Crawl Space Question

[Leslie Martin]

I have a portion of my encapsulated crawl space that is all but inaccessible (15 x 15 inch access port to a four x 18 inch area that is under the area of my front door) and the builder did a half a$$ job putting down the vapor barrier. Can I simply place foam board insulation down and pour concrete over the top of it to seal it or even better simply pour three or so inches of concrete to seal it up? The base is dirt but it is blocked by the footers and porch foundation so there isn't any heavy water infiltration into it but it has the normal crawl space dampness.

Thanks

Leslie Martin