@missgrit closed cell = high density foam underlay with vapour barrier/retardant properties. A Good floor starts with a Good underlay. Ever wonder why laminate floors sound and feel so BAD? The answer: bad underlay (bad = cheap). I've seen a high end underlay (not very squishy...should be quite dense) make an $0.89/sf laminate feel "high end". And I've seen high end laminate made to feel cheap because the owner wanted the $0.20/sf "white foam" underlay that you get at discount flooring stores...the rolls are in the back and out of the way. To buy them you almost have to go into the back alley with a hand full of dollar bills under the cloak of darkness. It is pretty nasty low end stuff.
Make sure you have the concrete levelled out as needed (usually 3/16" over 10ft is flat enough). Moisture mitigation is not cheap. Make sure you use some of the best vapour barriers you can get your hands on (read: spend money).
Even when everything is done right...there can still be issues with moisture. Have a long, hard look at your humidity in the basement (leave a sensor down there that you check/measure/record periodically). Make sure you have the ability to DEHUMIDIFY the basement AT WILL. Some basements are so moist that a dehumidifier running 24/7full tilt for WEEKS still can't drop the humidity below 60%. That's when you realize that engineered hardwood may not be a good idea.
What moisture tests were done? What are the numbers? How many places were tested? Each floor has its own requirements and each underlayment has its own requirements and each Self Levelling Compound (SLC) has ITS own requirements. If you don't know what floor you are using and you don't know what underlay you are using and the floor hasn't been leveled (I'm assuming as you haven't mentioned that), what number was given to you that said "Everything is OK"?
A concrete slab that has a hydrostatic pressure reading of 5lbs/1000sf over 24 hours sounds great. Until you realize the product you want to use REQUIRES 3lbs/1000sf over 24 hours (dry as the Sahara in July). That 5lbs/1000sf over 24 hours is no longer "OK". It is too high for the flooring/product of choice.
Q
well that sums it up!
Q