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I have an unconditioned garage with a few water lines near the exterior wall and above a garage door.

I live where the temp could drop to 0 one or two times a year, but in the winter is mostly about 20 degrees for 1 month. ( It doesn't freeze in there, but it is cold)

ceiling

My current plan is to put R15 faced insulation against the floor for ease of install and then another R30 batt - then put the drywall back up.

My questions are this:

  1. Would foam be a better insulator in this situation?
  2. Would an expensive garage heater be a safer option than insulation?
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    Is the water in those pipes used often? Even the best insulation only reduces heat lost, does not stop it. 50 degree water just sitting in pipes will lose heat and freeze. Do you need water in those pipes in the winter? Heating the garage only decent idea if the pipes are in the heated area(not surrounded by insulation on a cold wall).
    – crip659
    Sep 29 '21 at 15:06
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    Heating the garage would be a great idea, especially if you do things in the garage (change oil, woodworking, random projects). I picked up a natural gas heater and will be getting it installed shortly for doing all of the above things. I'll probably set it to maintain ~60° in the winter. Just warm enough to work in a sweat shirt. That supply of heat would definitely keep things from freezing.
    – FreeMan
    Sep 29 '21 at 15:24
  • @crip659 Yes, water is for a bathroom everyday (except if I want to go on vacation, for example) Yes, water in pipes is used 365 days/yr.
    – Marinaio
    Sep 29 '21 at 17:21
  • Those pipes aren't "near an exterior wall", from my perspective. Either your garage freezes or it doesn't. How you prevent that is up to you. This question is either a matter of prerogative or too broad.
    – isherwood
    Sep 29 '21 at 19:06
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    Keep in mind what you're trying to insulate against. Sometimes insulation makes the problem worse. What's the heat source? Where is heat lost? Also, I'm not sure what your statement about insulation against the floor means. What floor?
    – isherwood
    Sep 29 '21 at 19:07
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If the garage never freezes and the space above these pipes is heated then it "should" be okay -- the biggest risk may be that there can be air leaks around the overhead door, so the local area there above the door could get colder than what you've measured elsewhere in the garage.

Remember that insulation slows heat transfer -- both heating and cooling. To make a little more heat available for keeping those pipes warm you could omit insulation above them. Heat from the floor above would then more easily come down to the pipes.

If there is significant air leaking around the overhead door a gasket or weather strip could be a big help.

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