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Chris Lewis

According to :
I would get a piece of plastic automotive grade wire loom and put it
around the PEX or just get some of that round flam insulation intended
for hot water copper tubing that is available at any DIY place. It's
like $1 for a 6 foot length.

"flam"? Was that a typo? Normal foam pipe insulation isn't
heat proof by any stretch. There are fiberglass pipe insulations
that'd work, but these are harder to find.

Or secure a wad of fiberglass, or better still, rock wool in betwixt.

[I wouldn't recommend a metallic foil, because there's a good
chance it would flatten out over time, and the end result would
be worse (metal conducts heat better than air) than what you have now.]

Building code requires a 1" airgap or insulating material
(usually a wad of fiberglass) between romex and hot air ducts.
This is not just an heat issue, but a vibration one. A vibrating
duct will eventually wear through romex sheath if they're in contact.

Here we're talking about an exhaust duct, and the temperatures
are bound to be higher than a hot air duct. (though, with a
high efficiency unit, the temperatures would be about the same)

Building code talks about permissible gaps between flues and
flammable materials. PEX is likely flammable for code
purposes (wire insulation almost always is).

It may be worth a visit to the library to read a copy of the
local building code to see what it says. A house inspector
would probably know, and if so, could certainly tell you what
local "good practise" is. A building code inspector certainly
would.

Apropos this - one of my SILs recently bought a house.
The first piece of flue from her mid-efficiency furnace
is a 2"-ish pipe encased in a 3" diameter shell. This
connects directly to the outlet of the exhaust blower.

Aside from vibration abrasion, this probably wouldn't
be an issue if in contact with building wire or a plastic
pipe.

_However_, the inner flue only "engages" the blower housing with
_less_ than 1/8" of tubing overlap! It's not otherwise
physically connected. One slight inadvertant knock, the pipe
pops off the blower without it being visible, and the furnace
is venting directly into the basement.

Yow!

A mandatory must-fix ASAP.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.