Plumbing in Knee Wall

[Gary in NJ]

Long-time lurker, first-time poster.

I'm finishing the upstairs of my cape-style home. It will be about 1,700 sq-ft when finished. I have all of my heat registers sweated with PEX fittings and they are installed on the walls. I'm planning on running the Ox Barrier PEX tubing inside of the side attic knee walls. I'm concerned about:

1. Freezing
2. Lowering the water temperature. This is a 225' zone.

What is the best way to insulate & protect the tubing? I could run the PEX along the top of the floor joists (2x10) which are fully insulated and then place R-19 batts above the PEX. I've also been thinking about running PVC conduit along the wall/joist intersection and sealing the conduit with expanding foam.

Is there an accepted practice in this type of installation?

 

[DaveHo]

Is freezing really a concern? I mean, if it's cold enough to freeze, you'll be using the heat anyway and hot water won't freeze.

-Dave

 

[Sponsor]

 

[Gary in NJ]

I would assume there will be times (such as when the heat is reduced from 68 to 62 when going to bed) when there wont be any water flow in the loop, maybe an hour or so. Can the water go to freezing from 180 in an hour if the attic space is a zero? I doubt it.

I'm also concerned about the temperature drop from the first baseboard to the last. This loop has 50' of baseboard and 180' of PEX.

What I'm thinking of doing is building an insulated box behind the knee wall for the PEX travel from baseboard to baseboard. If this is overkill, I have better things to do with my time. I don't want to go fixing problems where they don't exist.

 

[Terry]

Don't put any insulation around the pipes.
Remove all insulation below the pipe, and lay fiberglass batts over them.
Between the ceiling joists on either side, lay two more batts, keeping space open below the pipes.
The heat from the home must be able to get to the pipes, and then you trap it.
A pipe in an uninsulated attic will freeze otherwise.

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Attached Images

 

[Gary in NJ]

Thanks Terry. That was the information I was looking for. A much easier solution to implement then what I was about to undertake.

 

[everest923]

Is freezing really a concern? I mean, if it's cold enough to freeze, you'll be using the heat anyway and hot water won't freeze.

-Dave

On this same subject; I have a cape cod and have the second floor finished and it includes a bathroom. The PEX pipes for the bathroom supply lines keep freezing. I have installed heat tape and added the round foam pipe insulation on the ends of the pipes where the heat tape stops and they are still freezing. Is it possible that having the entire length of the eaves and soffits ventilated is allowing too much air to enter the knee wall area?

Of course the bathroom was installed in the northwest corner of the house (where the wind comes from) and there was never any wind wrap installed between the siding and sheathing. The pipes to the sink are run right up to the outside wall. These are the pipes that typically freeze. HELP!

Thanks,
Everest923

 

[Gary in NJ]

I love it when an old thread of mine gets revived. Over eight years later, I can tell you that Terry's solution has worked 100%. I took it another step further. I drilled all of the joists about 1 inch from the drywall and then capped my 16"x 10" "tunnel with R-28 above the joists. On the runs along a joist I simply strapped the PEX to a 2x4 layed flat and covered that tunnel with R-28. I've never had a freezing problem, even when the temperatures dropped below 0F. Those 16x10" tunnels stay quite warm, dare I say hot.

There are two places where the PEX runs above an unheated space (above my front porch and in my garage) for about 8-12' each run and I still don't have an issue. For the porch I was able run the pipe along a vertical knee wall and I insulated a 12x12" boxed tunnel with R-28 on the 2 exposed sides. I did the same for the garage run, but there is NO heat source in the garage. Luckily my garage is insulated and even when it drops below 0F my garage will stay just above freezing.

 

[everest923]

I love it when an old thread of mine gets revived. Over eight years later, I can tell you that Terry's solution has worked 100%. I took it another step further. I drilled all of the joists about 1 inch from the drywall and then capped my 16"x 10" "tunnel with R-28 above the joists. On the runs along a joist I simply strapped the PEX to a 2x4 layed flat and covered that tunnel with R-28. I've never had a freezing problem, even when the temperatures dropped below 0F. Those 16x10" tunnels stay quite warm, dare I say hot.

There are two places where the PEX runs above an unheated space (above my front porch and in my garage) for about 8-12' each run and I still don't have an issue. For the porch I was able run the pipe along a vertical knee wall and I insulated a 12x12" boxed tunnel with R-28 on the 2 exposed sides. I did the same for the garage run, but there is NO heat source in the garage. Luckily my garage is insulated and even when it drops below 0F my garage will stay just above freezing.

Good to hear it worked out for you!
I've never had my heat pipes freeze as Terry mentioned in his reply, the water stays hot enough to keep them from freezing. It's the water supply lines to my bathroom fixtures that I have a problem with. The contractor ran them along the rafters and my flooring runs all through my knee wall (crawl space) so burying them in the floor joists isn't an option.

 

[Gary in NJ]

Having your supply pipes running in the knee walls and crawl space is no different than running them along an outside wall. Do the pipes run through the heated space of the first floor? I sounds like you are going to have to turn the pipe to finish its run in the heated 2nd floor space. No a big deal for a toilet or sink, perhaps unsightly for a shower. You may need to pull up the floor (or cut into the ceiling of the 1st floor) for any long runs.