Insulating PEX heating

I am planning on running a PEX 3/4 inch heating line and return for 11 feet in an open crawlspace under a small room to a new attached room for heating purposes. Can anyone advise me as to the best way to insulate this from the New England cold; or am I crazy to do it this way?

Sounds risky to me. I'm assuming that you will be insulating the floor. I'd consider building a chase for it right under the floor - far enough away so any nails, etc., won't be a problem, then putting the insulation over that. In that manner, it is right next to the floor, inside the insulation, it has a better chance of not freezing. If the water is flowing, it would take a really cold day to freeze it up especially if you were running hot water heat through them. Problem comes if you decide to shut the room, turn off the heat, or lose power...then it could get ugly. PEX is unlikely to split if frozen, though. Fittings, if there, can and will.

I installed an azel controller with min/max settings in my floor
with a relatively tight range, it cycles regularly (embedded sensor in floor) - i would venture that just insulating the PEX with pipe insulation will maintain enough heat to keep it from freezing if exposed to those conditions in that situation -

if you were wondering about heat loss from the PEX, don't - it only transfers heat "well" when attached to something (a heat sink) - so if the crawl space is warm, you should be fine.

You might do well to run an anti-freeze such as Hercules Cryo-Tek to protect your system.

Only run antifreeze if necessary. You need to derate the boiler about 20% verses pure water as it doesn't transfer heat as efficiently.

In New England in an open crawlspace its only one Ice Storm and Power Outage away from freezing.