18 inches is not sufficient for any pipes with water. Pipes
typically should be 3 feet under for reliability, human
safety, and pipe protection (even from rodents) as well as to
insulate the pipes. Insulation only 18 inches down makes a
perfect nest for rodents. Pipes should also be installed so
that if the heat is turned off, pipes will not freeze. 18
inches is not sufficient for electric wires feeding the
building. Why then would it be acceptable for heat pipes?

Meanwhile, if 6 inches of insulation was not sufficient for
the house, then why would anything less be sufficient for
pipes that are even hotter? Let's assume those pipes have 6
inches of insulation. That means less than 1 foot of dirt?
To be effective, the insulation must not compress which is
another reason why the pipes should be deeper - not subject to
compression forces of overhead traffic only 1 foot away.

wrote:
I have been helping my aunt excavate her back yard, in order to expose
hot water pipes, that run about 30 feet from the house to the garage.
Both the hot water heater and the natural gas boiler are in the garage.
The pipes are not insulated and are just about 18 inches below grade,
in Cambridge, Maryland (8 feet above sea level). Last year's heating
bills were astronomical, and my Aunt thinks that a lot of heat was
radiated into the frozen ground, before it ever reached her house.

My question is, once I expose the pipes completely, what is a good
method of insulating pipes in sandy and wet soil. Cutting the pipes are
not an option, thus we need something to retrofit. The insulation must
withstand backfilling, vast temperature changes, and moisture. What can
be used on this job?

Note: The house was built in the 1940's. In addition to fixing the
pipes, my Aunt is also insulating the house's ceilings and walls.