Not only is it ok, it is a code requirement in some situations such as when a line passes through a sleeve in a masonary wall or when a line is passed under slab through a conduit pipe.

Get a cat!

Paul.

Try stuffing it with pieces of a stainless steel potscrubber then foam it.Mice won't chew through that stuff.

rjoh is right on. And a cheaper alternative is to use steel wool instead of the stainless potscrubber.

Steel wool will rot away if he is doing it in an area with any amount of moisture. thats why I suggested the stainless steel potscrubber.

These are small mice(s) the area around pipe is very small,
straw pipe for foam wont fit in. Will try to stuff steel wool, first with screw driver of popsicle stick. Than fill and clean off after hardened. Thanks for the heads up.

If that is a copper pipe you should not stuff steel wool in the cavity. Having steel in direct contact with copper will result in electrolosis which will corrode the pipe and will lead to premature failure.

Not true. Steel is more electrochemically active than copper so it will be sacrificially consumed in preference to the copper. Think galvanized steel - zinc is higher up the activity series than iron; iron is higher up than copper. If anything, the steel will protect the copper against corrosion!

Now if you mix copper and iron pipes, then that's a whole other issue.

Paul.

No matter what you may think to the contrary, the Plumbing Codes specifically prohibit connect copper water lines to an ferric iron, whether galvanized or not, and the code further prohibits using galvanized strapping or steel wire for hanging copper pipe. Copper pipe may not touch any metal ductwork either.

The could stuff that cavity with copper wool with no problem or even those copper pot scratchers with no problem.

But what the hell do I know about running copper, I am only a master plumber with 40 yrs in the trade

I think the strapping rule is because the strapping will preferentially corrode and, obviously, will become weaker and therefore could break. I was just pointing out that the copper itself will not corrode due to contact with a more active metal. As for mixing ferric and copper pipes, this is certainly disasterous. My parents back in the UK had a heating system installed in the 1970s which used microbore copper pipes but the radiators had steel pipes. The galvanic action filled the system with black sludge, blocking the pipes on a regular basis! A horrible mess to clean out. Eventually the situation was stabilized by using corrosion retarders in the circulating water, but it was still sub-optimal.

Paul.

Try Roxul insulation - the mice hate it, it is moisture resistant and the stuff doesn't burn

Here is a link that might be useful: Roxul Insulation

Bronze or brass wool is available at boating stores. I shouldn't react with copper