Can I use thicker than 3mm underlayment?
Teragren normally advises use of a 3mm (1/8″) as being the thickest underlayment should be before most foam materials become overtly flexible. This can cause stress to the tongue & groove joint, resulting in separation of the planks or damages to the joint.
There are also a couple exceptions. These are cork and rubber (SBR) underlayment. Both have excellent long term support without losing their core abilities to flex less than their foam counterparts. Personally, I have utilized both extensively. Both in my own home and for various projects. These were all successful projects that I received satisfaction reports on, well after I would have expected them.
With the Silent Walk, you are slightly over our normal 3mm recommendation. But, based on my knowledge of the material and it being only slightly thicker; I would think any risk, is minimized. That’s my professional opinion. Teragren doesn’t specify underlayment types or manufacturer. Only the guidelines regarding thickness.
I’d be confident in your selection.