Flooring Forensics: Underlayment products and issues - Nov 2020

By Lew Migliore

Flooring installation failures related to underlayment performance, or lack thereof, have been steadily increasing. In most cases where we’ve been involved, the underlayment was not considered part of the problem, though it was in fact the entire reason for the failure. The costs for correcting these failures reach into the hundreds of thousands of dollars.

If you’re going to use an underlayment beneath a flooring product and over a substrate, you must understand what you may have going on with the substrate and how the flooring material will perform or interact with the underlayment. Any flooring failure we’ve ever seen could have been prevented. It’s a matter of knowing your materials and chemistries, choosing the right product, and on occasion, gambling as to how much money the alternatives will cost.

Let’s start with the fact that underlayments are not the substrate.

The substrate is the base of the horizontal plane-usually concrete or wood-that the flooring products and underlayment are going to be installed on.

Underlayment goes between the flooring material and the substrate to perform some type of function, be it for moisture mitigation, comfort and insulation, or sound attenuation.

There are a number of underlayment products for flooring made from various materials such as crumb rubber, vinyl foam, polyurethane, polyethylene, polypropylene, synthetic fiber, felt and cork, or a combination of those.

Many of these underlayments come with a film or membrane on one side, which is supposed to stop moisture from affecting the flooring. Some of these underlayments work well and others not so much, for various reasons.

It’s astounding to see some of these products marketed for use with flooring materials that have incompatible chemistry. Many are touted as stopping moisture from passing through. But what then? For starters, the moisture gets trapped beneath them and you have a host of other problems such as mold and mildew. If you try to circumvent science with a membrane on a slab and then glue an installation over it, the physics and chemistry within the slab will have other ideas. No matter what adhesive you think will solve your problems and what you might put in between it and the underlayment to try and stop the effects of moisture, the moisture will always win.

We’ve seen every membrane there is fail at some point where enough moisture is in play. They certainly have their place, but you can’t just put a bandage on a hemorrhage and expect the bleeding to stop. One of my favorite sayings is from famed astrophysicist Neil DeGrasse Tyson: “The good thing about science is that it’s true whether or not you believe in it.” Marketing words do not change the laws of physics and chemistry.

Underlayment can be installed loose-laid, glued to the substrate or have the flooring material glued to it. And failure can occur when two incompatible products are glued together. For example, never use crumb rubber underlayment with a vinyl flooring material. The two products are completely incompatible and will react. Crumb rubber from recycled car and truck tires contains oil, phthalates and solvents, which are petroleum-based and contain heavy metals, reinforcing agents and stabilizers. They most likely contain about 200 other chemicals that can also cause problems. These substances can compromise the integrity of PVC (vinyl) flooring products and the chemicals can leach out of the crumb rubber and create havoc with the vinyl flooring materials it comes in contact with.

The incompatibility between SBR (styrene butadiene rubber) and PVC plasticizers is well known. When plasticizer leaves the PVC construct and enters the SBR, the typical reaction is that the rubber swells. This process also affects the PVC, causing it to react. That reaction is often a lifting or doming of the material. With the crumb rubber underlayment, the reaction causes the vinyl flooring to dome, and this reaction is the same in each installation with whatever PVC product is installed over it.

Most large flooring manufacturers have now banned the use of crumb rubber underlayment under their PVC flooring tiles.

And it’s the reason buckets for SBR adhesive say it can’t be used with vinyl flooring. That should be enough to convince you these two materials are like trying to blend oil and water. We’ve done more work on this subject than anyone in the industry and we can show you a lot of very scary and convincing pictures to keep you away from these things.

Again, the glue won’t work between these two materials; the vinyl flooring is going to react, and the flooring contractor is going to get blamed for the failure.

Some issues with underlayments relate to the billowing or lifting of the underlayment from air beneath wood substrates, especially if joists are boxed for the HVAC runs. The forced air in the runs can escape from between wood subfloor joints and lift the underlayment membrane and the flooring material.

Trapping of moisture beneath the underlayment, especially if it has a nonpermeable type membrane, can be very problematic. Moisture vapor from concrete is a gas. When it encounters the membrane, it has to go somewhere, so it will lift the membrane. It then condenses to water, and in the process carries along its inseparable partner, alkalinity. Together, these two components can react with adhesive that is used to glue the underlayment down and other chemistries that may be in the underlayment.

There can also be a reaction of the flooring to the underlayment, such as I described previously with the PVC and SBR chemistry. There can be incompatibility of the adhesives with the underlayment, especially if the underlayment contains any bitumen, carbon black or release agents. We see this more commonly in multifamily installations, where the underlayment is glued to the substrate and vinyl plank or tile is glued to the underlayment.

We also see moisture-mitigating underlayments that don’t work. These products are used in lieu of a complete moisture mitigation process. They can lift the flooring material up and trap water beneath the underlayment. If the underlayment has an organic content, this can contaminate the concrete substrate. The result is having to replace the entire installation and mitigate the substrate.

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