NOTHING. Sigh. You have purchased a CoreTec product with a backing. That means you are done. You can purchase a CoreTec product WITHOUT the backing. That leaves you free to use 3mm cork underlayment. You are not allowed to work with underpad with 97% of all vinyl products.

If you want insulation you need to work with a floor that allows it = LAMINATE. A HIGH END laminate ($5/sf) can handle 12mm cork underlay (1/2"). That is serious insulation! All for $1.49/sf.

If you insist on the CoreTec Plus AND insulation then you MUST work with a rigid subfloor system like DriCore. It offers a floor raise of 3/4" (19mm) Then the thickness of the CoreTec 8.5mm. This will cause a 1" floor raise.

Do you have the ceiling clearance for this? How about the stairs?

Personally I would go with 6mm cork underlay (1/4") and a 12mm (1/2") European laminate (Lico, Kahrs, etc) and be done.

If your basement has moisture issues (either from above or below...it doesn't matter) you will loose whatever floor you put down. That includes the CoreTec. Yah. No one tells a purchaser that is the cost of a 'little flood' in the basement.

Coretec allows a second underlayment up to 3mm on this particular flooring. We don't need the heavy duty insulation of Dricore (though we have the ceiling height to accommodate if we did). Moisture is not an issue.

I've researched extensively and spoken with Coretec customer service multiple times. Using a second underlayment may not be standard practice but it is fairly common and within warranty. Curious to hear directly from people who have gone this route.

Hi Lafala - I am in the middle of basement reno and considering to purchase Coretec vinyl plank with attached underlayment which is pretty thin. I wanted to put another underlayment on top of the concrete to really keep the cold out. Did you end up going this route and are you happy with your decision? Which underlayment did you go with? I was also considering Dricore with Coretec on top but not sure if I need to do that.

I put 2mm rubber under my cortec vinyl floor, it made it much more solid and deadens the sound. I bought it at the isostore. If you have moisture issues, you might not be able to use it.

Thanks Scotty! Do you have any issues with the cork squeaking against the rubber? How about the locking mechanism?

No issues with squeaking

I’m installing this exact setup and my husband wanted an underlayment even though our LVP has a cork backing. contractor installed a 6 mil plastic underlayment all over. on concrete and sub floors, but not on top of original laminate.