If your attic is insulated on the outer walls so that its conditioned space, the PEX pipes should be fine.

My attic is conditioned and has foam insulation on the gable walls and roof deck, and the temp difference between the attic and the house is only a few degrees. My lowest outside temp is about 15 degrees Fahrenheit, but even then the attic would never get close to freezing.

PEX is also less likely to have freeze damage than copper pipe, since it can expand and contract (to a certain degree) without breaking the pipe.

Bruce

If the Pex pipes freeze, will they survive? I've been told by my plumber that they won't break.

"We live in an area that can get below 0 Fahrenheit occasionally." (OP)

AND

"My lowest outside temp is about 15 degrees Fahrenheit, but even then the attic would never get close to freezing."

And?

We put a digital temperature gage in the space and it got down to the low 40s. This was when the outside temperature was in the 20s. It hasn’t gotten colder than the 20s yet this winter so we haven’t been able to test the area during a really cold period.. I’ve heard pex is less likely to burst but that it can weaken and leak at connection points.

Your attic is insulated but NOT conditioned. It will eventually attain the outside temperature. If the floor in the picture is above a heated space, then some heat will make it through to the floor/ceiling . Put some foam insulation arounds all the pipes just to be safe but PEX has some expansion capacity so you should be fine.

PEX got a bad rap when it was first introduce because of leakage at connectors but that was fixed long ago.

When living in houses with pipes in the attic, we dripped the pipes when the temps were below 28 degrees. That's dripped, not run, the farthest faucet from where the water enters the house. Since the lows are typically at night we also would flush a toilet in the middle of the night.

Regarding PEX leakage at connectors, my builder ran each PEX line from a manifold in the basement to the fixture, with no connectors.

When it gets cold outside, mice start looking for ways in doors.

So, PEX main enemy is rodents outside of connectors failing. Usually repairs to rodent chews are done using connectors... either that you have to replace the whole run.

Copper doesn't suffer from this fatal problem. Mice can get into any structure the opening no more than the size of a quarter in circumference, maybe smaller.

Why you dirty rat? yeah my sentiments exactly.

rodents love PEX.

If you require added heat, a space heater is probably overkill (and you wouldn't want to run it unattended).

Depending on the size of the space and the required heat, a simple incandescent bulb might be enough to keep the space above freezing.

You only need a minimal amount of heat (if you need it at all).

Is the bathroom on the other side of that wall? I suggest covering the area with a fiberglass batt insulation stapled to the studs. It would be easy to add a few additional studs to 16 inches on center to make it easier to install the insulation. The idea is to trap the heat coming through the bathroom wall. It is the same idea as a water piper installed in an exterior wall.

I second mike_home’s suggestion. You need to provide an insulated barrier between the attic space and the pipes that will utilize heat from the conditioned space. Simply putting insulation around the pipes in the attic is of minimal help since such insulation doesn’t provide warmth...it only increases the time before freezing occurs.

How long ago was this reno? Can you ask the previous owners if they've ever had an issue or if they did anything to mitigate the problem?

You may be worrying about something that isn't a problem.

I can only report my experience with my first house years ago. I never had a problem with the pipes freezing in the attic...until the year they did freeze during a cold snap. The same with my present house: no problems for the first 10 years until a pipe in the kitchen wall froze.

I would go ahead and put that foam pipe stuff on the hot water line anyway. I'd be more concerned about the copper fitings than the PEX.

If the attic exterior walls and rafter bays are insulated and the walls and floors that the attic shares with living spaces is not insulated, the attic is considered to be "conditioned" which is not the same as "heated" and is unlikely to reach the temperature of the outside air. If it gets really cold outside you might change the attic insulation to foam. You could also hang batt or rigid insulation over the attic side of the pipes or create a double wall with insulation on the attic side.

Preformed foam insulation by itself will work but there is a risk if you don't use the water for an extended period of time.