Hello,

My basement currently has 2×4 wall construction against 2″ of rigid foam with mineral wool in between the studs all faced with drywall.  I have limited headroom so I am going to use DMX 1 Step as an underlayment with LVP on top.    

DMX says to leave an air gap around the perimeter of the product where it’s along the wall and then to install the baseboard about 1/8 inch above the LVP.  This is all supposed to let any vapor diffuse and  dry to the inside.  However, this seems to be contrary to everything I’ve read on this website.  

I was thinking that instead of leaving the air gap around the perimeter, that it would be better to seal along the perimeter with low expansion spray foam.  The idea would be to create and air tight seal for the underlayment.  Looking for any feedback.  Any problem with the method I propose?  It would seem that my method is more in line with building science, but I am not sure why DMX expects an air gap around the perimeter with their product. FYI, the house is 36 years old. I suspect there’s no vapor barrier under the basement floor slab. Climate zone 4.