In 95% of cases 6 mil polyethylene is the right choice. Here is an article for someone who might be in the 5%.
Underlayments are a complex topic with most flooring professionals because the conversation can easily run from scientific, sound-proofing membranes with real testing and dSTC ratings, all the way to witchcraft stories about whether some chemicals in the plastics can affect the outcome of your flooring project. To avoid the latter let's focus the conversation on the main purposes of underlayments:
1) Moisture Resistance - If your flooring project is housed above a moisture-prone area like a crawl space or concrete slab, a moisture barrier can help keep moisture from collecting on the under-side of the flooring planks. 6 mil polyethylene is the go-to choice for almost all flooring assemblies, and there are generally three kinds of cases:
a) Moisture in Slab: Cases where you have a concrete slab beneath your hardwood floor. Concrete is known to wick moisture up towards flooring materials (even on an upper level), and so a moisture barrier is almost always recommended. 6 mil polyethylene can be taped down easily and cost-effectively to provide this function.
b) Occasional Humidity Below OSB: Cases, such as living spaces over crawl spaces or basements, occasional increases in humidity are expected, but should not result in condensed water. Like with concrete subfloors, 6 mil polyethylene is both easy and cost-effective in preventing excess humidity from reaching the hardwood flooring materials.
c) Endemic Condensing Moisture: Cases where wooden structural supports beneath the floor are exposed to condensation/moisture. In these cases, you really should remediate the moisture condition before installing a moisture barrier and ANY flooring materials, because these areas are prone to rot. Placing a hardwood floor with a moisture barrier above these spaces is not going to solve the underlying problem and could make it worse by trapping excess moisture beneath.
2) Sound Barrier - In some homes and multifamily residential contexts, traditional hardwood floors and tile are known to transmit noise from upstairs areas to dwelling spaces beneath. Often, this is the case because the flooring is permanently affixed to the subfloor and noise can reverberate from above to down below through nails, glue and mortar. With a Steller Floor, this is substantially less of a concern because the flooring is not permanently affixed to the subfloor, and air gaps are part of the assembly. That said, some HOAs and local codes still require sound barriers between the finish flooring material and the subfloor to err on the side of caution. Materials we know and have worked with:
a) Quiet Walk LV. We have extensive experience with this material, and it is strikes a good balance between being sound deadening and providing enough rigidity to aid in a quick-and-easy install. Assembly of our flooring is slightly more difficult than it would be on a firm subfloor, but certainly is manageable and final results are good.
b) White Foam Underlayment. This is a cost-effective option if you absolutely must add a barrier underlayment, and you want something thicker than 6 mil polyethylene. In most cases, Steller Floors perform just as well or better over a rigid subfloor, but we have seen good results with this material as well.
c) Cork. We only have a small amount of experience with cork underlayment, and the results were that it is generally "too squishy" to install our flooring quickly. Some clients have had success pre-assembling sections of Steller Floors on a firm subfloor and then moving the section to its permanent area over the cork.
d) Extremely "Squishy" Acoustic Foam. Since our flooring snaps down, it can't be assembled on materials that have "too much squish." This includes some thicker varieties of quietwalk and foamed rubber materials. Our materials are extremely difficult to install over these underlayments, and so we do not recommend them unless you are willing to go the extra mile and put another layer of plywood on top of them and then your Steller Floor on top (which is extreme - but has been done).
Considering something not listed? There seems to be a never ending list of new underlayments available! The awesome part of a Steller Floor is that if you have some extra time, you can always test a small section of flooring over the underlayment you choose and decide if it will work for you. If you decide against it, disassembling the flooring and using it again during your install is a breeze.
Also feel free to reach out to our support team at support@floorsbysteller.com, and we can look over product specs with you to see if we can anticipate issues with any potential materials.