I have just put a 4-season sun room over a concrete pad. I used 2 x 6 directly on the pad and extended it about 2' beyond into gravel/dirt. It has 5/8 decking on top and is insulated with 2 layers of 2' foam board underneath. I am using Allure vinyl flooring ...my question is, should I use a thin foam underlayment underneath the flooring ? I don't think any moisture will be a problem but wanted to check.