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I plan on running PEX pipes both hot and cold across 16 feet of unheated attic space above a foyer. The foyer is 16' long and is off the kitchen, it isn't heated directly but is part of the house; it is usually colder than the rest of the house, mostly due to the cat door and slider the dogs go in and out of. There is insulation along the ceiling, underneath where I will be running the pipes.

What is the best way to keep the pipes from freezing? Would an insulation pipe jacket be enough to keep the water in it from freezing or should I also run a long heating cable? I also considered insulating along the roof but it is a tight crawlspace and not much room in there to work. This is in New England so some days it gets pretty cold.

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"What is the best way to keep the pipes from freezing?" - Don't run them through cold space. –  The Evil Greebo Dec 20 '11 at 12:48
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Let me just say this bluntly - some very good friends bought a house with a pipe in the attic. Over Christmas, while they were away, it burst. Their whole house was destroyed. Every wall had to be redone, kitchen, living room where it rained - completely destroyed (incl electronics), all the flooring trashed. They were in a hotel for months. DO NOT run water lines OUT SIDE. EVER. –  The Evil Greebo Dec 20 '11 at 12:50
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@Evil: I have seen some pretty nasty scenes from burst pipes in walls etc. About 5 yrs ago, I went into a place that had 4 inches of pure ice on the floor of the entire first floor after the furnace quit. Yep it was a mess. Took two days just to melt the ice with Salimanders before my plumber could even start looking for the splits. Found like a dozen split copper pipes, heat and water supply. Gotta love living in the north country! –  shirlock homes Dec 20 '11 at 16:02
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In climates with freezing weather, don't run pipes through attic space. Some dum bum SoCal transplant built a house that friends of mine bought. All the plumbing was above the ceiling. We're talking Southern Oregon, West of the Cascades here, not exactly the icehouse of the nation. Needless to say, the water damage after one 20 degree cold snap would have been hilarious in some Laurel & Hardy production, or a Charlie Chaplin movie. You don't want to face it in real life. Apparently the previous owner got by heating the house and leaving the attic access ladder down during cold snaps. –  Fiasco Labs Jan 23 '13 at 21:41
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The Evil Greebo has a point, but one benefit of PEX is that it's designed to not burst when frozen. You still don't want it to freeze up, for sure, but it shouldn't catastrophically fail like copper if it does. –  DA01 Jan 23 '13 at 22:09

5 Answers 5

You are definitely running a risk of freezing these lines in an unheated space. It is never wise to run water lines in an outside wall or above an insulated ceiling in an unheated space.

Perhaps you can do one of the following:

  1. Run the lines under the floor in the basement (not unheated crawl space) where freezing will be much less likely.

  2. Run the lines under the insulation in your ceiling next to the heated drywall. If you have to drill holes in ceiling joists, be sure they are at least 2 inches from ceiling to prevent accidental puncturing from drywall screws. Apply the insulation over the lines.

  3. Thermostatically controlled heat tape can work, but be absolutely sure it is installed properly and if possible use a heat tape that you can use pipe insulation over. Not all heat tapes allow use of pipe insulation due to overheating of the PEX.

  4. If you do have to run in this area, be sure there are no drafts that can come to bear on the tubing. Freezing is always faster if a cold draft blows on a water line.

I wish I had a foolproof method for you to use. Just be cautious, monitor the conditions and have a water shut off handy. Although PEX will handle a lot of freezing without bursting, a leak is going to be a very expensive fix and a mess to clean up. Good Luck

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+1. Option #2 is what we do for sprinkler lines in the ceiling of the top floor condos. The fiberglass insulation simply lies on top of the pipes like a tent, hopefully trapping enough heat to keep them from freezing. –  BMitch Dec 21 '11 at 0:25

Pex in the attic simply needs to be run BELOW the insulation. Put it against the ceiling drywall, and it will never get particularly cold.

The problem is, lots of installers don't do this. My contractor actually went to some trouble to hang the pex up high. I had to go through and undo all the clamps and put it down below the insulation, but it wasn't too big a deal. Get it LOW and make sure there's more insulation above it than below it, and it will be fine in all but the most ridiculous of climates.

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I have PEX in my attic and it was not properly secured to the rafters. This has caused some sections to rise above the blown in insulation. IT WILL FREEZE, mine freezes everyday if I don't use the fixtures in each bathroom at least sometime between dusk and dawn for a good while.

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Additional information as this winter tests out badly routed plumbing. PEX that gets water frozen inside it stretches and expands over time.

The weak point is fittings. Depending on the fitting, it can crack, the crimp rings can be stretched leading to a leak from the fitting or the PEX slipping off under pressure. They don't use the type fittings where they expand the PEX to go over a plastic fitting around here so I don't know how those hold up.

Keep the fittings out of the freeze zone, and summer cabin owners recommend having a gravity drain so the pipe doesn't grow or else charge the system with RV antifreeze.

My former neighbor had a blowout in the soffit in his new house built quite recently. It was a couple weeks ago where we had a snap down into the teens, a fitting let go. As a plumbing system, PEX is a lot more forgiving, but not invulnerable.

If you do freeze up, PEX is an insulator, it will take longer after heating the containing area for water in it to melt. Be careful with any sort of direct heat application (should I have to say no torches?).

Why water lines are being run through a soffit area, only the builder knows. Cheapness, ignorance, either unforgivable.

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If you are using PEX, you can run it through your attic. PEX will NOT burst unless you live in a place that gets like 50 below. And it does not freeze as readily as metal pipe. We used PEX once and ended up replumbing our entire house with it. You can put foam insulation tubing on it to make you feel more secure, but you do need to give it some slack which causes it to snake. Still, the foam tubing insulation works great, but under our house - which is open - our PEX has been fine.

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Frozen pipes are a problem even if they don't burst. Insulation might slow down the process, but if there's no water moving, they will still eventually freeze. –  Steven Jan 24 '13 at 3:00

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