18 inches in unacceptable even for water pipes in Maryland
AND unacceptable for buried utilities. Furthermore, 18 inches
is not sufficient to protect those pipes. Again, even
superior grade insulation must be at least 6 inches for spaces
that are at lower temperatures than that pipe. Two reasons
are provided in that sentence that says others are
recommending only minimal (marginal) insulation. Insulation
installed shallow in the ground will not be as sufficient. If
putting less insulation on those pipes, then don't even bother
with more than 2.5 inches in the attic - heat losses in pipes
close to the surface will be that great.

This little amount of insulation on that heat pipe is for
pipes that remain in about 50 degree F earth. Pipes are
buried deeper also so that temperature is closer to 50
degrees. Earthing at just under 1 foot is colder and requires
significantly more insulation than others have recommended. A
pipe in Maryland should be buried deeper as is standard for
other utilities - if for no other reason because others
recommended so little insulation.

How deep the gas pipe? This is often a function defined by
the local gas company.

Greater burial depth is for reliability, safety, code
requirements, and so that pipes can be so lightly insulated as
others have recommended. 18" is rarely deep enough in cold
weather states - except where the installer does not care.

Chris Lewis wrote:
I wouldn't take w_tom's comments about burial depth _too_ seriously,
other than considerations for frost depth. Especially since you
have a cold water line here, you need to check that 18" is
_below_ your frost line. It might be. But it certainly ain't
here (it's more like 60" here).

18" might be enough for Maryland, but I don't know.