Insulation during repipe slab to attic

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Nathans

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Hello friends,
I am repiping a Southern California house after several slab leaks. We are going with copper and re-routing everything through the attic.

I have read several threads here about preventing frozen pipes by removing all insulation from under the piping and instead insulating on top, in order to keep the pipes as close to the envelope of the house as possible. Obviously, I am not worried about freezing here in Southern California, but I am interested in avoiding the heat trapped in my unconditioned attic space. The temp up there in the hot summers can easily reach 130+ and it just feels awful when that hot water comes out of the cold tap.

I am curious if the above method of insulating would help in my situation as well? I am a bit worried about condensation as well, so I'd also like to hear from anyone with opinions on whether I should wrap the pipes in foam insulation tubes as well when run beneath the batts of insulation.

Thanks all.
 

Breplum

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Definitely you should insulate the pipes with the thickest insulation you can afford and have them under the insulation if you can.
Ignore the part about removing the conditioned-side insulation, unless you have, perhaps, an insulated roof assembly.
 

wwhitney

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I have a slightly different take.

Cold water pipes don't need insulating from the interior conditioned space (unless condensation is an issue, in which case they need a vapor tight insulation). They can benefit from insulating from the outdoors, to avoid freezing or extreme heat. So keep them on the interior side of the thermal envelope whenever possible.

I take it this attic is getting insulation at the ceiling joists? Then thermally you'd be best off laying the pipes in the middle of the joists bay right on top of the drywall, with all the insulation on top of it. But I imagine that's not a good idea plumbing wise. You could secure the pipe 1.5" - 2" above the drywall, either with a wrap of plumber's tape around it suspending it between joists, or a 2x block across the joist bay against the back of the drywall, with the pipe on top. Keeping it away from the ceiling joist itself avoids the thermal bridge presented by the joist.

Then if you're insulating with loose fill, don't worry about the 1" to 2" of insulation underneath, assuming you're using 6"+ of insulation. For example if there's 1" of insulation underneath and 9" of insulation on top, then the pipe temperature should end up about 10% of the way between the ceiling temperature and the attic temperature.

If you're insulating with batts, then I think it's reasonable to put the entire batt on top of the pipe, rather than to split it. Just don't compress the batt, let it puff up higher than its neighbors. [Unless you're running a second layer of batts cross wise above the first layer, then a little compression isn't going to hurt much.]

Cheers, Wayne
 

Nathans

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Thank you both. It's a flat run parallel to the joists-- I think the suggestion to use a 2x block to support the pipes and leave the bay mostly empty underneath makes sense.

I did take some crude temperature readings over the course of a week in September, with 3 sensors: 1-house, 2-attic-below-insulation, and 3-attic-above-insulation. It was impressive how effective the insulation is at delaying the transfer of heat.

The attic-above would top out at 105° at which point the attic-below was only 78°, just a bit above the ambient temp of the house interior of 76.7°.

The true test will be this summer I guess, but I expect the temperature differential to scale in a similar fashion.

I still plan to insulate the hot pipes (to retain energy), but am not sure if necessary for the cold lines? The only reason would be to avoid condensation which may be less of a concern in Southern Ca dry air, in a conditioned space.

The other concern with insulating the pipes directly is the chance of obscuring the location of any future leaks down the road. With insulation in place on the pipe, the water could travel a fair bit away from the source. Or, maybe not given water always finds a way out fairly quickly.

Thanks again breplum & wwhitney.
 

wwhitney

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I don't see any reason to insulate your cold pipes from the interior space in your location. As discussed, it makes sense to insulate them from the hot attic if you don't want inadvertently heated water.

As to the hot pipes, if they are going to be well in the middle of say 10" of cavity insulation, I don't see any upside to wrapping them in pipe insulation. I'm not aware that the pipe insulation R-value per inch is particular better than cavity insulation. If they are going to be in an uninsulated area, or would otherwise be up against a poor insulator like wood framing or drywall, then pipe insulation as separator seems beneficial.

In you're using batts, it would be somewhat annoying to have the hot pipes in the middle of the cavity insulation, you'd have to split the batts (or just plan on stack two layers in the first place, at least in those bays.) So then it might be easier to use pipe insulation and put them next to the cold water pipes. But if you're using loose fill, you could put the pipe up higher in the bay and just blow insulation all around it.

Cheers, Wayne
 

Terry

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If you're worried about high temps in the attic, then the blanket approach that we use for cold temperatures should work too.

index.php


Just consider that cold means hot in San Diego.
 

Jeff H Young

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I like to insulate hot and cold in attic What I call overhead water is noisy or can be and copper is noisier than Galvanized so its win win insulate for temp. and for noise reduction sometimes certain areas are almost or are impossible to get your body into and strap so the insulation helps . Repiping is tough work and its hard to get everything perfect So I really like to insulate I doubt youll ever say darn I shouldnt have bothered
 
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