Hello,
I've installed quite a bit of 3/4" solid hardwood flooring in my home a few years ago and used the forum for advice. Good results back then. Now I'm back with a new install. While adding a carport to our home, I included an upstairs bonus room above it. The subfloor is 3/4" Advantech + 1/2" OSB layer. I needed to meet a specific height at the top stair riser so I settled on a 3/8" engineered Bamboo click floating flooring. To meet the desired height for code, I needed to build up the floor a bit. I installed 30 lb roofing felt over the subfloor & purchased Roberts 70-190 Super Felt Premium Felt Underlayment. I installed the felt with the film side down (next to roofing paper) instead of facing up as per the installation instructions. I have the majority of the floor laid out (280 sq. ft) before I noticed the oversight. My plan is to reverse course and flip the felt over as per the instructions. But before doing so I wanted to see if the pros out there had any thoughts given there is a combination of vapor barriers in play. I needed the extra 1/16" of thickness from the 30 lb roofing paper. I was not using it as a vapor barrier per say. Given this is a second floor install, I was not too concerned with a moister issue. My main concern now is the floating aspects. Will the floor float properly with the felt side next to the bottom of the floor? The floor has a plywood veneer bottom, a MDF core and a bamboo top veneer. This is my first floating floor over felt so I have no prior experience to base anything on. I can reverse the installation. The click together makes this easy enough. But if it is a waste of time, I would prefer not to undo all the work thus far. Sorry for such a novice question. Everything I find on the web points to the film side facing the bottom of the floor but I also find some information that points to installs with the film side down.

Forgot to add one more comment... I assume having the felt exposed to the bottom of the flooring would allow moister from the living space to accumulate in the felt. Thus exposing the plywood veneer layer to prolonged periods of moister? Not sure if that is a super big issue given this is a climate controlled space but we are in the South where Summer time humidity can climb. The space is controlled by a mini-split. I have seen humidity as high as 60% worst case. It runs around 40~50% if the system is running frequently.

To be safe....maybe I'll stay the course and reverse the underlayment. Still interested in hearing other opinions if someone wishes to chime in. Thanks