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Discussion Starter #1
I have a an half-insulated rim joist (?) that the previous owner did a poor job on.

What do I need to do here? Just stuff some more R12 up in that hole and fill it?

 

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I would be hesitant putting any insulation in there with that plumbing in that space.

Unless that is a hose bib that you can drain prior to winter, putting insulation in that cavity will make that piece of plumbing much, much colder. You could get freezing where you didn't previously have it (depending on the area and nature of that supply piping).
 

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Discussion Starter #3
How would insulation make the pipe colder?

The pipe is part of my hot water radiation system btw.
 

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Even though it is a hot water pipe, you still would want to leave some of the area around the pipe with less insulation. Heat gets turned off and you get a massive cold spell and oops. I realize it may be unlikely, that's why you get to make the decisions, you are there :).

As for that pesky floor joist, it is close to the outside wall, thus if it disappeared no one would notice. But rather than removing it (I know I'll get yelled at) I wouldn't hesitate to turn it into a 2x6, or at least some of it too gain access. It somewhat depends upon what is above, hot tub or empty wall space. I have a similar pesky joist which I'm closing in on and it will meet with the jig saw at some point.

If 2x6 is too drastic, then 2x8 with a sister. (Why are they called sisters?)

Bud
 

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The insulation (should you spray foam it or stuff batting in there) will dramatically reduce the warm room side air that can reach the pipe and they exterior wall.

By doing so, the cavity and pipe will become much colder.

If you have enough room and can get your hands in there (doesn't appear as such) you could stuff some insulation behind the pipe and in between the rim. Leave the pipe exposed to the interior air in this case.

Otherwise, my recommendation, leave it be.
 

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Generally, in freezing climates there should be no insulation between a water pipe within an exterior wall and the interior wall surface.

Ideally there should be a more or less (empty) triangular space with no insulation around the pipe, as wide at the interior wall as the pipe is deep behind. For a pipe that went straight through the rim joist and exterior wall to a hose bibb, the empty triangular or conical or sideways pyramid space would go all the way to the rim joist. Batt insulation would be run behind pipes in the wall with a triangular wedge section cut out for the pipe if desired to make the batt fit in the space better. Fill insulation should not be used around pipes in the wall or joists.
 
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