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Is it possible to insulate piping in a vented attic by “bringing it inside the envelope”?

DIYJester | Posted in General Questions on

Hello I have a home that was built by a bunch of meth-heads and their friends apparently. I’m in Zone 4, near STL.

When they ran the PEX from the manifold in the utility room, they just laid all of the tubing on the top of the roof trusses and sprayed insulation over them. For one section that goes to the ice-maker they put a single layer of the foam insulation over this.

Every year since I’ve moved in all of my water lines except my kitchen sink will freeze if there are sustained temps below about 28 F.

I’m looking for ways to bring this piping “into” the homes boundary without having to remove it all and find ways to re-route in in interior walls. I would first remove the old insulation covering the pipng and beneath it to expose it to the back of the drywall to allow it to only be insulated by air and the 5/8″ gypsum below it. I thought about using foamboard tray for these to run in. I would cut it long ways about 6″ tall and make and 1.5″x 4″ deep notch ever 24 inches to slide over the rafters, one for both i sides of the run. I would then seal all the gaps between drywall and trusses with spray foam. Next I would lay a few inch wide piece of foam over this assembly and seal it as well. I would then cover this all with 8″ or more of loose cellulose. The current attic only has 8″ over non-cathedral areas, and looks to only be about truss deep over the cathedral portions. I planned on blowing more in till it was almost 16-18″ deep myself.

In all of this I was thinking up adding ventilation channels as they only let air flow through every third or fourth rafter bay. I have never notice ice or moisture damage in the attic, so I’m not sure it’s worth the effort to add the foam baffles to every bay. To me this would insulate the top plates better, and allow me to fill the fluffy stuff in deeper.

Thanks in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    The abbreviation STL had me stumped. Once again, I was sent out into the wilderness to do abbreviation research. I'm guessing "St. Louis," but I'm not sure.

    As you know, plumbing pipes don't belong in an attic. If it is impeccably executed, your plan might work. But you have to get rid of every scrap of insulation that now exists between the pipes and the drywall ceiling, and you have to do a perfect job of air sealing your rigid foam box. Will it work? I'm not sure -- it depends on how conscientious and careful you are.

    Would I attempt it for a client? Probably not. I would re-route the pipes indoors.

  2. Dana1 | | #2

    STL is the airline code for St. Louis MO airport, so yes that's right. Would LAX be as confusing? :-)

    With enough insulation over the tops you woudn't even need to remove the insulation below to keep the pipes from freezing. How much insulation is there between the PEX and gypsum, and how much space is there to work with above the PEX?

  3. GBA Editor
    Martin Holladay | | #3

    Lovers of abbreviations and acronyms are going to have to look elsewhere for support. I'm not backing the trend. I will continue to champion clarity rather than saving a few keystrokes -- a practice mostly promoted by people who type with their thumbs.

  4. DIYJester | | #4

    Dana, where they are currently routed, I could probably get a 12" or so packed in once I install proper soffit venting to help hold the insulation. The majority of the run is perpendicular to the trusses as you can see from the pictures. The trusses are 2x4 pre-engineered, so I can't drill them either. The cavity beneath them would be just over 3.5" counting the OD of the piping.

    I understand this would not be the best fix. There are two main things preventing me from just pulling to all into interior walls.

    1. I would need to remove drywall, fix, run lines etc. about 75' across the house from the current MANABLOC (brand, not abbreviation) cross linked polyethylene (PEX) manifold to the new MANABLOC in the additions basement. All of the addition piping is in interior walls since I build this portion. This is definitely doable, just a pain, but it would be worth not freezing the lines, and if I ever sell the house I can know I didn't do as the guy before me and lie.

    2. The design of the original home makes running these lines in all interior walls not possible without building a soffit in our entryway. See the pictures below. I have to bridge an area about 7 feet wide where there are no interior walls. The opposite side of the house as you can see is all windows and headers, and would still require me to cross that attic again to get back to the manifold.

    The pictures might help. The red rectangle is where there would have to be a soffit build, and the red line sort of points to the direction of the run. The MANABLOC sits behind the wall where the arrow ends in a utility room.

    The mess: that's what happens when you take on all new windows, 4" exterior foam, 4" Roxul sub-grade 4' on existing structure, all while adding on 1150 sq. ft including moving all of the mechanical to a new basement alone, all while traveling for work 60% of the time. I was hoping to get it done in a year, but April of 2017 is the deadline and its not going to happen.

    Martin, sorry for the abbreviation. I am not a thumb typist, I just happen to come from, and still work in industries where we use them daily as a norm, military, chemical, power generation, and nuclear. Granted we don't speak most of them. How about this one RAGAGEP, a very common one used in the chemical world, Recognized and Generally Acceptable Good Engineering Practices. We usually speak this abbreviation.

  5. GBA Editor
    Martin Holladay | | #5

    St. Louis has a more mild climate than northern Vermont, where I live and work. I am very conservative when it comes to frozen pipes. It only takes a very small air leak when the outdoor temperature is -25F to cause a frozen pipe. In St. Louis, your attic is likely to be much warmer than mine.

    Dana said your approach will work. He's probably right. Good luck.

  6. Expert Member
    Dana Dorsett | | #6

    With a foot of (air-retardent) cellulose on top and 3.5" of fluff below the pipes will be less than 25% of the way into the insulation.

    Even if the attic temperature dropped to -20F for a full day (hasn't happened since the last ice age) and the indoor temp was only 60F (an 80F difference) the temp at the pipes would less than 1/4 of the difference below 60F- it would be above 40F,and well above freezing.

    Air seal the ceiling, blow a foot of cellulose over it and you'll be done. If the rest of the house attic has less than a foot of insulation, blowing enough cellulose over all of it to bring it to at least 12-15" total makes comfort & economic sense.

  7. DIYJester | | #7

    So what I'm hearing is make sure the PEX tight to the top of the trusses and just fill it up, not worrying about indoor heat helping. I like that answer as I planned on trying to get R60 up there when I'm done. Right now even the deepest areas aren't 9" deep based on a 2x4 and 2x8 stacked for the attic entrance.

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