. Many installers will make the mistake of mixing their patch thin because the patch dries too fast during the application to the underlayment. The rapid drying can be overcome by dampening the underlayment joints with a damp rag to slow the absorption rate of the wood allowing the filler to be applied without thinning the patch or it setting too fast. Installers should never mix their patch directly on the underlayment or skimcoat the entire area of the underlayment panel. Remember, you do not want to put too much water into the wood. Moisture, in many forms, is the cause of many problems.

Sanding of patched joints: After the joints that have been patched are dry, the underlayment joints must be smoothed to eliminate any irregularities. This is usually done by sanding (either by hand or machine), but, regardless of the method used, all ridges and lumps must be removed to eliminate the possibility of show-through.

Adhesives: Adhesives also add water to the underlayment. Your adhesive choice should also be selected based on the effect it will have on the underlayment. All adhesive makers have specific requirements for the proper trowel notch for application of their adhesive over porous floors and underlayments. Equally important is the proper open time. Open time allows moisture to evaporate from the adhesive, which allows the adhesive to develop body. Otherwise the moisture is trapped under the material and forced into the underlayment. This causes the wood fiber in the underlayment to expand and the joints to show through.

In residential remodel situations, the underlayment is often placed over existing material. These materials serve as an effective vapor barrier that prevents moisture migration from beneath the underlayment. Since moisture is the cause of these problems, moisture must have been introduced into the underlayment during the installation phase.

Before the flooring is installed ask yourself if you have eliminated all possibilities of excessive moisture from being.