We have recently used the 42 5100 under our 20 MIL liner and I will have to say it's a must. We will be using this product under our crawlspace liners from here on out.

1) GOOD: This is not like roofing "42" , and shouldn't be. 42 has an organic fiber base which will decay if continuously wet, as underneath a vapor barrier. This synthetic product is just what is needed as a vapor barrier underlayment. 2) OK: I got this to protect my vapor barrier from sharp-edged gravel which was already covering my crawlspace floor. As such, this product is OK, but not as thick as I expected. It is like an extra-thick landscape fabric.

This is some really good durable stuff.

Product is of good quality

I would highly recommend buying this product when doing a "dry lock". I would buy again!!