Correct Glue for Foam Outsulation?
With a light touch on the screws you will be able to attach the numbers. Drill the surface with a slightly smaller bit than the threads, not the shank of the screws. An 1/8" bit may be the right size... Run the screws in and if you like put a small dab of clear caulk behind the number to seal the screw to the number and the screw to the hole so water may not get in. As light as the numbers are, the screws will keep the numbers at the right elevation and spacing and the way the caulk will hold, the numbers will stay until somebody removes them.